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Old 12-07-2010, 16:41   #1
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Catamaran

I my be a fool for asking this?
But can you build a catamaran out of two 25 or 30ft boats?
Being the same boat.
Like two 27ft catalinas
If not why??
Please give reasons why or why not?
I think this could be a good topic!
Thank to all
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Old 12-07-2010, 17:37   #2
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Sure you could - but why? It would be better to use boats with easily removable ballast - ie. bolt-on keel or water ballasted; it's not needed on a cat. You also don't need a full keel - a centreboard design would be better. Wharram designs are basicly bungeed together, so you could easily do the same thing.
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Old 12-07-2010, 17:48   #3
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You of coarse could but it would be an abortion. Catamaran design basics include low wetted surface and high length to width ratio hulls which can exceed normal hull speed constraints. You don't need form stability in the individual hulls so why accept the compromises that come with it? Certainly the ballast is un needed but you still must find a means of lateral resistance. Too many problems to make something that won't perform up to your expectations. Dave
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Old 12-07-2010, 18:45   #4
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Some of the reason I was thinking of this are.
There is a ton of very cheap Boats out there!
Maybe try to recycle some of these insted of going to the dump to rot for ever !
"GOING GREEN" and doing something people have not thought of.
I'm not going to the race so that would not worry me. I do want it to be safe.
And maybe cross an ocean.
I agree with the one post "don't need a full keel - a centreboard design would be better" Also thought of just cutting them off!
I just think with some or maybe lots! Of work you could use this 1 inch thick fiberglass for a better reason then landfill!
Also wouldn't you be able to carry a lot of weight this way?
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Old 12-07-2010, 19:27   #5
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Technically with the right engineering you could connect 2 monohull boats together. The biggest issue will be the large forces, vertical, horizontal, twisting etc. on the structure connecting the hulls.

Then you would need to decide whether to run two masts or eliminate the masts from the monos and make a mast step on the connecting structure. The forces on the mast will now have to be engineered to survive.

Then you have to decide if you are going to have a deck house between the hulls or not. One of the attractions of the cruising catamaran is the large deckhouse/salon. This structure now has to be engineered and the additional forces have to be engineered into the structure that connects the hulls.

At the time of design, layup and building all of this engineering is integrated and planned together. That allows the structure to be lighter than if everything is done after the fact as an "add on."

When you are done you will have a catamaran that is heavier than others for the LOA with a deeper draft unless you have done the work and engineering to redesign or remove the keels from the original hulls.

When you consider the cost of all this work and material you might be better off with a bigger mono or a purpose built cat.
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Old 12-07-2010, 19:31   #6
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Of course it is possible to do what you suggest - but at what cost - and long would the project take? Mono-hull hulls have a very different shape to that of a multi-hull and are contain much heavier material. The type of bridge deck used in Wharram catamaran designs could be envisaged as the method of joining your 2 mono-hulls together.
All 'wayout' ideas have certain merit, but in today's world the core of the idea must be practical and cost effective.
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Old 12-07-2010, 19:41   #7
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Deck house

Yes I would build a deck house.
I was thinking of a four post frame with support in the mono hulls front,back, and the front and back of the deck house.
Also was thinking of keeping the two mast in place as sails are abundant
and very cheap used!
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Old 12-07-2010, 19:53   #8
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This thread was duplicated in multihulls and tech support. I merged all the posts together in one place to avoid confusion.

The tech support thread has been closed and deleted.
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Old 12-07-2010, 20:01   #9
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Thank you

Thank you

Just to let people know
I would do the work myself. Fiberglass,steel work,welding,plumbing,electric,wood work, soler panels.
I am very handy can't see the work taking more than year.
Would one mast be of any benefit?
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Old 12-07-2010, 21:24   #10
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The Wharram catamarans use more or less identical hulls and attach them together with large wooden crossbeams forming a bridgedeck.

It would take lots of custom fiberglass work to join two identical monohulls. It's not impossible, but it would probably look aweful. You would never get your money out on a project like that

A more productive approach might be to go the trimaran route and use an unballasted monohull with two amas from a large beach catamaran. It would look less weird, and might even sail reasonably well without having to modify the mast and rigging in a major way. It would be a monohull on training wheels so to speak.
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Old 12-07-2010, 23:41   #11
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Quote:
Originally Posted by dogface View Post
Some of the reason I was thinking of this are.
There is a ton of very cheap Boats out there!

Ok Start with 2 $10,000 Catalina 27s - When you are done you have two galleys, two heads, two identical hulls unless you gut and refit the insides - there's a cost equation here. You also have to add in all the costs of the bridge and deckhouse and maybe alterations required to get across and into the 2 separate hulls. What about the cost of rigging? Tying the two rudders together - what are the rudder loads. Two engines? What about the engine control system.

"GOING GREEN" and doing something people have not thought of.
I'm not going to the race so that would not worry me. I do want it to be safe.

I'm an engineer although not a marine engineer. Making it safe requires some understanding and planning.

And maybe cross an ocean.

This ties into the making it safe part.

I agree with the one post "don't need a full keel - a centreboard design would be better" Also thought of just cutting them off!

This statement prompts a lot of thoughts
What do the Catalina hulls weigh?
Without keels?
What is the weight aloft with 2 masts?
Is the weight aloft OK?
What is the sail area?
How do you safely control 4+ sails?
How far apart do you space the hulls for stability?
What are the stress implications of a 25 foot beam with an 8 foot bridge deck?

Also wouldn't you be able to carry a lot of weight this way?

No one is really trying to discourage you but I don't think it is as easy as getting out the sawzall some epoxy and banging a couple of hulls together.

Could you do a bodge job? Yes. Would it be interesting? Yes. Would it be a novelty? Yes.

Would it be transpac worthy? Not if you bodge it together without some solid engineering work.
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Old 12-07-2010, 23:54   #12
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The first thing Rudy Choy learned was the significance of asymmetric hulls. You guys are a bunch of mono-hullers!!!
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Old 13-07-2010, 10:48   #13
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Quote:
Originally Posted by Randy View Post
The first thing Rudy Choy learned was the significance of asymmetric hulls. You guys are a bunch of mono-hullers!!!
Although it would be interesting to figure our just how widely spaced the bridgedeck would have to be between the hulls to prevent the bow wake from smashing into one another and creating you own mini tsunami beneath the bridgedeck. Catalina 27's have about a 9 ft beam. My guess is that you'd need about 12 feet between the hulls to prevent the aforementioned so you'd have 9+9+12 = 30ft beam on a 27 footer. So that means you'd have to come up with some new fangled system for dealing with all the inevitable pitchpoling that would take place. Or you could just make sure she's very well sealed and leave her turtled after the pitchpole and ride it out in your very own sub-maran.
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Old 13-07-2010, 11:20   #14
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You are not a fool for asking, but you would be a fool for doing it.

Why?

Hull shapes are wrong and the engineering and redesign involved to get from "bodgy" to "barely workable" to "decent" are huge (not to mention the $$$)!

Next!

Fair Winds,
Mike
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Old 13-07-2010, 11:49   #15
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Perhaps a better approach, if you are desperate to recycle heavy fat hulls, is to go Proa.
The existing rig would be fine, just need a slim second hull of sufficient ballast and floatation to keep the mother hull mainly vertical.
You will still need to get rid of the deep keel to allow the boat to slide in excessive winds.
Otherwise it's a much cheaper solution, and maybe one longer hull is cheaper than two shorter ones.
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