Prop Size Please help

[sebas54]

Okay, I know that this has been covered over a thousand times but I forget things very easy and I was unable to find it.


My engine is over revving. I have a 1987 gm 4.3l alpha 1 in a 1980 aquasport 22 feet. The recommended rpm is, I think 4400-4600. the engine goes to about 5200 and only go about 27 mph. Right know I have 15 x 15 prop. I forget the rule about the realtionship betwwen the size of ther prop and the rpms.


Please help, I want to make my boat fast and more efficient.


Thank you all.

 

[cooter2506]

Re: Prop Size Please help

From what I know you go up in pitch when you are over rpms and lower if you are under. But are you sure your not have a slipping prop. 27mph at that rpm seems low. But it is a big boat for a v-6.

 

[Silvertip]

Re: Prop Size Please help

The first thing is to make sure the tach is accurate. Many a prop has been changed because a tach was not calibrated correctly. Use a shop tach to verify it's accuracy. Your current wide open throttle rpm is 600 rpm too high and with a 15P prop it is easy to see why. To lower the rpm you need a prop with three inches less pitch (150 - 200 RPM/inch). Now this is where things can get confusing. IF you use an 18 or 19P from from the same manufacturer as your current prop, same design, and same material, that would "theoretically" put rpms in the 4600 rpm range. If you use a prop from a different manufacturer it will have different design factors so what it does will be hard to predict. Complicate matters even more if you switch from aluminum to stainless or vice versa and then throw four blades into the mix.

 

[steelespike]

Re: Prop Size Please help

Is the prop in excellent shape?Al.? SS? 3blade,4 blade?
You might want to check that the tach is acurate.Probably not way out it just seems a little high in relation to weight and size of the boat.
OK here goes. Rule of thumb 1" pitch = about 200 rpm.increase pitch decrease rpm decrease pitch increase rpm.That is quite a bit of boat at 2,200 dry and a deep V. I believe 170 hp.,not an awful lot. A 19 would put you at about 4,400
Might struggle out of the hole or when loaded up.
If you usually carry a bunch you might want try a 17.
I think I would verify the tach first.

 

[sebas54]

Re: Prop Size Please help

Thank you all very much!!

It currently has a aluminum prop that is brand new.

I will stay with aluminum because of the cost but the prop size I should get is 17-19 pitch by what. What should the other dimension be?

How do I check the RPMs for accuracy. Where can i find a shop tach.

Thank you

 

[sebas54]

Re: Prop Size Please help

What about the diameter?? What size should I get???

 

[boat1010]

Re: Prop Size Please help

You would usually stay with the same diameter on the prop and change the pitch. But like some of the others are saying, check to see that the tach is correct. The pitch angle is what determines the RPM at WOT. I would try a 17 and see what that does for ya. If you are really looking for speed the stainless steel is going to give you the most. Sure they are expensive but what isn't. It you take care of them they will last a long time.

 

[steelespike]

Re: Prop Size Please help

No need to pick a diameter it is a product of the design.Select a 17" pitch for your motor and the diameter will follow.If you have a marina you frequent they may let you borrow a shop tach.I believe an automotive tach will work.