I thought I'd share something with you that may really help this situation if it plagues you.

For years the sled I have cavitated when I'd hit slightly rough water, likely removing some amount of life from my engine evertime it reved because it reved quite high at times.

I read an article somewhere... can't locate it now but they discussed the properties of water under extreme pressure and extreme vacuum. One of the problems with water under extreme vacuum is that it develops air pockets or holes when a rapid change occurs from no vacuum to a high vacuum state. In the article they also discussed water flowing over and by sharp, rounded, and flat objects and the difference in water properties as opposed to air properties with regards to flow.

The grates on a jet produce both extreme pressure and extreme vacuum. Extreme pressure on the front side of the grate because the flat surface and likewise extreme vacuum on the backside, well because the impeller is pulling water past that grate like… (use your imagination here).

Ok so armed with the information from the article I got to thinking about the grates on the bottom of my outboard jet. I removed all the plates and took them to the grinding wheel (12” grinder). I formed a rounded leading edge on each plate, which will help eliminate a vortex forming. Incidentally a sharp leading edge can cause flow redirection problems, kind of like harmonic waves. I also formed a knife edge taper on the trailing edge (the edge closest to the impeller) of each plate. This shallow taper will allow the water to slip smoothly past the plate without forming an air pocket on the back side of the plate. Actually the ideal shape would have been a ‘tear drop’ but I settled for a very shallow hollow ground trailing edge. It took about four hours to grind all the material away on the eight plates but I can attest it was worth every minute of the task.

The jet runs noticeably smoother and the cavitation problem I had before is dramatically improved. As a mater of fact while you probably can’t completely eliminate cavitation on an outboard jet I would imagine that I have seen an 85%-90% improvement. Last time out the jet only cavitated three times and when cavitation does occur it doesn’t even spin up more than a couple hundred RPM, because the air pocket is evacuated almost before it develops. I think I gained a little more power too but this is harder to quantify.

Good Luck!

Legal disclaimer - your mileage may vary but it really helped in my case!

TC,

Cool! I like it. Seems like anything that keeps air out of the jet unit is a good thing.

I only get very occasional cavitation with my boat, and I've figured it was due to choppy water interfering with delivering the smooth laminar flow of water to the jet intake. You can treat cavitation to some extent with a thing that looks like a trim tab off the bottom of the transom. That's what I had on my previous boat. For some reason, I don't seem to need it on my present boat.

Sincerely,

Salmo g.

Overall, sounds like an improvement.

Just a thought...since it is easier for water to go through, is it is easier for debris to make it by the grate and into the impeller?

there is also a set of "fins" made by specialty jet that bolt onto the outside of the shoe. You fill the tiny gap between the fin and shoe with silicone. These fins are angled away from the shoe so they do a couple of things, 1) reduce caitation by forcing more water into the shoe, 2) help the boat turn more sharply by reducing slide, 3) help the boat get on plane quicker. I recently purchased a set for my flat bottom alumaweld and they worked wonders.

TC could you post a picture of your grate ?

To answer a couple questions...

Jim - Haven't noticed any difference but then again I don't usually suck debrise up. I am KEENLY aware of one thing though. If I do suck debris up I am going to to be VERY carefull removing it - as I said it's a knife edge on the trailing edge.

Centerpin - I'll get pics. I'll have to remove a plate or two to get pics. It'll have to wait a couple days.

Sounds like you have yourself a great business idea. I would pay several hundred dollars to have that done if I knew it would eliminate almost all my cavitation. I've been plagued with it for several years. Put a dam on my shoe and it helped a little but performance also suffered a little. Thanks for the tip. Don't know if I will ever have the time to do that but nice to know it works.

Here is the plate I put on the back of mine, it stopped the cavitation. I extended mine out to the sides of the boat so it would get up on a plane quicker.


Fishhead5

I recently bought a used Merc 90 from Stevens Marine.
It too has the side fins on the foot, but it has something I have'nt seen before.
On the rear, lowest part of the foot, at the base of the grates, it has a sharpened plate, about 1" wide, going side to side, angled down to catch more water.
I have'nt not gotten out on the river yet to check it out, but it makes perfectly good sense to me.
The guy who owned it before me must of known something about jets, because he had put in a polished, 4 blade stainless impeller.
Has anyone else seen this on a foot before?

I have that extra piece and fins on my jet and I love it. I have only owned one jet, which is the one I have now. I love it and feel that I get alot of power out of it due to the modifications described above. I believe it does help alot G-Man! Just my 7 1/2 cents.

saw a guy on the skagit with one. He said it help the boat get on plane better. He had 4 guys in a 20' sled with only a 80 merc jet and got on plane no problem.

Great ideas guys, it sure would be nice to see some more pics (TC, Remdrake, G-man, rln). With gas prices soaring it would be nice to maximize the efficiency of our jets. That’s sort of funny, “maximize the efficiency of our jets.”

TC – nice work on the oar blades, a true craftsman.


LT