"Designing" a double-handed racing dinghy

I did some crude update of the drawings in Powerpoint, sorry:

- Front deck, side decks (30 cms wide) and a rear tank that could be half height.
- Some "tunnel" to support the daggerboard case, carry the control line cleats,
- Stays, Mast, Boom, some control lines, tiller, extension, main sheet briddle.
- Position of rigg is defined by distance between shrouds on the donor hull.
- transom hung rudder, but 6 m hull length really seems to be pushing it. A rudder under the hull like for come IC/AC canoes or a shorter tiller with some linkage to the rudder would have advantages but also add a lot of complexity.


- Boom 90 cm above single floor

 

All sails pitch the bow down. There's not an intrinsic difference between pole kites and sprit kites or even no kite at all.
I suppose one could suggest that fuller bows are better for slower boats with less efficient rigs, so as all the rest of the boat has evolved to make boats go faster, the hulls have evolved to suit higher speeds. I suppose if I were designing a boat pottering round creeks and inlets, without speed as a priority, I might rethink my normal enthusiasm for straight waterlines. But there's something about sitting on the side watching a fine bow slice through the water with minimum fuss, but then there's also something about an old school boat with a foaming bow wave and the noise of the water.

But why should the mast stay in the same position relative to midlength? Move mast and foil forward and give more room to the crew. While I can see some arguments for having a small aft overhang I don't think there's much of a case for adding as much aft as forward.

 

You could move the mast forward and keep the shroud base the same if you gave her vestigial winglets. OK, this is far more winglet flare than you would probably want, but the principle is the same and it was the quickest photo to find. A nice straight bulkhead from the wingtip to the mast foot would be all the strngth needed.

 

Yes, it would be only 10 cm per side. But the rear "overhang" behind the rigg would we very long and look very strange. From a visual point of view I like the forestay set back a bit a lot it also helps the visual proportions.

However your comment lead me to try simply compressing the design from 6 m to 5 m overall length which would push the point with the required width forward enough:


A few cms more draft would restore the required underwater volume, wetted area would be lower, can-of-worms open...

 

All sails pitch the bow down of course. But I naively believe that a 26 m2 symmetric kite pushes more than a 12 m2 main and the combined force vector of a 24 m2 asymmetric kite with its tack 4,25 m in front of the mast should be angled upwards somewhat more than the vector of a 26 m2 symmetric kite with its tack 1,5 m in front of the mast. Or maybe there was a completely different reason that changed the int14 hull shapes after allowing asymmetric kites in 89, I don't know.

Re "watching a fine bow slice through the water" I have to say that watching videos of current IC canoes is mesmerizing and somewhat inspired going to 6 m hull length

I do not regard "room for the crew" in itself as an issue here, I am doing ok on a RS Feva. I am concerned about not being able to use the existing room, no matter if due to being forced forward to allow tiller movement or being forced forward even in front of the mast to lift the transom out of the water.

There are some dinghy types that are ok with my 90 kg as crew, but as a helm it becomes really difficult. So the "rear overhang" would mainly be there to move back the center of buoyancy. At the same time it increases waterline length, but obviously a longer bow would achieve the same.

 

Your mast looks like it is about 500mm too far forward.
SHC

 

500 mm back would make it look more "normal".

However it would also push the shrouds and therefore the seating position of crew and helm back: At the end of the day it would make it somewhat similar to the RS800 (dashed lines)

On an RS800 the crew spends so much time in front of the mast that the crew's trapeze wires are routed in front of the spreaders instead of behind the spreaders - that is exactly what I would like to avoid and trying to re-invent the RS800 does not seem very useful.

 

@Dolfiman

This picture gives a lot of detail, maybe it could help to "calibrate" your modeling?

The wetted surface area is very low...

 

"Overlay" of the 6 m hull with the new Melges 19. Mainsail size is similar, Jib is significantly bigger.

 

Overlay of the 6 m hull and the Australien Lightweight Sharpie:

 

Happy New Year to everyone!

This project is not dead - yet.

In case someone else is willing to "play along" these are the measurements that define the "stations" and thereby the hull "design":

(Design draft: 0,15 m)

A B C D
Width Width Height Height
"Underside" Deck level Overall Chine to deck level

Station 0 (Bow) 0 m 0 m 0,35 m 0,35 m

Station 1 0,206 m 0,54 m 0,425 m 0,325 m

Station 2 0,38 m 1,04 m 0,47 m 0,3 m

Station 3 (Midships) 0,5 m 1,4 m 0,5 m 0,275 m

Station 4 0,56 m 1,6 m 0,47 m 0,25 m

Station 5 0,6 m 1,56 m 0,425 m 0,225 m

Station 6 (Transom) 0,6 m 1,4 m 0,35 m 0,2 m

 

Happy new year to all, and Ok to "play along", here is my tentative attached, a design approaching as close as possible your proposal above.
However I was led to modify some of the input data, mostly : Hull body draft Tc 0.12 m instead of 0.15 m in order to obtain a moderated displacement of 337 kg which should be compatible with a dinghy weight of 177 kg + a heavy crew of 160 kg , and also to reduce the exit angle at transom ; Deck beam at transom : 1,32 m instead of 1,40 m ; Bottom panel max beam : 0,68 m instead of 0,60 m ; hull draft position at 55,4 % Lwl from transom instead of 50%, bottom line based on a curvature regularly decreasing from forefoot to aft transom (where the curvature is zero).
Some significative output :
Beam waterline = 1,07 m >>> giving a moderate initial stability, but a very good dynamic stability 0° - 25° thanks to the chine
Waterline half entry angle = 14,5° rather good
Bottom line exit angle at transom = 3,2° very good
Sf = 4,74 m2 >>> a sinkage or elevation of 1 cm corresponds to more or less 48 kg.
LCB with appendages = 2,775 m (46,3 % Lwl from transom) compatible with crew usual range of positions
Speed prediction for 12 Knots of wind, with 2 sailors of which one at trapeze
** Upwind : ~ 6,2 Knots
** Beam reaching : ~ 8,8 Knots
** Downwind with Spi : ~ 7,6 Knots (and ~ 12 Knots possible with 20 Knots of wind)

 

In my "peer group" most males are typically 1,8 m to 1,9 m / 6 foot to 6 foot 3 inches tall. So 80 kg is rather "light" than "heavy". That's why I targeted 200 kgs as the upper end of the crew weight range. The lower end of the targeted range of 150 kgs is only achievable with a mixed crew. However as that will be the crew weight most of the time there is maybe some value in decreasing draft to ensure full waterline length at 150 - 160 kgs as you are proposing.

It looks like you managed to keep the same relatively narrow waterline despite a wider "flat" by raising the chines midships and increasing the angle of the outer panels. The angle of the panels seems to be constant over the lenght of the boat, did you do this intentionally?

Would you be willing to share why you choose a wider "flat" and a narrower transom?

Thank you!

 

I initally chose a relatively narrow width of 1,6 m to achieve a sleek look and make transport easier.

I now "simulated" the widest station with some cardboard and 2 chairs

Being 6 foot 3 inches tall it felt kind of "too small" assuming some kind of longitudonal "spine" through the center of the cockpit that would divide the total width in two halves. Either I need to increase overall width significantly or the "spine" needs to be low enough to position my feet above/beyond it when hiking....

If the dinghy would feature a double floor the crew's feet would be above any "spine" below the upper floor, anyway. Then again the current internal cockpit height (distance from hull underside in the "flat" area to deck" of nearly half a meter felt quite comfy when just sitting on deck level...