Prop pitch change

[Rumbaut]

My 19' boat runs great with an aluminum 3-blade 14-1/2 x 19" propeller. If I change the propeller to stainless steel, do I have to buy one with a different pitch? Thanks...

 

[WesNewell]

Depends on what your rpm does with the ss prop. As long as it stays within the wot rpm range then it's fine.

 

[Rumbaut]

Thanks, but I was wondering if a stainless prop, which is stiffer and does not flex like aluminum, as a rule of thumb required a reduction in pitch to achieve the same performance as I am getting with the 19" pitch aluminum prop. The idea is to know this before spending $500 on a stainless prop and then finding out that I should have changed the pitch.

 

[dingbat]

Thanks, but I was wondering if a stainless prop, which is stiffer and does not flex like aluminum, as a rule of thumb required a reduction in pitch to achieve the same performance as I am getting with the 19" pitch aluminum prop. The idea is to know this before spending $500 on a stainless prop and then finding out that I should have changed the pitch.

"Rule of Thumb" only applies when it does.....

Prop Design, Cup, Porting, etc. all play into how a prop performs on a given hull. I've had props that swapped 1 for 1 and I've had props that required a change in pitch.

Keep in mind that SS prop are typically "performance" props. Most are designed to/for a specific need or function. You would get a better answer if you where more specific as to what prop your currently running and what props are currently under consideration.

 

[Rumbaut]

"Rule of Thumb" only applies when it does.....

Prop Design, Cup, Porting, etc. all play into how a prop performs on a given hull. I've had props that swapped 1 for 1 and I've had props that required a change in pitch.

Keep in mind that SS prop are typically "performance" props. Most are designed to/for a specific need or function. You would get a better answer if you where more specific as to what prop your currently running and what props are currently under consideration.

Currently running an OMC 3-blade 14-1/2Ø x 19"pitch aluminum prop on a 19 ft. family runabout with a semi-V hull powered by a 175 HP outboard with 25" lower unit. Boat planes in 4-5 seconds with six people on board and cruises at 33 MPH at 3500 RPM. What I want to know is if I would have to change the pitch on a s/s propeller in order to get the same results, so I'll know what size prop to purchase. I live overseas and no local vendor is going to loan me a prop to try out nor will they exchange it for another size prop if I purchase it and it doesn't work. I want the s/s prop because it resists dings and scratches better that aluminum and it looks nicer.

 

[Scott06]

Currently running an OMC 3-blade 14-1/2Ø x 19"pitch aluminum prop on a 19 ft. family runabout with a semi-V hull powered by a 175 HP outboard with 25" lower unit. Boat planes in 4-5 seconds with six people on board and cruises at 33 MPH at 3500 RPM. What I want to know is if I would have to change the pitch on a s/s propeller in order to get the same results, so I'll know what size prop to purchase. I live overseas and no local vendor is going to loan me a prop to try out nor will they exchange it for another size prop if I purchase it and it doesn't work. I want the s/s prop because it resists dings and scratches better that aluminum and it looks nicer.

Really depends on which prop you are thinking of getting and what your current Aluminum prop does in terms of WOT rpm (3500 is way low for wot, assume that is cruise speed or tach is off). If you aregetting to the top of reccomended rpm range for engine AND you buy a basic stainless prop like a merc vengeance i would stay in a 19” . If you were currently low in rpm and weregoing to a big stainless prop with a lot of cup like a mirage, tempest, enertia, or revolution 4 id go down an inch or two in pitch.
if you can post current wot rpm and wot rpm rangefor the engine , what type of prop you are considering and what performance issue you are trying to solve we can probably help narrow things down.

 

[Rumbaut]

Really depends on which prop you are thinking of getting and what your current Aluminum prop does in terms of WOT rpm (3500 is way low for wot, assume that is cruise speed or tach is off). If you aregetting to the top of reccomended rpm range for engine AND you buy a basic stainless prop like a merc vengeance i would stay in a 19” . If you were currently low in rpm and weregoing to a big stainless prop with a lot of cup like a mirage, tempest, enertia, or revolution 4 id go down an inch or two in pitch.
if you can post current wot rpm and wot rpm rangefor the engine , what type of prop you are considering and what performance issue you are trying to solve we can probably help narrow things down.

Thanks Scott. I never go WOT, that's why I mentioned cruising speed. And the s/s prop I am considering is an OMC SST. I guess I'll just have to bite the the bullet and go trial and error with different pitches. Thanks again.

 

[dingbat]

I never go WOT, that's why I mentioned cruising speed.

it’s doesn’t matter how fast you normally travel. You prop a boat to obtain the manufacturer’s rated WOT rpm. Guessing your WOT is around 5500-6000 rpm. Check your manual.

Outboards are low displacement, high rev motors. They need rpm to generate power. Otherwise, your lugging (overloading) the motor badly. Worst thing you can do with a motor

Guessing your looking at used props?

OMC has been out of business for sometime now. Most the good stainless props are now Mercury

 

[Scott06]

Thanks Scott. I never go WOT, that's why I mentioned cruising speed. And the s/s prop I am considering is an OMC SST. I guess I'll just have to bite the the bullet and go trial and error with different pitches. Thanks again.

As mentioned elsewhere you want to prop it to hit in the correct WOT rpm range so more or less for prop testing. The big issue you will have if you dont know it currently is you dont know where to go with your next purchase because you have no baseline. My brother did the same thing because his tach was broken and bought a prop that didnt work.

Its your time/money but my advice would be to find this out first, or if you need a prop due to damage get a basic aluminum 19" pitch like Black max or Michigan Vortex. Get your baseline rpm and you can make a reasonably informed decison. You seem reasonably happy with the performance already...