Cavitation burn? prop modifications.

[Frank Acampora]

OK! You guys have seen this boat before.

However, due to a plastic tailcone malfunction, I "donated" my favorite bronze racing prop to the river.

Now, using the aluminum stock prop (10 3/8 X 13 cupped), I am getting what looks suspiciously like cavitation burn (circled in black) on all three blades. This never happened with the two blade bronze prop.

When the tank (under the deck in front of helm) goes about half empty, the boat lightens up just enough that she will start to mildly bounce and "gurgle" a bit, like it is ventilating some, but speed picks up.

The anti-vent plate is up about midway in the tunnel because that it where the bronze prop ran best--in fact, I was thinking of raising the engine a bit more with that prop.

So, not wanting to lower the engine, the question becomes: What can I do with the prop? Will additional cupping help? Or, will I just have to live with the burn? Getting any decent props for this engine is a difficult search and expensive so stock props are the choice since it is a recreational boat, not a die-hard racer.

Hoping Dhadley will have some input here.

10 foot Cougar Cub, Chrysler 55 Horse, 5000 RPM, 45 MPH.

 

[Faztbullet]

Re: Cavitation burn? prop modifications.

Live with it as its air bubbles coming off gearcase mating line. If gearcase was a little bit more fluid dynamic(less drag) you wouldnt see this

 

[Scott Danforth]

Re: Cavitation burn? prop modifications.

aluminum is softer than bronze or stainless.

 

[Texasmark]

Re: Cavitation burn? prop modifications.

Looks like it is occurring at the interface of the bulge (water pump housing, upper gear box ?????) which very well could provide turbulence. The only engine I ever had cavitation burns on was on their larger engine, the 85. It had this reverse twist at the front of the blade and it would burn about an inch behind the leading edge of the blade right against the housing. I notice that the burns you refer to are in a different place and the prop doesn't have the reverse hook on the leading edge.

Mark

 

[Faztbullet]

Re: Cavitation burn? prop modifications.

10 foot Cougar Cub, Chrysler 55 Horse, 5000 RPM, 45 MPH.

Something is off big time on this setup... The power to weight ratio must be off on motor as its only putting out around 30hp.

Lee Motorsports
As a former owner and driver of three of these 10' Cougar Cubs with 25XD Mercs back in 1984 - 1986, these boats are very fun to drive. With a 25 Merc, trimmed out in the #5 tilt pin hole, you can get 43-45mph with a re-pitched aluminum prop to 18"p

 

[Frank Acampora]

Re: Cavitation burn? prop modifications.

Thanks, guys. I never noticed where the burn was in relation to the parting line. One of my in-process modifications is to install a "Stump-Stabber" lower unit. This lower has the nosecone extended forward to a point and should be a little more slippery. You can see the paint worn off the gearcase nose from water impact-- this lower has never been in contact with the bottom, not while I have had it and the nose paint was there when I started to run it.

Fazt: Yeah, power to weight IS off. Remember the Merc put out more than 25 HP and the Chrysler is no powerhouse--but it is what I have. The engine weighs 160 and I weight the same. Then, add a full sized car battery and a full tank of fuel--We are up over 400 pounds burden and she sits low in the water. So, I am satisfied with 45 MPH---for now. Eventually I will need to power her with an engine for which I can get a selection of props.