cavatating just before plaining

[chippy]

I have a 1988 Stratos 201 XL with a 1988 Mariner 150 MagII. The boat runs excelent at speed(56mph @ 5800rpm) I have a High Five 22p on it.I have a hydrofoil on it. BUT when I give it the gas it starts great then right before I get on plain it starts to cavatate(14mph)(rpm's jump up 1500-2000 at this time) till I get some speed up about (20mph). Then It's normal. When I trim it all the way in at idle it don't run all that well. so I trim it up a little. It cavatates when trimed all the way in too. I'm thinking X dimention??? At this time I don't know where the cav. plate is to the bottom of the boat but I'll check it tomorrow. Thanks for all the help you guy's and gal's give me

 

[rickdb1boat]

Re: cavatating just before plaining

The cavitation plate height is key right now. Find that out before we go further.. But since it runs well at other speeds, it could be other problems with your set-up. Any problems with fast turns in the water? Does it cavitate(Ventilate) then? Have you tried it with the foil off and how does it react?

 

[Chris1956]

Re: cavatating just before plaining

Chippy, Does that prop have exhaust ventilation holes? Although I do not think this could be the cause, I thought it might be a contributing factor. The RMP you cite would be max exhaust and min speed.<br /><br />If you do have the exhaust holes, you can plug them with SS bolts (nuts on the inside), that just fit the holes flushly. Then try it again.

 

[LubeDude]

Re: cavatating just before plaining

Do you by chance have a jack plate? If so, it may be too high. I have a High Five on my Bass Boat also, and it has the biggest vent holes Ive ever seen and it is fine. try taking the fin off and trying to plane off with the engine trimmed all the way down.<br /><br />I also can guarantee you that if you put smart Tabs on your boat you will not even recognize it afterward. Your boat will jump on plane and wont act like its shooting for the moon on take off. You can plane at very low speeds also. You can through the hydofoil in the Garage sale pile.<br />--------------------------<br />

 

[chippy]

Re: cavatating just before plaining

Thanks for all the help. The boat without the hydrofoil on it did the same thing. the only diff. now is it planes sooner and stayes on plane at a lower speed. The prop does have the small vent holes in it. I still have not checked the cav. plate to the bottom yet but I'll let you all know the results soon. There is a trim spot that I can leave the motor in and it works great. at that spot the rpm's are 5700@50 mph. A little trimming and I get 55mph @ 6000 rpm. Thanks for all the help you guys are the best

 

[chippy]

Re: cavatating just before plaining

I got the measurement today and the cav. plate is about 6 inches above the bottom of the boat. But, there is a step on the bottom that is about 16 inches forward of the back of the boat where the motor is mounted. That is the part that I measured from. I can lower my jack plate about an inch and a quarter. if I lower it all the way down will I loose speed? I'm thinking about lowering it down and give it a try. If it don't work,I can jack it back up. What do you think? Or should I just live with it? The boat runs excelent as it sits,but It cavatates just before plaining. Thanks for all the help.

 

[chippy]

Re: cavatating just before plaining

bump

 

[rickdb1boat]

Re: cavatating just before plaining

I would drop it down an inch and see what happens. Couldn't hurt..

 

[LubeDude]

Re: cavatating just before plaining

I think that we are on the right track, after lowering the engine some, check your RPMs again at WOT full trim. You may need to use a higher pitch prop. Im running a High Five prop also, but my speed was better with a four blade but I like the hole shot of the High Five.<br /><br />I went back and reread the whole post, Obviously this is a NEW boat to you, Some cavitation is normall, The only time you really have a problem is if the engine actually loses all grip and completly blows out. A little spinning of RPM is just the vent holes in the prop doing there job. If it dosnt blow clear out, your Jack plate set up may be optimal but if this is true, you need to step up on pitch at least to a 24P High Five.

 

[chippy]

Re: cavatating just before plaining

Lube,Mercury marine's prop's guide said a 21" pich high five was the best prop for my boat for what I was using it for. I got a 22" pich on it. I had a Laserll 20" pich when I got the boat. It did the same thing. I'm going to lower the jack plate 3/4". I'll let you know how it works. Thanks for the help.

 

[LubeDude]

Re: cavatating just before plaining

Prop guides, are just that, "a guide" Its just to get you in the ball park. Trial and error gets you the home run. Lowering the engine will drop your RPM and hurt your WOT speed some, the bigger pitch prop might hook up better, lower your WOT RPM, and maybe give you more speed with the engine where it is. See if you can find one to borrow.