wrong prop ?

[ironman55]

I recently bought my first power boat it's a 1979 18' Mark Twain bowrider v-sonic, I am having issues with getting the boat up to speed, sometimes I can with a bit of coaxing get her to plane off nicely, other times though especially in rough water or after running at low speed for a while I can't get anything out of her, my friends tell me it's a cavitation problem, the motor is a mercruiser mcm 470 stern drive 170 hp, the prop is a 4 blade 13.5" dia. with a 22 pitch composite blade the previous owner replaced the original due to damage. When I first start out I can usually get her to plane off nicely at about 1500-2000 rpm, but in rough water or after a lot of low speed cruising, or when I have more than 2 people in the the boat I cant get anything, I start to ease on the throttle and then when give her more gas I lose momentum and the engine just winds out, I have already checked on the hub, it's definitely not spun, I am thinking I would like to go back to the factory prop which I am guessing is a 3 blade model but I am unsure as to the correct diameter and pitch, anyone have an idea what that might be ?

 

[jestor68]

Re: wrong prop ?

Your Mercruiser 470 has a 1.84 gear ratio and a WOT rpm range of 4000-4600 rpm.

That gear ratio and rpm range lends itself to larger diameter props. No 13.5" diameter prop will work well on that boat.

Drop that plastic prop in the nearest trash can and get yourself a proper Mercury or Solas aluminum prop to start with. The Mercury prop selector calls for a 14.5 X 21. Solas makes a 15-3/10 X 21 that should be even better for acceleration to plane, if your 34 year old motor is healthy enough to pull the added diameter.

That motor is happy topping out at 4200-4300 rpm; as it's a low rpm torque motor.

Make sure you have the outdrive trimmed all the down(in) when starting and cruising at slower speeds.

 

[ironman55]

Re: wrong prop ?

Your Mercruiser 470 has a 1.84 gear ratio and a WOT rpm range of 4000-4600 rpm.

That gear ratio and rpm range lends itself to larger diameter props. No 13.5" diameter prop will work well on that boat.

Drop that plastic prop in the nearest trash can and get yourself a proper Mercury or Solas aluminum prop to start with. The Mercury prop selector calls for a 14.5 X 21. Solas makes a 15-3/10 X 21 that should be even better for acceleration to plane, if your 34 year old motor is healthy enough to pull the added diameter.

That motor is happy topping out at 4200-4300 rpm; as it's a low rpm torque motor.

Make sure you have the outdrive trimmed all the down(in) when starting and cruising at slower speeds.

Thanks Jestor,
picked up the new prop today will hopefully be on the lake on sunday.