Prop effect on plane speed?

[nichollb]

Hello,

I recently upgraded my old 1991 19' I/O bow rider that had a 190 hp 4.3L, to a 2005 19' I/O bow rider 220 hp 4.3L MPI.

The question that I have is about effect of the prop on the plane speed of the boat. The old boat would plane out at 12-15 mph and hit a Max speed of 35-38 MPH. The new boat planes at 20-22 and hits a Max speed around 50mph.

I have young kids and like to do a lot of towing tubes/wakeboards

I spend a lot of time between 15 and 20 mph and very very little time at 40mph +.

Will changing prop pitch get the new boat to plane out down closer to the 15 MPH that I want or is that more of a function of hull design?

Any suggestions on what I could try. photo is current / stock prop

 

[H20Rat]

Re: Prop effect on plane speed?

So think of the thrust coming off a prop. It is never 100% directly behind the prop, it is more of a cone of thrust. If a prop has a wide cone of thrust, some of that is going to hit the anti-ventilation plate and will lift the stern drive up. (other actions also at work here) That in turn lowers the nose, and makes you plane quicker. Of course, any thrust that isn't perpendicular to the waters surface is also thrust that isn't moving you forward, so you will lose some top speed.

If you add smarttabs you might be able to get the best of both worlds. The tabs lift the stern without directly pushing the bow down, and you can retain your top end.

 

[nichollb]

Re: Prop effect on plane speed?

Thanks for such a quick response,

So is what your saying is yes I think? With a different prop I should be able to cruise around on plane at say 15mph depending on how heavy we are loaded exc.

I under stand that I will be trading low end get up and go for top end speed for what we do that is a fine.

Any suggestions on what might be a good prop to try?

 

[Bondo]

Re: Prop effect on plane speed?

Thanks for such a quick response,

So is what your saying is yes I think? With a different prop I should be able to cruise around on plane at say 15mph depending on how heavy we are loaded exc.

I under stand that I will be trading low end get up and go for top end speed for what we do that is a fine.

Any suggestions on what might be a good prop to try?

Ayuh,.... I agree, for slower planin', go with a set of trim tabs 1st,....

Then you can play with the prop,... maybe a 4 blade in abit lower pitch,...

But do single steps, so ya can See the differences each feature makes,....

 

[Texasmark]

Re: Prop effect on plane speed?

Agree on the advice. Many respondents on here have just gone to 4 blades and dropped the pitch, which usually increases the diameter slightly but that's prop engineering science. They not only improve their hole shot, but less strain on the engine and lower planing speeds. Other thing is ability to control the boat at those slower near planing speeds. The boat doesn't run up and fall back. Rather you get a good steady controllable response.

On your numbers, the shape of the hull makes a big difference in those numbers in addition to just hp and thrust. Shallow semi-v hulls will plane out faster and stay on plane easier. Deep Vs with high dead rise at the stern, especially with an I/O will be sluggish to get out of the water (hole shot) but their advantage is smooth riding in rough water so you can't have it both ways. With water toys they will stay low......and smooth.

I'd go with a 4 blade alum for about $100 on here. If you don't make it the first time, you can buy some more of a different pitch till you get it right and you aren't out a lot of bucks. Then if you get tired of dinging up the alum, get a SS and be done with it. I know what tabs are, how they work, what it takes to install them, especially on complex variable hulls, and their effectiveness. Being lazy the choice would be the prop first........yes I know the big guns on here have preached get the setup right first then diddle with the prop. Well I gave you my reasons.