Cavitation
 

Greetings,

I've a 22' D&M sailboat. It weighs in at 4000 lbs with 2000 lbs of
ballast. Right now, I have a Johnson 15hp motor with a 20inch shaft on
the transom. The boat floats pretty much on its lines.

I'm experiencing a lot of cavitation when underway in moderate seas (2-4
ft) and am wondering if this is standard for an outboard powered
sailboat or if perhaps I need a longer engine shaft. Also, would
moving the tilt pin forward or backward help?

Any thoughts or comments would be greatly appreciated.

Thanks,
Nelson

Cavitation
 

Subject: Cavitation
From: Nelson Kirsch

Greetings,

I've a 22' D&M sailboat. It weighs in at 4000 lbs with 2000 lbs of
ballast. Right now, I have a Johnson 15hp motor with a 20inch shaft on
the transom. The boat floats pretty much on its lines.

I'm experiencing a lot of cavitation when underway in moderate seas (2-4
ft) and am wondering if this is standard for an outboard powered
sailboat or if perhaps I need a longer engine shaft. Also, would
moving the tilt pin forward or backward help?

Any thoughts or comments would be greatly appreciated.

Your experiencing airation which is not uncommon in that type of boat. It
sounds like you need a longer engine shaft. Or a way to lower the outboard you
already have.

Capt. Bill

Cavitation
 

Hello Nelson,

First let's get the correct terminology. What you most likely have is prop
ventilation, where surface or exhaust gas enters the prop stream and reduces
the thrust. Cavitation is the boiling of water on the low pressure side of
a prop under very high thrust conditions, often damaging the prop. This
doesn't happen on low hp motors.

see:
http://www.powerboat-training.co.uk/...rs-ventilation
,%20cavitation.htm

Look for a way to get the motor lower in the water, usually by moving its
bracket, assuming its tiller still has room above the transom. Otherwise
you need a longer shaft. Normally the motor is best set for zero
tilt....prop shaft horizontal.

Brent
www.bensonsails.com



From: Nelson Kirsch
Organization: AT&T Worldnet
Newsgroups: rec.boats.cruising
Date: Thu, 04 Sep 2003 13:08:39 GMT
Subject: Cavitation

Greetings,

I've a 22' D&M sailboat. It weighs in at 4000 lbs with 2000 lbs of
ballast. Right now, I have a Johnson 15hp motor with a 20inch shaft on
the transom. The boat floats pretty much on its lines.

I'm experiencing a lot of cavitation when underway in moderate seas (2-4
ft) and am wondering if this is standard for an outboard powered
sailboat or if perhaps I need a longer engine shaft. Also, would
moving the tilt pin forward or backward help?

Any thoughts or comments would be greatly appreciated.

Thanks,
Nelson

Cavitation
 

Nelson Kirsch wrote:

Greetings,

I've a 22' D&M sailboat. It weighs in at 4000 lbs with 2000 lbs of
ballast. Right now, I have a Johnson 15hp motor with a 20inch shaft on
the transom. The boat floats pretty much on its lines.

I'm experiencing a lot of cavitation when underway in moderate seas
(2-4 ft) and am wondering if this is standard for an outboard powered
sailboat or if perhaps I need a longer engine shaft. Also, would
moving the tilt pin forward or backward help?

Any thoughts or comments would be greatly appreciated.

Thanks,
Nelson

If the prop is lifting out of the water in those conditions, you'll want
to lower the motor mount or get a longer shaft, but 20" is already
pretty long.

Cavitation, depending on what exactly you're experiencing, could be
because the prop's not down far enough and surface air is being drawn
in, or possibly that you're simply trying to drive the boat too fast, or
that you have too aggressive a prop. Your engine is nearly as powerful
as ours, you're half our weight and we only use about 5 hp at cruise
power (5.5-5.8 knots, depending on the conditions). If you're trying to
keep to that sort of speed, you might be pushing against hull speed.

As another scenerio: We have an inboard and the prop and engine are just
about perfectly matched to the boat, but we've been stopped twice by
boat wakes or waves and the prop cavitated: Spun in its own air pocket
(or something) and didn't have any real connection to the water. Had to
throttle back for a second to re-establish thrust. Under normal
conditions, we could pull stumps with our 16"x10 3 blade prop (had to,
once), so I doubt there's anything we can do to prevent that.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/