Outboard Well vs. Slide for Outboard Mounting?

There are a few ways to mount outboards on a catamaran. Here are a few pictures to get us started. I'm trying to select the best way to mount my outboards so that...

1) The props don't aerate or breach.

2) The power head of the outboard doesn't go under water.

Here are some examples.

Which is best? Do you have an even better way to mount them?

#1 - Deep Well Right In Hull



#2 - Seawind Style



#3 - Like 44C


#4 - This thing


#5 - The Sled Mount


Any ideas on what the best way to do this is so that I have no aeration of the prop and the power head stays above the water at all times - even in the event of a total slam of the bridge deck? (AKA, a catamaran belly flop?)

 

The only suggestion I have is an (or two) inboard(s)...

-Tom

 

Ok, Tom. I'm accepting donations.

 

I believe the in hull well will "work" best, the set up in Jeff Schionnings designed boats seems very effective, however here in NSW Aus for commercial use our Maritime Authority dont like it & want the instalation to be very well ventilated & drained(petrol) as in similar to a transom mount- the Seawind style set up is approved by them. On private vessels you can have either, with the through underwing set up the outboard should be close to the hull as practicable & at about the center of pitching. I had a 33' cat with a central outboard nacelle- as small as possible to shield it- worked well but would sometimes ventilate in a small chop & a bit under powered with 9.9 Yamaha 4 stroke. Regards from Jeff.

 

Wondering the same thing. What about on rails where they can be lifted vertically? I know it's more complex but. . . removes the power head submersion issue and allows for deeper immersion?

Next question is location -

Mounting them just behind or in front of the aft portion of the cockpit beam seems common, so what about -

if accommodations allow, mounting them directly behind the bridge deck where they can be lifted through the cockpit floor? Closer to the pivot point, but further from the rudder. Better immersion/less aeration. Thoughts?

With both of the previous locations they would need to be as widely spaced as possible - against the in board portion of each hull.

 

When Gold Coast was building boats with outboard power Roger was using a central mounted sled. The engines were just aft of the after crossbeam so they were a little closer to the pitch axis than the transoms, they could easily be retracted and if they started to ventilate you just dunked them a little further.

My MacGregor 36 had dual 9.9 Yamaha's that were on transom mount vertical sliding brackets. That arrangement worked well and I only had ocassional ventilation. You would have thought that well spaced engines would have good slow speed manuvering, not. Even a good high thrust outboard doesn't have squat for thrust in reverse.

Steve

 

I don't think a sled is appropriate for your intended use. Pic 1 looks good. Inboard power better. Saildrive maybe.

Sled is fine exactly as pictured for occasional kicker use. You need dead reliable primary propulsion available right now.

In the Keys, the big cat headboats hung new motors every nine months. In essence, they leased their power. I'd investigate this sort of arrangement and design to accommodate the stuff that's available under these programs.

 

Two owners of catamarans with outboards in wells that I spoke with both said that they would not do the same again. They said that the wells are full of condensation and are experiencing a lot more corrosion as a result.
Option #1 wells are not enclosed so would be fine.

#3 AlanM has had the boat in the water for over 12 months now, he is very happy with the set up. Also the performance and economy he gets with the 20 HP Hondas is excellent. Send him a PM.

Another person you may want to contact is Alan Carwardine, sorry but I dont have his new email address. You should be able to track him down easy enough as he is the new Asian Manager for Lewmar/Nevtac based in Phuket.
The reason why I think it be worth talking to Alan is that on his previous boat he had the honda 20's mounted on the rear beam with simple vertical slides.
His new boat has 30hp tohatsus and are mounted behind the saloon entry bulkhead. Be worth knowing the reason for change, with the 30hp motors he is getting 15kts but the boat is much lighter than yours.

 

Thanks for the input here, Andrew.

I liked those open wells best too, but people elsewhere kept saying I'd be getting my engines swamped. Didn't seem to likely to me, since they'd be up when I was sailing in huge following seas. Plus, I never really see following seas go up and over catamaran sterns. They usually rise up with the wave.

Only snag in that plan is my rudders are located as in the diagram side profile of the boat in the first post of this thread.

So... I'd have to locate the outboard just forward of those rudders in a deep, but still open to the stern, well.

Good compromise?

The one thing I can't get over with Alan's setup is what happens when you slam the bridgedeck. By definition, when it kisses the water, the engines are fully submerged. I just can't get past that, even if his installation is working well for him.

What is the actual weight of the boat with the 30's that is getting 15 knots? I'm about 5,398 kg (5.8 tons) in weight, although the boat's waterline allows it to displace 8,315 kg (9.1 tons). I plan to keep her as light as possible.

 

A pair of Honda's with prop cowls on a sled:

http://www.goldcoastyachts.com/catamarans/earlier/Earlier-GC46-Esmeralda.pdf

Steve

 

Cool, Steve. Thanks. That looks like the most KISS approach. If I did that, I'd do a pair of seeds further outboard to help with maneuvering in tight areas.

 

Actually most of the Gold Coast Cats are twin engined on center sleds, but they add a twist by making them steerable. They put a regular steering wheel just below and off center and inside from the "real" steering wheel that steers the outboards. You can see in the previous post's link that the hondas are turned.

This does not give you "spin in your own length" control but it does give you some real neat control abilities.

One sled is simpler to build, twice as easy to use, lighter and less in the way than two.

Remember, none of these outboards work worth a damn in reverse.

Steve

 

Alan Carwardine builds his boats light, I am guessing his loaded displacement is 4500- 5000kg for his new 12.6m.
You have plenty of separation of your hulls so personally I would not bother with open wells as in #1. I would just hang them on vertical slides behind the rear beam or better still behind the saloon entry bulkhead if layout allows.
I am guessing that on Alan C's last boat where the outboards were on the rear beam he must have had some ventilation and hence the midship location on the new boat.

 

Yes, the layout would allow for outboards behind the saloon entry bulkhead. I think it's pretty much a wide open set of choices at this point, which doesn't make this any easier. I can do anything with these outboards.

 

What is Cardawine making these boats out of?

Now that I am building one, I am not at all understanding how you could build a boat of my boat's size (LOA and Beam) and make it any lighter.

Are these using carbon fiber?