Morgan Ohlson wrote:

[...]

But, if making The Big effort to design and buid a dinghy I will not let
it be bad in any sense since it would be a waist of time and money.

Versatile, balanced, stable, reliable, roomy...
...wich doesn't seem to outrule a fast under waterline-body.

2 x 40kg pivoting centerboards (for low D)
water ballast
outboard well
at the weight of maximum 70Kgs (empty 17½' x 6½ hull).

...some luxeries wich naturally will slow the vesell down, but not more
then nesessary.

From the beginning I thought of some skegs wich are now outruled. Only
true foiles under WL!!!

Thos 70 kg's will not get you much boat. assuming a sg of 400 kg per m^3
those 70 kg will get you a stack of about 9 sheets of 6 mm ply. If you look
at other designs in this lengt range, you'll find that 9 sheets is pretty
low. But what are the ballasted boards and water ballast for? preventing
you to get on a plane? the ballasted boards will only do much good in a
capsized situation, but do you really want the boat to get get up again
after it has thrown you out? Sure it will lower the center of gravity
somewhat, but in the end it is only the vertical projection of the mass
that counts. moving your body around is much more effective then leting the
boat fall over.
Water ballast will make your boat more stable. At the cost of (at lot of)
space inside, it will save you from putting heavy ballast or floorboards
in. But it will probably prevent your boat from getting on a plane as it is
too heavy.

Maybe a catamaran style boat is what you are looking for? no need for water
ballast, can be built with less material (actually it is less boat anyway)
and will go faster than a dinghy, is more stable while still allowing a
narrow waterline.
Downside is that you will sit more exposed on it and that you will have much
less carrying capacity.

Some designers who's designs i like are Iain Ougthred (sp?) and John
Welshford. Youmight have a look at their designs first before designing
your own.

The Teal book is quite a good read, IMHO, but you should be able to find
some books focussed on smaller boats.

I designed and built (or better built & designed) my own folding dinghy, but
would go for plans next time. It takes too much time to think out the next
steps if you do not have a plan. and the cost is offset by the chance you
end up with a boat that is not what you thought it would be.

--
vriendelijke groeten/kind regards,

Jelle