Fuel Economy at Cruising Speeds 17Pitch vs 21Pitch prop

[comstox]

I asked this same question last year and it turned into a 5 day long question and answer session that barely led me in the right direction. (Mostly because of my lack of boating and technical knowledge, but I DID learn what WOT meant! (Believe me, I was grateful for the input, but I think I had over-asked my question.) Today I'm simply asking a simple question that SHOULD lead to a simple answer. As a matter of fact, I would like to ask if the length of your answers could be kept to a minimum, and if that's not possible because of too many technical factors, let me know if what I'm asking is not answerable in a few words.
We run an older fiberglass speedboat with a 1984, 75Hp 2-Stroke Mercury Outboard. We mostly cruise at just above plane speed of 25 - 35 Mph. IF WE SWAP OUT our 21P prop for a 17P prop AND COME UP ON PLANE and cruise that sweet 25 -35Mph speed, WILL I USE LESS FUEL?
Okay guys, Let's have it!

 

[ahicks]

Short answer = No.

 

[dingbat]

No. It takes X amount of horsepower to push a hull at speed Y.

1 horsepower = 2,544 btu /hr
1 gallon E10 = 120,096 btu

 

[Sea Rider]

As OP hasn't mentioned any rpm whatsoever, assume the boat doesn't count with an installed tach, right ? if so, need to buy one and install it as a starter...

Once installed go for a wot run on flat calm water cond, report max rpm achieved with current prop, check if within min-max wot rpm range factory stated for that motor. Correct RPM working range is more important for any motor than achieved speed which is secondary..

If wanting to achieve the best fuel economy while cruising you need to prop your motor to run towards max wot rpm range as loaded, if with a fixed load much better going towards a spot on prop maximization. Once motor is revving at its max wot rpm range, go full throttle for a fast plane, once on plane reduce throttle to a position that will maintain combo riding on perfect plane and motor revving smoothly and happily.

Happy Boating

 

[beerman2]

No: smaller pitch prop means more throttle to maintain same speed as higher pitch. Be careful to not over-rev your engine with smaller pitch. Pitch equals amount of movement thru the water with each revolution of prop. 17 pitch= 17 inches. Happy boating.

 

[GA_Boater]

No.

 

[jimmbo]

Larger pitch means higher load, and therefore more throttle to attain and retain a given rpm or speed.

Fuel Economy at WOT might be a bit better with the higher pitch, as the rpms are lower and that the carbs wide open, so the amount of air being being sucked in would be less. Then again if the VE(volumetric efficiency) is higher at the lower rpm,(closer to the peak torque rpm) the airflow might be the same.

 

[Sea Rider]

OP has not stated if boting lightly, medium light or heavily loaded nor wot rpm range achieved with any of both 17 and 21 pitch props, so expect theoretical answers. Without knowing wot revs and load posted thread is going nowhere...

Buy a tach LOL!!

Happy Boating

 

[Scott Danforth]

pick the prop that gets the WOT speed in the window its supposed to be in. then cruise speed is what you feel comfortable cruising, just on plane

lugging motors is bad, not only will it cost you more fuel, it will also kill your motor.

fuel economy and power boating are a dichotomy.

so with a 17p prop, what is your WOT RPM? with the 21p, what is your WOT RPM?

 

[QBhoy]

Need to know more details if rpm at certain speeds currently.
If the 21” is currently well suited and in the rpmrange at wot.....the answer is no...the 17” will not be better on fuel at the same speed.

 

[jimmbo]

And if the 17 is correct for WOT rpm, then the 21 is very poor choice in every case as it will be overloading the engine

 

[comstox]

Guys I am planning on reporting back after this weekend with the findings on RPM @ WOT. I did locate my manual and it say that 5500 is my max WOT RPM. At this point I KNOW that the prop I have is too big, but I'll try the 17P prop and report back as soon as I can.
Thanks guys!!!

 

[Sea Rider]

Need to know more details if rpm at certain speeds currently.
If the 21” is currently well suited and in the rpmrange at wot.....the answer is no...the 17” will not be better on fuel at the same speed.

Nobody knows for sure till the OP posts his wot rpm achieved as loaded. If the 21 pitch prop is theoretically achieving full wot 5500 rpm as factory stated, the 17 pitch besides being awful in fuel economy will overrev the motor real bad towards kaboom territory, LOL!!

Happy Boating

 

[QBhoy]

Nobody knows for sure till the OP posts his wot rpm achieved as loaded. If the 21 pitch prop is theoretically achieving full wot 5500 rpm as factory stated, the 17 pitch besides being awful in fuel economy will overrev the motor real bad towards kaboom territory, LOL!!

Happy Boating

I think I pretty much just said what you did, haha.

 

[Faztbullet]

That motor will turn 6000-6500RPM all day long

 

[comstox]

That motor will turn 6000-6500RPM all day long

I hope not! My manual says 5000-5500 is peak @ WOT!

 

[comstox]

That motor will turn 6000-6500RPM all day long

I'm thinking you're saying the 17P will allow the RPMs to get too high? Manual says 5000-5500 is MAX @ WOT.

 

[jimmbo]

That motor will turn 6000-6500RPM all day long

And it would be making less than it's rated HP at those RPMs

 

[ahicks]

What seems to be lost here is the fact that the prop YOU like best is a going to be a compromise of hole shot/pulling power, cruise, and wide open throttle performance. It is absolutely impossible to get a prop that will do all best possible! Decide where your priorities lie, and prop for YOUR priority!

If the engine rated at 5500 max turned up 6500 with a prop that worked well for YOUR priority, use that tool between your ears and don't give it that much throttle! It's not rocket science....

 

[Sea Rider]

I hope not! My manual says 5000-5500 is peak @ WOT!

You can prop the motor right to run 7 K wot revs if wanting to, the question is how long will the motor last. Can run 5% max over max wot rpm safely. Much better is running towards its max wot rpm as loaded.

Happy Boating