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Catalina/Capri 25/250 Sailor's Forums

General Sailing Forum

Cavitation

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gop711
1st Mate

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Armenia
83 Posts

Initially Posted - 03/10/2008 : 12:41:10

Jim,

I presently sail on an lake in middle Ga. I have a 1989 wing keel.

Last year we put 1000 miles on the boat. With retirement in site, it is my hopes to take the boat to the coast and the Bahama's.

My concern is with the motor. I have a 9.9 long shaft Honda. I am thinking it is about 23". Do you have any problems with cavitation coming into or out of channels?

Do you have any suggestions on how to avoid excessive cavitation?

Thanks
Greg

Edited by - on

gop711
1st Mate

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Armenia
83 Posts

Response Posted - 03/10/2008 : 12:42:46

Sorry about putting the post in the wrong place.

It was suppose to go into Jim's post about the distance he has sailed this pass year.

Greg

Edited by - on

stampeder
Master Marine Consultant

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1608 Posts

Response Posted - 03/10/2008 : 13:18:37

AS a rule, you want the longest shaft possible. I have a Honda 9.9 Xtra long shaft, I think it's 27"

By cavitation, are you referring to the propellor making contact with the surface of the water?

Edited by - on

JohnP
Master Marine Consultant

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1520 Posts

Response Posted - 03/10/2008 : 13:27:27

Keeping the prop in the water is difficult with outboards on our boats in heavy seas. When I've needed to motor in big waves, I've had to constantly rev down after the stern rises on each wave. Sailing is much more pleasant than motoring in heavy seas.

If your plans involve crossing the Gulf Stream in a Catalina 25 from Miami, it may be critical to time the crossing to avoid opposing north or northeast winds, so the sea is reasonably calm.

A 35' or 45' boat with an inboard engine would handle that crossing more comfortably, but once you're in the Bahamas, a C25 would be great if the weather is good.

Edited by - on

Sloop Smitten
Master Marine Consultant

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USA
1181 Posts

Response Posted - 03/10/2008 : 16:04:55

Greg,
I have the same engine and in those conditions you either come out of the water or easily swamp the motor. I have not found a cure other than to try and sail when those conditions arise. As others have said, the best solution is a longer engine. Keep your RPM's as low as possible to minimize the imapct.

Edited by - on

gop711
1st Mate

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Armenia
83 Posts

Response Posted - 03/11/2008 : 09:38:34

Thanks for the input.

It has been my experince on other boats that you can not always sail into a channel, when you want to, based on wind or tide.

My concern with cavitation, when the prop comes out of the water, was two fold. One possible harm to the motor, low rev idea would help on that one. Second is it a big problem, with people that sail in the ocean with a C25 with an outboard? Do you have trouble maintaining control of the boat?

Thanks
Greg

Edited by - on

Dave5041
Former Mainsheet Editor

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USA
3758 Posts

Response Posted - 03/11/2008 : 10:19:30

Just for clarity, cavitation is when the prop rpm's are to high for the boat speed - you see a long line of bubbles/foam in the wash and develop little thrust (not just the exhaust bubbles). Lifting the prop clear of the water from pitching is just that. It happens to any boat in the right situation. It is a function waterline length, hull shape, distribution of mass, and prop depth; the only component we can address is prop depth. You can reduce pitching by adjusting speed, adding some sail when motoring, and approaching waves at a different angle.

I also intend to cruise the Bahamas in the future (I'll have a new 4 stroke by then), and have no qualms about crossing the stream. As said previously, the issue is wind direction. Patience for the right weather window is essential. I read an interesting article about crossing the stream and cruising the Bahamas in a Compac 16. They waited until they could follow the back side of a high and had no problems crossing in light southerlies. The Compac line are great boats, but a somewhat lesser boat with half again as much waterline like our Cats should make make the crossing even less of an issue.

Edited by - on

OLarryR
Master Marine Consultant

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USA
3442 Posts

Response Posted - 03/11/2008 : 12:25:09

Cavitation is a term generally used for pumps that are air bound or have turbulence internally created from a number of conditions one being that the water upstream of it has significant resistance exceeding the pump's ability to pull a suction. The result is sort of similar to what you are experiencing and that is instability in the flow of water around the propeller. That sometimes creates abnormal wear of pump internals (bearings, shaft, propeller) if done for a long period of time. How long is a good question but i would think it is the bearings that take the beating in these instances.

The other negative consideration to watch for is the water to cool the outboard. If the propeller sometimes cavitates, then you may sometimes also have the water intake very close to the water surface and it may suck some air resulting in inefficient cooling of the outboard. Rcmd check your outboard's manual for which plate is always to be below the water surface to ensure adequate water cooling.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 03/11/2008 : 12:57:52

The term for when air is drawn into the prop, like when it breaks the surface, is ventilation.

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 03/11/2008 : 13:12:51

You'll find that open water motoring, as in the ocean, is generally less of an issue than an inlet. A C-25 sails (or motors) up, over, and down ocean swells, and pitches somewhat in wind-waves on the ocean. Inlets, however, create "standing waves" as wakes and wind waves reflect off the jetties and intersect, very likely lifting the prop out periodically. Paradoxically, that can sometimes be alleviated by increasing speed so the boat plows through the standing waves more and is tossed around by them less. Also, with more speed under power, a C-25 tends to squat at the stern, lowering the prop further.

Overrevving is unlikely to be alleviated by cutting back the throttle--it can happen even at 1/4 throttle when the prop comes out. Newer outboards have built-in protection against damaging levels of overrevving.

With my 27.7" (extra-long) Honda, I positioned my new bracket so that the anticavitation plate was something like 5" below the waterline. I usually didn't need to lower it all the way to that point, but it virtually elminated cavitation in Long Island Sound chop and wakes. My P.O. had a "long" shaft (22") engine with a bracket that put the plate at the waterline. If I walked to the foredeck with the engine running, it would suck air into the cooling port.

If you plan on using the engine for extended cruising, I'd seriously recommend an XL shaft. Lowering a long-shaft sufficiently can expose its powerhead to getting dunked, and can cause the mounting plate to drag, splashing water up on the powerhead.

Edited by - Dave Bristle on 03/11/2008 13:18:57

Dave5041
Former Mainsheet Editor

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USA
3758 Posts

Response Posted - 03/11/2008 : 13:27:17

Yes, ventilation is the term for air being drawn into the prop, usually when near the surface or only partially submerged. Cavitation occurs when fluid flow cannot feed a high speed impeller like the prop. The bearings in the lower unit should not be affected since they are lubricated independently of the external water flow, although over revving is a possibility. Cooling is the real issue since the vacuum created on the front side of the prop can pull exhaust gas, water vapor, and possibly air into a foamy mix flowing over the water intake. The issue here is the prop sometimes being completely out of the water. I don't if there is a specific term for that, but we all know its not a good thing unless you're on an airboat.

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 03/11/2008 : 13:42:20

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dlucier
 The term for when air is drawn into the prop, like when it breaks the surface, is ventilation.<hr height="1" noshade id="quote"></blockquote id="quote">Ya, ya...

But the term "cavitation" was around before you were born, and at that time referred to air drawn down creating a "cavity" around the prop. Since then, somebody apparently redefined it to refer to a molecular effect in water that can damage props at something like 8-10K RPMs, which can be a problem for turbine-powered racing boats. But some of us predate turbine-powered boats.

Edited by - on

Dave5041
Former Mainsheet Editor

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USA
3758 Posts

Response Posted - 03/11/2008 : 13:59:19

I think "cavitation" refers to the prop creating a cavity in the water. A submarine 500 feet down will cavitate if it suddenly goes to high rpm's at very low speed.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 03/11/2008 : 14:29:29

Dave,

I don't make up the news, I just report it!

Edited by - on

stampeder
Master Marine Consultant

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1608 Posts

Response Posted - 03/11/2008 : 14:59:05

That's why I was asking for his definition of cavitation.
My understanding of Cavitation is: the result of a change of volume in a liquid, which can be caused by a change in the speed of a propellor, introduction of air, change of speed of a object moving through a liquid. Methinks that the definition of the word Cavitation has been modified over the years to mean (include) the action of a propellor making contact with the surface and sucking a bunch of air downwards to create a cavity in the water AND/OR when there is a lot of air being expelled by the propellor due to misalignment or other usage error.

The answer for Mr gop711, IMHO is that he really needs a 27" OB shaft, otherwise, he's got to deal with the potential damage one does to dunked OBs.

There are lots of Sailboats out there with imperfect OBs.
A possible solutions is a trade-in. There is a ready market for good used OBs.

Edited by - on

Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 03/11/2008 : 15:06:53

It always makes me wonder what the story is behind any non XL shaft OB on a sailboat. I think if I were give an L free I would sell it and use the money to get an XL rather than use it. Outboards are like technology, if done correctly they become invisible. If you are having to worry about your OB for any reason get one that will become "invisible". ...and practice sailing into your slip anyway.

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 03/11/2008 : 17:00:12

Right, Frank... I sold the 7 yr. old Honda L ("Classic") for $1100 (close to what somebody paid for it new) and got a new Honda 8-XL Power Thrust with electric start for about double that. Invisible!

Edited by - Dave Bristle on 03/11/2008 17:02:03

JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 03/11/2008 : 17:10:19

I had the Honda 8 HP Long shaft. I had some problems far offshore, and it was easy to foul the idle jet. The problems occured when motorsailing and the motor was on the low side, also problems occur when there is no wind or wind on the nose and you need to motor directly upwind in rough seas - with frequently pulling the prop out of the water or risking dunking of the motor.

After sailing to Santa Rosa island and experiencing 25 to 30 knot winds and rough seas, I determined I would not return without a XXL shaft, so I bought the Tohatsu 9.8 25 inch shaft with electric start and installed it on my 2 spring Garlick motor mount.

Zero problems since, never needed to operate the motor mount on the lowest setting, no risk of dunking, almost never pull the prop out of the water in rough seas. I had this motor on my "Crew of 2 Around Catalina" race which ended in me motoring back to the island at night in a thunderstorm in the worst conditions I have ever experienced at sea. Zero problems, started in 2 seconds and ran all night without pulling the prop, dunking the engine, fouling the idle jet, or anything else. Seas were 5 to 6 from all directions, winds were 0 to 30 from all directions, it was very scary and I was glad to have the Tohatsu on board.

Other benefits:

(1) alternator makes useful power even at low RPM
(2) never fouled the idle jet even in rough seas. The underside of the Honda is open to the sea and I think it is easy for water to spash or surge up. The intake of the Honda carb points down. This is prone to problems. Never had a similar problem on the Tohatsu.
(3) I seem to get much better MPG with the Tohatsu, perhaps because the prop is well below the hull in clean water.
(4) Love the electric start, yet still has a easy to use pull start.
(5) Love the extra HP when you need it.

Downside to the Tohatsu

(1) Much heavier, hard to lift on and off the boat with one person. Still easy to handle on my motor mount.
(2) When the boat is heeling past 40 degrees towrds the side your motor is on, and the motor is tilted up all the way, some waves can lift the motor, then the latch will disengage and the motor will swing down all the way. In really rough conditions I found it necessary to keep the motor latched down.

Edited by - on

gop711
1st Mate

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Armenia
83 Posts

Response Posted - 03/12/2008 : 13:01:54

Wow, I think I have become a cavitation expert now.

As far a having the XL model and not the XXL, all I can say is that I have to dance with the partner I brought to the party. This is the motor that came with the boat and the boat came from the ocean.

Lessons learned, other than cavitation, is that I will be in the market for a xxl shaft motor and the Tohatsu will be on my short list. May look at a Mercury as well.

Thank you for taking the time to respond.

Greg

Edited by - on

Dave5041
Former Mainsheet Editor

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USA
3758 Posts

Response Posted - 03/12/2008 : 13:32:28

Good choices! That is my short list as well. All of the manufacturers make good reliable engines, choices are made by price, feature sets, and personal bias, evidenced by how many people swear by their chosen brand. I'm not sure what an XXL is, usually standard is 15", long is 20" and XL is 25" measured from the inside top of the mounting arms to the cavitation plate. Manufacturers names may vary and length may vary by an inch or two, but it is pretty standardized to simplify boat/engine design When it comes time, be sure to follow the link from this site to OnLine Outboards, even if you buy somewhere else

Edited by - on

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