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Old 12-08-2014, 08:58   #1
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catamarans & nacelle pod propulsion

I recently finished construction of a 63' 'wharram type' cat. It's powered by an outboard mounted in a central nacelle pod, about 2/3's of the way back from the bows. Even though the outboard cavitation plate is 20 cm below the static waterline, the propeller still sucks air and cavitates when running at speed.

I'm considering a prop guard/kort nozzle kind of attachment around the prop to contain the thrust, but questioning whether it'll do any good.


If anyone has any insight to this problem I'd greatly appreciate it
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Old 12-08-2014, 09:11   #2
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Re: catamarans & nacelle pod propulsion

Welcome, any pics of the boat? So the prop is staying underwater but still cavitates?
What size motor and prop, maybe to small a prop, too much rpm?
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Old 12-08-2014, 09:26   #3
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Re: catamarans & nacelle pod propulsion

Thanks for the reply,
The prop is well below static waterline, but obviously it's catching air when the bows waves come together under the boat at 9 to 10 knots at just over half throttle. The attached pics show the pod before completion, but it gives the idea. I originally mounted the 90hp engine in a normal prop depth position, but it sucked air at 6 knots,, I have now added a 5' shaft extension kit, but I still can't get over 3000rpm with out sucking air and over revving
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Old 12-08-2014, 09:31   #4
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Re: catamarans & nacelle pod propulsion

Have you tried a 4 blade cupped prop. Some of the alu props have no cup
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Old 12-08-2014, 09:35   #5
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Re: catamarans & nacelle pod propulsion

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Originally Posted by Oceansurfer View Post
The prop is well below static waterline, but obviously it's catching air when the bows waves come together under the boat at 9 to 10 knots at just over half throttle.
I think that is your issue, you have identified it
Maybe use a hydraulic jack plate like you see on Bass boat so you can lower and raise the engine at will?
Your prop is ventilating, not cavitating
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Old 12-08-2014, 09:37   #6
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Re: catamarans & nacelle pod propulsion

There are many of course but this is what I'm talking about
CMC PL-65 Transom Jack Plates by T-H Marine : Cabela's
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Old 12-08-2014, 11:01   #7
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Re: catamarans & nacelle pod propulsion

Years ago we put the "Handler" kort nozzle on our Honda 50 because we had the same problem on our cat. Worked like a charm. The Handler is no longer being produced and I'm not sure how well the new prop guards will work. The Handler was aerodynamically designed and had about 1/8" between the props blades and the kort nozzle.
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Old 12-08-2014, 14:15   #8
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Re: catamarans & nacelle pod propulsion

Sounds like the motor is in the wrong place relative to the wake.

Depending on the speed, each hull will create a bow wake and they will meet in the middle. Before or after that peak will be a trough. It sounds like you put the motor in the trough. In a static condition, there is no trough so the motor is plenty deep enough. The faster you push the deeper the trough becomes.

An extension or other lowering option will help to a degree but as soon as you go faster you need more depth. I would be hesitant to push the overall engine too deep as it could flood when you go into reverse or get dunked in choppy conditions.

Is this the mounting location that the design proposed or are you otherwise deviating significantly from the design specs?

Can you see underneath safely while at speed? If you can find the peak and move the engine forward or aft to match it, that would probably be the most effective option.
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Old 12-08-2014, 14:34   #9
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Re: catamarans & nacelle pod propulsion

Valhalla, I definitely get what you're saying about the colliding wakes. The pod position is as was designed, but there have been several things that question the design....

It's very difficult to see what's happen down there when running, but am I to expect the peaks and troughs to stay in about the same relative place as speed increases, or decreases?
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Old 12-08-2014, 16:22   #10
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Re: catamarans & nacelle pod propulsion

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Originally Posted by Oceansurfer View Post
Valhalla, I definitely get what you're saying about the colliding wakes. The pod position is as was designed, but there have been several things that question the design....

It's very difficult to see what's happen down there when running, but am I to expect the peaks and troughs to stay in about the same relative place as speed increases, or decreases?
No. The wavelength of the bow wave varies with boat speed.

(The whole concept of "hull speed" centres around this changing wavelength where at "hull speed", the wavelength is the same as the waterline length of the boat.)

Consequently, at some specific speed, the troughs of the bow waves will coincide at the location of the prop.
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Old 12-08-2014, 17:46   #11
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Make a rectangular fiberglass plate about 12 by 30 and atach it to the cav plate. It will not allow the prop to suck air from the surface. Make sure and trim the motor so the cav plate is parallel to the waterline. Also should probably go to a smaller pitch and you.will.see less suction from the prop.blades. and less stress on the engine resulting in longevity.
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Old 12-08-2014, 17:57   #12
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Re: catamarans & nacelle pod propulsion

One of the nice things about cats is the ability to have two motors widely spaced one each hull to give better maneuverability and redundancy,
so you might just consider getting two smaller outboards and attach
them close to each hull.

Still didn't see any pictures, would like to see the boat and hopefully a website/blog with the build process, I read every single one I can find. Thanks.
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Old 13-08-2014, 14:45   #13
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Re: catamarans & nacelle pod propulsion

Hey everybody,
Thanks for the input. I'm going to try the cheapest fix first with a whale tail, then maybe a nozzle. I think Valhalla and stuM have hit it on the head with the coinciding bow wakes (troughs) changing position as the speed increases. The boat has a 14 knt potential with the 90hp and I'd really like to see it. I'll let you guys know how it goes in a couple of days.

I don't see here that my attached pics show up?? If they do you'll see where I photo-shopped in a mast where the mast will go... And the old pod pic doesn't show the transom.. I better get one of my kids to help me with this
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Old 13-08-2014, 15:26   #14
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Re: catamarans & nacelle pod propulsion

Quote:
Originally Posted by Oceansurfer View Post
Hey everybody,
Thanks for the input. I'm going to try the cheapest fix first with a whale tail, then maybe a nozzle. I think Valhalla and stuM have hit it on the head with the coinciding bow wakes (troughs) changing position as the speed increases. The boat has a 14 knt potential with the 90hp and I'd really like to see it. I'll let you guys know how it goes in a couple of days.

I don't see here that my attached pics show up?? If they do you'll see where I photo-shopped in a mast where the mast will go... And the old pod pic doesn't show the transom.. I better get one of my kids to help me with this
had not realised it was a 60 ft cat,my good friend hans klarr drives his 70 ft cat with just one 50 hp long shaft mounted in one hull in a well in front of one of the rudders.

Hans Klaar is an extraordinary sailor, a modern-day Moitessier sailing "Ontong Java" on Vimeo
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