Prop Pitch - Will I gain any Speed?

[Bassn7]

I have an 18 foot fiberglass bass boat and a 1981 Mercury 50 HP 2-stroke with a 10 inch 12 pitch prop. I plane up and max out at 29 mph. Fine for my fishing needs. If I move up to a stainless 15 pitch...will I gain speed? Still plane up? Worth going to a better prop? I was hoping to get up to 35mph. Thanks for your help.

 

[harringtondav]

The best advice will come with a little more information. Mainly your motor's current rpm at wide open throttle. If you don't have a tach on your instrument panel invest in a photo tach. They're inexpensive. You'll have to remove the motor housing, put a small piece of the included reflective tape on the flywheel and a helper to either drive the boat or run the tach. Without your engine's rpm info, it'll be a trial and error guessing game. Expensive if you can't return your new prop.

 

[racerone]

It takes a small atomic bomb to gain 6 mph on an 18' boat.

 

[QBhoy]

My natural instinct would tell me your current speed would be around or not far off normal for that set up. Likely meaning it’s propped not too terribly. To go another 3” of prop might see it be a total dog out the hole.
I might also mention that although not impossibly unlikely, it is unusual in general terms to see a 15” or odd number pitch prop (generally) that’s out the box for your engine and gearcase size. Only speaking in general and usual terms. Just make sure it’s the right suitability for your gearcase maybe.

 

[jimmbo]

Another 25 - 30 hp would get you that 6 mph. A prop change isn't going to do it. 29mph on an 18 ft with a 50 is good, actually very good

 

[QBhoy]

Another 25 - 30 hp would get you that 6 mph. A prop change isn't going to do it. 29mph on an 18 ft with a 50 is good, actually very good

What I was thinking. I’ve known 14ft fletchers with a 50hp to do around 30, and going well. Typically fast hulls on them too.

 

[WesNewell]

If your boat weighs less than 1000# and has a good fast hull, you might get 35 mph out of it, BUT, everything would need to be properly mounted. And that would start with a proper engine mount height. I've got a 60 hp on a 17'5" boat that weights 995# dry. Bought new it only did 31 mph with a 4 blade 12p prop. Just replacing the prop with a 15p ss 3 blade prop got me up to 40mph, but had to trim it so high it looked like a porpoise in the water and the ride at that speed was really rough. With it trimmed down enough for a smooth ride, the most I could get was 35 mph. After raising the engine up 1.5" (2 holes), I got a smooth ride at 38 mph and could hit 40 mph with just slight porpoising. But the engine rpm was below rpm range. Going to a 14p prop brought that back up with the same results in speed.

 

[Sea Rider]

Without a tach won't see the light at the end of the tunnel any time soon...

Issues to consider to gain more speed or performance : Hull must be immpecable clean, motor in top running state sitting at the sweet transom height spot, well trimmed and deck load well distributed..

With motor running at max wot rpm range will achieve top hole shot with slight less top end speed, running middle wot rpm achieves slight more top end speed with a slower hole shot. Can't have both top scenarios with current prop. Dial a prop that will make the motor to run towards the middle wot rpm range, for that purpose will need to know the factory min-max wot rpm range that motor revs at in ordr to play safely with prop pitches.

Dropping 3 pitches from current installed prop will decrease wot rpm dramatically towards the lugging side of the motor which is nada bueno. Dial a prop that will make the motor to run towards the middle wot rpm range, for that purpose need to know the factory min-max wot rpm range that motor revs at in ordr to play safely with prop pitches.

Happy Boating

 

[QBhoy]

Without a tach won't see the light at the end of the tunnel any time soon...

Issues to consider to gain more speed or performance : Hull must be immpecable clean, motor in top running state sitting at the sweet transom height spot, well trimmed and deck load well distributed..

With motor running at max wot rpm range will achieve top hole shot with slight less top end speed, running middle wot rpm achieves slight more top end speed with a slower hole shot. Can't have both top scenarios with current prop. Dial a prop that will make the motor to run towards the middle wot rpm range, for that purpose will need to know the factory min-max wot rpm range that motor revs at in ordr to play safely with prop pitches.

Dropping 3 pitches from current installed prop will decrease wot rpm dramatically towards the lugging side of the motor which is nada bueno. Dial a prop that will make the motor to run towards the middle wot rpm range, for that purpose need to know the factory min-max wot rpm range that motor revs at in ordr to play safely with prop pitches.

Happy Boating

Hi searider. I’ll let you edit the “spot the mistake” in your post, haha. I know fine well you didn’t mean it, given your experience though. Just being a cheeky bugger. Forgive me !