Propeller size for 1977 mercury 1150

[tavacska]

My boat is 1977 boston whaler montauk
outboard: 1977 inline 6 1150
propeller: solas 13.25 x 17
RPM: 4600
Boat runs very good now, no issue so far.

So I need to change 17 pitch to 15 pitch, which looks very low to me.
Any comments please? Thanks.

 

[racerone]

It is a Whaler.----But foam in the hull could be waterlogged.

 

[Chris1956]

NADA says boat weighs 950#. Engine is 300#. Trailer is likely about 300#, but it's weight should be avail at the trailer OEM web site.

You might weigh is, as 17P prop is what I would expect would work fine. Tacho is accurate? Motor is running on all cylinders?

 

[tavacska]

Tacho is accurate. Calibrated with a signal generator.
Motor running sounds very healthy. Idling trolling very good too. It can jump to run right away.

I haven't tried different trim though. Does that play a big difference on RPM?
I didn't play with trim very much. Just trim it to bottom and run it all day.

 

[Chris1956]

Try trimming it while on plane. You are looking for 5500RPM.

 

[Texasmark]

You didn't say what length. BW www site (https://www.bostonwhaler.com/boat-models/montauk/170-montauk.html) shows a performance curve with a 115 and 90.

I'd think that your 17P with the 2:1 gearbox would work very well. Go fast.com prop slip calculator numbers with the speed as posted on the BW site of 33 at 4600 RPM shows an 11% slip (https://www.go-fast.com/Knowledge-base/Find-your-prop-slip/Prop-slip-calculator). With the estimated 2500# (per BW site) fully loaded weight I would say that you are where you want to be and 11% slip sounds about right.....to me......except for one thing.....your RPMs need to be up around 5500 with a light load and at least 5000 with your normal load. So dropping down in pitch is a great idea and on this site, 150-200 RPM change per inch of pitch is normally used.

Go fast calculations (https://www.go-fast.com/Knowledge-base/Find-your-prop-slip/Prop-slip-calculator) indicate that your dropping to a 15P should get you the same performance in the upper RPM band. It will improve your hole shot and cruising speed will be better stabilized just above planing speed.

On the proper trim comment, an absolute must to find the sweet spot if you want max. performance. Very big performance enhancer.

 

[racerone]

Yes , dropping pitch will increase RPM.-----But there is a need to determine condition of the engine.-----Post compression test numbers.----Does spark jump a gap of 3/8" on all leads ?-----Carburetors confirmed clean ?-----Carburetor fully open at full throttle ?-----Fuel pump diaphragm ruptured ?----Water pump impeller replaced as it needs a new one about every 3 years.-----How did the boat run for you last season.?