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Old 28-04-2018, 18:10   #1
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Converting power catamaran to sailing catamaran

I have silly question.

I've heard about dismasted cats converted to be power cats. That would be easy - just discard all "extra hardware" and you all set - you have slow power catamaran.
What about other way around?

Here's why I'm asking this question.
There's power cat, 50-something ft. I was told it has Tennant hulls, but cat is highly customized, not something "standard". It has unique layout, I simply love it. Not just inside layout, but overall layout of the boat. No sailing cat have anything close to that layout. I mean - to buy sailing catamaran and convert it to something similar in terms of layout will most likely cost more than to build new one. Unfortunately, I'm not feeling like building boat from scratch, for many different reasons, at least for now.

Hulls are displacement hulls, so there are no problems with that.
Pricewise...Let's put price of the boat aside for now. Let's say it's free.
I'm willing to do ALL or most modification work myself. I'd use used mast of proper size.
I'm engineer, but not a naval engineer.

So the question is - what would it take to create proper compression post with adequate reinforcements, mounting points for chainplates etc. on such big boat?
I especially would like to hear opinions from these who had built their catamarans from scratch.
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Old 28-04-2018, 18:32   #2
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Re: Converting power catamaran to sailing catamaran

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I have silly question....

So the question is - what would it take to create proper compression post with adequate reinforcements, mounting points for chainplates etc. on such big boat?
I especially would like to hear opinions from these who had built their catamarans from scratch.
It a silly question, but taking it seriously... the answer is IT DEPENDS. A sailboat is NOT a motorboat with a stick stuck on top.

A lot of power cats are really just sailing hills without the mast. The hulls were designed from the get-go to support a mast and the associated rigging loads.

I am guessing it is very unlikely the hull you are looking at has such things. The loads imposed by a catamaran's rig are HUGE, and not to be trifled with.

Consider, just for a second, the downward force on the mast. A properly tuned rig will be pushing that mast down into the hull with many TONS of force. It can't just be "stuck somewhere".

A rig is also not a simple matter if the boat is to actually be able to sail. Where do you put the mast so the sailing force balances and you don't have crazy weather helm? or worse... lee helm?

I am sure if you have enough people answer you here, you will get the answer you want to hear... but it will be wrong.
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Old 28-04-2018, 20:57   #3
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Re: Converting power catamaran to sailing catamaran

Many power cats will also have a narrower overall beam because they don't have to maintain lateral stability with the forces of a huge sailplan.
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Old 28-04-2018, 21:37   #4
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Re: Converting power catamaran to sailing catamaran

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It a silly question, but taking it seriously... the answer is IT DEPENDS. A sailboat is NOT a motorboat with a stick stuck on top.

A lot of power cats are really just sailing hills without the mast. The hulls were designed from the get-go to support a mast and the associated rigging loads.

I am guessing it is very unlikely the hull you are looking at has such things. The loads imposed by a catamaran's rig are HUGE, and not to be trifled with.

Consider, just for a second, the downward force on the mast. A properly tuned rig will be pushing that mast down into the hull with many TONS of force. It can't just be "stuck somewhere".

A rig is also not a simple matter if the boat is to actually be able to sail. Where do you put the mast so the sailing force balances and you don't have crazy weather helm? or worse... lee helm?
It's all understandable. I have no illusion that this can be "few weeks job".

But it's also clear that there are no hulls under the mast. So all the forces transferred to the bridgedeck and then to the hulls.
These who had built their cats from scratch for sure would know what's involved in creating these reinforcements.
Same applies to chain plates etc. I'd like to hear from them.
I know that I should study some building plans of similar sized sailing catamarans to understand these things, but since it's just silly thought for now, why not to ask these who has hands-on experience?

I can see it from different point view. Many catamarans were damaged by Irma, dismasted, have chain plates ripped off, destroyed front cross beams, structural damage. Many of them (especially expensive ones) will be rebuilt (this is regardless if you guys like it or not. It's a fact). These who will do it will face similar challenges - re-creating and reinforcing mounting points for mast and rigging.

And yes, catamaran in question has good size beam.
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Old 28-04-2018, 22:47   #5
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Re: Converting power catamaran to sailing catamaran

Almost all modern boats, especially sailboats and even more multihulls are designed fully for specific load cases.
Not only are the laminates and structure designed for these, interior components form an integral part of that structural layout. Bulkhead sizes and locations, compression posts....
Apart from that hydrostatics are designed in a way that the boat floats properly on its lines.

Now add a fullsize rig to a 50ft(!!!) powerboat. The forces of this rig will be very big.

The changes needed will most likely be huge.
New layout (to care for structural bulkheads).
Very big issue to accommodate the added weight of such a big rig with sails and associated structures at their proper fwd. location. Your hulls will have not enough underwater volume fwd.
Adding sizeable bridgedeck reinforcements.
Adding keelsons or center board and associated structure.
Adding all sailing related gear.

All those items will be heavy, weigh the boat down, make it heavy and a poor performer under sails.

Unless you think of a broomstick sized rig don't do it.
Costs will be huge, performance will be poor.

Nonononono, not a good idea.
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Old 28-04-2018, 22:49   #6
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Re: Converting power catamaran to sailing catamaran

The forebeam has to be able to handle forestay loads. It would need an A frame and bridle. Where it attaches to the hulls also has to cope. On my boat that's a lot of unidirectional fibreglass.

The mast beam is a seriously strong structure. There are major uni glass webs top and bottom. The bottom webs run right across the bridgedeck down to the keels. The bulkhead itself is also very strong, with a 25mm core and quadraxial glass. This is secured with overlapping double triaxial glass taping.

Then there's the chainplates, laminated into another serious bulkhead, 25mm core, quad glass, with double triaxial glass tapes overlapping.

Then the rear beam, where the traveller is mounted. Two bulkheads, with uni glass top and bottom, double triaxial tapes again.

Headsail sheet lead tracks have 10mm backing plates.

Winches, deck organisers, stand up blocks, rope clutches etc, all require backing plates.
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Old 28-04-2018, 23:19   #7
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Re: Converting power catamaran to sailing catamaran

Oh yeah, and it would probably need bigger rudders.
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Old 29-04-2018, 00:30   #8
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Re: Converting power catamaran to sailing catamaran

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The forebeam has to be able to handle forestay loads. It would need an A frame and bridle. Where it attaches to the hulls also has to cope. On my boat that's a lot of unidirectional fibreglass.

The mast beam is a seriously strong structure. There are major uni glass webs top and bottom. The bottom webs run right across the bridgedeck down to the keels. The bulkhead itself is also very strong, with a 25mm core and quadraxial glass. This is secured with overlapping double triaxial glass taping.

Then there's the chainplates, laminated into another serious bulkhead, 25mm core, quad glass, with double triaxial glass tapes overlapping.

Then the rear beam, where the traveller is mounted. Two bulkheads, with uni glass top and bottom, double triaxial tapes again.

Headsail sheet lead tracks have 10mm backing plates.

Winches, deck organisers, stand up blocks, rope clutches etc, all require backing plates.
Thanks, that's kind of reply I was hoping to see. So basically - bulkheads help to spread the loads, right? And they would be least of a problem, relatively easy to build on existing boat, right? Forebeam installing would require serious reinforcement of the bows, but it's still doable, right? Traveller beam not as strong as mast beam, I suppose?
So major problem would be creating of mast beam and question whether boat will be happy with all additional weight?

Hull core is Airex R63

What would be your estimate - would such project make sense at all assuming it's unique boat and only other option would be to build sailing catamaran with similar layout from scratch?
It even has some features you were talking about, extra shallow draft, for example

The only production catamaran which can be "somewhat" adapted to similar layout is probably Majestic 530 Fly. Its design so flexible that you can create almost anything out of it...But it's $1M, waaaaaaay over my budget.

BTW, traveller track on this Majestic is on cockpit roof.

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Old 29-04-2018, 00:59   #9
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Re: Converting power catamaran to sailing catamaran

It's pretty impossible to say how easy or difficult it would be. IMO, depending on what structure is already there, the mast beam would probably be the most difficult part of the project. On my boat it is installed in one piece, with the top uni web laid up after, and the decks fitted on top.

Getting a bulkhead in in one piece with the decks already in place could be quite difficult, and would certainly require the removal of a lot of the fitout.

But it would be better if you could get hold of the plans and get a multihull designer to look at them. Maybe there's already a suitable bulkhead in place.
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Old 29-04-2018, 01:07   #10
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Re: Converting power catamaran to sailing catamaran

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Getting a bulkhead in in one piece with the decks already in place could be quite difficult, and would certainly require the removal of a lot of the fitout.
For sure, lots of cutting would be involved.

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Maybe there's already a suitable bulkhead in place.
I don't think so
This was my first question - I asked to connect me with designer.
Here's reply I've got from broker

"I don't think it would be prudent to put a mast on her. She is already a mash-up of Malcolm Tennant, her builders choices, and her owner's choices so no real designer (hulls are Tennant)."
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Old 29-04-2018, 06:09   #11
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Re: Converting power catamaran to sailing catamaran

Easiest solution is to put an unstayed mast in each hull with a wishbone boom.
No deck fittings apart from 2 mainsheet blocks, no fore beam, traveller or chainplates. Easier to hoist, lower and reef. The main beam only sees bending (no compression), so a lot easier to strengthen.

Cheaper would be an unstayed mast in one hull, if you could live with the assymetry.
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Old 29-04-2018, 06:34   #12
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Re: Converting power catamaran to sailing catamaran

It's obvious you REALLY want to do this, and will keep looking until you find someone who says it is a good idea. I am pretty sure you will not find someone who has done what you want--successfully--but heck life is short, just do it!.

I hear the name "Flyin Hawaiian" is available.
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Old 29-04-2018, 08:18   #13
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Re: Converting power catamaran to sailing catamaran

consider converting to a kitesailing yacht see wingit.com or skysails.com---way smarter than trying to put on a standard rig
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Old 29-04-2018, 08:24   #14
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Re: Converting power catamaran to sailing catamaran

Malcolm Tennant power at hulls have made excellent sailboats, is "Athena". However to convert a powerboat to a power sailor requires a different, modern, mindset. In the case you describe the best solution is a pair of free standing rigs, one in each hull. These are way more efficient than conventional rigs and easier to retro engineer for conversion in any power vessel. The Tennant already has good, though not fantastic, leeway resistance so it probable that engineering for centerboards may not be necessary. Because the rigs have less windage then conventional rigs they need not be huge, and because they have no stays they are superior downwind so extras are not needed.
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Old 29-04-2018, 08:30   #15
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Re: Converting power catamaran to sailing catamaran

It should also be noted that reinforcement of the entire vessel to spread stay loads is not necessary with free standing rigs, resulting in a lighter end product. I am astonished that this approach was not more encouraged in this forum.
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