lets talk about a Footy for a moment and see what it is needed to sail ‘normally’.
Here below the ESTEREL 4 calculations.
It start with the general formula in blue that is composed of two parts, heeling force and righting force
Followed by the list of parameters and continuing with the proper calculations in Red, last data is the Bulb weight.
From the drawing:
Wind that day was blowing at >10kt.
The Hull Displacement of 529g
The Sail Area of 1150cm²
The Design Bulb of 303g
The ratio is not very high since 303 / 529 = 57.2% - could be far better if > 62/63%.
The drawing contains also the formula using the Bernoulli Law and Righting arms at 30° of heel, therefore the distance from CE to LCB and from LCB to Bulb CG are included.
The calculation shall provide at the end the BULB weight needed for the sailing conditions set above.
No difficult to apply the same simple calculations for different Sail Area and Wind Speed (always in m/s ²).
The results obtained with 5.5m/s Wind Speed is that the Bulb shall weight 401g while the design was fixing it at 303g.
Not possible to increase the Bulb Weight without sinking the boat. The amount of sinking (in mm) depends from the Water Plan surface.
Two possibilities: decrease the Sail Area or sailing with lower Wind Speed.
The original design is already critical since the Weight Ratio is only of 57.2%.
To increase performances, one should develop a larger Hull Displacement, not an easy task for a small boat like the Footy and his Box.
Assuming, nevertheless, a Footy design with a DSPL of 650g and 400g Bulb, the Ratio will be 61.5% .
Construction weight little bit more than 226g since larger Hull/Deck.
If Wind will increase above 12kt the boat will heels more…and “nose down” not very far away !
This simple formula do not account for sea waves and gust !
Certainly my ESTEREL 4 Footy is not the Queen of Seas, but among the others is not bad if sailing up to 8kt and not more !!!
This simple formula can be applied to any model !