Catalina 350 International Association

Since the problem is almost certainly in the hull design ie the prop is partially shielded by the thick stern section and far too close to the hull surely a redesign or for boats already built either grind down the thick section in way of smooth water flow or, as in my case, instal a feathering prop that sits further out from the hull. I can proceed at max rpm 3000 with only slight cavitation
Hugh

Hugh - interesting statement about how the problem is "most certainly in the hull design....prop partially shielded by the thick stern section...too close to the hull"

Can you point out a little of what you are talking about in the following picture:


There's another picture from a slightly different angle in an earlier post by me in this topic.

It seemed to me that there was quite a bit of clearance between the prop tips and the hull and that the prop was extended quite a ways into the stream - but what do I know, I'm no marine engineer.

The photo's are from a recent haulout on April IV - just before she was pressure washed, so you are seeing some slime on the bottom.

It would seem the top of the blade will always be running in disturbed water flow as it is sheilded by the thick moulding the shaft runs in.
Would be interesting to fair the fibreglass above the prop shaft to allow a smoother water flow past the blades.
Your pic shows the problem well as I see it.
Hugh

I have a pic of my prop taken today.



It shows that it is a much greater distance away from the hull than your 2 blader.

Hugh
#315

Hugh - Jerry posted a set of instructions for posting pictures - it's contained in a post in this topic, titled "props, zincs, etc."

There also is another picture I posted that gives a better perspective of just how much space there is between the blade tips and the hull. The picture in this thread distorts the perspective a bit and makes it look closer than it really is.

Does anyone know the beam at the waterline and the hull draft excluding the keel?

The original 15x9 propeller is being questioned (relative to transmission failure and severe wear on the thrust washer) and based on program a 14x8 or 14x9 would seem to be right but I want to make sure the input data is correct.

I noticed the "new" propeller offered by Catalina was a 14x10 but can't get the pitch up that high.

Duncan

Rest assured a 14" prop of any pitch is not the answer. I had the 14-10 and it had cavitation burn on it before it was replaced. 14" of any pitch is to small. The answer is to get the blades further back from the deadwood. This can be done with a feathering or a folding prop, or by extending the drive shaft.

We tried the 14x8 prop over the summer and as you predicted (and I half expected) the speed was off getting only around 4 knots at 2400rpm and having to run around 2700 rpm to get 5.5 knots or so. The other concern was that engine would go far to easily to WOT. In discussing with Ron Campbell (Campbell sailor prop) it was agreed we had probably been too conservative and he repitched it to 14x9 and slightly cupped the blades. The latter was his idea alone to prevent slippage and was probably what made the difference.
Last week with modified prop she ran comfortably at 5.7knots at 2400rpm and over 7 knots at 2900rpm (we actually had a wake).
Stopping the boat has also been improved so we have good progress.
With regard to cavitation I'm not hearing or feeling it but to be honest I'm surprised that cavitation on a prop driven by a 35hp engine would be of a magnitude to actually cause any damage to itself. Typically on a pump we would not be concerned until you get well above a 100hp driver. Not to say it's not happening, just not enough energy.