What prop size do I need?

[sebas54]

Hi, I have a 22' 1980 Aquasport with a 4.3l engine 175hp I/O.

I corrently have a prop size of 15.25 x 15.
My max speed at 4450rpm is around 26mph.

I dont have trim tabs but,

Does this sound right?

I have to buy a new prop and would like to get some more speed from it, if possilble.

I appreciate all of the help.

Thanks

 

[rndn]

Re: What prop size do I need?

That's an awful big boat for a V-6 engine so that may be all the speed you're going to get. Your boat weighs 2200#.

 

[Bondo]

Re: What prop size do I need?

I corrently have a prop size of 15.25 x 15.
My max speed at 4450rpm is around 26mph.

Ayuh,......

It sounds like you're either Way Down on Power,.... You've got the Wrong Ratio Outdrive,....
Or,....
Your hull is Waterlogged.....

Even with the 15" prop, you're Not pulling very high rpms.......
Matter of Fact,.... It's Barely into the recommended RPM Range......

 

[sebas54]

Re: What prop size do I need?

SO,

What do I do?

Change prop,
compression test,
and how do I figure if it is water logged and the type of gear ration??

When I bought it last year the boat always leaned to on side more than the other about an inch or two. You could see the water line when the boat was out of the water. Owner told me it was because of the the sterring was on that side and something else. thanks for all the help.

 

[sebas54]

Re: What prop size do I need?

should I get a larger or smaller prop??

 

[tallguy63]

Re: What prop size do I need?

If you want to check for a water logged boat, drill or use a hole saw somewhere where it won't be seen and find out. You might want to consider drilling into some of your foam to see if it is soaked. That can account for a lot of weight. You might want to do the same to one of your stringers on the heavy side of your boat, but use a small longer bit. I just rebuilt my boat from the hull up and the soaked foam and rotten wood accounted for a lot of dead weight. good luck

as for prop size...i'm still trying to figure that out for my boat...

 

[b_lish1966]

Re: What prop size do I need?

I'm by far no expert on boats, but have learned alot in the past year by boating itself and talking to people on iboats and others as well, having said all that I believe you should get more speed from that boat, as a rule of thumb the lower the pitch the lower the speed. One thing you can do to check the weight is take it to a scale and have it weighed, it's really worth it because you might be dealing with a water saturation issue. now if the weight is ok then I'd say change your prop pitch to say 19 or 21 three blade, stay with aluminum if cost is an issue also and also since you don't know what to expect from the changes, I believe for one digit change upwards in pitch you will gain 100-200 rpm so the increase will bring you closer to what your rpm range should be.

 

[rndn]

Re: What prop size do I need?

I'm by far no expert on boats, but have learned alot in the past year by boating itself and talking to people on iboats and others as well, having said all that I believe you should get more speed from that boat, as a rule of thumb the lower the pitch the lower the speed. now if the weight is ok then I'd say change your prop pitch to say 19 or 21 three blade, stay with aluminum if cost is an issue also and also since you don't know what to expect from the changes, I believe for one digit change upwards in pitch you will gain 100-200 rpm so the increase will bring you closer to what your rpm range should be.

I'm sure this poster means well and I really don't want to bash his free advice, but going up in pitch is the absolute last thing you need to do. Your motor is already at the minimum rpm's at WOT and going up in pitch will make it even worse. If the boat is not water logged and the engine is running properly, then you need to decrease in pitch to get the rpm's up. Your motor will run at the correct rpm's and will perform to the best of it's ability. One inch of prop pitch gain, say going from a 17 inch pitch to an 18 inch pitch will reduce the rpm's by about 200. The opposite is true for going down in pitch from your 17 to say a 15 you should increase your rpm's by about 400. You may actually get more speed by reducing the pitch, but your engine will run where it was designed to run.
By the numbers it appears you should go to a 13" pitch which is really really low, which points to something not right either with the boat and/or motor.

 

[r81gsr95]

Re: What prop size do I need?

I'm by far no expert on boats, but have learned alot in the past year by boating itself and talking to people on iboats and others as well, having said all that I believe you should get more speed from that boat, as a rule of thumb the lower the pitch the lower the speed. One thing you can do to check the weight is take it to a scale and have it weighed, it's really worth it because you might be dealing with a water saturation issue. now if the weight is ok then I'd say change your prop pitch to say 19 or 21 three blade, stay with aluminum if cost is an issue also and also since you don't know what to expect from the changes, I believe for one digit change upwards in pitch you will gain 100-200 rpm so the increase will bring you closer to what your rpm range should be.

your rpms wont go up as you increase the pitch. Sorry but you are wrong.

 

[sebas54]

Re: What prop size do I need?

Thanks for all the info. I just purchased a prop 16 x 14. Hopefully this will help and everything will go well. I will post my result.

 

[sebas54]

Re: What prop size do I need?

I just got back from taking the boat for a test spin. The new prop 16 x 14 brings my rpms to 4700 but my speed was only 24mph. Should I go back to my 15.25 x 15 prop. Please any recomendation are greatly appreciated.

Thanks.

 

[sebas54]

Re: What prop size do I need?

Any Ideas???? Please.

 

[Dandelion]

Re: What prop size do I need?

A 16" prop seems like a really low pitch for your set-up... but if you're making 4700rpm at wide open throttle that's just about where you want to be. You may squeeze out a few more mph by going with a higher pitched prop, but you're going to lose rpm's, which means you'll be "over-propped", and lugging the engine, and that's damaging.