Cavitation, or perhaps prop hub slipping?

I have a '88 Yami 200HP 6 Cyl "Precision Blend" with which I am having two issues. I'll outline one here, and the other issue on another post.

I have a SS prop. I had the boat out on big water a couple weeks ago, 4 to 6 foot swells. Every time the bow would come down "heavy", the motor would act as if the prop was losing grip in cavitation. The RPMs would skyrocket, and the boat would stop moving until I would back off the throttle, and ease it back up.

I attributed this to some weird phenomenon due to my boat's configuration - it's got an extremely "stepped" hull, and I thought maybe that in combination with the big waves was causing cavitation.

But then I had it out on not-so-big water a few days later. While the motor performed much better, it did "lose grip" a couple times, in just the same way as before.

Question: Can anyone tell me definitively what's going on here? I've been told it's possible that the prop needs to be rebuilt - that the plastic ring that's supposed to "give" if you hit rocks may be causing the hub to slip - and I have dinged the occasional rock http://www.bassboatcentral.com/smileys/sick.gif.

Does it sound like that to any of the experts here, or is it a simple cavitation problem that I can chalk up to having an old boat with a weird hull?

If the motor runs well in calm water the hub is not slipped. I think the motor is mounted to high. Can you lower the motor down?

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by hurricane &raquo;</TD></TR><TR><TD CLASS="quote">If the motor runs well in calm water the hub is not slipped. I think the motor is mounted to high. Can you lower the motor down?</TD></TR></TABLE>

+1 http://xs-s.com/zf/images/smile/emthup.gif

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by hurricane &raquo;</TD></TR><TR><TD CLASS="quote">If the motor runs well in calm water the hub is not slipped. I think the motor is mounted to high. Can you lower the motor down?</TD></TR></TABLE>

There are 4 bolt-holes spaced vertically on each side, and the motor is bolted in the top hole - so no, it cannot be mounted lower without some kind of reconstruction.

But this gives some credence to my thought that the step-hull might be the problem. The transom is a full 12" from where the chines end, and the hull cuts up about 6" on center.

Here's a not-so-great picture of what I'm trying to describe...
http://i126.photobucket.com/albums/p...t/CIMG5509.jpg

So in essence, it could be said that because of that step hull, the prop is a full 6" higher out of the water in relation to the bottom of the hull as compared to the transom.

Do you think that's my problem? If so, solutions?

A jack plate may be a solution. It would allow you to lower the motor when you are in heavy seas. Maybe a four blade prop, it would provide better bite. But as long as the anti cavitation plate is aligned or slightly below the bottom of the transom you should not have those problems. The hull design appears to be a pocket which allows the motor to run slightly higher than the actual bottom of the hull.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by hurricane &raquo;</TD></TR><TR><TD CLASS="quote">A jack plate may be a solution. It would allow you to lower the motor when you are in heavy seas. Maybe a four blade prop, it would provide better bite. But as long as the anti cavitation plate is aligned or slightly below the bottom of the transom you should not have those problems. The hull design appears to be a pocket which allows the motor to run slightly higher than the actual bottom of the hull. </TD></TR></TABLE>

Thanks for the jack plate and 4 blade advice. So you're reasonably convinced that my prop hub is not slipping?

If you only have this problem in choppy water and runs fine in calm. When the hub is spun the motor will over rev all the time. But stranger thing have been known to happen.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by hurricane &raquo;</TD></TR><TR><TD CLASS="quote">If you only have this problem in choppy water and runs fine in calm. When the hub is spun the motor will over rev all the time. But stranger thing have been known to happen.</TD></TR></TABLE>

Yeah, it definitely does NOT over rev all the time. It performs fine most of the time, and the worst events were on big swells in choppy water.

Now I s'pose I have to figure out if I wanna spend the money on a jack plate for a 21 year old boat, when I'm not positive that it would cure the problem.
http://www.bassboatcentral.com/smileys/doh.gif http://www.bassboatcentral.com/smileys/sweating.gif http://www.bassboatcentral.com/smileys/praying.gif