"Designing" a double-handed racing dinghy

On another note, the dimensions of the existing daggerboard are as follows:

Chord length 32 cm
Thickness 3 cm
Overall "height" 147 cm
Height of rectangular "top part": 20 cm

Distance Mast - front edge 57 cm
(in the donor boat for the rigg)

So the daggerboard case should at least be 20 cm high to contain the rectangular part. I believe 25 - 30 cm would be typical for a selfdraining "Skiff" type dinghy. (daggerboard case height more or less equals height of false floor)

For a single-floor design I guess the daggerboard case needs to be above the fully flooded waterline after a capsize, so water does not rush in again while trying to bail.

I am trying to guestimate how much foam would fit into the first two and the last meter of the hull (i.e. outside the cockpit) to keep the waterline as low as reasonable. If the daggerboard case would reach up all the way to deck level it would hinder the crew crossing the boat over it swiftly. And the vang needs to pass over it when tacking and gybing...

Is it a fair assumption that the heavier crew member could stay in the water acting as "sea anchor" and keeping the bow pointing into the wind while the other crew member bails, so the foam would have to support only half the crew weight?

 

Not really, no. In more serious conditions it may need to be one to bail, one to sail (well control the boat anyway).
The trick with using one person as sea anchor is for them to wrap their arms round the windward shroud, and not keep the bow head to wind. That will keep the boom to leeward out of the cockpit which makes bailing far easier and safer. It also means that the crew weight stabilises the boat in roll, and its far easier to recover them into the boat.

 

Thank you for pointing this out, it absolutely makes sense.

(I had more than my fair share of capsizes, but I never had to bail in anger as those dinghies either had false floors or massive side tanks.)

 

This week-end I got the recommendation to shorten the design to less than 5 m for various reasons (Transport, storage, weight,...).

If I combined this with
- Dolfimans approach to move the point of highest draft forward for a flatter run aft, compensated by a wider "flat" in the rear sections,
- a false floor combined with a higher sheerline to restore some distance between cockpit floor and deck,
- some flare in the upper panel to gain some extra overall width, i.e. righting moment and space in the cockpit,
- and gggGuests reccomendation to chose a flexible carbon rigg in order to improve gust response
the result would actually look fairly similar to this: VX Two: High‑Performance Double‑Handed Sailing Dinghy

I would like to deliberately stay away somewhat from existing designs to avoid ending up with "something similar, just worse" but it does not feel sensible...

 

At this stage of your thoughts, I think you should preliminary fix a hull geometry and go for a first round of the structure distribution, materials and scantlings in order to have a consistent weight spreadsheet of the dinghy (bare hull and fully equiped). After such stage, it would be more easy and consistent to optimise the length & beam issue.

Attached a V5 version at L 4,80 m x B 1,68 m to illustrate your above evolution and gives some figures for such design.

Another example of a dinghy built with ply panels, Kitoo simple. This video shows some insights of the structure (at 1:12 , 1:49, 2:12, 2:26 2:35 3:15 3:43) :
Kitoo simple dériveur en kit. www.kitoo-simple.com

 

Some first weight estimation:

47 kg for rigg, ruder, daggerboard, fittings, blocks, ropes etc. - as compared to 34 kgs in your estimation for the 6 m hull if I got it right

12 kg for all kinds of "connection elements" within the hull structure
37 kg for the outer shell and sidedecks in 4 mm ply.
34 kg for bulkheads and a "spine" in 12 mm OSB with generous "cut outs" up to 50 % of the respective area

83 kg for the bare hull structure (no fittings etc.)

47 kg for rigg, ruder, daggerboard, blocks, ropes etc. (as compared to 34 kgs in your estimation for the 6 m hull if I got that right)

130 kg Total

That would allow meeting the 295 kg displacement condition with a 150 kg crew and wet clothing and leave 50 kg as margin for error before meeting the 388 kg displacement condition with the 200 kg crew.

I believe 83 kg would be also easily achievable in epoxi-foam-sandwich for a professional dinghy builder - if I can ever afford one


My initial goal would be to (first?) build some kind of "open prototype" (it would be "mule vehicle" in automotive land...) to test if my ideas are complete nonsense, find attachment points for the jib sheets, check the ergonomics for hiking, and so on.

As quick as possible and as cheap as possible which might be done after a few test sails, instead of something I could pass on to my not existing grand children.

So I planed with
- bulkheads every 1 m, -
- a kind of longitudonal "spine" that will "split" around the daggerboard,
- an angled bulkhead connecting mast foot and shroud plates ("Y-bulkhead" in German...)
- no fore or aft deck, just "sidedecks" from bow to transom and up to 30 cm wide
- foam panels on the floor in front of the mast and on the last meter of hull length to provide buoyancy.

I can see this going terribly wrong... fortunately our small lokal lake does allow taking stupid risks...

 

This is a suggestion not specific to sailing, but aimed at baling small boats that need regular bilge draining; and you mentioned an electric pump back in the thread a bit.

I needed a compact solution that could be moved between craft.

Many modern bilge pumps run on a range between 12 and 24 volts and I use power tools with 18V batteries that provide a lot of electrons in a compact size. I found adapters for them online to feed power to the pump, and if your pump has a float switch you won't need additional switching. Put the tool battery and it's adapter in a waterproof container (food storage works), with a cable gland to slow water entry, onto a sealed Deutz connector, with a light flexible outlet hose to throw over the side.

This was the prototype I used for kitchen sink testing, but in the field it should have the battery more protected.

 

Yes, I was wondering if such a pump could be an alternative to a false floor or self-bailers if I manage to limit capsizes to say once per sail. Thank you

The set-up would need to be ready for full submersion. Maybe a small motorcycle battery or a NiMH would be a little bit less of a concern than such a powerful Lithium battery. If it would be mounted close to the daggerboard case its additional weight would not hurt so much...

 

Its about 70 years since Paul Elvstrom developed the self bailer and they work very well.Why would anybody choose to add the weight and complication of an electric pump to a boat that is sensitive to extra weight?

 

Its also about 70 years since the 505 dinghy was developed. With bailers and it also works very well as a whole

My "rationale" to at least think about alternatives would be:
- There is a risk of cutting yourself if you forgot to close them before crawling onto the daggerboard while righting. (And it is easy to forget something when you just capsized...)
- If you are my size it is pretty easy to accidently step on and critically damage them.
- They need significant boatspeed to work at all which is difficult with a flooded boat in inconsistent wind (505 is different with the huge side tanks that significantly reduce the amount of water remaining in the cockpit after righting)
-If boat speed is not consistent you permanently need to reach to the cockpit floor to open and close them even with a little bit of water in the cockpit. Reaching into the cockpit when a gust hits you is not convenient...
- Yes, they a pump+battery would be heavier. But the weight could be located in a "good" place, i.e. midships down low and create some tiny amount of extra righting moment at least.

 

Talking very little extra mass for a pump with a small drill battery.