Depth of propellor

[Fitzroy]

Any thoughts or ideas out there on minimum propeller depth beneath the surface of the water. I am rebuilding a 38' steel sailboat around 22000 Ibs with fin keel and skeg hung outboard rudder. The shaft log has been installed at a 10 degree angle and terminates within 4' from the stern of the boat and 22" below the waterline. The 11/4"shaft will swing an 18" propeller which will leave 13" of water covering the propeller with no boat heel. i have not had the boat in the water and am thinking that this location of the propeller will cause it to cavitate especially in certain sea conditions. I would appreciate others thoughts or knowledge on this situation. I have been measuring other boats in the yards and the only time this situation is replicated is on boats of around 30' at 24" in depth and 3' in from the stern. A Westsail 32 or similar design had the prop cutout right at the end of the full keel at 24" down to center of shaft and located right at the stern of the boat with an outboard rudder.

[gonzo]

Cavitation is caused by too much load on the propeller blades. You may be refering to ventilation, an alltogether different problem. With the propeller under the hull and so much forward, ventilation is not likely.

[Fitzroy]

I stand corrected, I meant ventilation. What about the fact that the tips of the propeller may only be covered by a foot of water when heeling. Will this not cause ventilation?
Are there any design minimums for this situation?

[gonzo]

I think that the problem would not come up too often. If the boat is heeling so much, what do you need the engine for?

[Fitzroy]

I was thinking of a bar crossing situation under power with a following sea

[gonzo]

I see, you mean if the boat roll over and gets some air under. It would be on extreme situations, and as soon as the boat rolls back, the propeller should grab again. I don't think it would totally ventilate though. Also, it would not be good seamanship to cross a bar in those conditions. Unless you are dismasted, lowering all sails cuts down your options. Sails will dampen the roll too, which may prevent that problem.

[Fitzroy]

Yes thanks for your input. I moved the log aft to create more room in the boat. Originally it was a lot further forward and lower by about 4' and 16" respectively and im having doubts now!

[gonzo]

How did the log aft free more space. Did you shift the engine aft?

[Fitzroy]

Yes by 3'

[gonzo]

I suppose on a heavy boat like that it shouldn't make much difference to the trim. One solution is a Kort nozzle, but it may create more drag.

[Fitzroy]

Yes having a wheel this size is enough.

[Fitzroy]

What is your connection to this blog gonzo?

[gonzo]

I joined a few years ago when I was living in Milwaukee.

[michael pierzga]

Pitch in a seaway is a consideration. Cavitation. Also having prop wash , interference with water flow across the rudder, whether under power or sailing, affects the efficiency of the rudder. Ive sailed two identical sisterships. On one ,the pro shaft was well forward, on the other to accommodate interior living space requirements, the engine and prop were moved well aft and close to the rudder. The boat with its prop close to the rudder was much harder to steer...once your hand came off the steering wheel, she would turn. That yacht with the close rudder prop arrangement also burned up more autopilots.

Oh gawdelpus,

Michael when will you stop to tell us your unrelated lies and drivel?