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Old 07-13-2018, 03:41 PM   #11 (permalink)
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Have you looked at checking proper engine mounting height and tried different props?

You're a little underpowered probably. I have a '96 Boston Whaler Dauntless 15 with a Honda BF60 4-stroke and get 8-9mpg. I think my boat is bigger, has a sharper bow and more deadrise (less efficient). Mine tops out at 35mph, planes at a little over 10mph and cruises comfortably at 20mph/4ish KRPM.

If you can find something with a bit more umph but not much heavier, that may change your ride significantly. What is the hull rated for?

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Old 07-15-2018, 06:53 PM   #12 (permalink)
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I was actually reading up on this subject earlier, and chances are boat efficiency is very closely related to airplane efficiency. One big thing I found is that jet drives are not as efficient as props. Just like planes, you will gain efficiency from large diameter 2 blade props, more blades=more turbulence.

No idea on hull design and water interference yet, haven’t looked into that enough.


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You're a little underpowered probably. I have a '96 Boston Whaler Dauntless 15 with a Honda BF60 4-stroke and get 8-9mpg.
That is really disappointing. I believe the BF60 engine is closely based on the 1st gen insight engine, and comes with lean burn.
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Old 07-16-2018, 10:53 PM   #13 (permalink)
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Quote:
Originally Posted by Hersbird View Post
https://newatlas.com/yanmar-dtorque-...utboard/51675/

Its great but $25,000+ for 50hp buys an endless supply of gas.
I would say this closer to the ecomodder solution:

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Old 07-17-2018, 12:47 AM   #14 (permalink)
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here is a little calculator to play with. It really depends on the type of hull. You can underpower (or throttle down) and efficient displacement hull and get great results. Just like slowing down a car because of aero drag but it's much more drastic with water. A 30 foot displacement hull has a theoretical top speed of 9.2 knots if you put 81 horsepower to it, but to go 8.3 knots on the same boat you only need 38 HP. If 5.5 knots was enough it only takes 8 HP.

Now on a 20' planing hull it takes 210 HP to reach 35 knots. Dropping down to 75 HP will go 15.2 knots. Down below 40 HP it turns into a poor displacement hull design and goes slower on the same power as the bigger heavier 30' displacement hull.

once you know the horsepower required for what speed you can figure engine efficiency and get a MPG.

Boat Speed Calculator

I also plugged in my ship the USS Carl Vinson for fun. The US Navy always says 30+ knots, but if you put enough HP to that hull it is capable of 65 knots. That would require almost 1.8 million HP. Well it doesn't have that much but it does has an advertised 260,000 HP which would be good for 37 knots (which is pushing 43 mph). I can't confirm or deny that number but physics is physics.
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Old 07-17-2018, 03:49 AM   #15 (permalink)
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I just wonder why boats are always cut off flat in the back rather than achieving a teardrop shape? Shouldn't the pointy end face aft? Liquid fluid dynamics must not directly mimic gas fluid dynamics.
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Old 07-17-2018, 01:02 PM   #16 (permalink)
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Quote:
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I just wonder why boats are always cut off flat in the back rather than achieving a teardrop shape? Shouldn't the pointy end face aft? Liquid fluid dynamics must not directly mimic gas fluid dynamics.
Displacement hulls do have rounded backs.

For a planing hull the design objective is to get as much of the hull out of the water as possible to reduce drag.
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Old 07-17-2018, 03:31 PM   #17 (permalink)
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Can anybody explain how to use that calc for a 15' flat bottom? lol
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Old 07-17-2018, 10:19 PM   #18 (permalink)
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What's a boat?
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Old 07-18-2018, 09:09 AM   #19 (permalink)
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What's a boat?
A 1969 Deuce-and-a-quarter.
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Old 07-19-2018, 08:34 PM   #20 (permalink)
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It's easy to get great fuel economy from a boat. You just need the right boat, like a canoe, kayak, or a sailboat.

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