Prop Question

A

Andy1804

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Jan 17, 2017
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Hi.
i have a Honda 135hp forstroke 2015 model.
its currently set up with a 3 blade 14.5 x 19p which gives a cruise speed at 25knts at 3900rpm. 'wot' is 5200rpm and 33knts fully loaded.
im on the understanding that the rev band for these motors is between 5000 & 6000rpm. ive been told the engine needs to be in the upper rpm range around 5700rpm. i don't want to loose the current cruise speed. im thinking of a 4 blade 19p prop but not sure on diameter. as ive read the diameter per inch either way can increase/decrease by around 400rpm. so if i got a 13.5 x 19p would i loose cruise speed.

appreciate any feedback tia
 
alldodge

alldodge

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Rule of thumb is 19P 4blade = 21P 3blade

Don't know the gear ratio but the prop slip is bad. Using 2.5:1 ratio it showing 45% slip

Might want to try a 14.8 diameter 17P 3blade
 
A

Andy1804

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alldodge said:
Rule of thumb is 19P 4blade = 21P 3blade

Don't know the gear ratio but the prop slip is bad. Using 2.5:1 ratio it showing 45% slip

Might want to try a 14.8 diameter 17P 3blade
Click to expand...
i would loose cruise speed by going down to a 17p, i would like to try and keep that
 
alldodge

alldodge

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With a 2.15 it just means there is more slip

Andy1804 said:
im all for cruise. i don't want to loose that. if i kept the 19p and reduced diameter how would that work out.
Click to expand...

Think of it this way. If the motor is in neutral and the motor is reved up, it takes next to no fuel to do that. Put a lower pitch prop on and the rpms go up but there is less load so no more fuel used, and maybe a bit less from no working so hard.

With the higher then normal slip, how is the drive setup on the hull?
What boat is it and is the vent plate at or slightly above the bottom of the keel?

Did the boat use to have a 2 stroke with same HP?
 
R

racerone

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Props and simple things can be confusing to a novice.-----Facts for you.-----Did my own test years ago.----Same boat , same motor, all done within 1 hour.----- With 19" pitch 5800 RPM and speed 54 MPH.-----With 21" pitch 5400 RPM and 54 MPH.----With 23" pitch RPM at 4900 and speed 54 MPH.------Andy----With a 17" pitch prop your RPM will go up.---Speed of your boat will likely stay the same.-------No need to believe this information.-----But run to your local friendly boat shop or research some more on line !!!
 
A

Andy1804

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racerone said:
Props and simple things can be confusing to a novice.-----Facts for you.-----Did my own test years ago.----Same boat , same motor, all done within 1 hour.----- With 19" pitch 5800 RPM and speed 54 MPH.-----With 21" pitch 5400 RPM and 54 MPH.----With 23" pitch RPM at 4900 and speed 54 MPH.------Andy----With a 17" pitch prop your RPM will go up.---Speed of your boat will likely stay the same.-------No need to believe this information.-----But run to your local friendly boat shop or research some more on line !!!
Click to expand...
I tried a 14.5 x 17 and lost all cruise speed. it went from 25knts to 22knts.....@25knts i was getting 0.7ltrs per mile. @22knts i was getting 0.9ltrs per mile.....i understand that dropping 2 pitches will increase rpm. the diameter of the prop makes a huge difference. i believe approx 400rpm per inch of prop? (is that correct?) my thoughts are sticking with the 19p and dropping the diameter? my boat is a British built Raider18
 
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Andy1804

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My options. im looking at.
14.8 x 19p 3 blade.
15.0 x 17p 3 blade.
15.5 x 17p 3 blade.
14.5 x 17p 4 blade.

i believe that the 14.5 x 19p is the same as a 15.5 x 17p?
 
Texasmark

Texasmark

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Once you get in a range of props, like 13+ for 75-125 hp engines and 14+ for 135-250 engines for a couple of examples, whereby the diameter is limited by the allowable distance between the prop shaft and the Anti Vent plate, the change in diameter has to do with the blade shape and geometry, and blade area for the intended purpose and the pitch for tuning your WOT RPM for the application. Pitch and performance are the rule. Solace has a good tutorial with their props. Take their SS 3 blade and 4 blade of the same diameter range and read how they "tune" the prop for the desired result....in short the 3 is usually tuned for top end and the 4 for midrange cruising or heavy loads. Also you might www prop design dissertations where they discuss the things of which you are interested.
 
flyingscott

flyingscott

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Andy1804 said:
im all for cruise. i don't want to loose that. if i kept the 19p and reduced diameter how would that work out.
Click to expand...
Racer is right about dropping pitch. Speed usually doesn't change that much because the lower pitch prop spins faster. I would also bet your fuel economy won't change much if at all because the 17 will be easier for the motor to spin.
 
A

Andy1804

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All. thank you for you're input it seems that a 17p is the way forward! can i ask what diameter would you go for?
 
Q

QBhoy

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alldodge said:
With a 2.15 it just means there is more slip



Think of it this way. If the motor is in neutral and the motor is reved up, it takes next to no fuel to do that. Put a lower pitch prop on and the rpms go up but there is less load so no more fuel used, and maybe a bit less from no working so hard.

With the higher then normal slip, how is the drive setup on the hull?
What boat is it and is the vent plate at or slightly above the bottom of the keel?

Did the boat use to have a 2 stroke with same HP?
Click to expand...
Prop slip is 13% as far as I can see.
 
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