Guys,

We have a choice to make I'm afraid.

I won't go into details right now but because of the short hull length this F12 will have a more narrow crew weight range then larger catamarans. We need to decide where to place this range.

I worked out the underlaying scalings law and as such can give you all a feeling of what we are talking about.

As the reference I put forward an F18 sailed from 115 kg to 180 kg, with 155 kg being optimal. Lets assume that all F18 crews in this range are more or less competitive and experience enjoyable sailing, meaning, neither heavily overpowered nor sluggish and underpowered.



If the F12 is exactly 12 feet long = 3.66 mtr then its characteric weights (similar to F18 above) are :

range = 31 to 50 kg with 43 kg optimal


if the F12 is 3.90 mtr long then its characteric weights (similar to F18 above) are :

range = 49 to 72 kg with 63 kg optimal.


if the F12 is 4.00 mtr long then its characteric weights (similar to F18 above) are :

range = 57 to 82 kg with 73 kg optimal.


All above boats have been scaled to not perform worse then the F18 when not looking at the other issues associated with scaling like the pitching moment problem (= dive happy).

Additionally, from the point of drag it is always better to sail an boat underweight then overweight. Meaning that being 10 kg overweight is just as bad as being 20 kg underweight. The overweightness just hits harder.

Be sure to note how only 0.1 mtr (= 4 inches) difference in hull length, (from 3.9 to 4.0) moves all characteristics weights up by 10 kg's ! 0.1 differenc in length is nothing on a F18 but it is the world on a F12.

The point of this post is that we need to decide what the F12 should carry in crew weights a choice here will have a direct impact on the required MINIMAL length of the hulls.

Please everybody give your opinions on this, I'll provide my own opinion at the very end of the discussion.

Wouter

I know nothing about working out such figures, but my instinct says go for the greater buoyancy. It can still be called a F12, even if the length is closer to 13ft. When I see a Hobie 14 with it's hull so low in the water, I get worried about a 12ft cat. Even though I know that the hull shapes are different.

Don't look at the H14.

Picture yourself an F18 or Spitfire / F16.

I'm using the same scaling laws as I did with the F16's

In principle this would lead to a F12 with the same performance as the F18, despite being much smaller. But for true equality it would have to be fitted with jib and spi. Sadly there is a thing that I call the Quad-problem and it limits the amount of sail area you can put on the F12 before it turns into a right submarine. Therefor we are not going to achieve full F18 performance. But I have a feeling that we'll discuss this soon enough.

Wouter

Stick with 3.6m don't get so hung up on performance. Make the hulls wider to get the increased crew weight. A 12ft boat will be lighter and more practical.

Gareth

Gday

12ft = 3.660m, so I would go 3.7m max, I wouldn't go anymore than that, it will start getting further from the objective if you go more.

I don't think theres any need to make them comparable to F18's, F16's or whatever to terms of performance, remember theres a lot of things that they have that this F12 won't.

Regards

F12 Great Idea!

I’d guess drag is not the big challenge as short boardless cats are a much more difficult to tack, especially without a jib! Rocking horse effect is another matter. What makes you think this design will be any different?

Ease of tacking is an essential element in training new sailors and one reason why the Opti is so popular (less run away boats!).

I suppose a hull design using ultra low profile boards would be make a major difference in the handling characteristics.

A MAJOR objective, IMHO, would be no boards of any kind. Skegs only.

By the way, has anybody ever heard of a "shoe"? An exremely low-profile "keel" that has been used on cruising catamarans? When I say "low profile," I mean it is only a few inches deep below the bottom of a 35-foot-long boat. It has kind of a fat, upside-down-T shape. The boat we sailed pointed very well with VERY little leeway, and it tacked on a dime. Would that work on a smaller boat, too?