i have the spifurl as supplied by FP. I don't know the size. It has a Facnor FX2500 continuous furler. Answers to your question:
TWA - True wind angle, TWS - True wind speed.
Optimum at 120 TWA. We reach speeds of max. 70-75 % of TWS, even 80 % below 6 knots TWS in flat water
. With the square top sail. e.g. 4 knots of boat speed in 5 knots TWS.
Can be used until 100-110 TWA with flat water
. At 100 TWA hardly better than the jib, because it is too round.
Good at 140-120 TWA. Useful at 150 TWA, but large coverage by the mainsail. Useless at 160-170 degree TWA due to coverage. OK at 180 in goosewing of course.
b. Size - don't know. But bigger is not useful I think. Only by attaching a spu higher up the mast
, but maybe that is not allowed due to forces on the mast.
c. Taking the spi to the bow for deep wind angles would really be useful, but it has the following problem:
It is i.m.o necessary to be able to remove the spi in the windshadow of the mainsail, for safety
in increasing wind. As a consequence you have to be able to move the spi from upwind to centre position when deployed. I would not know how to solve this safely with the continuous furler and double lines at the bottom of the spi. Any suggestions?
There are (at least) 3 option for mounting the spi off-centre:
1. Using double lines at the bottom, one to the bowsprit and the other to the luff hull
2. Using a track for the spi, like on the Gemini 105
; very attractive but needs to b engineered well;
3. Just flying the spi off the luff bow. I think Lori has told us on the forum that she did this once, using a spi from her previous Gemini
d. Furler or sock: Furler is nice, works well. Can be done shorthanded, deployment can be done single-handed, for furling
it is nice but not necessary to have someone who lets go the sheet while the other furls.
And sometimes we set the spi with the jib still on; don't know if this works with a sock.
But sock is simpler and cheaper.
e. Continuous furler:
Risk with a continuous furler is unwanted unfurling, so I tend to always remove the furled spi when switching to jib, except in light winds when I expect to need the spi again.
And a single-line furler would need a lot of line for a spi which rolls around the forestay, especially if the spi is furled from the centre like with the spifurl.
I hope this helps.