Outboard frame/platform across sterns on a 30ft Cat

[JasonDS]

Hi, I'm restoring my Iroquois Catamaran and it came with an outboard attached to a parallelogram-shaped box bolted to the transom so it hung lower.
I want to change this and have a framework crossing the stern hulls (a span of about 8 ft) which I can use for multiple outboards (I have 2 at the moment) and to help getting in and out of the sea by having a planked (diving platform) area. This also means I can lower the height of the outboards as they tend to cavitate in choppy seas and even leave the water.

Both outboards are already long shafts.

THE QUESTION IS: What problems would I have in big seas with a low framework? It would be about 6 inches above a calm sea and be made of stainless steel 316 50mm angle (L shaped) in a rectangle shape with two cross beams. I'm going to use planks of wood to span the platform but these would be removable if the sea state got bad. I'm not worried about strength as the design is very strong, but about waves hitting the bars going across. Would it push the boat around in a following sea, would slow it down?

I don't want build it and find it's a waste of time and could endanger myself and my boat.

Thanks in advance. PS I will reply to questions, but I'm away for 3 days so I will reply when I get back.

[Howaya]

The last thing you want is to add too many heavy objects in the ends of a boat. Outboards aren't exactly light. But you can always experiment with wood two by fours before committing lots of time and expense with stainless steel. If you cover a 12' long 2 x 4 with fiberglass and epoxy, this will be strong enough and will span the eight feet you mentioned. Thru-bolt it to your transoms with good backing plates and you'll have a cheap way to do sea trials.

[Ex-Calif]

I googled a bit and found many pictures of the transom setup. Most installs are on the aft bulkhead between the hulls. I did not see any case where the outboard was mounted at the end of an ama - It seems this is what you describe for your setup.

I saw one builder mod that extended the deck aft making the aft bulkhead unavailable for engine mounting. Photo 3.

Moving the engines aft - a span across the stern of the ama's would be counter intuitive for me - boat pitching would make cavitation of the props worse.

There is one person with a pretty extensive blog describing how he is modifying his setup up with tilting mounts - His is twin engined as well.

Resurrection of Vivace: September 2012

His boat is in photo 5 & 6 - In fact what he is doing sounds uncannily like what you propose and I am wondering if you are this guy - LOL...

Photos of your transom would be enlightening.

[JayCall]

Ex-looking at the one you mentioned, the last oic, it looks like it would not be too difficult to make the wood piece spanning the hull on some sort of hinge or swivel where it could be swung up when the outboards are nnot in use. Depending on weight distribution, you could probably even construct it so the outboards could stay attached when in the up position.

[Ex-Calif]

I think that is what that guy is intending to do. Reading his blog he is mocking up the system in wood then will build in angle iron and activate with electric trim. You can see he is adding remote helm stuff (controls) already.

[JasonDS]

Thanks for the replies. My boat setup at the moment is like photos 2 and 3. Yes I could create something like the last picture but I would like to know how this would effect the handling whilst down. Sometimes you need to use the engine when in less than calm seas. Photo 6 would be an extreme example as it looks a foot high where as mine would be 2 inches. On holiday at the moment but i have internet! Just takes ages to type on a phone!

[sandy daugherty]

The Iroquois doesn't offer much room or aft buoyancy for two engines. With the engines so close together maneuvering using opposite throttles would demand much higher power settings and multiply the consequences of one running away or not shifting at the wrong time. Gemini tried it with non-stellar results. I would hazard a guess that you would be giving up a lot of controllability going to two non-steering engines from a single steerable one.

Are you looking for more speed under power?

[JasonDS]

I have two engines but one is 30hp and the backup is 8hp. I'm not worried about maneuver ability but whether the frame would cause problems in a rough sea if waves hit it. E.g. if running in a sea, would a wave broach the boat because of the frame? And going to windward, would it act as a brake and slow me down considerably?

[Ex-Calif]

I am still not sure how you plan to construct the frame.

What is the problem with simply mounting them on the aft bulkhead as per like photo 2?

[JasonDS]

Ok, the shape is a rectangle 8ft by 2 ft there will be two beams crossing the 8 ft so it will look like 3 rectangles joined together Like below

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x....x....x....x
x....x....x....x
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This will be bolted across the two hulls about 5 inches above the waterline and about 8 inches below The beam that is shown in picture two which has the outboard attached to. From this frame, i will hang 2 Outboards and create a platform with planks.
I think the problem is i called the beam that spans the two hulls the transom. Is that incorrect?

[thinwater]

Unless this is fared WAY forward you are going to slam fiercely into every wave when the chop gets up. This is a problem with single engines, but with a board all the way across, it will be like a brake.

Stiletto eventually started making diverters to go in front of the engines. PDQs and many others have a shaped cowl in front of each shaft. IF you are going to do 2 engines (bad idea as Sandy pointed out) you will need a wave-breaker in front of each and it will still be bad.

As for removing something if the sea state gets bad, 6 inches is too low almost all the time, and when things get bad I can't see you hanging off the back to remove something. It will just be removed when it breaks.

You really don't need a back-up engine. most sailboats have 1. You have sails and an anchor.

[Ex-Calif]

If I get the main point it is you want a platform that you can stand on?

I am with thinwater - This will hang low and will be in the water a lot.

[JasonDS]

Ah, back to a normal keyboard!

Yes, To stand on, to help climb in and out especially with diving gear.

So, based on what thinwater said, I should get rid of one engine, to reduce weight. I can't afford to replace them either one with new and they're both quite old.

I should then design my platform so that it can be raised some how. Should I have the engine raise as well or should that be fixed? I imagine when you really need to use the engine, you don't want to be playing with a platform!

Thanks for the replies, keep the ideas coming!

[thinwater]

Originally Posted by JasonDS

Ah, back to a normal keyboard!

Yes, To stand on, to help climb in and out especially with diving gear.

So, based on what thinwater said, I should get rid of one engine, to reduce weight. I can't afford to replace them either one with new and they're both quite old.

I should then design my platform so that it can be raised some how. Should I have the engine raise as well or should that be fixed? I imagine when you really need to use the engine, you don't want to be playing with a platform!

Thanks for the replies, keep the ideas coming!

The 30 hp should be plenty on its own. 15 hp would be enough for this weight, but 30 will be nice. If you really want the 8 hp for back-up, consider simply stowing it below. Just a thought.

Because transom mounted motors can be really hard to reach, I did consider a platform, but abandoned the idea. But if it is low enough to help with dive gear, it's too low. I would consider and upgraded transom ladder first. I have upgraded ladders and been pleased.

[Ex-Calif]

Going back to this photo.

It looks like he has a pivot bracket at the port end. It looks like the center bracket is a standard engine tilt and the pivot point is lined up with the end brackets. The span he is making should then pivot up and down.

What if the end brackets were actually sliding channels. And then two center channels could be added.

The platform and integral engine mount would slide vertically up and down rather than tilt. You could then devise a lifting mechanism - wire, pulleys and maybe a winch mounted between the ends.

Crank the winch and the entire platform raises vertically out of the water, on vertical slides, maybe with rollers to reduce friction, including lifting the engine. The slides would have to extend above deck level probably and might be ugly unless designed well. The verticals could actually be incorporated as the aft end of an arch system for a bimini and solar mount. (ain't getting carried away with someone else's time fun - LOL)

This could be designed and fitted but again you are adding weight at the end of the boat. Although it is the weight of 4 slides, associated pulleys and wire and the winch. You could save weight if you designed it for a 15hp instead of the 30 you already have.