SS prop for Bayliner Cobra 1800???

[Ama]

Hi to you all, I have been reading this forum quite a while and now I would need help to choose correct prop for my Cobra.

It has 140hp Johnson VRO with Solas Rubex3 prop and it?s 13.2"x21" aluminium. It has Hydrofoil that I installed cause it planes much better with it, effect to top speed i haven?t noticed. Possible I cant trim it as high as I should.

So, the boat runs now little over 50mph at wot with rpm around 5000. If I try to trim it more the bow starts to bounce.
It feels like it shold go 5500rpm, am I correct?

So what I?m asking is should I dump hydro or not and what would be optimal ss prop for that? Same size as aluminium is now or what? Which model, Omc Viper, Mercury?s props? Top speed is needed

Thanks!!!

 

[craze1cars]

Re: SS prop for Bayliner Cobra 1800???

Do you have a manual for your engine? If yes...what is the WOT rpm recommended range for your motor? Rarely is it a fixed number like "5,500." Usually it is a range. You did not tell us the year of your motor, someone here might have access to the info if you share the year.

After you know the range, if you discover that you need to raise your top speed RPMS, go down to a smaller pitch with your replacement prop.

With regards to whether you should lose the hydrofoil, nobody can answer that question any better than you can, after a pair of test drives. Most foils are retained by four nuts/bolts. So just remove it and go for another ride. If you think it handles better, do not reinstall. If it's worse, reinstall.

Since this sounds like an outboard, you have another adjustment besides trim. You can try remounting your engine higher or lower, and see how that impacts your handling and porpoising. Normally higher = faster...unless you mount it too high and your prop starts to slip out of the water...then drop it back down a hole and that's the highest you can run it for your hull.

Personal experimentation is the best way to answer these questions. I will let others give their opinions on which props are "best". I haven't done enough back-to-back testing with props on different types of boats to be able to give any valuable input on that topic.

 

[Ama]

Re: SS prop for Bayliner Cobra 1800???

Engine is -88

 

[craze1cars]

Re: SS prop for Bayliner Cobra 1800???

Is there a data plate somewhere on the engine that provides RPM range?

Motor is too old for my resources so i can't find what it's supposed to be. Maybe someone else can find it if you can't.

 

[Dhadley]

Re: SS prop for Bayliner Cobra 1800???

A 1988 140 is a V4 looper which means the target rpm is 5800, even 6000 is OK. The limiter is probably at 6200 but you can get the numbers off the power pack to verify. If you're using ethanol fuel you may not get to 6000. Good motor, tons of torque.

 

[craze1cars]

Re: SS prop for Bayliner Cobra 1800???

Well....with that knowledge of a 5,800 target, you need to gain 800 rpms...

Brings you all the way down to a 17 pitch prop in order to probably get close. But then again maybe a 19 will let it spin fast enough to get it up into its powerband and find a few more...

This all assumes your tach is accurate, engine is in excellent condition and in perfect state of tune, your engine is at proper height and trimmed properly, etc...

Experiment with everything I mentioned. But it sounds most definite that you will need to drop to a lower pitch to get your RPMS up.

 

[Ama]

Re: SS prop for Bayliner Cobra 1800???

Well ok...

Tach is new Faria, I installed it few months ago. What is about correct height for engine, say from the bottom of the boat where should cavitation plate to be? When transiting from aluminium to stainless steel, is there something to think for? Is same pitch same thing for both props?

 

[Dhadley]

Re: SS prop for Bayliner Cobra 1800???

If the tach is a Faria then we need to verify the rpm with a shop tach. They are fairly well known for being off.

 

[Ama]

Re: SS prop for Bayliner Cobra 1800???

So, have anyone idea or experience for engine height for this boat? I bought for it 13 1/2 * 20 Omc Raker prop, and the engine is now on its lowest position?

 

[steelespike]

Re: SS prop for Bayliner Cobra 1800???

For your boat to do 50 The tach would have be at about 5400.
Using the antiventplate(just above the prop)for reference it is suggested it should be even with or slightly above
the bottom.I usually suggest raising it to a point just before venting becomes a problem.
If the raker can get the boat out of the water and the rpm up to 5800 the speed would be about 51 at 7% slip.
Remove the foil before anything and test. Add the new prop and test then raise the motor and test.

 

[Ama]

Re: SS prop for Bayliner Cobra 1800???

Ok, I just bought also jackplate for it, vertical 4" and setback 4". Should it be mounted like when in center of vertical adjustment cav plate is level with hull?

 

[steelespike]

Re: SS prop for Bayliner Cobra 1800???

I've never used a jack plate but it makes sense to have it installed so you could adjust either way.
You may want to raise the motor first so you get an idea of a good height to start from.The jack plate will require more height when you add it.