"Designing" a double-handed racing dinghy

[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.]
>>> Actually, I kept unchanged your heights chine to deck level, i.e. your D values 0,35 m at bow, 0,275 m at 35% Lwl (at approximately your station 4) and 0,20 m at transom. It is your Cmax value which is decreased from 0,50 m to 0,47 m according to hull draft 0,15 m reduced to 0,12 m.
>>> The angle of the inclined panels is the last input parameter I adjust in order to have displacement = weight, and the flat panel final shape and beam results from this process.

[The angle of the panels seems to be constant over the lenght of the boat, did you do this intentionally?]
>>> Yes by construction of my application focused on a simplified case of multi-chines dinghy, i.e. with 5 developable panels including :

• a flat bottom panel
• 2 inclined chine panels with a constant angle
• 2 vertical (or quasi) topside panels
• a plumb bow with a forefoot at water level (for the design displacement)

Here for V3 version, this angle is exactly 25,06°

More info on the application : Gene-Hull Dinghy Chines 3.1 | Boat Design Net

The file for V3 is attached, stations offsets and developed panels are given in the dedicated « offsets » sheet.


[Would you be willing to share why you choose a wider "flat" and a narrower transom?]
>>> they cannot be input data in my application, but output. I considered that they were not imperative specifications but orders of magnitude, and so I actually stopped my iterative process when I estimated that my values were sufficiently close to yours.

[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...]
>>> Good to first carefully consider your ergonomics, and then we can return to the design of a compatible hull.

 

Using @Dolfiman 's latest stations, the dots on the port hull.

 

I make a correction to my previous message : yes TS19, you are right for the chine at midship which is slightly raised in the V3 version/ your data (0,2448 m instead of 0,275 for the height chine to deck level) and this help me generate a better hull in my oipinion. I took the opportunity to compare the two versions with either a concav chine line (V3) or a straight chine line (V3,1) all other things equivalent, here attached.

 

Attached you'll find pictures of my quarter scale cardboard model

 

About your thoughts for the cockpit arrangement, two examples to illustrate various options :
** Hadron H2 with a very interesting interview of Keith Callaghan and photos of the cockpit with the option of a central longitudinal buoyancy tank + open transom but no full double bottom « to provide an ergonomic sitting out position – no ‘straight-leg’ sitting out required! »
Hadron H2 – A New Development in Singlehander Design | Hadron Dinghies Ltd https://hadrondinghy.com/about/
** Canot, built in wood, also with a double bottom, open transom and a generous free-board to compensate the height of the cockpit floor.
A faire soi-même : Le canot de bow https://www.voileetmoteur.com/voiliers/actualite-voile/a-faire-soi-meme-le-canot-de-bow/173026

Both have the advantage , in case of capsize, to have the daggerboard at water level (see Hadron H2 photo at the end of the article) facilitating the recovery. More free board can be envisaged for ergonomic purpose, say ~ 40 cm at midship while maintaining 35 cm at bow and transom.

 

To me it looks like my "artificial" constraint of a "straight" line as well as your "build constraint" of a constant angle both have a significant impact on the design

I tried 14 cm draft and got 340 kg overall displacement to be close to your assumed 337 kg:

Wetted surface area: 4,47 m2
Width waterline: 1,01 m
LCB from transom 2,74 m
Angle of entry: 13,3 ° (At least, if one would trimm the bow down by 1 cm, 90° otherwise )

So the "resulting" numbers are still very close

 

I moved our chairs around a little more:

If I would "perch" on the sidedeck with my bum just outside the gunwhale, my legs comfortably angled and my toes pointing to leeward (instead of upwards, which is supposed to be bad for knees), I would need 1,1 m in width from gunwhale to whatever spine structure is located in the middle of the cockpit. With some kind of 20 cm wide "box" (daggerboard case + crew seating position for light airs + place for blocks&cleats) that would yield 2,4 m overall width. So firmly in Merlin Rocket/National 18 territory. I guess extending the angled panels up to gunwhale level and omitting the vertical panels would roughly get me there. Normal "sitting" (= not hiking out) would be compromised by the feet standing on the angled outside panels and not on the low "flat" in the middle anymore.

The other way round at 1,6 m overall width a single hiking strap in the middle of the cockpit would be perfect. However that would either require
a) Laser Standard style straight leg hiking "on top" of a standard daggerboard case, which would be really bad for knees
b) a compromised angle between upper and lower legs in combination with a lower daggerboard case which would require a self draining cockpit for any water coming up through it, Normal "sitting" would also be compromised due to the lower vertical distance between cockpit floor and side decks.
c) the daggerboard case magically disappearing. The German "Ixylon" dinghy features a smaller centreboard on the inner edge of each side tank which keeps the cockpit free.... (Please see F10OAP0uLZOVEZQE_600x600@2x.png (1200×439)), but that creates other issues...

 

The H2 is the brain child of an experienced dinghy designer who cut his teeth in a development class - a huge difference to someone just being able to fire up a software package for cruising yachts like the designers of some recent production dinghies and I will never get even close to that level of refinement. I am a big fan of self draining cockpits, but I believe the trade-off concentrating the buoyant volume midships to ensure the centreboard staying close to the water surface despite the hull being pretty wide for a singlehander is absolutely valid. And even more so for the target demographic, which will also benefit from the more knee-friendly hiking position, of course.

 

I did not know this design, despite spending way to much time surfing the internet

 

Width waterline 1,01 m , hum hum ... not a lot for a confortable intrinsic stability. To be objective on that subject , I did a quick dedicated study by using V3 version and varying the input Tc (hull body draft) from Tc 0,10 m to 0,15 m, step 0,01 m, at constant displacement 337 kg. Here attached, with Tc 0,14 m (your value above), I obtain a width waterline of 1,023 m (close to yours 1,01 m) and it is at the frontier between a moderate and a poor stability in my opinion.

 

Je suis désolé....

I tried to round the 1,097 m waterline width that Delftship calculated and failed:

At 295 kg displacement:
Waterline beam: 1,05 m

At 340 kg displacement:
Waterline beam: 1,1 m (not 1,01 m)

At 388 kg overall displacement:
Waterline beam: 1,14 m

I would be tempted to go to something between 0,9 and 1 m for a low crew weight condition, though, if the wind at our local gravel pit would be more stable in general.

 

I do like that the Canot is a DIY-design that does not follow the typical "traditional" style like most of the other plans.

But it looks and sails (in the videos) like a miniature open keelboat/daysailor, but with the keel gone missing.

Due to the exessively high cockpit floor only a small immersion of the volume will be sufficient to maintain the 160 kg of the boat. The daggerboard will be far above the water surface when the mast would be horizontal - the complete opposite of the Hadron H2. However the mast will of course be pointing downwards in reality leading to fast inversion.

 

If I assume a 5 cm "minimum height" of the cockpit floor above the lower edge of the transom (for structural reasons and to ensure some volume => buoyancy in case a crew member is entering over the transom) and 5 % incl. incline to ensure that water flows to the transom reliably when the hull is level, I end up with

5 cm + 4 m cockpit length * 5 % = 25 cm

height of the cockpit floor at the mast compared to the lower edge of the transom. => This would be only 10 cm below deck level.

So to maintain at least the "magic milk crate height" (30 cm) I would need to increase freeboard from 35 cm to 55 cm.

Maybe no incline but strategically located selfbailers would be an alternative. That would allow 30 cm cockpit height at 35 cm freeboard.

Or some kind of "inverse sheerline" that would maintain the cockpit height in the first half of the cockpit but keep the ends at 35 cm freeboard.


30 cm cockpit height allows sitting/perching/hiking with nearly straight legs in steady wind, but if you need to move quickly, you need to keep your feet close to you/bend your knews as much as possible.

 

I outlined attached a version with a double bottom at Z + 5 cm / design waterline. This cockpit floor can be flat, with opening at transom + one (or two) drain hole(s) in the fore part, just behind a bulkhead at ~ X 420. Volume under this floor is 0,317 m3

 

Some background comments to my specific projects that would not apply to a "generic" design:

- I plan to use the daggerboard of our current dinghy , so the case will need to be only as long as the chord length of the daggerboard.

- The mast is too short to reach down to the cockpit floor so the maast foot will need to be at deck level and the cockpit will end just behind the mast foot at about 4 m from the transom.

Regarding "self draining":


- A simple hole close to the mast would create a nasty fountain at higher speeds when the flat around it starts creating lift due to "over pressure".
- Any water in the cockpit would need to be "guided" to either the whole or the open transom. For example contemporary "Pirat" dinghies feature a double floor PLUS selfbailers at the widest point of the cockpit at both outside edges, so water will collect at the self bailer on the "lower" side when the boat heels to leeward and is sucked outwards. It not, the self bailer can be closed. An only partially self draining RS400 features a kind of self bailer close to the mainsheet cleat and the cockpit floor is "falling" towards it from all directions respectively. Even from the transom as the cockpit floor is pretty low and allows for a comfortable seating position. The RS Feva drains the crew area through the daggerboard case as the thwart is a feature of the hull moulding and separates it from the helm area which drains through the transom.

I sailed many different self draining dinghies and getting the water out after a capsize can require a very precise hull trimm to really make the water flow towards the transom opening as intended by the designer