RPMs, Bow Angle, and Fuel Economy

[Jason2tpa]

no one talks about bow angle or measures it, just trim until you get it right for the load and conditions.
If you have a trim gauge you can look at it and come back to that if you ever have the same load and conditions, but just bumping the trim until it feels right is what most do.

Somehow we've gotten away from the purpose of this thread. I'm completely familiar with the function of trimming the engine and its effect on how the boat responds. What I am not familiar with is the effect, if any, it has on fuel economy. I posted the question in hopes that someone does know, and then I would know for the sake of knowing.

[TownsendsFJR1300]

Somehow we've gotten away from the purpose of this thread. I'm completely familiar with the function of trimming the engine and its effect on how the boat responds. What I am not familiar with is the effect, if any, it has on fuel economy. I posted the question in hopes that someone does know, and then I would know for the sake of knowing.

If it's trimmed correctly, the highest speed for the given RPM's, that's the most efficient for MPG. THAT is the bottom line....

(It doesn't matter where the bow is).

[Jason2tpa]

Back about 1980, my other neighbor had a 17' Sutphen speed boat ( Home ) with a 200 hp, Evinrude 2 stroke. (check the link, the 17' restored boat is what my friend had)

We'd take it out, it'd probably do 55, 60 MPH, guesstimating (No GPS/speedometer).

When he nailed it from idle, the nose rose I'm guessing 3', then flat out
hauled a$$..

He would trim the engine up and leave EASILY a 15' tall, 30' long rooster tail!! I don't remember if it had a tach.

From a distance, you couldn't see the boat, just the rooster tail and we didn't have half the Manatee idle zones back then either.

It beat the snot out of you but it was a blast!!!!

Talk about skipping atop the waves!!!!!

Man that sounds like too much fun! It has been amazing to see technology grow at such a rapid pace. Topping speeds of 60mph+ used to be unheard of for center console boats 30+ft. Nowadays these go-fast offshore fishing boats are hung with trip 300's and reach speeds near 80mph. But until you've experienced moving at speeds 60+ on a boat with you sitting 1-2ft above the waterline, like you did with your neighbor, man you have no idea. 60+ in a flats boat or speed boat feels like you're riding lightning–with a white knuckle grip on whatever you can hold onto.

[TownsendsFJR1300]

But until you've experienced moving at speeds 60+ on a boat with you sitting 1-2ft above the waterline, like you did with your neighbor, man you have no idea. 60+ in a flats boat or speed boat feels like you're riding lightning–with a white knuckle grip on whatever you can hold onto.

Oh yea...

http://www.sutphenboats.com/resource...article_sm.pdf.

They make a 21' speed boat, 5 seat-buckets,
max sized engine 502 cubic inch MERC inboard, 525 HP.

94 PLUS MPH.... $120,000K

[Jason2tpa]

Oh yea...

http://www.sutphenboats.com/resource...article_sm.pdf.

They make a 21' speed boat, 5 seat-buckets,
max sized engine 502 cubic inch MERC inboard, 525 HP.

94 PLUS MPH.... $120,000K

Man that's a sweet boat. But if i'm gonna spend 100k+ on a speed boat might as well get a 40ft cigarette style. That way you and the ladies can duck under into the cabin.

[TownsendsFJR1300]

A 40' new cig boat you can easily spend 4x's that...

Useless (IMO) but to go fast, spend a bunch on fuel and maintenance..

I wouldn't drop 50K on a 21 foot anything!!

[99yam40]

If it's trimmed correctly, the highest speed for the given RPM's, that's the most efficient for MPG. THAT is the bottom line....

(It doesn't matter where the bow is).

I agree,

I would think that once up on plane you drop back on throttle to the RPM you want to run at and trim to get get the ride and RPMs to the best and then readjust throttle if RPMs get away from what you were wanting to run at.

I also would think having the trim not set properly for the speed you are running would be lugging the motor some.

To tell the truth I never thought about fuel use while lugging a motor much.
Carberated motors adjust fuel into the motor using the air movement thru the carbs.
The more air flow the more fuel.
the higher the RPMs the more air will flow,so more fuel.

Now fuel injected motors may adjust more for the load on motor if it has enough info on what the motor if doing.
I do not know much about the different injected motors to say whick one will do any of that.

Maybe this is more of the type of input the OP was looking for

[cpostis]

[QUOTE=Jason2tpa;129898]Here's one for y'all intellectuals to mull over:

RPMs are directly proportional to increasing the throttle, and fuel economy has a disproportional response to the same increase in throttle. On plane at a steady 2500 rpms, you increase throttle to hold steady at 3000 rpms. The result is you reduced fuel economy.Yes

Here's my question:

On plane at a steady 3500 rpms, you increase your bow angle. What results is less drag and a corresponding increase in rpmsAssuming the bow has risen and then lowered to slightly above the pre-plane condition. You did not increase throttle, yet you increased rpms. What fuel economy response do you expect? Load would decrease, forward speed would increase, thus better fuel economy

Does fuel economy remain consistent with the throttle position at 3500 rpms?Yes, if load remains the same as well
Does fuel economy increase because of less load? Yes
Does fuel economy decrease because rpms increased? Again, yes if load remains the same[/QUOTE


However, I think we are all saying there are many variables considering attitude of the boat dealing with fuel economy. When your bow is too low, you are "plowing" the water placing more load on the engine. When your bow is too high, your engine is straining to keep the bow at that level (maybe "porpoising", the bow raises, then drops because the motor cannot hold the bow at that level).

Basically, you want to have the angle of your propeller pushing as much forward speed as possible with as least hull on the surface of the water at the least angle. Bottom line, least load on your engine. When you are on proper plane, not only is the bow higher in the water, the stern is as well. Now consider sea conditions, you want to have your boat in the proper attitude to get best fuel economy and safe/comfortable conditions, a compromise.