Guess on MPH increase, prop pitch.

[hoofbite]

Yeah I know "get on the lake and see!" but does anyone have a guess on a MPH? I finally got my 19 pitch reconditioned looks like new!! I swapped out it from a a beat up 15 pitch that I managed 32-33mph @ 5500 rpm on the gps. Any idea of what I might gain in MPH. FYI it's a 1976 75hp chyrsler. Same boat same weight 2 guys totaling 400lbs.

 

[fucawi]

Re: Guess on MPH increase, prop pitch.

19 pitch is way too much unless its a hydroplane.......19 pitch is what you put on a 115 hp.....your revs will drop about 800 so what you gain in the pitch you loose on the rpm 15 pitch sounds about right.....the way to get more speed is a bigger outboard .

 

[jerryjerry05]

Re: Guess on MPH increase, prop pitch.

The 19 will KILL the gas economy.It can cause overheating and damage.Use it as a spare.
Oh yea it will probably slow you down.

 

[JB]

Re: Guess on MPH increase, prop pitch.

The 19 will KILL the gas economy.It can cause overheating and damage.Use it as a spare.
Oh yea it will probably slow you down.

Yup. Might well kill the engine, too.

 

[steelespike]

Re: Guess on MPH increase, prop pitch.

While the engine model list is incomeplete here it only goes to 18" with a 75 hp.
Yes the 19 on your setup will be like trying to haul your boat and trailer up a never ending hill with your car locked in overdrive.
My info indicates the 75 is rated for 4,750 rpm. I would suspect your tach isn't accurate,
reading high.I think your 15 repaired may help bring the rpm down.In the mean time test the 19 and confirm your tach is accurate against a shop tach or other reliable source.

 

[Maclin]

Re: Guess on MPH increase, prop pitch.

That is a BIG jump in pitch in my opinion, and I will explain why I think that. I had a 55hp on a 15 foot trihull with 11 pitch prop. That hub spun so I bought a 12 pitch and sent the 11 off to get fixed.

• With the 11 it could hit top RPM's and speed (not quite 30) in 7-8 seconds.
• With the 12 it never hit the top RPM's even with just me in the boat (with the wind, downhill,etc..).
• With the 11 I could have 2-3 people in the boat and towing one skier (2 skis, slaloms were always a load) or tube or kneeboard. Heavier skiers I could only have 2 smaller people besides me in the boat.
• With the 12 it could barely get up on plane with just 2 people in the boat, took a long time. Skiing was out of the question, kneeboard was just ok.

Not trying to be a downer, just some real world numbers for you to correlate.

 

[Nathan k]

Re: Guess on MPH increase, prop pitch.

I am just repeating other people?s opinions here... stick with your 15.
If your tach is correct then try like a 16 and (maybe) a 17
The best way to do it is to make sure your tach is working correctly and to decrease rpms by about 200 go down a pitch and to increase by 200 go up. Then get the rpms to what the manufactures recommend.
You should also try different make props, which can add the odd mph.

Good luck!

Nathan