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Catalina/Capri 25/250 Sailor's Forums

Catalina 250 Specific Forum

Cavitation woes

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jbkayaker
Captain

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USA
299 Posts

Initially Posted - 05/04/2011 : 17:45:34

I normally sail on a large inland lake, where I've had a few cavitation problems, but nothing like I experienced recently in the Charleston (SC) harbor. A squall and a tidal current stirred up some waves that had my boat hobby-horsing so much that the outboard motor prop was sucking air about 50% of the time.

How deep is the top of your outboard's propeller (when sitting still in calm water) and do you have cavitation problems ? Do you have anything (like a curved or flat plate) installed to reduce cavitation?

Jim Butler

Edited by - on

superbob
Navigator

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USA
200 Posts

Response Posted - 05/04/2011 : 18:15:28

Jim:

You're missing one important piece of the question that may bring more intelligent answers than mine: What's the length of your shaft (the outboard, that is). Therein might lie the problem. Most of us own OB's with 25" shafts. Some try to get by with 20" shafts.

And, what's your OB mounted on? An adjustable bracket or to space provided on the stern?

Edited by - on

delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts

Response Posted - 05/04/2011 : 20:40:23

Jim,
We originally had a Johnson Sailmaster 8hp on SL, but replaced it a couple of years ago with a Tohatsu 9.8hp. The Johnson would cavitate badly if we got into some chop, especially if I had to go up on the foredeck to take down the jib or something. With the Tohatsu and a Garelick mount, that no longer happens. Here's an out of the water photo to show you how deep the shaft is below the waterline:

The prop is roughly 1-1/2' below the waterline or so. When it's in the up position, I can still maneuver the boat as the prop is just barely in the water. I can tilt it half way up and get it completely out of the water.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 05/04/2011 : 21:56:38

Ventilation occurs when surface air (or exhaust) gets sucked into the prop as when the prop nears or breaks the surface of the water. The anti-ventilation plate on the lower unit above the prop prevents ventilation, hence the name.

Cavitation occurs when a liquid (in this case water) vaporizes on the surface of the prop with the resulting bubbles collapsing which sometimes causes pitting on the prop's surface. For boat props, this damage is generally the only sign that cavitation is occurring.

These are commonly confused terms (including by me for the longest of times

Edited by - dlucier on 05/04/2011 22:04:58

Stinkpotter
Master Marine Consultant

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Djibouti
9082 Posts

Response Posted - 05/05/2011 : 06:08:43

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dlucier
 [u]...These are commonly confused terms (including by me for the longest of times

).<hr height="1" noshade id="quote"></blockquote id="quote">It's also confusing because outboards once had "anti-cavitation plates" before that distinction was made (I think some time in the 1970s or '80s). You'll see it in older owner's manuals.

If you've ever gunned your outboard when at a near stand-still, and heard it rev up to red-line RPMs while losing its grip on the water, you might have been experiencing cavitation (which in turn might have been induced by ventilation). The evidence is clear on a painted aluminum prop--the paint is missing around the trailing edge of each blade, and that edge might be pitted.

BTW, my anti-ventilation late was about 5" below the C-25 waterline, and the prop very rarely sucked air, even in substantial Long Island Sound chop and large stinkpot wakes. I could have put it a few inches lower (the last stop on the bracket) but almost never did.

Edited by - Stinkpotter on 05/05/2011 06:14:19

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 05/05/2011 : 07:55:04

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Stinkpotter
 It's also confusing because outboards once had "anti-cavitation plates" before that distinction was made (I think some time in the 1970s or '80s). You'll see it in older owner's manuals.<hr height="1" noshade id="quote"></blockquote id="quote">

Older owner's manual? The illustration of the Honda 9.9 outboard features in the 2001 owner's manual points out the anti-cavitation plate and in the installation section states, "The anticavitation plate of the outboard motor should be 0 - 2.0 inches below the bottom of the boat. With the boat in the water and loaded, the anticavitation plate should be about 3.9 inches below the surface of the water."

In the 2002 edition, the same illustration was changed to read anti-ventilation plate as was the text in the installation section. So actually, it wasn't all that long ago.

Edited by - on

Steve Milby
Past Commodore

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USA
5904 Posts

Response Posted - 05/05/2011 : 08:30:14

Jim, when it happened, were you motor-sailing, or were your sails down? If you were motor-sailing, I'd suggest you lower the sails and see if that helps. An outboard engine is situated so far aft that, in a very short, steep chop (which appears to be your situation) even a deep prop can be raised above or very near the surface. Usually, if you can steer downwind, it will help keep the prop in the water. [When the boat is in the wave trough, the prop will be immersed. As a wave approaches from astern, the stern will be lifted by the wave, and the prop will remain immersed. By comparison, when motoring to windward, when a wave rolls under the boat, it lifts the bow first. When the wave reaches the stern, the bow drops down onto the back of the wave, and that lifts the transom into the air, and sometimes out of the water. That's what causes the hobby-horsing, and, if you turn downwind, it usually goes away.] You also have an option to drop the anchor and wait for the conditions to abate. A change in wind direction or speed could alleviate the problem. If a localized squall is passing through, the worst of it could be gone in 15-20 minutes.

Edited by - on

jbkayaker
Captain

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USA
299 Posts

Response Posted - 05/05/2011 : 10:48:26

I was motor sailing with the genoa furled at first and losing ground toward the aircraft carrier which is permanently grounded in the Charleston Harbor. Dropping the main didn't help except to make the boat manageable when I turned to roughly 40 degrees off downwind to run into the narrow entrance of a lee shore marina about 100 yards away.

I understand the several ways to get the propeller deeper. My first question is how deep seems to be good enough for rough conditions.

My second question is how effective at compensating for too little depth are the $89 add on propeller shrouds made by Garhauer Marine :
http://shop.sailboatowners.com/prod.php?50097/Power+Thruster+for+Sailboat+Outboards

My current setup doesn't work so I'm wanting to find out what does.

Edited by - jbkayaker on 05/05/2011 10:51:22

Steve Milby
Past Commodore

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Response Posted - 05/05/2011 : 12:02:29

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by jbkayaker
 My first question is how deep seems to be good enough for rough conditions. <hr height="1" noshade id="quote"></blockquote id="quote"> I don't know of any outboard motor that has a shaft that is deep enough for especially rough conditions. I've never seen one that wouldn't cavitate or ventilate in really rough conditions. My long shaft Merc certainly did. The susceptibility of outboard motors to this behavior is one of the factors that make small, outboard-powered, trailerable sailboats unsuitable for rough weather. The outboard motor becomes unreliable, because you can't keep the prop in the water, where it can propel the boat. An inboard engine puts the prop deeper and farther forward, where it can continue to provide drive.

If you're in that situation, your choices are to continue sailing as long as you are able, or to try to drive the boat to windward under power, taking the risk of ruining the engine, or to develop a "plan B." Plan B means to either run downwind under power, or anchor somewhere until the conditions moderate.

From your photo, your engine appears to have a shaft that is about as long as they get, and, if yours raises out of the water, I doubt that any engine can be found on the market that won't do the same. It's the nature of an outboard engine, and one of it's drawbacks.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">My second question is how effective at compensating for too little depth are the $89 add on propeller shrouds made by Garhauer Marine :
http://shop.sailboatowners.com/prod.php?50097/Power+Thruster+for+Sailboat+Outboards<hr height="1" noshade id="quote"></blockquote id="quote">I've never used one of those, but, as I understand, they are supposed to increase thrust. If the prop lifts out of the water, they won't work any better than your bare prop.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">My current setup doesn't work so I'm wanting to find out what does.<hr height="1" noshade id="quote"></blockquote id="quote">A boat with an inboard engine.

Edited by - on

Stinkpotter
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Djibouti
9082 Posts

Response Posted - 05/05/2011 : 18:32:16

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Steve Milby
 ...From your photo, your engine appears to have a shaft that is about as long as they get...<hr height="1" noshade id="quote"></blockquote id="quote">Did I miss something? I only saw David's photo, which has the prop as deep as I've ever seen. And I hadn't seen an adjustable bracket on a C-250 before.

How deep is yours, Jim? (Specifically, how far below the at-rest waterline is the anti-ventilation plate?)

I agree that the Garhauer "tunnel" is unlikely to alleviate the problem.

Edited by - Stinkpotter on 05/05/2011 19:07:29

Nautiduck
Master Marine Consultant

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3704 Posts

Response Posted - 05/05/2011 : 20:25:16

Holy crap David! That thing is deeper than the keel!!

If you cavitate or ventilate or whatever it would mean you were nearly vertical nose down.

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delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts

Response Posted - 05/05/2011 : 22:00:32

I spent a lot of time in AutoCAD optimizing the depth before I ever drilled a hole. I figured the keel was about 2' deeper, and roughly the same with the rudder, so I'm not too worried about hitting anything with it, or at least nothing on the bottom that I haven't already plowed into with my keel.

The funny thing was, I had no intention at all of mounting it on a bracket, it was going on the transom, until I figured out that it couldn't turn inside the transom cut out (the engine width is different from the one Paul had measured for me due to model year changes). I scrambled to find a solution and the Garelick was it.

The unfortunate side effect was to completely change the geometry of the hard link I'd designed & made for our previous outboard, and I've never been able to make a suitable one for this geometry. I do have an idea, but it involves eccentric cams and lever locks, so I'm not sure it'll ever get built.

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John Russell
Master Marine Consultant

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USA
3444 Posts

Response Posted - 05/06/2011 : 06:38:42

I have the same motor and mount as David and my prop has moved air above the Lake Erie chop.

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jbkayaker
Captain

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USA
299 Posts

Response Posted - 05/06/2011 : 15:22:19

I haven't had an opportunity to measure the depth of my anti-ventilation plate since it's 3 hours away. Estimating from memory I'd say about 6 inches. I gave up my plan to do some short offshore runs after having problems in the harbor. Steve M. is right, I'm going to need a bigger boat (with an inboard engine).

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Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 05/08/2011 : 08:50:57

Steve is of course very right about the limitations of outboards. The discussion is reminding me once again how much I enjoyed the water ballast model on the Great Lakes. It sits lower in the water than the wing keel model and suffers less in the way of hobby horsing. It suffered some of the problem with a long shaft (20") but very rarely with an extra long shaft (25").

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