Setup after prop change

[piperdriver]

I have a 1992 javelin 389 Fish/ski with a 1985 Evinrude 150XP. It had an older 3 bladed SS prop. 14 1/4 X 21. It was in pretty sad shape. I replaced it with a Rubex HR4. 14X21. It brought the WOT RPM'S right in line. My question--out of the hole I am trimmed all the way in (down), as the speed comes up I start trimming out(up). With the old prop I could trim up to the point of blowing the prop out. With the new prop I trimmed as i usually did, and i could never get trimmed up to the point of blowout. Without seeing my setup, let me ask, is it possible that the new prop is holding so well that I am reaching the limit of the engine trim? I have been suspecting that i needed a jack plate, and I think this confirmed it. It looks like the anti-cav. plate is a little below the bottom of the hull.

 

[Dhadley]

Re: Setup after prop change

The AV plate should never be below the keel / pad (when parallel). That prop would not be our first choice for that hull but if you can use 100% positive trim and not blow out then the motor needs to be raised. Generally speaking - very generally - to get the most out of a good 4 blade you'll most likely need a jackplate. No need to pay for prop technology you're not going to take advantage of otherwise.

 

[piperdriver]

Re: Setup after prop change

Ok, for future referance, can you give me an opinion on what prop would have been a good choice? After reading and talking to some floks, i thought that this was the right prop. There is a lot I don't know about this type of setup. This is the largest O/B that I have owned. Now that I have the prop, what can I do to get the most performance out of my setup? Let me just say this---it is a HUGE gain in performance compared to what was on there. I wold be happy with it as it is, but even I know that there is more to be had, and I do want to get the most out of what I purchased. So, What can I do?

 

[Dhadley]

Re: Setup after prop change

We're kind of shooting in the dark since we don't know what the top rpm is. Based on your info about using all of the trim range without venting it's a good guess the motor can be raised. If we were doing the set up we'd raise the motor even without a jackplate.

 

[piperdriver]

Re: Setup after prop change

Let me give you the info on RPM'S.
OLD PROP---WOT trimmed up to just before blow-out 5900 RPM
NEW PROP---Fully trimmed up as far as it will go--4900-5000.
The RPM was increasing gerat as speed was building. I really think that the RPM'S would come up more if the motor was raised. Keep in mind, nothing changed as far as load, setup, nothing changed but the prop. To make sure i was trimmed up to the max extent, I came off of plane without trimming back down. Shut the motor off and hit the trim switch. The motor immediately came up in "TILT" mode. Just for referance I put it back to the max "TRIM" re-fired and launched from that setting. It struggled, but came on plane, and once on plane, speed and RPM came up as it had been. I think that the motor is as high as it can go on the mount holes.

 

[piperdriver]

Re: Setup after prop change

This was more of a setup question than a prop question to begin with. Dhadley, you did say that a jackplate was recommended. You sound familir with my setup. What setback plate would you recommend?

 

[piperdriver]

Re: Setup after prop change

Dhadley, thank you for all the help. You sort of left me hanging. You said that the prop was not a good choice for the setup. What prop would be a good one? I gave you the RPM info that you needed. Do you need anything else? A jackplate is something that is needed according to you. What setback? I realize that you have NOTHING else to do during that day othere than answer goof-ball questions. Seriously, and further info would really be appreciated.
Thanks--

 

[Dhadley]

Re: Setup after prop change

Sorry for the delay but work keeps getting in the way. While that prop would not be our first choice it doesn't mean it won't work. Before adding a jackplate we'd try raising the motor on the transom. The old "one change at a time" rule. You have a pretty good baseline, raise the motor and record the changes.