under proped or what?

[yootles]

I have a new yahama 150 hpdi 2 stroke engine on a 1988 18.6 boston whaler outrage. I can not get the engine to hold rpm between 2000 and 2900. top end is 5800 rpm and otherwise the engine runs fine. with the 17 pitch prop i kinow i am a little under proped but is it normal that under propping won't allow the engine to hold a certain rpm. The boat jumps out of the hole and settles at 3100 or so . if you pull back on the throdle to say 2800 it holds for a minute AND then slowly drops the rpm to 2000. this makes it impossible to run rough water off a plane. The mechanic says i need a 21 pitch prop , it just doesn't make sence to me. help!!!

 

[steelespike]

Re: under proped or what?

I think your asking too much to expect it to stay on plane at 2800. A brief check of 8 boats running similar 150 hp motors
6 of the boats stayed on plane at 3000 or 3500. 2 were around 2500 but one was a flats setup and the other had after planes
to keep it on plane. Do you trim down when you reduce throttle? Have you tried moving weight forward?
A 4 blade could help maintain plane.also improve hole shot handle better. Also I think a 17" will control your rpm.
Top speed may be reduced.

 

[yootles]

Re: under proped or what?

I am running a 17 pitch prop. I am not trying to maintain the boat on a plane. I blue water fish in the boat and have to go at idle speeds when its rough or fly out of the water at planning speed, there is no in-between. my goal is to ride bow high when its rough but that takes an rpm range or 2400-2800 that I don't have. the dealer says its because i am under propped, that doesnt make sence to me

 

[Jlawsen]

Re: under proped or what?

I am running a 17 pitch prop. I am not trying to maintain the boat on a plane. I blue water fish in the boat and have to go at idle speeds when its rough or fly out of the water at planning speed, there is no in-between. my goal is to ride bow high when its rough but that takes an rpm range or 2400-2800 that I don't have. the dealer says its because i am under propped, that doesnt make sence to me

Your dealer is correct, you are under propped for what you're trying to do but the down side to that is that you have a two stroke. With a 5800 rpm WOT your motor appears to be correctly propped but if you increase the pitch to a 19 and then get a prop with less cup you should be able to accomplish your goal without lugging the motor. I would get on the phone and talk to a few manufacturers. Stilleto makes a sweet Bayrunner II 4 blade that might get you where you want to be. Ask them but I think a dropping an inch in diameter but keeping the same pitch in a Bayrunner might be the ticket. Talk to them though, they have far more experience that I do.