Cavitation

T

theleverbeef

New member
I am having trouble with cavitation on my 1982 22ft cdory cruiser. It has a 70hp Johnson long shaft. It wants to cavitate and rev up when turning even at moderate speeds. It is worse in any amount of seas. Any suggestions?
 
C-Dory did not make a cruiser model in 1982. Angler only.
I had a 1983 C-Dory angler for 26 yrs. The only time I had cavitation problems was in a big following sea.

Is your motor tilted so it is vertical in the water, cavitation plate ok???, prop blades ok??. Could it possibly be the prop clutch rubber slipping.

That 70 hp was probably the original engine as that is what was installed on them at the factory. 3 cyl. 70 hp Johnson.
 
Typo, it's a 1988, I do not have a cavitation plate, the wheel is like brand new, and i run the motor at the recommended spot on the trim gauge indicator. It powers up nicely and runs fine even with a full load; no slipping. But as soon as I give any rudder that is more than a mild turn, she cavitates and revs up. Any way of checking the slip clutch on the wheel?
 
Typo, it's a 1988, I do not have a cavitation plate, the wheel is like brand new, and i run the motor at the recommended spot on the trim gauge indicator. It powers up nicely and runs fine even with a full load; no slipping. But as soon as I give any rudder that is more than a mild turn, she cavitates and revs up. Any way of checking the slip clutch on the wheel?
 
A 1988 has a completely different bottom design from the pre 1985 models so I do not have any knowledge of those.
A good prop shop could check your prop slip clutch rubber.
 
Good idea! Thanks. Any other suggestions would be much appreciated!. I've seen some mixed reviews on cavitation plates; anyone had any luck with one.
 
You do have a cavitation plate, All motors do. you may not have a aftermarket wing that attaches to your cavitation plate. I would see if the motor is set on the highest set of holes in the bracket. if you look where you motor mounts to the transom you will see two rows of holes with at least 4 (two to a side) bolts in them. You should have your motor set as far down on the transom as possiable. what you are trying to achive is to have your cavitaion plate level with the bottom of your hull when the motor is trimmed all the way down.

You also said the the motor was trimmed up to the recommended spot? recommended by who?? the marks on the gage mean nothing. there just there to give you a guide. Trim is set by the driver for the conditions, which are always changing just at the trim should be.

I hope I was clear and helpfull
 
Correct, I do not have an aftermarket plate, the motor sits on the transom, as low as possible, and I use the trim guide indicator, for what it is; a guide. I use the tach and my ears for everything else to get a nice ride depending on conditions. I have owned and run boats all my life and have not experienced this before. I am curious if a aftermarket cavitation plate might help.
 
It might help but the lack of one in not the problem. Not sure why this is happening, would have to drive it. Might be the prop as others have said. Not the right size, which is different then not the right pitch. How much space between the top of the prop blade and the motor above it? Any time my 27 start to rev up on a turn I just lower the trim a little and know that that is my max amount of trim.

on edit: how does your cal. plate line up with the bottom of your hull?
 
I bought the boat back in May; it was in mint condition and I'm the second owner. I put it in the water and haven't pulled it out since. I'll pull it and investigate some of those things. It was dealer installed and the older gal I bought it from doesn't recall any issue like I'm having. Also, if I'm stopped, turn hard over and throttle up it does the same thing, acts like its cavitating and revs up; back the throttle down, steer straight and it bites just fine. I can run around 5400 turns which tells me prop size is correct. I'll usually back it down to 5 or 52 when cruising. Again no issues until I give a little more than easy rudder. Thanks for your feedback.
 
I bought the boat back in May; it was in mint condition and I'm the second owner. I put it in the water and haven't pulled it out since. I'll pull it and investigate some of those things. It was dealer installed and the older gal I bought it from doesn't recall any issue like I'm having. Also, if I'm stopped, turn hard over and throttle up it does the same thing, acts like its cavitating and revs up; back the throttle down, steer straight and it bites just fine. I can run around 5400 turns which tells me prop size is correct. I'll usually back it down to 5 or 52 when cruising. Again no issues until I give a little more than easy rudder. Thanks for your feedback.
 
It would be very unusual to put the engine hard over to one side and then give full throttle. Also since these boats are almost flat bottom, (the 1988 boat hulls are the same as current models), they will slip side ways far easier than a conventional V hull or a boat with a keel. Perhaps you are pushing the limit of the turn?

If not then as Tom suggests blade tip clearance, too much cup or rake in the prop, or a slipping hub could be the issue.

If you topping out at 5400 then I would run the boat in the mid to high 4,000 RPM range--not over 5,000 all of the time. What speeds are you seeing--and what is the diameter and pitch of the prop? Aluminum, composite or SS material?
 
I'm on a tugboat right now but will see if my wife can get me my prop specs. I normally don't have reason to go hard over and throttle up; was doing some fiddling trying to figure out when and why it does this, last time home. I guess I haven' owned the boat long enough to know it's true limits. Finding out would seem like a good thing. Just towing my young kids on the tube, going quite slow and moderate turns and it would respond as decribed earlier. Bummed us out!. It will not tow my barely over 100 lb wife on a single ski. It'll cruise comfortably at around 28, the fastest I've seen was around 32. I'll get that info! Very much appreciate the help. Marc
 
I've lost a rubber propeller insert on a Johnson before, and indeed one on a Honda 150. The engine winds up and the boat doesn't go. The insert can be replaced by a propeller shop at a nominal cost..

Cavitation is where, for various reasons, the water pressure around the prop drops below the water vapor pressure and air bubbles form. To cause this on your boat after lo these many years, something must have changed and I don't hear that from you. I would assume from what you're saying, that what's actually happening is that the drive shaft is slipping inside the rubber insert.

It's a reasonable place to start, and cheap as things go.

Boris
 
Rodger, the boat was in a shop unused the previous 3 years before I bought it in May of this year. I have a spare wheel, new in the box that came with the boat . I think I'll try that first. Thanks for all your help guys! When I get home of the boat, I'll put the spare wheel on and if I still have issues, see if anyone has any more suggestions.
 
I replied in the wrong forum I think; the boat sat in a shop the previous three years before I bought it in May of this year. A brand new prop came with it, when I get home off the boat I'll try it and see if that makes the difference. If that fails, I'll check in and see if anyone has anymore suggestions. Thanks everyone!
 
theleverbeef":3cjsivr1 said:
I guess I don't know how these things work!
Click to expand...

theleverbeef,

At the page bottom, left corner are two "buttons" the left one is "new topic" and the right one is "reply".

The "reply" will put your answer into the current thread. The "new topic" will start a new thread in the same forum. You can change forums by going to the top of the page and click on "forums" and go down the list to find the appropriate forum, click on that and go through the resulting list to see if someone has already started a similar topic thread to what you are wanting.

Can find a whole lot of interesting "stuff" that way.

Good luck and enjoy.

Harvey
SleepyC :moon

JC_Lately_SleepyC_Flat_Blue_055.thumb.jpg
 
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