cavitation with bracket

[natchalkley]

so I have now had my 23cc in the water for about a month and a half, and I have noticed that I get a lot of cavitation at cruise/high speed. it is a 250 suzuki on a stainless marine bracket. I have played with the trim/tilt but it doesnt seem to do much for it. do I need to think about dropping the motor a hol or two?

[gofastsandman]

Hey Nat, Scroll down to the post "Need help w/ porpoising"
Bushwacker knows the formula for bracket setback and cav plate height. Is it a flotation bracket?

[Fr. Frank]

Are you cavitating or ventilating your prop? There's a big difference, and different fixes.

[Michael Vezzosi]

That has got to be one of the more/most difficult questions I've read on this site. Had enough of a hard time answering that one by watching videos of my race boat...that question was two versus three blade props. I'm sure that it is a very legitimate question but I would have a very difficult time trying to answer it. Call me stupid...I'm not doubting anyone...just stating the obvious, I hope. Vezo, Part II.

A cavitating fire pump is introducing air into the impellars of the pump vanes, generating heat, causing the molecules of the water to compress, heat up, expand and ultimately pop causing catastrophic break down of the steel impellar vanes, and in lesser terms, the pump itself.

I am definately not sure of the difference between cavitation and ventilation, but would like to learn. This should prove a profound difference between the two.

P.S. After 28 years in the fire service, I hope my boss' aren't reading this... Part II.

[Bushwacker]

There are some subtle differences in the physics between the two situations. Cavitation is a phenomenon that can occur in any pump or propeller where the suction effect drops the LOCAL static pressure below the vapor pressure of the liquid, i.e. the water literally boils. It's well documented in textbooks on pumps and propellers. As the fluid moves along the blade from low pressure area to a spot where local static pressure is above the vapor pressure, the bubbles suddenly collapse. This collapse can be quite violent, enough to move metal! The suction (convex) side of a prop blade can look like it's been shot peened if it's experienced extensive cavitation. Pumps tend to cavitate under conditions of high head pressure and low flow, as occurs when you close down a discharge valve. I believe this condition can occur on high pitch props under sudden acceleration. It's also analogous to stall on an aircraft wing when the loading or angle of attack gets so high that the flow separates over the top of the wing.

Fr. Frank may be able to explain Ventilation better but I believe it's simply the case of a prop that's close enough to the surface that it sucks air and looses it's "bite" on the water; typically happens when you have the engine trimmed up fairly high and then make a sudden hard turn. Some high performance outboard props have holes in the hub just upstream of the suction side of the blades to dump exhaust gas to intentionally cause ventilation during hard acceleration. Once the boat gets up to speed, there's enough suction in the wake of the hub that the exhaust goes out the hub instead of out the vent holes. This allows engine to quickly rev up into it's power band, similar to a high stall speed torque converter on drag race car.

[Fr. Frank]

Excellent descriptions, Denny.

[natchalkley]

from the descriptions there, it seems that it is ventilating. I have tried trimming down, but it doesnt seem to do much for it, other than slow it down.

[Fr. Frank]

What kind of prop are you using?
How is it cupped?
Do you have anything mounted to the transom within 8" either side of the lower unit?
Ditto anything mounted to or through the hull within 8 inches of the centerline and within 8 ft of the transom?
What is your setback?
How high is the cavitation plate above the keel at neutral (parallel) trim angle?

[Snookerd]

Well said Denny! My 900 lbs of motors on my Armstrong bracket have jack plates that allow way too many variables to adjust for all different speeds and motor trim positions. I am not even using them right now. They are set all the way down and I will ventilate if the motors are not trimmed all the way down on take off.

[Bushwacker]

You might want to check your engine mounting height (distance of cavitation plate from bottom of boat with motor trimmed such that plate is parallel to bottom). Standard mounting height with a bracket is to raise cav plate 1" above bottom for each 1' of setback. Don Herman set mine at 2.5" for the 30" setback of the Hermco bracket. That's just a starting point however; I was able to raise mine another 2" from there without getting significant ventilation in hard turns unless I have motor trimmed higher than about 70%. (100% is max uptrim, where it switches to tilt mode)