Author Topic: Engines, propellers, rpm, and speed  (Read 3667 times)

Offline Dave Wright

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Engines, propellers, rpm, and speed
« on: September 16, 2016, 09:12:29 PM »
quote author=Cannon link=topic=3738.msg48264#msg48264 date=1474075996]
 Could someone # me some pointers on RPM vs speed?
[/quote#

We are lacking a good data base for Tolman skiff performance. Our reports are scattered over two forums and about 15 years. Let's collect what we know and post it in this thread. Post your performance data. No data, don't post until you get some. Keep it simple and informative. Identify the boat and it's weight if you have it, otherwise describe, for example, heavy construction, light construction, per the book, etc. Identify the engine and it's horse power rating. Identify the propeller by number of blades, matetial, diameter, and pitch. Importantly, identify the propeller manufacturer - different manufacturers do different things.

*So, for example my old 18 foot standard:

18 foot standard with cuddy at station 9, 3/4 ply cockpit sole, heavief construction.

40 HP Tohatsu two stroke, older style, oil injection, new engine

Propeller: Tohatsu, aluminum, 3 blade, 11.1 Dia  X 13 pitch

Performance new, one occupant, light boat, six gallons gas

RPM          MPH
1000         3.7
1500         5.5
2000         7.2
2500         8.7
3000        12.1
3500        16.8
4000        19.5
4500        23.9
5000        28
5400        30.1 

Dave Nolan has posted similar data over the years, it is effectively lost in the chafe.

I will post my Widebody data after I get it sorted. I'm using a 2016 new 70 HP Yamaha 4 stroke. I specified a Yamaha  3 bladed  aluminum prop, 13.5 X 14. To my surprise I couldn't exceed 4200 RPM at 25 MPH . I'm mounting a different prop next week.

Chris Collins had similar issues with the same engine  and same prop, a 13.5 X 14 on his 18 foot standard. In his case he couldn't exceed 4800 RPM top.  However when he mounted a Quicksilver Black Diamond  3 blade aluminum prop 14 X 13 he reached 6300 RPM.

PROPELLERS MAY HAVE SAME LISTED PITCHES BUT DIFFER GREATLY.

Post your data here.

Offline tananaBrian

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Re: Engines, propellers, rpm, and speed
« Reply #1 on: September 17, 2016, 08:48:17 AM »
No data from me - posting anyway:  Please include fuel burn rates or mpg info if you have it as well.

bd

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Offline pfithian

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Re: Engines, propellers, rpm, and speed
« Reply #2 on: September 17, 2016, 11:32:35 AM »
Numbers for Ken Chase's and our Jumbo are on the Wikipedia page.

Here are some revised numbers for mine, showing the comparison of the old 115 vs 135 HO E-TEC motors.
Made It Jumbo 25
Skiffkits No. 7025 1/2009
Build Start:  3/2009
Hull Flipped: 1/31/2010
Maiden Voyage:  9/16/2011

Offline Cannon

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Re: Engines, propellers, rpm, and speed
« Reply #3 on: September 17, 2016, 08:16:44 PM »
Funny, but sea conditions play a big part in this, speed can vary 5 knots depending on currents and swell.
2003 Yamaha 225 6 cylinder 4-stroke
5600 rpm 18-20 knots
5800 rpm 22-24 knots
Average fuel burn has been consistently 3.51 MPG
SOLAS SS three blade 17" prop
Great Alaskan 28' started June 2015, launched August 2016
Remember, the ark was built by amateurs; while the Titanic was built by professionals.

Offline Dave Wright

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Re: Engines, propellers, rpm, and speed
« Reply #4 on: September 28, 2016, 01:34:26 PM »
So I removed my aluminum 3 bladed Yamaha aluminum propeller, 13.5 X 14, from the Yamaha 70 horse 4 stroke on my Widebody. With this propeller I could not exceed 4200 RPM @ 25 MPH.

After measuring this propeller, and using the Yamaha prop selection guide which suggested a 9 inch pitch Yamaha prop, I decided to not use this particular series of Yamah propellers.

The Yamaha F70A top end operating band is 5300 to 6300 RPM. When I built this Widebody my goal was to achieve a day cruising,  occasional overnighting, bird watching, now and then fishing boat. Much like the Blue Jacket design, I set a goal of 15 MPH cruising speed and a top end of 25 MPH. I wanted to achieve this top end at close to 6000 RPM. The engine has speed limiters so overspeed is not a real worry.  If I'd wanted faster performance I would have bought a 90 HP or larger engine. But I like smaller engines and the Yamaha's listed dry weight of 271 pounds for the 20 inch leg attracted me.

So, I decided to go with a Mercury Quicksilver Black Diamond prop because I liked the low price, I thought that the Flo-Torq hub adapter kit would protect the shaft better than the little plastic shear rods in the Yamaha prop, and I felt that the measured average pitch of the Blacl Diamond prop more closely fit the advertised pitch than the Yamah prop that I was first using.

What pitch to use? I wanted to go from 4200 RPM max with the Yamaha prop to 6000 RPM with my new prop. A gain of 1800 RPM. Some rules of thumb suggest 200 RPM per inch of pitch. Those rules fail to consider aspects of propeller shape and variations in pitch across the blades by various manufacturers. I decided to try the Black Diamond 3 X 14 X 11 aluminum prop. This prop had much greater blade area than the original Yamaha prop - roughly a 1/4 inch band of increased area over the entire blade perimeter along with the 1/2 inch larger diameter. It also had more cup to the blade tips, and its average measured pitch was closer to its listed pitch of 11.

With this prop I was able to achieve 15 MPH at 3600 RPM. I was able to top out at 28.1 MPH at 5900 RPM. I believe that my motor should be raised one hole, and that my top,end speed will increase a small amount.

So the 3 inch pitch decrease resulted in a 1700 RPM increase. This shows that 200 RPM per inch rules of thumb are problematic and fail to consider propeller design differences among manufacturers.

I'm happy with my new propeller. The boat is a 21 foot Widebody with pilot house. It isn't lightly built. There were two of us aboard with 12 gallons of gas, anchor, lines, and all standard equipment plus lunch..

The 4 stroke 70 horse Yamaha will suffice, and meet my needs, but if you want terrific acceleration and higher top end you need a more powerful engine.

Early this morning the wind was blowing strongly against a river current of at least 2 MPH. People ask about pounding.  This morning any attemp at speed into the wind resulted in plunging and slamming.  Would it pound? You bet it would.

Offline Kobuk

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Re: Engines, propellers, rpm, and speed
« Reply #5 on: January 02, 2021, 03:56:09 AM »

We are lacking a good data base for Tolman skiff performance. Our reports are scattered over two forums and about 15 years. Let's collect what we know and post it in this thread. Post your performance data. No data, don't post until you get some. Keep it simple and informative. Identify the boat and it's weight if you have it, otherwise describe, for example, heavy construction, light construction, per the book, etc. Identify the engine and it's horse power rating. Identify the propeller by number of blades, matetial, diameter, and pitch. Importantly, identify the propeller manufacturer - different manufacturers do different things.

...

Post your data here.

22' Widebody with cuddy and lightweight pilot house.
Rigged weight, dry: ~2000 pounds gross
Main: Yamaha F70
Kicker: Yamaha 9.9HT

2200 pounds estimated gross
Mercury Black Max 3 x 14" x 11 pitch
3800 RPM = 10 KTS
4000 = 11 transition to plane
4100 = 12
4250 = 14 on plane
5000 = 18
5500 = 20
6000 = 22
6300 = 24 (This motor is governed at 6300 RPM to prevent over revving).
Feels a little under propped.

2950 pounds estimated gross
Mercury Black Max 3 x 14" x 11 pitch
2000 = 5
3000 = 6.5
4000 = 11
4500 = 13
5200 = 15.5
5500 = 18  happy efficient plane
5600 = 19
6000 = 21
6300 = 22.5 WOT
Still feels under propped. (And still easily hitting the governed rpm speed).

2600 pounds estimated gross
Mercury Spitfire 4 x 13.8" x 13 pitch
3000 = 9.2
3500 = 12
4000 = 15
4500 = 18
5000 = 21
5500 = 23 WOT
Feels over propped and working too hard.

Estimated 2800 pounds gross
Mercury Black Max 3 x 14" x 13 pitch
2000 = 6
3000 = 8.2
3600 = 11.4 on plane
4000 = 13
4500 = 16
5000 = 19
5200 = 20
5800 = 23.5 WOT
Feels good, handles really well.

Estimated 2800 pounds gross
Mercury Vengeance 3 x 14" x 12 pitch
2500 = 6
3000 = 7.4
3500 = 9.5
4000 = 13 transition to plane
4500 = 16
5000 = 19
5500 = 22.5
6000 = 25 WOT
With smooth, easy acceleration this prop will ventilate a little bit transitioning through 4000 RPM and produce a little shudder; it will ventilate aggressively/slip with maximum acceleration from a standing start.  Otherwise, performance and handling, and fuel efficiency are excellent.  Average fuel consumption with typical use is averaging well under 3 gal/hr.

This winter I've installed a Permatrim AV plate which I expect will eliminate the ventilation issue. I'll report again after next trip.

2021 update:

22' Widebody with cuddy and lightweight pilot house.
Rigged weight, dry: ~2000 pounds gross
Yamaha F70 with Permatrim AV plate installed, fully broken-in with ~100 hours on the hobbs

Estimated 2400 pounds gross
Mercury Vengeance 3 x 14" x 12 pitch, with Permatrim AV plate
2500 = 6
3000 = 7.4
3500 = 9.5
4000 = 13 / transition to plane
4500 rpm = 16 kts, 3.3 gph / 4.85 mpg, 83.9% hull efficiency / 16.1% prop slip
5000 rpm = 18 kts, 4.0 gph / 4.5 mpg, 85% hull efficiency / 15% prop slip
5500 = 22.5
6000-6100 wot rpm = 24-25 kts, 5.8-6 gph / ~4 mpg, 94% hull efficiency / 6% prop slip

4500-4800 rpm is extremely efficient economy cruse, and the skiff handles great.
Prop ventilation issue during max acceleration from full stop was resolved with addition of the AV plate.
« Last Edit: October 23, 2021, 11:04:33 PM by Kobuk »
22' Widebody
Started: 3/2019
Flipped: 6/2019
Floated: 6/2020