FishyFish Tolman Alaskan Skiff and Boat Building Forums

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.

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.

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...!

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.

Quote from: KenB 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:

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.)

Yep,
So I do hear the beer ‘a talken here

I'm cracking one now...but i'm gonna keep my mouth shut.

“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.