Trim adjustment for max MPG

[Boatingbob]

At a given RPM what rules of thumb might there be for optimal trim for best MPG? I have heard:

1. Trim at which nose is lowest.

2. Trim at which turning the steering wheel is easiest.

3. Something else I can't remember.

Thanks.

Bob

 

[Bamaman]

I would think it would matter which kind of toons you have--with or without lifting strakes.

Two toons without strakes--I've always run with the engine parallel with the toons. Two toons are displacing the water--plowing through it. Ever notice how two toons will just run so fast--and just won't go faster?

If you've got lifting strakes, you've got to throw the horsepower to it--and trim the engine up to get the nose out of the water. That's why they take heavy horsepower--not minimal. Getting the nose up is how you get the speed and efficiency out of the strakes.

 

[ericscher]

Would you be mad at me if I said there wasn't one?

To mangle an old saying, a boat is a hole in the water you pump fuel into.

Somewhere around 1/2 of your max rpm, and it'll vary quite a bit, you'll find your best cruise speed; which is the best speed for gallons/mile. Your trim won't matter enough to worry about.

 

[kaydano]

Bamaman - Your comment about "two toons going just so fast, and not going any faster" reminded me of something I read regarding displacement type boats (2 toons). I thought it was wrong until I read your comment. There is a mathematical formula (for displacement boats only) that determines the max speed of the boat, beyond which adding HP doesn't get you any more speed. You can add all the HP you want, but the max speed changes very, very little. The curve (HP vs Speed) goes asymptotic (an asymptotic curve is essentially any math equation in which the result will continually approach a value, without ever reaching that value). It's just like penalty after penalty of "half the distance to the goal" will never score a touchdown. While all this hit me as hard to believe (because more HP is always faster, right???) it fit your comment to a T... Displacement boats hit a max speed wall.

For displacement boats, the max speed is dependent on the length of the boat, not the HP. This also hit me as strange, but that's what it said. The formula says the longer the boat, the faster the max possible speed.

To go any faster, you have to get up on plane (and no longer be "displacing" water) and reduce the contact patch between the boat and the water. At that point, adding HP does adds speed.

I suppose eventually you hit another max speed wall due to air resistance. At that point, you need a flux capacitor. You will find how to add one of those to a pontoon boat in another thread on this site.

 

[Wild and Free]

Having had 2 Benningtons I have found that if the trim gauge is even close to accurate then half to about 3/4 up is the best performance depending on load and conditions no matter the rpms if tubing and turning hard then I need to get to below half trim to keep from ventilating or cavitating the prop..