Prop pitch and size change

[Newyota]

Can a beat up 10.5x11 be changed to a 10x13 economically or at all?.Rpms at 5900plus on 25hp.Should be 5500 max I believe.

 

[F_R]

go for it

 

[jakedaawg]

Yes but why reduce the diameter? That defeats part of the pitch change...

 

[Newyota]

Yes but why reduce the diameter? That defeats part of the pitch change...

As a side bar I am running a rock cage on my motor.Would this effect the rpm's if it has some cavitation?Guess I need to take it off and go for run to see.And looking at parts book it should be 10x13

 

[F_R]

Yes but why reduce the diameter? That defeats part of the pitch change...

Not really. It is customary to add more blade area when reducing pitch in order to push heavier loads. Reduce blade area (drag) when increasing pitch for lighter, faster boats. Carry the theory a step further, you would get rid of a blade (run a two-blade) with high pitch for very fast boats, like a race boat.

 

[jimmbo]

Yes but why reduce the diameter? That defeats part of the pitch change...

Many Prop makers would reduce the Diameter as the Pitch increased. In the past year or two there seems to be a real Fixation about Diameter. Let the prop engineers worry about it. Myself, I try to get smaller Diameter props, hoping to increase the pitch, and I would be foaming at the Mouth if Mercury would reintroduce 2 Blade Props I could fit on my I/O

 

[jakedaawg]

Yes guys, I thoroughly understand and agree with what you are saying...my point was a little different though...

we aren't talking about a champ boat or an f1 or a siebold. This is presumably a 25 johnson. He wants to lose 400 rpm. A .5 diameter reduction gains him ~50 rpm. That's all I was saying. The 2 in pitch change leaving the diameter alone will give him ~negative 400 rpm in theory. 200 per in of pitch on the smaller wheels. I doubt it will actually be that much with a re-pitch, more like 250 or so. On the smaller wheels diameter seems to change more effectively and predictably for me. Smaller dia. More rpm but you lose some hole shot. Until you get into the super high rev razor sharp stainless alloy race props from liquid jesus and the like. Those are a whole nother art and science when your talking motors spinning north of 9500....

 

[Newyota]

Yes guys, I thoroughly understand and agree with what you are saying...my point was a little different though...

we aren't talking about a champ boat or an f1 or a siebold. This is presumably a 25 johnson. He wants to lose 400 rpm. A .5 diameter reduction gains him ~50 rpm. That's all I was saying. The 2 in pitch change leaving the diameter alone will give him ~negative 400 rpm in theory. 200 per in of pitch on the smaller wheels. I doubt it will actually be that much with a re-pitch, more like 250 or so. On the smaller wheels diameter seems to change more effectively and predictably for me. Smaller dia. More rpm but you lose some hole shot. Until you get into the super high rev razor sharp stainless alloy race props from liquid jesus and the like. Those are a whole nother art and science when your talking motors spinning north of 9500....

Thanks for the info,so do you agree that motor should max at 5500 and I should try and repitch or get another prop to get down as close to the max as possible.I don't think many small outboard owners put tachs on there motors,but due to other issues last year I did and saw the rpms and concerned it may be too high.

 

[jakedaawg]

I dont know what motor you have, you should prop to attain close to max rpm with an unloaded boat. My prop shop charges me $25 to go up or down 2 inches of pitch.

 

[Newyota]

go for it

I just found an old post of yours from 2014 someone verifying the rpm range on the 20-25 of my same year.You discuss the 25 being able to sling a steeper prop etc.

 

[Sea Rider]

Reducing 200 wot revs going one pitch up does not always works, will depend entirely on boat's weight and current load. Have seen on 25-30 HP motors +, - 500 wot revs with just one more/one less pitch with same diam OEM prop. Only a wot test run will determine if motor was able to reduce 400 wot rpm going up 2 full pitches as usually loaded.

Happy Boating

 

[JerEazy]

I dont know what motor you have, you should prop to attain close to max rpm with an unloaded boat.

correction - max RPM under normal load.

as said above: Smaller motors react more to pitch changes than larger motors.

buy yourself a tiny tach and set it up and find out what RPM you’re actually getting before doing any changes.

But: prop shops normally will go +/- 2 pitch on a prop. So if you are getting near 6k on a 5-6k max RPM and 5.5 is the sweet spot, go ahead and have it refurbed +2 shouldn’t hurt too much. But you need accurate numbers first.

 

[54bobby]

Can a beat up 10.5x11 be changed to a 10x13 economically or at all?.Rpms at 5900plus on 25hp.Should be 5500 max I believe.

what year is the engine? the 22 cubic 25's turned 6000 at wot with the factory prop and i think that was a 9x10

 

[Newyota]

1985 motor..I had thought it was max 6000 at first, but found the old post F R HAD IN 2014 concerning a 1985 like mine and he quoted 4500 to 5500.

 

[Newyota]

correction - max RPM under normal load.

as said above: Smaller motors react more to pitch changes than larger motors.

buy yourself a tiny tach and set it up and find out what RPM you’re actually getting before doing any changes.

But: prop shops normally will go +/- 2 pitch on a prop. So if you are getting near 6k on a 5-6k max RPM and 5.5 is the sweet spot, go ahead and have it refurbed +2 shouldn’t hurt too much. But you need accurate numbers first.

Thanks.Learning a lot on the replies here...I have a beat up 10.5x11 that needs repaired so I am going to get it repaired and have the pitch changed to 13.

 

[jimmbo]

It might be cheaper just to buy a new 13, instead of repairing and repitching. I don't believe in repitching, especially AL. It involves bending/twisting the blade. All the blades would need to be bent/twisted exactly the same or there will be problems.

 

[Newyota]

It might be cheaper just to buy a new 13, instead of repairing and repitching. I don't believe in repitching, especially AL. It involves bending/twisting the blade. All the blades would need to be bent/twisted exactly the same or there will be problems.

Thanks for the feedback, I thought I would try a shop and dropped it off already...New props are around $90 or so.Repair and repitch half that so no biggie.

 

[Newyota]

what year is the engine? the 22 cubic 25's turned 6000 at wot with the factory prop and i think that was a 9x10

So is a 1985 a 22 cubic?

 

[F_R]

No.

 

[Newyota]

Reducing 200 wot revs going one pitch up does not always works, will depend entirely on boat's weight and current load. Have seen on 25-30 HP motors +, - 500 wot revs with just one more/one less pitch with same diam OEM prop. Only a wot test run will determine if motor was able to reduce 400 wot rpm going up 2 full pitches as usually loaded.

Happy Boating

Now, can someone smarter than me tell me why I went to the river today and decided to change my tilt pin from top hole to middle hole and I had a lot less wind today and going up river full throttle I was at max 5800rpm and last weekend hitting almost 6000rpm with tilt pin on upper hole and a descent amount of wind considering I am in a 14 foot boat.River did drop a foot since then.Curent may have been less.