"Designing" a double-handed racing dinghy

Just a remark about the shape of your sheer line top view : I think that you tend to put the maximum width and the maximum curvature at the same X position. It is not optimal in my opinion, it is better to put the maximum curvature at say 50% L while the maximum width is at say 32% L (from transom). The curvature is then more evenly distributed with a lower maximum, so too for the waterlines. Here attached drawings to highlight on this difference.

 

It is hard for me to tell the difference in your attachment, so I tried to grossly exaggerate what I understood. Do you recommend something more like the blue outline in contrast to the red outline?

 

I am not sure to understand what you question with your lines, are they support lines to build splines ?
I show in the doc attached a simple method to build a regular sheer line with its max curvature at 50% L, I think this makes more clear my suggestion.

 

I tried to understand the impact of your proposal on the outline of the deck so I exaggerated the "curvatures" at one third and mid length to "creases". My impression was your proposal would yield an increase width at mid length - which would improve the righting moment.

 

I threw the quarterscale cardboard model into a local pond and loaded it to an equivalent of 300 kg displacement.

This generated three learnings:
1 - The hull floated as expected for 400 kg, I assume that the hull skin was not rigid enough and was pushed inwards slightly, thereby loosing volume/buoyancy.
2 - quarterscale means we would have had to move the model at half of the speed of a real dinghy to get a realistic wave pattern. That was not possible with a simple cord...
3 - When we applied an asymmetric loading to achieve a typical heel for lightwind conditions ("fourth mode"), the model wanted to continue a sharp turn, even if the angle of the hull to the pulling cord was already at 45 ° => A real dinghy would show a very pronounced "weather helm".

1 and 2 could be improved by using another scale, e.g. 1/8.

Does anybody have suggestions how I could change the shape to reduce the "self steering" in 3? Would a somehow "narrower" transom help?

 

I hesitated to answer you for the point 3 , so my general comment by hoping this can be what you with :
** weather helm main cause is the misalignement, in projection in the top view, of the sails force vector and of the resistance vector, because of the heel inclination with the sails center of effort (CE) a lot higher than the center of lateral resistance (CLR).
** this gives a yaw moment toward windward, so-called the « weather helm »
** … and this can be more or less accentueted if the CE and the CLR themselves are not fixed in the heel process, in particular for the CLR if the hull body not only heels but also trims.
** the usual compensation to recover the vectors balance is to put more incidence for the rudder, so the name « weather helm », up to Lareral resistance +Rudder sum of vectors be again aligned with the Sails one. A bit of weather helm can be optimal (with a good feeling at helm) as long as the rudder wing works close to its best lift/drag ratio, i.e. not stalled. Then you recover more lareral resistance at a low drag cost. The contrary, i.e. a leeward helm, is to avoid , both unpleasant feeling and more lateral drift .
** the « weather helm » was a critical issue for the classic long keel sailboat, with both a rudder attached to the keel and mainsail with long boom extending far back, made of cotton canvas which could deform over time and cause the CE to move back.
**now with aft suspended rudder and more modern keel wing and sailplan, the issue is fortunately a lot less critical for a naval architect although still uneasy to calculate (where are exactly the CE and the CLR, how they move with heel), rule of thumb based on accumulated experiences still prevails.
** when the phenomena still appears to an unmanageable point, it is by breeze, to my knowledge I never faced or heard of a weather helm problem by light winds. So to answer you, I will not worry of what you may have feared.

 

Maybe this is an misunderstanding:

To me "weather helm" is if a dinghy turns to windward for any reason while the tiller is straight.

In our "test" the hull had no rigg, no daggerboard and no rudder, so any weatherhelm was caused by heel angle/the hull shape itself, like in this video at 2 min 10 secs:

Jollensegeln: Steuern mit dem Mannschaftsgewicht

If we would sail the boat with leeward heel the effect you explain would come on top of this and make things worse (i.e. requiring the "handbrake" turned on even more) as dinghies are typically designed to be "balanced" at zero heel as targeted in most situations. (Scows and some Foilers excluded, of course)

In light winds however some leeward heel is intentionally induced to reduce wetted surface area and to ensure boom and sails staying to leeward due to gravity.

In other words my concern is very different from weather helm in strong winds caused by sails not holding their shape, crew unintentionally letting the boat heel to leeward due to bad trimm/lack of righting moment, main fully eased, etc.