Proper prop size for my boat?

Question

I have a 16.6 ft fishing boat, with a 40hp johnson motor on it.

I'm having problems planing out with 3 or 4 people in my boat.

My current prop is 11.5 x 19, I asked at a couple boat stores and got similar but different answers.

What should I be looking for. Also what will happen to my revs(Current 4800 max when planed out), speed, and gas mileage if i go down in prop size?

Thanks.

Answer 1

You generally would not decrease the size but go to a lesser pitch. If I read you right, you have a 11 1/2 " diameter and 19 " pitch, what is called "oversquare". What is your redline?  Are you near your recommended RPM? A move to a 17 " pitch prop (or maybe less?) would be a good start if you aren't getting enough RPMs. Of course, I don't have any info on your boat- weight, hull shape, etc. Whether or not you are already at your correct RPMs , you should also pay attention to TRIM. Weight distribution and trim tabs can work wonders for presenting the least amount of resistance to acceleration and therefore planing, not to mention fuel economy. SO in order: Revs would increase, speed would increase if you weren't planing well or in rough water or with a heavy load, but would be less in smooth water with a light load, and gas mileage? If getting the correct pitch gets you going more efficiently, it could increase economy, but carrying more people will need more fuel of course! Have you considered "Doel -Fins"? These foils attach to the cavitation plate and have shown an improvement in the planing characteristics on my 11 1/2 ' whaler with a 25 hp engine on it. Planes quicker and stays planing at a lower speed. Finally, 40 horses may be not enough to push a 16 foot boat with 4 people all that easily! Good Luck- Carlo

Answer 2

general rule, one inch of prop pitch will change RPMs by 200.

here's a link to a prop pitch calculator:

http://www.boatpropellers.com/propcalc.h...

running on plane will improve your fuel economy and speed.

I usually advise against hydrofoils except on underpowered boats -- I suspect you would benefit.

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"I also use this motor for trolling, If i drop down to the 17 size, Will this effect my trolling speeds dramatically?"

You might not even notice the difference while trolling -- on the other hand, if you're needing to slow the trolling speed down just a little bit, it just might do the trick.

Answer 3

Was one of the recommendations: "Get a bigger motor!"  The rpm range you are dealing with would allow for a one size downward pitch, ie 17" but the real issue is this boat will only plane out a little better.  It's the load that's just a little more than the engine can handle.  If you are going to fish with 4 people you need a bigger boat.  The used boat market in boats is buyer friendly right now, so ......buy!

Answer 4

I agree with Richard. You have too much weight (boat, gear and people) for your engine HP. Your rpms could be higher with a different pitch but won't solve the weight problem.

Answer 5

Try a 17 pitch bring the RPM up to 5200 you will go faster and have more low end power>