The only dumb question...

... is the one never asked, right? So, here it is:

What are the odds that someone could take two exactly duplicate mono-hull 39-foot flat bottom swing keel boats that they own and turn them in to a cat by adding a bridge deck? Performance does not overly matter as long as it is still functional and usable. Are cat hulls so much different from mono-hulls that this would make a poor boat?

Thanks,
- John

 

The advantages of a cat over a momohull are both sail-carrying and narrowness of hull. Certain monohulls might gain the ability to plane at lower wind speeds. Those would be ones having hulls that normally plane at higher wind speeds.
Multihulls are wave-piercing hulls rather than planing hulls. They haven't got enough planing area aft--- in fact, if you look at a Hobie 16, there isn't much difference between the hull forward and aft.
Your speed might occasionally increase due to sail-carrying increasing at low angles of heel, but unfortunately, your light-wind efficiency will suffer due to far too much wetted surface and one of your original mono sailboat sisterships would likely sail right past you in such conditions.
It's assumed you'll be lightening up the hulls, as your leeward hull is going to get pushed down considerably by the weight of the windward hull pressing down on it when on the wind. The leeward hull would have to be exteremely light in order to plane under such conditions. That would mean a lot of weight loss involving virtually the entire two hulls, something that costs a lot of moneyand effort. That is, unless the two hulls are already extremely light and strong.
In other words, I'd bet the boat model you are thinking about doing this with isn't that light, and probably isn't very narrow. you'd also be up against an entirely new rig, which would be very expensive, being much larger than the original rigs.
And on and on...

 

I think I understand...

So what you are saying is that a displacement hull is not for multi. In my case, I am thinking of taking light, narrow Bolger-like Sharpies. These are 39 feet long and only 8 feet wide but that still may be bit wide for a real multi-hull. These are symetrical, only about 10" depth when the balast is removed. I also saw some cats with two masts (one on each hull) which made me think that this would work if the hull shape is ok.

If we didn't get the speed advantages, could we at least keep the same speed as the mono which is about 8 kt?

 

It sounds about as impractical as can be. Can you tell me why you are thinking about this? You'll get stability and upright sailing, but at what cost?
A cat specifically designed with two rigs is an entirely different thing.
Even at eight feet, the width is twice that of a cat of that length.
Why do this? Domestic tranquility?

 

Rather then fiddle with design concepts out of your realm of current understanding, how about building a 40' catamaran from a set of plans, developed by someone who understands why these suggested types of "contrivances" are ill conceived.

I mean no offence, but the nature of your questions suggests only a very basic comprehension of the physics and design concepts necessary.

There are many multi hull specialists around and many successful designs to choice from, even those with biplane rigs on them.

The cost of a good set of plans will be just a fraction of the overall costs associated with a project of this scope. It's considered the first material investment, so cutting corners here is just as unwise as doing the same on planking stock.

 

Exactly

Yep. Domestic tranquility and using what we have to make something bigger for a growing family. If it isn't practical, we'll look in another direction. Some things in life and even in engineering are that easy. Some aren't. Oh well.

Thanks so much for all of the advice, however!