Help selecting replacement prop

7lazy77

Petty Officer 1st Class
Joined
Feb 28, 2011
Messages
226
I just recently purchased a 2004 Chaparral Sunesta 236 deckboat from Texas & brought it up to Wyoming. There is a difference of about 3800' in elevation, & I knew that I would have to re-prop to compensate for the elevation difference. Boat has a 300hp 350 Mag MPI with a Bravo 3 outdrive with a 26p prop. Got a chance to take the boat out for the first time today & at WOT I was reading 4000 rpm at 40 mph & it took about 10 seconds to plane. Any suggestions on what I should down-prop to? I was thinking a 24p, but not sure if i would need to go down to a 22p?? Was also wondering if changing out a dual prop can be done by a "novice" person like myself. Any help is greatly appreciated!
 
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ihearth2o

Petty Officer 2nd Class
Joined
Aug 18, 2014
Messages
187
uh, ya...I would say you're over propped. I believe the recommended rpm range for your engine is 4800-5200 so you've got quite a bit of play there. Rule of thumb is for every inch of pitch, rpm will change by 200. Lower pitch means high rpm and vice versa. You want to be at the top end of your range without going over. At 4000 rpm wot, you could conceivably go down to a 20p prop. Never seen a B3 pitched that low but I live at sea level and this is actually the first time I've heard of elevation being a consideration in selecting prop pitch.

Did you take your wot reading with a normal load? You sure that your tach is set to 8 cylinders?
 
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jestor68

Commander
Joined
Jun 12, 2012
Messages
2,308
uh, ya...I would say you're over propped. I believe the recommended rpm range for your engine is 4800-5200 so you've got quite a bit of play there. Rule of thumb is for every inch of pitch, rpm will change by 200. Lower pitch means high rpm and vice versa. You want to be at the top end of your range without going over. At 4000 rpm wot, you could conceivably go down to a 20p prop. Never seen a B3 pitched that low but I live at sea level and this is actually the first time I've heard of elevation being a consideration in selecting prop pitch.

Did you take your wot reading with a normal load? You sure that your tach is set to 8 cylinders?

HP loss=elevation X 0.03 X HP at sea level/1000.
 
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