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Old 08-27-2018, 04:29 PM   #31 (permalink)
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I just sold my pedal boat projects, but they were winning races right out of the box. For me, the key was propeller efficiency. While studying the possibilities, I decided to try out a model aircraft propeller, and stayed with them. For motor power, you'd probably want to copy one in metal or carbon fiber, but if you are going for a dragon tail, there's room to gear it right.
I was recently appalled to learn that marine architects are still using the Betz limit in propeller calculations. This is usually quoted regarding windmill efficiency. It applies to tugboats, and pulling contests against stern wheelers, but once a vessel is under way, we need the Froude numbers, which take into account the steady supply of undisturbed water. By pushing gently on a large mass of water, we do much better than with a smaller diameter prop, which is always pushing on water that is running away. The situation is even worse with jet drives, but both are selected for their ability to run in shoal conditions. A dragon tail is easy to lift, or even operate surface-piercing at need.

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Old 08-27-2018, 04:30 PM   #32 (permalink)
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Wouldn't changing to a 4-stroke outboard produce a substantial increase in MPG? Not to mention less stink.

Or turn off the gas engine and use an electric trolling motor. :-)
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Old 08-27-2018, 06:38 PM   #33 (permalink)
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Quote:
Originally Posted by hayden55 View Post
Boats are pretty thirsty an inefficient. I'm looking into ideas to get my 1538 1970s fisher water rover with a 1989 yamaha 30hp outboard to get better mpg. What ideas have you guys tried?
Got a picture ?
Google turns up some really squarish shallow draft boats


A newer engine will likely get you better mileage

Clean the hull
Paint & polish it really smooth & glossy

Add "stern plates"
Dunno what they are really called, but they are plates added aft of the stern, along the underside of the hull, and extend the apparent length of the hull




Increase the fineness ratio: make it longer, or narrower
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Old 08-27-2018, 06:53 PM   #34 (permalink)
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For displacement hulls , long boats go faster i.e. better MPG .
But since you are stuck with the boat you have - fussing with the prop and how you 'manage' your ride makes all the difference.
Virtually no 2 boats are the same - load, engine, prop etc etc.
So what works for you probably won't for the next man.
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Old 08-27-2018, 07:06 PM   #35 (permalink)
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Trim tabs are handy for trim adjustments, and are a cheap way to lengthen a hull in plane. However, width is much better than length for planing, for the same reason that gliders are wider than fighters.
Displacement hulls have the unique problem of producing waves, which gives rise to a given hull speed often quoted as 1.37 X the square root of the waterline length in feet, to get the speed in knots beyond which great increases in power barely return any more speed, until the boat planes over it's bow wave, if possible.
Hydrofoils are quite feasible as add-ons, and increase both efficiency and available speed in a given sea condition. A main wing can be almost flat, carrying over 90% of the load, with a small canard having it's angle of attack controlled by a little spoon follower on the surface, provide a simple, stable and efficient configuration.
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Old 08-27-2018, 07:42 PM   #36 (permalink)
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Clean and smooth the bottom. Wax it to make it slicker. A friend's Father had a 3/4 cabin Larsin with a v-8. We cleaned and waxed the bottom. End result was 7 mph faster.
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Old 08-27-2018, 08:06 PM   #37 (permalink)
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Quote:
What ideas have you guys tried?
The only thing I've tried that might apply is putting a bellmouth on the air intake.

https://www.physicsforums.com/thread...r-work.678202/

Depending on the hull architecture and performance envelope maybe a surface-piercing propellor? You might have to raise the transom.

I like the M-hull. Maybe you could get some benefit by strapping surfboards vertically on either side of the bow (sort of a trimaran) to kill the bow wave.
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Old 08-27-2018, 09:03 PM   #38 (permalink)
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Free: Try raising motor height, experiment with weight placement in the boat, and if you have power trim, try trimming out just a bit. Figure out your most efficient cruising speed and try and operate at that speed as much as possible.

$$: get a propeller that brings gets you to max rpm @WOT

$$$$: buy a newer motor, consider the evinrude DFI 2-stroke E-TEC line, they are lighter than 4-strokes and get equivalent, if not better FE. From a performance perspective they blow 4-strokes out of the water.
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Old 08-27-2018, 10:46 PM   #39 (permalink)
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I have a 14 alumicraft and Yamaha 9.9 4stroke. 2.5 gallons seems to last for days. I think weight is your biggest issue.
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Old 08-28-2018, 11:27 AM   #40 (permalink)
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Quote:
Originally Posted by hayden55 View Post
Boats are pretty thirsty an inefficient. I'm looking into ideas to get my 1538 1970s fisher water rover with a 1989 yamaha 30hp outboard to get better mpg. What ideas have you guys tried?
Add a sail!!! Duh!!!

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