Cavitation and Ventilation....?

[nala]

Im fitting Hydro Foil Fins to the Cavitation plate..of my Parson 6hp 4stroke outboard..hopefully this will minimise or completely cure it...?

I've added a link to see why it occurs,which will explain....

propellerhub.com/cavitation-and-ventilation.html

Im fitting Hydro Foils not just for that,it get's it up on the plane quicker and saves a bit on fuel too.(so you can throttle back..)

And probably won't notice it on my SIB,it also stabilizes the boat when turning...at speed : )

I did notice on the fins,by the mounting holes some cracks....which i think are just surface scratches......purchased from MES.

[philpot]

Oh the wonderful world of cavitation/ventilation. I am surprised if you are having any issues with a 6hp Alan but it can be an issue that shows itself on sibs more than rigid boats and air floors can be very prone to this.

It is not that unusual to have to jack up the transom height in order to set the cav plate just right for the boat concerned, a simple job, take some strips of 3 or 6mm ply about 10'' long by the thickness of your transom, ie 1.25''. A series of tests are required and keep adding pieces of wood until the problem stops.

It can also be overcome by placing the boat on the water after inflation and leaving for 10 - 15mins to allow the tubes to reduce air due to cold water, check the pressures again and top up with air as required. The floor pressure is critical and don't forget you can get air passing under the floor at speed which again will cause ventilation.

Sometimes placing a piece of ply wood under the floor and touching the transom so perhaps 3' wide by 12''/14''. this can change the water direction at the stern of the sib and improve the passage of water going through the prop.

It is not always easy to fix and it may well be intermittent which is a bugger. Your hydro fins are an aid to get the boat on the plane but having used them years ago, I found them to be okay but a pain in the rear when transporting the engine in the car because the prop shaft will not lay flat. This can be another issue as oil can run back toward the engine from the gear box and so it is very important to ensure the engine cowl/block area is ALWAYS higher than the prop.

Phil

[nala]

Thanks for that Phil...

I think its cupping and trapped air under the boat and when i accelerate it dissipates out the back...

And then drawn into the prop as the boat moves ahead causing increased revving and non-forward motion.

I can live with it,but i do feel its weight distribution too....and that area in front of the transom which is in the boat.

Which if you imagine a flag flapping in the wind......its a similar effect but in water.....the flag being the floor membrane.

Its happened a couple of times to which i am like a dog with a bone to find and sort out whats going on...

I'll post on the hydrofoil test.........when i get time

[philpot]

Yes I know what you mean with the Mexican wave going on below you but be prepared for a slow series of tests to find the problem area. Try the pieces of wood to raise the engine up and try that. It is a process of elimination.

How far below the transom is the engine cav plate, it may be too low.

Phil