Please help with pitch selection

[droach77]

I have a 98 17ft trophy center console with a 98 force 90 hp, currently has a 12.75 X 21 pitch prop. It is very slow getting on plane and will only tach 4000 rpms, all vitals are good..compression, fuel, spark. I am not sure what prop came on boat originally but it was not this one and i do not know if i should go to a 17 or 15 pitch prop. All suggestions are greatly appreciated.

thanks,

dave

 

[180shabah]

Re: Please help with pitch selection

What is the WOT range for that engine?

 

[droach77]

Re: Please help with pitch selection

Sorry for lack of info. The max rpm on motor is 5250, currently will not go past 4000, after on plane and good conditions it will run between 28 and 32 mph.

 

[steelespike]

Re: Please help with pitch selection

I'm pretty sure you need to drop some pitch but lets be sure about your setup.Throttle plates opening all the way?Antivent plate just above the prop slightly higher than the bottom of the boat.If you go to a 15 it should put you around
5,200 Just be sure your getting all you can first.

 

[droach77]

Re: Please help with pitch selection

Thanks for the input, the throttle plates open fully and i will double check the plate but i'm pretty sure its level with the bottom.

thanks

dave

 

[junior1113]

Re: Please help with pitch selection

21 is way to big for that boat motor combo. 17 would put you around 5000
lifting engine one hole should give you another 100 or so. is that a ss cupped prop or an alluminum? dont run that boat any more with that prop. motor luging is the rebuild mechanics job security.

 

[droach77]

Re: Please help with pitch selection

It is an aluminum prop, i dont plan on taking it out anymore until i get a new prop, overpropping is a hard on the motor.

Thanks for the help

dave

 

[droach77]

Re: Please help with pitch selection

I just realized the antivent plate is 2" under the transom and i need to come up a couple holes on the motor. It looks like it was already mounted in another set of holes and someone moved the motor down for some reason....how much affect was this on the motor performance?

thanks

dave

 

[junior1113]

Re: Please help with pitch selection

antiventalation plate being underwater 2 inches creates lots of un-needed drag. which will lower rpms and speed. most v-hulls are 1-2 inches above lowest portion of boat. still think the 21 is going to be too big

 

[steelespike]

Re: Please help with pitch selection

I've seen boats gain almost 2 mph with a 2" increase in height. Ifb your 2" below you might be able to gain 3" to about 1" above the bottom.
If your changing props now is the time for any changes for improved performance.
I think a 15" 4 blade would vastly improve your low to mid range performance ,allow maximum height,resist slip and ventilation(cavitation)

 

[droach77]

Re: Please help with pitch selection

Thanks for all the input guys. I bought a 13.25 x 17 prop last week, raised the motor up and i took her out yesterday. Comes outta the hole, great planes quickly, and i am getting 4700 rpms outta her now...but the throttle plates are opening to far so i have to adjust them and hopefully that will put me right where i need to be, speedwise is a lot faster, my speedo gets stuck at 32mph for some reason , but it handles a hole lot better.

Thanks again

dave