Author Topic: Jon boat with a Tolman Transom: jallii skiff  (Read 1690 times)

Offline KenB

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Jon boat with a Tolman Transom: jallii skiff
« on: November 10, 2021, 07:13:01 AM »
Totally hypothetical questions here...

If someone had a 9.9 hp toad motor, or even a new 4s 9.9... and wanted to make a scaled down tolman standard to fit that motor, what would the dimensions be?
- 12'? 14'?   If an 18' standard has a 25' transom and a jon boat has a 20 inch transom, then the jon boat is 14.4 feet.  14 feet feels long for a flat bottom skiff with a 9.9, but the 8° vee should make the jon tolman a little more efficient:

https://www.youtube.com/watch?v=iVumR3M1iN8

- when I think of those 14 aluminum jon boats, the transom/bottom corner is usually flat, and the bow pops way up before getting on plane...
- using jumbo (22' long, 6' at bottom), widebody (20, 5.5, and standard  (18', 5') numbers plus some math ('show your work' below), results in a bottom around 4.5', but wider is usually better so even going shorter than 14, probably 4.5 is about right.

(Kind of a fun aside... to curve fit I just wiggled the curve around until I thought it fit; mathematically, this process is by definition 'AI,' where machine learning, deep learning, etc are just different tricks for getting the line in the right place. Theoretically, AI is just a statistical method to approximate or curve fit. I did mine in 2 dimensions, while AI can add many more. That said, at least for right now I'm still better at picking out the 3 pictures of a stoplight.)

Other hypothetical topics:
- laminated 1/2 ply transom, but no 2x8
- shorter sides to save weight...
- ... but a spray rail to keep it a tolman. 
- 3/4 ply stringers to make the side seat/box things that jon boats have? Jon boat owners, are these side seats better than the ones that run abreast?
- no shelves, instead 1x3 rubrail, from maybe ipe decking. On the gulls and skiffs I have done, when wood rubrails are PL'd to the sides, they becomes kind of structural.
- no floatation at bow, but maybe a cubby. That 'bow stem to the top of the sides' triangle piece is probably needed.
- 1/2 ply for bottom? poking through other stitch and glue designs, perhaps 3/8 is enough, since both sides get glass. I think 1/4 is too thin for bottom, but maybe ok for sides. But any votes for a 1/2 ply bottom?
- Most controversially, again hypothetically, because of no shelves, maybe one laminated frame amidship, in order to spanish windlass the sides to the transom. The way I have done work skiffs and gulls is to attach the sides to the bow stem, let that cure, then bend the sides around a frame placed amidship, and finally attach sides to transom. On gulls, you can do one side at a time because of the strongback/frame, but with work skiffs you can do both sides at the same time; the spanish windlass! Cut the sides to fit the bottom, then tape all the seams. I use Renn's '2x4 blocks' trick to snug the sides/bottom and sides/transom to fit. On the gulls, the frame is then removed. On work skiffs, I've left the frame in, so I know where to put the bench seat. I actually use thule roof straps instead of a traditional spanish windlass, but anyway I also prefer epoxy and plywood to lapstrakes, too.
« Last Edit: January 09, 2022, 03:01:30 PM by KenB »
best,
KenB

"HOW CHEAPLY CAN A TOLMAN SKIFF BE BUILT AND JUST HOW  MUCH IS SACRIFICED IF COST IS THE PRIME MOVER?"
- Bruce Armstrong   

"I can tell you that either a nice BFT or a big YFT is an absolute riot on a Tolman. The boat is so light it's like the old man and the sea..."
- Dave Nolan (RIP)

Offline KenB

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Re: Tolman Standard Jon Boat? Jon Tolman Skiff?
« Reply #1 on: November 18, 2021, 02:08:46 PM »
Picked up my 9.9 today, $2300. 20" shaft, no tiller. Guess the jon boat is getting a console...

These guys did the side bench seats, too bad its flat bottom.  Could use 8 degrees of deadrise.
https://www.clcboats.com/peeler
- they went with 1/4 ply, then complained about the floor flexing.
best,
KenB

"HOW CHEAPLY CAN A TOLMAN SKIFF BE BUILT AND JUST HOW  MUCH IS SACRIFICED IF COST IS THE PRIME MOVER?"
- Bruce Armstrong   

"I can tell you that either a nice BFT or a big YFT is an absolute riot on a Tolman. The boat is so light it's like the old man and the sea..."
- Dave Nolan (RIP)

Offline Kobuk

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Re: Tolman Standard Jon Boat? Jon Tolman Skiff?
« Reply #2 on: November 18, 2021, 02:43:15 PM »
If you really want to venture down this slough you otter check out the boats guys build for the Yukon 800.  That's some crazy stuff.

Just because a floating object is created with stitch and glue construction doesn't mean it should be associated with the Tolman name though, especially a jon boat. IMO, that's cringe worthy, if not outright sacrilege. :o
22' Widebody
Started: 3/2019
Flipped: 6/2019
Floated: 6/2020

Offline jallii

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Re: Tolman Standard Jon Boat? Jon Tolman Skiff?
« Reply #3 on: November 19, 2021, 02:35:05 AM »
The idea to simply scale plans of a 20 -28 foot boat form to some 12-16 foot size will not work as perhaps you would like /or expect it to. Geometrically scaling linear proportions changes other factors to the power of 2, 3 or 4 depending on witch particular we consider.

If you for example change the length of a 20 foot by 6 feet to 14 feet the change would be as a persentage 6/20 = 30%.

However the corresponding:

Bottom surface area has 2 dimensions and the change vould happen in both dimentions.
30% = 0.3  change in 2 dimensions would mean 0.3 times original length multiplied by 0.3 times original breath. So you would need to factor in 0.3 *0.3 = 0.09 = 9 % change in surface area of the bottom. (One of the components of calculating power requirements) 

The total volume of the boat and the displaced underwater volume of water would change in 3 dimensions. So what the boat weighs and how much weight the boat can carry would also change in 3 dimensions and the corresponding change would happen to the power of 3.
0.3*0.3*0.3 =0.027 = 2.7%

The inertia effect of a certain part from a distance of x to the center of mass would change to the power of 4.

The above is an oversimplification of what happens with the intent to illustrate the problems. With the above I have tried to demonstrate the if you change the dimensions of the plans, then whatever is the end result of this operation the boat would not be the same or behave similarly.

Renns designs are exceptional in allowing some variations in length without big changes in the overall characteristics of the boat as a whole, but there are limits to acceptable changes. Go overboard and you are not happy with the results.

If you want a smaller boat its much better to take as a starting something that originally designed to that particular size.  There are plenty of FREE designs available that can be used with stich and glue ply-epoxy building style.

If you have trouble finding I can post a few to the forum that spesifically made for that.

I suggest you start by trying to define what you want from the smaller boat.
I like to find out and really understand things. A perfectionist, curious mind cannot stop learning, and picks up many things. I don't claim to be an expert. I'm not an engineer or a chemist by training. I make mistakes. If I manage to make something understandable I am happy.

IF you want to build a boat make sure you follow designers instruction. Do your homework and be safe.

Offline KenB

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Re: Jon boat with a Tolman Transom.
« Reply #4 on: November 19, 2021, 07:51:04 AM »
Kobuk - thread name updated. Point taken, but I am not sure I understand the cringe part, can you please explain? At the very least, can you maybe suggest another name?

Jallii - Did you click on my little diagram? I believe the phrase is "a picture is worth 1000 words."   

Fuck it here's my 1000 words... maybe you are just a little dense or something, but my premise is to take the three designs in the book, derive a best fit line as a scaling factor (THE WHOLE POINT IS TO INTEGRATE ALL THE VARIABLES YOU LISTED), and see what the results are. Unsurprisingly, you end up with about the dimensions of the classic aluminum jon boat. So the only part of your reply that is correct, I already did! You are clearly unfamiliar with boat building trends (do you even read wooden boat!?) but micro flats skiffs are a pretty hot design right now, a tangible example of why the rest of your math is wrong. You are ranting about boxes and cubes, but boats are made of curves. I bet that I have downloaded and built more free boats than you have... you get what you pay for. You advice is horrible, and your attitude is toxic! Let's cut to it, how many boats have you designed? How many have you built? I have designed 3 boats, and built over 10. Have you ever spent time talking with Renn about boat design? I have. He didn't write one book, then tell people "go download skiff designs online."  He wrote three books, he designed and built a diesel-based shallow draft tunnel hull. What I took from the conversation but also his writing is that he was constantly tinkering, pushing limits, and exploring common sense ways to address basic boat design in spite of the recreational marine industrial status quo; there is a reason he calls them shelves and not gunnels. Unless you have something nice or helpful to add, maybe contribute to some other thread!  You suggest that I didn't "define what I want from the smaller boat"... in a forum literally called "Free Boat Building Plans." And you are completely wrong about the origins of the skiff... go read the book again. The first chapter. The standard skiff was 16-20 feet, and is based on the classic flat bottom work skiff. The books don't have a 28 foot boat, for exactly the reasons you listed, you clown. Guess what, Im naming this skiff after you. Enjoy the Jallii Skiff!
best,
KenB

"HOW CHEAPLY CAN A TOLMAN SKIFF BE BUILT AND JUST HOW  MUCH IS SACRIFICED IF COST IS THE PRIME MOVER?"
- Bruce Armstrong   

"I can tell you that either a nice BFT or a big YFT is an absolute riot on a Tolman. The boat is so light it's like the old man and the sea..."
- Dave Nolan (RIP)

Offline Kobuk

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Re: Jon boat with a Tolman Transom.
« Reply #5 on: November 19, 2021, 10:33:00 AM »
Kobuk - thread name updated. Point taken, but I am not sure I understand the cringe part, can you please explain? At the very least, can you maybe suggest another name?
Sorry, I didn't mean to rub your fur backwards. Maybe I was just being a little AR.

What would Renn say about this?:

I can only imagine...!
« Last Edit: November 19, 2021, 10:48:54 AM by Kobuk »
22' Widebody
Started: 3/2019
Flipped: 6/2019
Floated: 6/2020

Offline Kobuk

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Re: Tolman Standard Jon Boat? Jon Tolman Skiff?
« Reply #6 on: November 19, 2021, 10:37:05 AM »
Go overboard and you are not happy with the results.
I'm going to borrow this quote and make a little placard for my skiff...
22' Widebody
Started: 3/2019
Flipped: 6/2019
Floated: 6/2020

Offline jallii

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Re: Jon boat with a Tolman Transom.
« Reply #7 on: November 19, 2021, 07:08:20 PM »
Totally hypothetical questions here...

If someone had a 9.9 hp toad motor, or even a new 4s 9.9... and wanted to make a scaled down tolman standard to fit that motor, what would the dimensions be?
- 12'? 14'?   If an 18' standard has a 25' transom and a jon boat has a 20 inch transom, then the jon boat is 14.4 feet.  14 feet feels long for a flat bottom skiff with a 9.9, but the 8° vee should make the jon tolman a little more efficient:



Totally unhypothetical answer here...
Starting the design process by fixing the engine power and taking a boatmodel thats designed to be in a totally different size category and scaling it down untill the boat is small enough to be handled by your chosen engine is not a good strategy. A much better strategy is to first define needs (2, 3 or 4 persons... etc) and take as a starting point a boat that resembles the one you want and then modify the design to fit your preferences. If the boat model is designed by a naval architect or an already existing professional build you can get the power requirements etc from the starting point. After your modifications you only need to estimate how much your modifications change the requirements from the starting point. This way you end up pretty close to what you want to. By large scaling you probably end up with something unexpected.

Hope this clarifies.

And YES there are boatdesigns available for free by professional boatdesigners and naval architecs. The design process for a boat includes necessary calculations that estimate desired characteristics. Without this info its not a design its a drawing of a boat.

Both William Atkin and W.D.Jackson were both experienced designers and trained naval architects. You will find many possible starting points on these sites.
http://www.atkinboatplans.com
https://www.diy-wood-boat.com
http://www.svensons.com/boat/

There are free versions available for these designs by professionals because they were originally published in magazines for DIY builders.

For example: 
You were looking for proper length for a jonboat type boat for a 9 hp engine... Here is one.
http://www.atkinboatplans.com/Dinks/Chatterbox.html 

It was actually published many times. There is an article by Mike O'Brien in Boat Design Quartely
The above Atkins site shows you the article content and has a reference to the website of the article publisher: http://www.boatdesignquarterly.com/  It was also published long time ago in MoToR BoatinG-magazine. One of the DIY magazines of the past. Its free if you can get a hold of one of the articles. The above sites have many of those published articles in PDF form.  So you get the drawings and the DIY build instruction with all the necessary information for building the boat in these articles. If you need more you can buy the plans.

Note also: In a proper design you get dimensions, max load, power requirements, expected behaviour and speed with the engine. Buildin material thickness... So its a proper design by a wellknown designer. William D Jackson also built every design he published. His designs are actually quite modern.

Even if the boat structure changes from plank on frame to plywood epoxy, the boat behaves the same if the total weight is about the same.  Most of these original designs have strenghtening frames etc... and very thin plywood as shell. If building from plywood-epoxy with glass on both sides and glued seams you can reduce the framing mostly or in some cases totally. Transverse framing can usually be taken out by compensating with thicker ply and epoxy filled glass on both insode and outside. Avoin reducing longitudinal support structure. Just use a bit thicker plywood and build the same way as you would build a Tollman boat. If you use Renns specs for a standard for material thicknesses, etc. then every boat thats smaller than it should be fine with specs from a standard. To be sure that you have strong enough scantlings, refer to  Dave Gerrs book. Use scantlings for plywood epoxy construction instead of plank on frame construction. This way you modernize the old design. The boat should still behave the way the original did, but you have your wood protected. If you decide to build exactly as old plans indicate, I suggest you still make sure that every wooden part is covered from all sides with glas and epoxy.

So from this boat and some others I come to the conclusion that the rigth answer to the boat length for a 9 hp outboard is about 10 to 12 feet with max 2-3 persons to get a decent speed. Some 14 foot boat with 4 persons migth still work, byt there is a risk of not having big enough engine. Small dimentional differences or the shape of windows etc dont matter. Big weigth differences do matter. A flat bottom has the smallest power requirement. Power demand increases as the V bottom deepens.

NO I do not call mere drawings of boats (even proper line drawings by people that have no clue about the principles that govern the process) as designs. They are mere drawings. Drawings can be good or bad. Bad is more probable by unknown "designers" unless its a drawing of a traditional design.

You are rigth keeping the L/B ratio steady you keep some of the characteristics stable, but in defining the needed power, the weigth of the fully loaded boat (displacement) is more important than LENGTH / BEAM ratio that you have tried to extrapolate. A higher L/B ratio would give a faster boat with same weigth. A beamier boat is slower and power hungry. Deadrise angle has a big effect on power requirements. But it all boiles down to how many HP you need to move your boat. Some think 25-40 LBS/HP is a good general estimate.

The point I tried to explain was that scaling a boat design distorts many things in numerous ways. Most often in ways most people cannot even imagine. Still some things stay the same and even that is a problem. You can Shrink the boat, but you cannot shrink yourself. Same weigth in a different sized boat changes everything. Therefore a different strategy is better.

I am sorry if I offended you somehow. That was not my intention. And I see no positive outcome in a discussion comparing the size of my third foot to yours. For the most part I dont understand you 1000 words, but if I have offended you, I apologize for that.
« Last Edit: November 20, 2021, 07:37:07 AM by jallii »
I like to find out and really understand things. A perfectionist, curious mind cannot stop learning, and picks up many things. I don't claim to be an expert. I'm not an engineer or a chemist by training. I make mistakes. If I manage to make something understandable I am happy.

IF you want to build a boat make sure you follow designers instruction. Do your homework and be safe.

Offline KenB

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Re: Jon boat with a Tolman Transom.
« Reply #8 on: January 09, 2022, 02:34:59 PM »
I checked out every boat building book at the woods hole library, stopped by the woods hole small boat museum a few times, and googled a bunch; I looked at every single boat option for a 9.9 20" outboard, but as I wrote in the first post in this thread, there is simply no design for a 14’-15' skiff that integrates the innovations of the Tolman design.

There are 3 types of hulls: flat bottom jon boats, micro-flats skiffs, and fisherman/work skiffs.

The flat bottom boats pound with any kind of waves, and micro-skiffs are surf boards, not exactly stable.
https://www.sportfishingmag.com/fishing-boats-super-light-microskiffs/
Bateau looks like they are all in on micro skiffs with their Extreme Flats series: https://www.boatbuildercentral.com

So that leaves work/fishermen skiffs. Here's the short list of boats I reviewed, closely.
Payson Diablo: https://www.instantboats.com/product/gypsy-15-x-4-4-copy/
Devlin Candlefish: https://devlinboat.com/product/candlefish-13-plans/
B and B Jessy Skiff: https://bandbyachtdesigns.com/powerboats/jessy-skiff/
CLC Peeler Skiff: https://www.clcboats.com/shop/boats/powerboats/peeler-outboard-power-skiff.html
Bolger SeaHawk: https://www.duckworksmagazine.com/r/plansindex/bolger/bolger.htm
Bolger Chippewa: https://www.duckworksmagazine.com/r/plansindex/bolger/bolger.htm
Atkin Sprite Skiff: https://www.woodenboat.com/boat-plans-kits/14-10-outboard-sprite
Wooden Boat Yankee Tender: https://www.woodenboat.com/boat-plans-kits/yankee-tender
Bowdidge Edgetracker: https://bowdidgemarinedesigns.com/edgetracker-365/
Abaco (Panga) Skiff: https://abacoskiff.com/14-skiff/
Lowe Utility V1460 Skiff: https://www.loweboats.com/fishing/utility/tiller/v1460.html

To me, the Diablo and Peeler are by far the best options. Jessy skiff is pretty good too (8° deadrise). I would have added the classic (Hunt designed) boston whaler 13, but that boat won't plane with anything less than 4stroke 20hp... and weighs as much as a Tolman Standard 18.

In shrinking the Tolman design, I noticed 3 things:
1) No rocker. No other boat does this, probably because no other boat has such a gradual bow curve; without rocker, the bow can dig in. This also means Station 6 is in the middle of the boat. Every other boat design shifts the console/stn6 slightly astern, likely due to rocker lifting the bow. (Yes I know that stations come from dividing the boat into 12 sections; go measure where everyone puts their consoles and compare to the center thwart on page 16.)
2) No framing. In order to get compound curves, all other designs use horizontal framing. Tolmans skiff don't need horizontal framing because the shelf (gunnel) is actually structural; it makes a box with the top of the sides. I first noticed this when I was PL cementing ipe to make gloucester gull gunnels... don't need framing if you make boxes in other places, and don't need frames if you build on a jig/strongback.
3) 8° deadrise. The picture of the table with the outboard is pretty useful here. Most designs (including aluminum utility skiffs) have flat transoms, e.g. no deadrise, because this gets you on plane more easily. But flat chines chine walk at speed, fall all plane when you turn, or worse:
http://rbsafety.d11nuscgaux.info/docs/2016_Safety_Advisory_TEXAS_FLATS_BOATS-BAY_BOATS.pdf

I really liked the CLC Peeler rear seats and console layout.
best,
KenB

"HOW CHEAPLY CAN A TOLMAN SKIFF BE BUILT AND JUST HOW  MUCH IS SACRIFICED IF COST IS THE PRIME MOVER?"
- Bruce Armstrong   

"I can tell you that either a nice BFT or a big YFT is an absolute riot on a Tolman. The boat is so light it's like the old man and the sea..."
- Dave Nolan (RIP)

Offline KenB

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Re: Jon boat with a Tolman Transom.
« Reply #9 on: January 09, 2022, 03:01:13 PM »
Totally hypothetical questions here...

If someone had a 9.9 hp toad motor, or even a new 4s 9.9... and wanted to make a scaled down tolman standard to fit that motor, what would the dimensions be?
- 12'? 14'?   If an 18' standard has a 25' transom and a jon boat has a 20 inch transom, then the jon boat is 14.4 feet.  14 feet feels long for a flat bottom skiff with a 9.9, but the 8° vee should make the jon tolman a little more efficient:



Totally unhypothetical answer here...
Starting the design process by fixing the engine power and taking a boatmodel thats designed to be in a totally different size category and scaling it down untill the boat is small enough to be handled by your chosen engine is not a good strategy. A much better strategy is to first define needs (2, 3 or 4 persons... etc) and take as a starting point a boat that resembles the one you want and then modify the design to fit your preferences. If the boat model is designed by a naval architect or an already existing professional build you can get the power requirements etc from the starting point. After your modifications you only need to estimate how much your modifications change the requirements from the starting point. This way you end up pretty close to what you want to. By large scaling you probably end up with something unexpected.




And if my grandmother had two wheels we could call her a bicycle. You are unhypothetically a clown! Your entire paragraph is a parenthetical edge case, it is literally a hypothetical statement. And your basic point is completely disproven by the fact that the original standard became the widebody, which became the jumbo.  I literally took the same approach the 3rd edition of the book does, but you decided to write a bunch of 'but whatabout' paragraphs that while not wrong (besides the ones that are...) are not exactly meaningful or helpful. Real boat designs get tested in the water, that's the biggest difference between Bolger and Payson... one of those guys actually used the boats he built. Referring me to Marine Architects is like telling me I need to call the operator to make a long distance phone call; times have changed.

I do not accept your apology, and instead name this boat design after you!  I give you the JALLII SKIFF. 


(If anyone wants dimensions, let me know.)
best,
KenB

"HOW CHEAPLY CAN A TOLMAN SKIFF BE BUILT AND JUST HOW  MUCH IS SACRIFICED IF COST IS THE PRIME MOVER?"
- Bruce Armstrong   

"I can tell you that either a nice BFT or a big YFT is an absolute riot on a Tolman. The boat is so light it's like the old man and the sea..."
- Dave Nolan (RIP)

Offline TFreeburg

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Re: Jon boat with a Tolman Transom: jallii skiff
« Reply #10 on: January 09, 2022, 08:13:02 PM »
Yep,
So I do hear the beer ‘a talken here
Snohomish Tom
My goal: 25' Jumbo
First epoxy: 10/3/17
Flipped: 6/15/2019

Offline Kobuk

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Re: Jon boat with a Tolman Transom: jallii skiff
« Reply #11 on: January 09, 2022, 09:39:34 PM »
Yep,
So I do hear the beer ‘a talken here
I'm cracking one now...but i'm gonna keep my mouth shut.
22' Widebody
Started: 3/2019
Flipped: 6/2019
Floated: 6/2020

Offline KenB

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Re: Jon boat with a Tolman Transom: jallii skiff
« Reply #12 on: January 10, 2022, 08:29:16 AM »
Not beer... just (tongue-in-cheek) frustration over being lectured to by some clown who thinks Renn Tolman was Naval Architect who designed a 26' boat!

I do not believe Renn Tolman sat at a drafting table lofting up silly curves then penning essays on them (Bolger); I think Renn built a lot of boats and MOST IMPORTANTLY USED THEM (Payson).  You can throw away all the theory, academic degrees, maxims, algorithms, and general blowhard pontification by going outside and making some sawdust. How many times in all the threads on this website has someone 1) come up with a great idea that improves upon the book  2) tries to actually do it and 3) comes back with their tail between their legs after realizing that the book pretty much gets things right... it happens more times with Tolman Skiffs than any other design I have looked at. Just remember the last edition of the book was published before outboards had electronic fuel injection.

Jallii writes "Totally unhypothetical answer here..." THIS IS LIE! Jallii then gives a reply not based on his own experiences building boats, not based on boats that actually exist (go look at the picture of my 8 degree skiff... THAT exists), and certainly not based on anything except hypotheses. You can judge for yourself what his motives are.

Bolger and Payson needed each other, Payson probably needed Bolger more. All because of who they were selling boat plans too, nothing much else. I attached some excerpts from a Bolger book below, which I think get at it, especially the paragraph where Bolger admits to screwing up a design. The 2nd paragraph on seahawk nails the Tolman crowd. The Hunt boat he is referring to is predecessor to moppie, perhaps the single most important boat design in powerboating: https://www.yachtsinternational.com/owners-lounge/it-all-began-with-richard-bertram

Anyway, what are peoples thoughts on my 3 big discoveries about the Tolman Skiff design?

The first two chapters of the book kind of allude to the deadrise, but I've never seen a discussion on rocker, or the lack of frames. Does anyone know if using shelves was a deliberate reason to not use frames? I stopped using frames on my gloucester gulls once I realized how stiff the gunnels make everything. I have used both 1x3 ipe decking ripped to 1 1/2" rounded over on 3 sides, and also laminated ply scraps. In both cases, the boat magically goes from a bit wobbly to as stiff as framed. I notice that the "long beach dinghy" also uses a shelf at the top of the sides to stiffen things, and lacks frames, while being built on a jig...  lots of rocker though, rocker is great for sailing and rowing. 

The one more thing I would add is that UMHW chine shelves are much more important than I first thought. Work skiffs often get the same effect by bolting three 2x6 runners to the bottom, to create some turbulence.
« Last Edit: January 10, 2022, 10:56:37 AM by KenB »
best,
KenB

"HOW CHEAPLY CAN A TOLMAN SKIFF BE BUILT AND JUST HOW  MUCH IS SACRIFICED IF COST IS THE PRIME MOVER?"
- Bruce Armstrong   

"I can tell you that either a nice BFT or a big YFT is an absolute riot on a Tolman. The boat is so light it's like the old man and the sea..."
- Dave Nolan (RIP)

Offline jallii

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Re: Jon boat with a Tolman Transom: jallii skiff
« Reply #13 on: January 22, 2022, 02:48:18 AM »
Quote
“The truth is incontrovertible. Malice may attack it, ignorance may deride it, but in the end, there it is.” – Winston Churchill

I have tried to avoid anything that could be interpreted as malicious or offensive. I have used only polite language and havent resorted to any ad hominem arguments as proper posting in any forum should. However... Well, I find it increasingly hard to avoid the interpretation that you have a need to somehow "prove yourself", "belittle me" or "win an argument". I could not care less about winning any arguments or pissing contests. I have tried to offer friendly advice and you are free to ignore everything I say.

Most introductory courses in epistemology would try to distinguish bethween different things that the word knowledge migth refer to. Its common to distinguish bethween "knowledge about things" and "how to -knowledge". If you read old classics like Plato you migth come across discussions about the kind of knowledge a maker of shoes and the user of shoes has and how that differs from the kind of knowledge philosophers and scientists have. I guess that at least some insigth about making shoes applies also to making a boat.

You seem to dismiss the kind of knowledge a naval architech has about making boats. I think thats a mistake. It even feels like arrogance. I find this dismissal of knowledge even dangerous. Of course the practical "know how" that comes by years of practical experience as a professional boatbuilder is extremely valuable, but so is the more theoretical kind of knowledge a naval architect gets by years of studying from books. I would not want to risk my life in a ship or boat that is not at least inspected by someone that really knows his stuff. Remember also, that any boatbuilding project involves hundreds or even thousands of dollars worth of investment. Dont you want to be sure about the end result before you commit to you r project? Both kind of knowledge is needed. Its also good to remember that it takes time and lots of different kind of learning experiences to become a professional craftsman. With this I do not intend to dismiss any of the learning experiences you have aquired sofar.

The thing that fascinates me about Tollman boats is not the boat itself but the work Renn has done to figure a way how a non-educated layperson with normal carpentry skills can build a decent boat with a reasonable budget. The boat itself is good or bad depending on what you expect from the boat. The goodness or badness of the boat depends on how well it is suited to intended use, that is what you need the boat for. It is suitable (or not) for someones particular use. There is no such thing as a "one boat to rule them all" type of goodness of a boat.

I still dont understand what upsets you so that you feel the need for this unfriendlines. As a friendly advice,  I tried to explain to you, that simply shrinking Renns standard or wideboby to a much smaller size gives you a boat thats significantly different from the original, because size matters a lot. Therefore I think that you get a more predictable end result if you start either from an existing boat with known specs or a proper design with known qualities rather than try to shrink Renns design. The length/beam relationship you tried to extrapolate is not as important as you migth think. The boat may resemple the original, but will behave differently.

The design process is basically a process where you try to find a balance of different things. The boat you get is a result of finding a proper balance of different characteristics that each depend on certain characteristics that can be expressed with different co-efficients and formulas. At different sizes the best balance is different. If you have a certain balance of things for a boat thats 18 feet long, you most certainly will not get the same balance if you shrink that boat to 14 feet. For example 8 degree deadrise for a 18 feet boat does not mean the same for a 14 feet boat even though the angle is the same, because other things change also. You can shrink a boat 22% in photoshop, but you cannot shrink yourself by 22%, so your 100% weigth has a different effect. Because the boat has to float, the basic Archimedes principle means that for each kg of weight, there has to be about 1 litre of underwater volume of the boat thats not occupied by water. This means that weight=underwater volume = displacement. This is the factor that affects most. Its wise to take these main measurements from a known source, either an existing boat that behaves like you want or a design that has the needed info. . This is the thing you need to determine first. Everything else is secondary to this. You can adjust legth beam ratio, deadrise angle etc to suit your preferences. That is: find a new balance. You most certainly will not get the same boat by simply shrinking it more than a little.

Its a mathematical certainty that a linear curve and exponential curves to the power of 2, 3 and 4 scale differently, so the further away you get from the original point, where these were in a certain balanced relationship to each other, the further apart they get from the original balance point. Thats why you necessarily get a different boat by scaling method. I will add a picture here to illustrate this. Here you have 4 mathematical curves. First you hace the brown linear curve, that could represent something linear like the length of the boat. So its 1 dimensional and therefore linear. The red line is exponential to the power of 2. Blue one to the power of 3. and green one to the power of 4. I have used the constant multipliers to get them to be close to each other at a certain point. This represents getting a balance of design parametres for a certain size boat. As you can see that the further away you get from the balance point the further apart those lines get from each other. They even swap relative places due to different dimensionality. Here these lines are just mathematical curves that ignore meaning, but the effect is the same when there is a reality behing design parametres that have a definite effect on some characteristic of a boat. So size does matter a lot. For example initial stability of a boat is pretty much an effect of boyancy of the underwater volume (length*width*breath) and its distance from the centerline (beam), so the unit would be meters to the power of 4 (m4). So "beamines" would be the most important factor in initial stability assesment of a boat. Shortening the boat from 20 feet to 14 feet would happen along the brown line, but the boyant force creating stability would change along the green line. To compensate for this dramatic reduction in stability you would need to increase the relative "beaminess" of the boat (the precise factor you tried to maintain)...    So why not start from something thats known in the first place.  As you tend to ignore my concerns I will attach some text from Dave Gerr (Nature of boats pages 89 and 90) on the laws on relativity and similitude as it applies to boatdesign. As its just a short clip it should fall into the appropiate use category. 


Thats just the way things are.
« Last Edit: January 22, 2022, 11:37:59 PM by jallii »
I like to find out and really understand things. A perfectionist, curious mind cannot stop learning, and picks up many things. I don't claim to be an expert. I'm not an engineer or a chemist by training. I make mistakes. If I manage to make something understandable I am happy.

IF you want to build a boat make sure you follow designers instruction. Do your homework and be safe.

Offline jallii

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Re: Jon boat with a Tolman Transom.
« Reply #14 on: January 23, 2022, 01:20:25 AM »
In shrinking the Tolman design, I noticed 3 things:
1) No rocker. No other boat does this, probably because no other boat has such a gradual bow curve; without rocker, the bow can dig in. This also means Station 6 is in the middle of the boat. Every other boat design shifts the console/stn6 slightly astern, likely due to rocker lifting the bow. (Yes I know that stations come from dividing the boat into 12 sections; go measure where everyone puts their consoles and compare to the center thwart on page 16.)
2) No framing. In order to get compound curves, all other designs use horizontal framing. Tolmans skiff don't need horizontal framing because the shelf (gunnel) is actually structural; it makes a box with the top of the sides. I first noticed this when I was PL cementing ipe to make gloucester gull gunnels... don't need framing if you make boxes in other places, and don't need frames if you build on a jig/strongback.
3) 8° deadrise. The picture of the table with the outboard is pretty useful here. Most designs (including aluminum utility skiffs) have flat transoms, e.g. no deadrise, because this gets you on plane more easily. But flat chines chine walk at speed, fall all plane when you turn, or worse:
http://rbsafety.d11nuscgaux.info/docs/2016_Safety_Advisory_TEXAS_FLATS_BOATS-BAY_BOATS.pdf

Some comments about these 3 things

1. The amount of rocker has very little to do with the placement of the stations or their number. Its common to divide the lenght of the boat to 10 or 12 equal parts, but you could divide it to any number you wish. You can also start counting from either bow or aft, witch ever you prefer.  Renn does not give full plans in his books, he just gives instructions on how to build the boat. The thing that matter from a perspective of boatdesign is the location of the midpoint.  I suspect Renn used his unique way of expressing things by referring to station 6 because wanted to find a way to make it possible to reference several boats of different lengths with a single expression. The midpoint would be ambiquous for several different length boats. Renn allowed for changing the length of the boat from the aft end. So by referencing a certain point as station 6 by counting from the bow, the front end of the would be referenced the same even if the aft end measurements were different. 
The placement of the console has to do with the distribution of weigth. By putting it too much towards the bow you risk getting a dynamically unstable boat, as referenced in the warning you posted. But it all depends on the weight of the engine and the placement of seats.

2. The lack of framing in Renns boats has more to do with using plywood and epoxy than the gunnels. You are right the gunnels form a strenghtening "frame", but so does the hard corner/corners  in chine and the "spray rail" and "bottom strakes".  They are all actually longitunal frames that are disguised in another function they perform. Renn does not mention this, witch is a shame, but thats why they are there actually.  Directing the flow of water and spray is just an additional bonus. Traditionally wooden boats had transverse frames and longitunal planks. Renns design has longitunal frames and plywood planks. Traditional boats didn't need much longitunal support as the planking itself was much stronger in the longitunal direction and was strong enough for that purpose, but they needed transverse support structure. As plywood is composed of layers of veneers that are glued together and have their fibres at 90 degree angles, it provides only half the strength in the longitunal direction, and the other half in transverse direction and lots of dimensional stability. So as a result of using plywood planking the longitunal direction needs a support structure, but you can pretty much neglect the transverse direction. The whole matter of how to use a flat material to form the shape of the boat is too complex to comment here, but there is much more to it than you refer to. Renn was innovative in avoiding traditional temporary framing structure/ stronback by letting the preformed parts themselves form the necessary support for the formation of the form. Thats quite an accomplishment without a CAD program.

3. Chine walk is a dynamic instability thats caused mainly by having the center of gravity in the wrong place and how the waterflow is in contact with the bottom and boyancy is distributed at different speeds and not with the deadrise angle by itself.

I like to find out and really understand things. A perfectionist, curious mind cannot stop learning, and picks up many things. I don't claim to be an expert. I'm not an engineer or a chemist by training. I make mistakes. If I manage to make something understandable I am happy.

IF you want to build a boat make sure you follow designers instruction. Do your homework and be safe.