Monohull dinghy with amas above waterline just for stability

The main problem is that you won't want the amas there once you've learned how to handle the boat well and how to right it after a capsize, so it's just extra expense, clutter and hassle. If you have a small ama there it can still be pushed underwater and will make the boat more unstable at those times rather than less, and there's a significant chance of the boat ending up upside down and being impossible to right. A simpler solution would be a masthead float or semi-inflatable sail to stop it inverting, and a pole with a relatively light lead weight on the end that can be swung or extended out in the opposite direction to the mast to right the boat. I'd look at building something like that into the centreboard such that the lead weight is a bulb kept at the bottom of that and can be extended down on a telescopic pole, while it could also be removed and replaced by a standard centreboard for racing. While not extended it would still provide lots of extra stability that would make sailing more comfortable for beginners.

 

A boat such as you imagine will have numerous negative unintended consequences for dealing with mainly 'what if' situations.
Simplicity is an asset in dinghies. If you want to trailer the boat it will have to have foldable or demountable amas. The sailing rig will have to be made heavier.
Boarding and disembarking at the dock will be a big bother. The boat will have handling issues as it flops on and off the amas. Weight placed away from the center of the boat affects motion, handling and stress on the rig..
With all due respect, learn how to sail a dinghy. You do not have the experience to improve them.

As far as safety, you want good flotation and the center/daggerboard trunk top opening to be above water when the boat is righted.
The following video is of a sister ship of mine and pretty much the gold standard for non self-righting sailing dinghies.
To save time start at 6:06.

 

OK, thank You for the explanation and the interesting ideas, now it is much more clear to me.

 

My first sail boat had a ballasted keel. Going to an unballasted dinghy sure took some getting used to!

 

First, I'll say that I find the general tone of Boat Design Net frequently arrogant and off putting. Fairly often, there are sound technical reasons for skepticism regarding proposed ideas (some of which are not old and common, but also not new). But often, reasonable ideas are treated as if they are crazy.

There is a thread here, having to do with a 70 tonne vessel being foil borne. Several respondents poo poo'd the idea, (as if it was ludicrous) and the original poster was taken aback. But there is nothing new about the idea proposed, and in fact much larger craft have been foil born. So most of the responses came out of ignorance, but were accompanied by arrogance... a repulsive combination. In that thread, one response claimed that the density of water was 64 lbs (as used in the 1/2 rho V^2 formulas for calculating lift). Of course, it is not, because those formulas need the mass density of water (about 2 slugs per cubic foot) not the mass density multiplied by 32 (ft/sec^2, the acceleration due to gravity) which is its weight density. I keep stumbling upon positively idiotic statements, such as in another thread, that VA (used the world over as a measure of apparent power in AC electrical systems) is a measure of battery capacity. And those statements are advanced as if the poster thinks they are correct.

So my first response to the OP would be "Yes you could build a dinghy with amas about 2' clear of the water. I built one like that, powered by a rigid wing, and was able to make it go a little less than three times the true wind speed. And of course, Moths have little mini amas a few feet off the water, and they are pretty fast too.

However, if used as just a last ditch effort to keep from capsizing, (instead of a device to gain large righting moments, eg., in my case, perching the skipper 10' off centerline), then I'd have to agree with those who would say "first learn to sail a small dinghy, and learn to avoid capsizing it. " In those classes in which capsize is common, (Laser, for instance,) good skippers learn to have the boat back to sailing in seconds... as the boat goes over, the skipper is already climbing over the edge to put weight on the centerboard. Having amas would interfere with that process, and add weight and windage, making the Laser much less fun to sail.

On my boat, hanging off the windward ama, with my butt 11 feet of centerline, doing 32 knots, was a blast. But had I buried the leeward ama, (also kept 2' off the water, if I didn't screw up) it would have created quite a spectacle.

There have been some dinghies with integral wings, (I can't remember the name of one... vector? Yes, that's it!) The Vector was not all that easy to sail, but fun for an agile skipper and crew.

But, to the OP: don't let me discourage you. Build one and try it out. But first it would make sense to sail 1. A Laser. 2. A small trimaran, like a Windrider. 3. A larger dinghy, like a Lightning. Spend about a year with each... or you might fall in love with one or the other quickly. A monohull with amas is a trimaran. better to sail a real trimaran. The Lightning makes a good family boat. Its beam makes it inherently stable, although with bad sailing it will capsize, and it's harder to right than a Laser. The easy-to-sail small trimarans, like the Windrider put the crew weight in the wrong place for speed and fun (the weight should be on the windward ama), but they are still fun to sail , and darn near impossible to capsize.

After spending some time with those three, then start drawing up a boat that suits your purposes. (or just buy a Hobie wave... etc, etc, etc, etc, etc. There are loads of small boats that are hard to capsize. ) If you want a monohull that responds like a dinghy, but won't capsize, buy a Hobie 33... they are fast an fun to sail.

Marchaj's book, The Aerohydrodynamics of Sailing is good for giving you an understanding of the issues involved in boat design, incidentally. Fair winds.

 

Thanks! I do appreciate Your answer. The only thing I would like to emphasize is that I think about a camper cruiser rather than a sport dinghy. I intend the amas to 1) prevent from capsizing, 2) give self righting when capsizing anyway (as the amas would be pretty high about waterline, they could prevent such a dinghy from stability in the inverted position). However I know my idea has a lot of cons. I have the Marchaj's book. It's high time to read it all!