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mroettersr
Navigator

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USA
148 Posts

Initially Posted - 09/13/2012 : 22:10:06

Just out of curiosity, has anyone used outboard motor mounted trim tabs to prevent cavitation and improve efficiency? They are usually used on outboard powerboats. I have a Honda 8 long shaft which works well except when we get some short chop on Lake Erie. There is some cavitation until the motor gets a good bite and the boat gains some speed.

Mike Roetter

Mike Roetter
'83 C25 #3568 SK/SR
Marblehead on Lake Erie

Edited by - on

Ape-X
Admiral

Members Avatar

USA
662 Posts

Response Posted - 09/14/2012 : 04:11:26

that's what the anti-cavitation plate is for. Trim tabs work at plane, so would not funciton on a displacement hull.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 09/14/2012 : 07:03:02

I also experience some ventilation (surface air getting sucked into the prop) when in chop, and as you noted, it usually goes away once boatspeed kicks in and the stern starts to squat a bit which thereby lowers the anti-ventilation plate further below the water's surface. Depending on conditions, I generally start out at lower rpms then gradually increase throttle as the stern starts to squat and hobby-horsing somewhat lessens.

Edited by - on

Stinkpotter
Master Marine Consultant

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Djibouti
9089 Posts

Response Posted - 09/14/2012 : 07:42:30

If you mean what the motor-heads call a "hydrofoil"--a fin that mounts to the anti-ventilation plate--I doubt it. That initial ventilation, or "sucking" (technically not "cavitation") is generally caused by a vortex just forward of the prop, where the fin would have no effect. The vortex is disturbed and goes away as the boat gains speed. (You can see this happening more clearly in reverse, where the vortex forms aft of the prop. If you had the fin, it'd probably form beyond it.) A fin won't change the "ventilation" caused by the prop being lifted out of the water, although it might deflect the spray somewhat...

Generally, the "trim tab" on an outboard is a little vertical tab on the underside of the anti-ventilation plate that can be adjusted to help counteract prop-walk in forward gear (theoretically). It's often made of zinc to also serve as an anode.

Edited by - Stinkpotter on 09/14/2012 07:50:16

awetmore
Master Marine Consultant

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USA
1144 Posts

Response Posted - 09/14/2012 : 08:39:09

How long is the long shaft on your Honda, 20" or 25"? With our 25" Nissan/Tohatsu and the engine mount set to it's lowest position the body of the motor is pretty close to the water and the prop is about 15" below the surface. I've only had it come out of the water once in this configuration, and that was in short and steep 3 or 4' waves (strong headwind, opposing current).

The worst outboard setup that I've seen is on this C-25 (which I saw in person up in the San Juans):
http://seattle.craigslist.org/sno/boa/3263559388.html

Fixed motor mount and a 20" shaft motor. The anti-cavitation plate was only about 1" below the surface when the boat was level in the water. The boat is cheap enough though for a new buyer to get a decent motor mount on there.

I've always wondered if the parts could be bought cheaply enough to convert a 20" shaft motor to a 25" shaft one. The components necessary are pretty minimal, a longer shaft, some cowling pieces, and the shift linkage.

Edited by - on

OLarryR
Master Marine Consultant

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USA
3477 Posts

Response Posted - 09/14/2012 : 09:08:46

You would think that they would sell a conversion kit to convert from 20" to 25" length and maybe some mfrs do offer one, however, on some outboards the extension piece and longer studs is not the only change. On my Honda XLS, the shaft is of a larger diameter than the Honda with the 20" length. I know this because the marina accidentally gave me the wrong water pump impeller when I was performing mtn this past winter. The XLS (25" length) outboard has a larger diameter shaft and therefore has a water pump impeller that has a larger inside diameter to fit onto the XLS. So...depending on the outboard, things can be a little bit more complicated considering offering a conversion kit. Check with your mfr.

Edited by - on

Stinkpotter
Master Marine Consultant

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Djibouti
9089 Posts

Response Posted - 09/14/2012 : 09:15:39

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by awetmore
 ...I've always wondered if the parts could be bought cheaply enough to convert a 20" shaft motor to a 25" shaft one. The components necessary are pretty minimal, a longer shaft, some cowling pieces, and the shift linkage.<hr height="1" noshade id="quote"></blockquote id="quote">...and the exhaust and cooling pipes. I looked into it (including labor), and compared to selling my old 22" Honda and buying a new 27" Honda, it made no sense. (I got electric start and high-thrust in the deal, so it wasn't quite a fair comparison.)

Edited by - on

delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts

Response Posted - 09/14/2012 : 09:38:46

An option other than a fin might be a [url="http://www.propguardmarine.com/how.html"]Kort nozzle[/url] for the prop. However, while this will likely increase directional thrust (giving you better control), it's unlikely to help with cavitation/ventilation. I've considered adding one to my Tohatsu, but since I've got an XLS shaft and I took great pains to get it as deeply mounted as I could, I think the only time we've experienced cavitation/ventilation was with the giant tug wake I described a few weeks back. I think that was largely because the engine bounced out of it's detent and up to the top of it's range in it's scissors mount where the anti-cavitation plate is only about 1" underwater.

Edited by - on

awetmore
Master Marine Consultant

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USA
1144 Posts

Response Posted - 09/14/2012 : 10:05:26

That Kort nozzle could be nice on my dinghy motor, where I've broken the shear pin by being a little too caviler when coming into a beach. I wonder if it would help with anything enough on a sailboat though. Directional control is already pretty good between being able to pivot the motor and having a rudder.

The discussion on modifying shaft lengths is interesting.

Edited by - on

delliottg
Former Mainsheet C250 Tech Editor

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USA
4479 Posts

Response Posted - 09/14/2012 : 10:33:48

Modifying the shaft length on a Tohatsu/Nissan didn't look like it'd be all that difficult. All the parts that would need to be lengthened come out with the gear case except the water tube.

You'd have to replace or extend the gear shift linkage, the drive shaft, add an enclosure for the works, and replace or extend the water tube.

Extending the shift linkage would be easy except that it disappears into the leg almost immediately after the join. I believe it was a rubber grommet it went through, so it might be easy to get the second joint past that. The drive shaft could be lengthened with a bolt on kit to the top, and I don't think there's any interference along the way. Oh, wait a minute, yes there is, there's a guide bearing in there about 5" above the gear box housing. So you'd need to get a longer drive shaft or come up with a way to make a secure joint beyond the guide bearing.

This just got hard, now you either have to take apart the gear case or take apart the drive leg to get at the guide bearing.

Never mind.

OK, so I Googled it, and here's a thread on [url="http://forums.iboats.com/showthread.php?t=577776"]shortening a shaft on an 8hp Tohatsu[/url]. Looks like the conversation is with a parts dealer, complete with parts listings and prices, so it looks like it's doable, just not as easy as I thought it might be.

Edited by - on

mroettersr
Navigator

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USA
148 Posts

Response Posted - 09/14/2012 : 19:32:27

Thanks for your comments. As I said, it was a matter of curiosity. I don't have the XLS - just the LS and the problem simply goes away once the boat is underway and the stern settles in.

Regards,

Edited by - on

Dave5041
Former Mainsheet Editor

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USA
3758 Posts

Response Posted - 09/14/2012 : 20:24:17

Lake Erie square waves make my well mounted 25" start coming out intermittently between 3 and 4 feet

Edited by - on

GaryB
Master Marine Consultant

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USA
4316 Posts

Response Posted - 09/15/2012 : 12:08:17

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Stinkpotter
 ...to help counteract prop-walk in forward gear (theoretically). It's often made of zinc to also serve as an anode.
<hr height="1" noshade id="quote"></blockquote id="quote">
Counter-act torque from the prop... When adjusted properly it relieves the need to hold onto the steering wheel to counter-act the torque of the spinning propeller.

Edited by - on

awetmore
Master Marine Consultant

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USA
1144 Posts

Response Posted - 09/15/2012 : 12:23:44

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave5041
 Lake Erie square waves make my well mounted 25" start coming out intermittently between 3 and 4 feet
<hr height="1" noshade id="quote"></blockquote id="quote">

Do you have photos of the well mounting on a Catalina 25? Is this a different boat?

Edited by - on

GaryB
Master Marine Consultant

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USA
4316 Posts

Response Posted - 09/15/2012 : 12:33:35

I think he means it's well (as in properly) mounted, not like down in a box (well) as done on some other types of sailboats.

Edited by - on

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