Will prop diameter affect speed?

I have a mercruiser 140 I/O. The old prop I had on there was a 15.75 x 17" and it is pretty damaged. I would go about 30 mph max speed at 4400 rpm. So I ordered a new prop to see if I could go faster, a solas 13 x 19". We tried it out and we probably maxed out at 10 mph. Is this normal? Our gear ratio is 1.98. So obviously, the smaller diameter spins too fast in the water, so if I get a higher pitch prop, say 21" pitch, what diameter should it be?

Re: Will prop diameter affect speed?

You do need to read the stickys at the top of the prop forum and get us as much info as possible.
We'll get you going.First of all increasing pitch is like shifting to a higher gear its harder for the motor to turn
if it can turn the 19 up to the right rpm it would normally be faster than the 17.
If your boat is the typical 18' or so with a 3.0 it would normally over rev with a 17" pitch.
Typical prop would normally be a 19 or 21" with speeds approaching 40 mph at about 4600 rpm.
Diameter is a secondary consideration and as pitch goes up diameter goes down.
I think your 13" diameter is intended for an outboard. Besides being too small its probably leaking exhaust into the prop stream.How did you select your prop?
Props for an I/O like yours would normally be over 14" in diameter and would vary with the pitch.

Re: Will prop diameter affect speed?

My boat is a 16.5 foot bayliner with a striaght 4, 140 horse. When I bought the prop, it said it was for pre-alpha, so me not knowing anything about props at the time, I bought it. I am just surprised that 2 inches shaved off of the prop diameter would make that much difference.

Re: Will prop diameter affect speed?

When you say 10mph...like full throttle - 10mph? Was the RPM at the rev limiter?

SS would this just be severe slip from lack of surface area/grip or with it being so severe probably cavitation? Little of both?

Re: Will prop diameter affect speed?

Yes, full throttle at 4400 rpm, 10 mph.

Re: Will prop diameter affect speed?

BoatDrinksQ5 said:

When you say 10mph...like full throttle - 10mph? Was the RPM at the rev limiter?

SS would this just be severe slip from lack of surface area/grip or with it being so severe probably cavitation? Little of both?

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Don't we know what year the boat is.And does it respond to up trim with the 17" prop.
My research indicates a 13" prop is not intended for a I/O As prop pitch goes uo diameter decreases even a 21" prop is 14" or over.
I can't be sure but a 13" prop probably doesn't match up to the gear case so the exhaust is fouling the prop
also could have a spun hub. Based on the rpm with the 17 either the prop is badly dinged up or the setup has problems..
It may be cost effective to have the prop repaired. Then do a test run and d get us the info.
With that little boat the 17" prop should rev off the chart. And a healthy motor and drive train should do well with up to a 23" prop.
at 40+ mph. Not good for skiing or a big load of people but good speed.

Re: Will prop diameter affect speed?

I have a 15.75 x 17P prop and it's intended as a heavy load prop, you don't have a heavy load so that prop isn't a good match. On the flip side, you replaced it with a 13x19P prop which is too small of diameter so you don't have a good match there either (probably lots of exhaust venting around prop barrel). As ST said, I/O props run around 14" diameter, more or less diameter depending on pitch, something like 15x15P or 13.25x23P.

Is the barrel of the prop the same diameter as the lower unit, it should be 4 3/8". If it's a 4" barrel prop, it's intended for an outboard (again, as ST said)

I checked Iboat's website and the only140HP Mercruiser 19P prop that's around 13" is a Quicksilver 5 blade prop, all others were 14-15" diameter. What is the part number of the Solas prop you bought?

If you go with a 17P prop in the correct diameter (14.5 to 15"), you'll probably be over the engine's WOT range, maybe right at the top end.

Something odd, with the 13x19P prop it's only reving 4400RPM at 10MPH, TONS of slip. I would think you should be getting more RPM out of the engine with a prop slipping like that??? Maybe 4400RPM is all the engine will turn under load with it's current tune. WOT RPM should be 4200-4600RPM on the 140HP Mercruiser, and it's best to prop at the top end of that.

Re: Will prop diameter affect speed?

Thanks you guys for all the info. The barrel of the prop is smaller than the outdrive, so it was intended for an outboard engine. I bought a new prop that is 15 1/4 x 21P. I think this will get me up to speed!

Re: Will prop diameter affect speed?

Based on how the 17 did I would say until we figure out why it did so poorly the 21 will be too much prop.
But give her a try.

Re: Will prop diameter affect speed?

wardfamid said:

I bought a new prop that is 15 1/4 x 21P.

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That's still a heavy load prop, a 21p prop's diameter should be right around 14", give or take 1/4" (Mercruiser 140 I/O).

I agree with SS and think you'll have performance issues with that prop, too much pitch (and diameter). Can you return the prop?

You don't have any good figures to run with, but I'll throw out the suggestion of a 14 1/2" x 19P prop. See how that runs.