Down wind rough water cavitation

SeaSpray · Posted: Mon Jul 06, 2009 7:57 pm Post subject: Down wind rough water cavitation

I experienced something this weekend I have not heard of or experienced before. I have Johnson 90 with the three blade aluminum prop selected by EQ marine for the CD22. It has worked well in all conditions. I also have a permatrim.

I was in conditions that required me to slow to 7 to 8 knots while going upwind into the waves. Not extremely rough, but 2ft+ fairly steep and close.

About an hour after making the upwind leg I was driving the same route downwind, down wave. I was able to do about 10 to 13 knots with only an occasional moderate bump coming off a wave into the next wave. I did have to trim the bow up a little from my normal bow down position to cancel strong bow steering when punching into the backs of waves. On a few occasions after the bump I would get brief cavitation with the motor rpms rising. I now have some small cavitation marks on the prop.

Has anyone had this type of cavitation while running in a straight line in chop? I am thinking that smacking into the next wave disturbed the water that the prop then passed through. Is this possible?

Any thoughts?

Steve

CAVU · Posted: Mon Jul 06, 2009 8:13 pm Post subject:

Steve,
I would guess that you were getting the prop a little high in the air at times. The combination of short steep waves and trimming the engine up a little may have let a little air in under the permatrim. I don't have permatrims so I have no experience with them-but with twins running downwind I have had this happen a few times. I either slow down or trim down a little more.

Levity · Posted: Mon Jul 06, 2009 10:32 pm Post subject:

Levity has developed some cavitation when running downwind in a 2-3ft chop. My interpretation was the hull was being driven too hard for the trim settings. Three adjustments brought the cavitation under control.
1. Slow down from 14 kts to 11 kts
2. Increase bow up trim with trim tabs
3, Increase engine (prop) down trim
Since this was an uncontrolled experiment identifying which or how much of the three adjustments provided relief is uncertain. However, the nearly simultaneous adjustments did make a real difference.
Mike "Levity"

SeaSpray · Posted: Mon Jul 06, 2009 10:53 pm Post subject:

Thanks,

Good to know it is not particular to my boat. Probably just driving the boat too hard for the conditions and the trim.

I did slow down to correct the problem but since the boat was handling well and the ride was acceptable I did not think it should be cavitating. Now I know it can.

Steve

Catch 22 · Posted: Mon Jul 06, 2009 11:41 pm Post subject:

im guessing that if the waves were at the right distance apart, as you hit one wave with the bow, it would slow the boat just a bit, to have the a wave at the back of the boat advance in relation to the prop, lifting the back of the boat and dropping the prop into a trough where it would cavitate for a bit, just a guess, on another note, what is a cavitation mark, ive never heard of that...

thataway · Posted: Tue Jul 07, 2009 12:00 am Post subject:

Some times even a 2 foot chop is enough to bring the lower unit out of the water--with the steepness of the waves. Generally one slows down to the speed of the waves in large seas--and may be necessary in these conditions. You want to keep the hull on the back of a wave; this revents the burying of the bow, or bow steering. Trim tabs, plus the Permatrim help with this. You may have had the engine trimmed up too far--hard to know.

I would be surprised that just a couple of cavitations causing burn marks on the prop. You may want to look at the position of the cavitation plate--should be about even with the bottom of the boat.