Eric

You've asked a hard question.

Firstly the boats you mention, Stealth, Taipan and Bim are not so diferent, They all sail as uni rigs with the same rated sail area, the Taipan and Stealth have 8.5 m masts with about 15 sq m of sail and the Bim has a 9 m mast with about 14 sq m of sail, both rigs will produce about the same power and will perform about the same.

The difference is that the Taipan and Stealth have jibs in addition and as such will produce more power and be faster in all conditions.

Next to comparing speeds ofunis to sloops with the same sail area.

The dificulty is that all cat rigs are a compromise, you have to use the same sails in 5 and 25 knots, look at a swing wing fighter plane, at high speed it need short fat wings to go fast but at low landing speeds it sweeps its wings out or it can't fly, likewise at high wind speed a short fat rig is faster and at low wind speeds a tall skinny rig is faster.

Lets take a fully compliant F16 with 15.25 sq m main 8.5 m mast and 3.5 sq m jib, 18.75 sq m in all, just over 6% less than an 18HT sail, so we'll put it on 9.4 m mast, that is a big sail for a 16 ft boat! We'll put the f16 17.5 sq m spinnaker on.

How will it perform.

Light winds <8 knots

The uni will blow away the sloop upwind, downwind it will be a little faster.

Light- med wind 8 - 12 knots

As the wind increases the sloops speed becomes closer to the uni, by the time the sloop is twin trapezing (9-10 knots) it will be slightly faster upwind, downwind the uni will be slightly faster although as the wind increases the speed will get closer.

Med winds 12 - 16 knots

From here on the sloop is going faster and faster upwind compared to the uni, at 16 knots it will blow away the uni, by now the downwind speed will be pretty much the same although if its choppy the sloop may well be quicker, at some point around this wind speed the taller unirig with a much greater pitchpoling moment will have to start backing off downwind and then the sloop will become quicker.

Med - strong winds and above

The sloop is quicker all round, the stronger the wind, the bigger the difference.

Efficency

The thing here is that the uni rig produces more power, the problem is that this power is generated half way up the mast, and most of the power is pushing sideways with only some of it pushing forwards.

So once you get to the point where the power is more than the crew can hold upright (either capsising sideways or pitchpoling forwards) then you have to de-power the rig, either by flattening the sail or by running deeper, either way you are not usng all the power available so the efficiency is going down.

So the unirig is more efficint at producing power, but produces it in a way that less of it can be harnessed.


Hope this helps

john