effects of pitch changing

[northmansx]

Here is my situation. I have an 85 merc 25 hp with 13 pitch that spun the rubber out of the prop the other day. I ordered a new one, had 3 choices but accepted the only one they had in stock which was turns out to be a 14.5 pitch.

What can i expect for power difference between these two props in terms of speed, and torque?

 

[RogersJetboat454]

Re: effects of pitch changing

Depends on how happy your motor was with the 13P.

Over-all, less hole shot power.
If the 13P was under-proped, might gain some top speed.
If the 13P was correct, or too much it's self, the new one will lug the motor which isn't good.

No prop shops in your area to put an new hub in your old prop?

 

[Texasmark]

Re: effects of pitch changing

+1...............

 

[northmansx]

Re: effects of pitch changing

there are no prop shops in my area at all, i am a 3 hour drive from the nearest town in the middle of nowhere. I asked the dealership if they re-hub and he said no they don't. I may get it re-hubbed from somewhere else though.

it seemed to push my 16' lund well until it was loaded then it slowed down quite a bit. In a perfect world though in relation to the same boat and motor, same load, same everything will 1.5 degrees make a huge difference? i won't be testing it until friday but would like to know kind of what to expect.

 

[RogersJetboat454]

Re: effects of pitch changing

In a perfect world though in relation to the same boat and motor, same load, same everything will 1.5 degrees make a huge difference?

Yep.

You say freighting down the boat with people and gear really slows it down, proping up (the equivalent to going to a higher gear in a manual trans) isn't going to help the situation.

Ultimately, if you really want to know how well the prop is working for your motor, you need to put a tach on the engine, and figure out where you stand at WOT loaded and unloaded. There is a acceptable WOT range given by the manufacturer. RPM's too high, and you're obviously over-revving the engine. Too low will lug the engine, and that's when you start melting pistons.

 

[jestor68]

Re: effects of pitch changing

The ONLY way to know what the correct prop is for your boat is to use a tachometer to be certain the motor operates within it's recommended WOT rpm range.

Guessing doesn't get it. The Mercury prop selector will help in getting a pretty accurate estimate; but then the suggested prop must be tested with a tachometer to be sure.

For reference, your gear ratio is 2.25 and your rpm range is 5000-6000 rpm.

An 11 or 12 pitch sounds about right in order to get your rpm between 5500-6000 rpm.

 

[steelespike]

Re: effects of pitch changing

Pitch is measured in inches.1.5" is almost 9% higher pitch.
What they said above and I don't think the 14.5 will be satisfactory at all.
Don't test it. See if you can return it for a full credit.
There are 10 or more 13" props listed here at iboats if you include stainless props.