Advice on prop pitch

[droo88]

Hi there I'm new here . I like this forum a lot, very informative!

I have a question regarding my current prop pitch and whether I should change to a lower pitch one.

My boat is a 13 foot displacement hull with a small cabin and powered by a 1984 evinrude 35 (10x15 prop). I normally cruise at 7 mph but at WOT the boat will reach 20mph and rpm is at 4200-4300.
Now the manual says that ideal rpm lies between 5000-5500.
My question is this. Should I switch to a 12 or 13 pitch prop? I rarely go at wot since fuel consumption suffers greatly. If I change to say a 13 pitch would I see a difference in both cruising speeds and WOT speeds whilst maintaining fuel efficiency?

Thank you for your help!

 

[steelespike]

Re: Advice on prop pitch

At wot your motor,and any 35, will use about 3.5 gph(gallons per hour) whether
pushing a houseboat or a runabout.You need a setup that gives the best speed for a given throttle
setting while remaining within the wot rpm.No need to operate it at its wot rpm it just needs to be able to reach it. If a motor can't reach its wot rpm rating it is working harder(lugging) at whatever throttle setting you selectThis can cause premature motor problems over time and not likely to be efficient.
Your not likely to find a prop that gives the best displacement economy
and the best wot economy.Marine engines need to be able to rev freely within their rated rpm.
I would think for displacement speeds you need a prop with large diameter and big blades.
To try to get the lowest slip numbers. Perhaps a 4 blade prop.
Its also possible that wot operation may need a similar style though less exaggerated prop.
It does appear you need to reduce your pitch by about 4" to gain about 800 rpm.
You will likely find that as you reduce pitch the diameter will increase I feel you need to pick one
one likely to work best for your displacement speed.
If your serious about the finding the best setup you need a flow meter.
The difference between a good setup and an excellent setup will likely be measured in tenths
or even hundredths of a gallon.

 

[jestor68]

Re: Advice on prop pitch

No matter what the boat, the motor needs to be propped to allow operation within it's recommended rpm range at WOT.

You need to gain about 800 rpm to prevent your motor from lugging or laboring all the time.

That would equate to 4 inches of pitch reduction.

A 13 ft boat with a small cabin? Is that a typo?

 

[droo88]

Re: Advice on prop pitch

Thanks for the replies. No typo lol, the boat has a very small cabin where I keep the fuel tank, lifejackets etc

Normally when cruising the motor is turning at about 2500 rpm which equates to 7mph (set to about half throttle). Would the engine still be laboring even at that slow speed?
If I were to install an 11 inch pitch prop, would my cruising speed be lower or would I gain speed since the rpms increase? Sry for all the questions but I am new at this.

Current fuel consumption at 7mph is approx 0.8 liters per mile. I haven't tried it out yet at WOT.

 

[jestor68]

Re: Advice on prop pitch

Your motor developes it's peak horse power at 5000 rpm.

If it can't spin up to it's peak hp rpm, it is laboring all the time except at idle speed. This causes higher than normal stresses on all of the motor's internal parts, shortening it's life.

With the proper propeller pitch, it will be turning a little higher rpm at a given speed(compared to now), but the motor will actually be working less hard and will likely not use any more fuel. I'm putting you near the bottom of your WOT rpm range, where you will experience the best fuel economy versus performance and motor longevity.

With the proper pitch, I doubt you will lose any top speed. In fact, I expect you might gain a little top speed.

The important issue here is the longevity of the motor. It will last a lot longer this way.

 

[droo88]

Re: Advice on prop pitch

Thanks for the informative replies. I think I will go either for a 10.5 x 11 3 bladed prop or else a 4 bladed for a bit more stern lift and thrust.
Thanks!