Went from 19" to 17" still 4000 rpm at WOT????

[loganfitz]

I have a 1990 Bayliner Capri Cuddy w/ a Force 120, I went from a 19" pitch to a 17" pitch, but I am only getting 4000 rpms at wot, it will do 40 mph, which is 2-3 more than the 19", any ideas??? Is something else wrong, it does 40 mph so I don't think it is a dropped cylinder?

 

[steelespike]

Re: Went from 19" to 17" still 4000 rpm at WOT????

I think you have a tach problem.Be sure all connections are clean and tight.
Note the position of the switch on the back; operate it a few times to clean the contacts.

 

[loganfitz]

Re: Went from 19" to 17" still 4000 rpm at WOT????

Behind the tach?

 

[jkust]

Re: Went from 19" to 17" still 4000 rpm at WOT????

A little bit of an apple and an orange here but I went from a 21" to an agressive 21" and I lost over 600 rpms and a lot of speed.

 

[loganfitz]

Re: Went from 19" to 17" still 4000 rpm at WOT????

I got the 17" pitch and it didn't gain any rpm, but a little speed?

 

[grneop]

Re: Went from 19" to 17" still 4000 rpm at WOT????

I'm far from been a expert, but base on other posts, if you drop the pitch you gain RPM, faster plane and lose speed.

 

[NHGuy]

Re: Went from 19" to 17" still 4000 rpm at WOT????

Is it possible the older prop had been reworked or repaired? Or was it in bad shape? Normally you will increase RPM, and often speed, as log as the RPM change is enough to overcome the pitch drop.
So definitely check the tach too. It could easily be out of whack.

 

[roncoop75]

Re: Went from 19" to 17" still 4000 rpm at WOT????

My thoughts, if you REALLY NEEDED to change pitch in the lower direction you'd need to do that because it was a bit tall (too much pitch). Being too tall means it would be slipping a little bit too much. You went to a 17", got a better grip on the water, lower slip, each rev of the new 17" prop pushes the boat a little farther than the 19" did because it was slipping so bad.

In the end your tach doesn't change because you got a better grip on the water. It's that grip that has you going maybe a little bit faster than you did with the 19".

IMO what your seeing could be just what you're supposed to see. If you want a little more revs you might have to drop pitch a little farther.

 

[steelespike]

Re: Went from 19" to 17" still 4000 rpm at WOT????

Behind the tach?

Most tachs have a switch on the back so it can be used on a number of applications.
The contacts sometimes become corroded and operating the switch sometimes cleans the contacts.You improved your performance so you surely aren't likely to have a motor idssue likle a dropped cylinder.40 mph is respectable though a gps speed is more accurate.
imo your only problem is a inaccurate tach.You may be able to fine tune your setup
but your motor seems responsive.

 

[bob johnson]

Re: Went from 19" to 17" still 4000 rpm at WOT????

where they both the same material???


how long has it been since you tuned up the motor???

is your linkage going all the way?

bob