That makes sense given a few assumptions:

1. That a boat's downwind speed is greater than its upwind speed.
2. That a boat sails the same distance upwind and downwind on a windward/leeward course regardless of sail plan.
3. That additional sail area in jib or spinnaker results in the same percentage speed increase upwind and downwind respectively.

Now, I agree that (1) an Isotope generally sails faster on a downwind reach than on a beat to windward. If you assume that a catamaran both tacks and gybes through 90 degrees, then it sails the same distance upwind and down. Therefore, in a W/L round-the-buoys race, more time is spent beating than reaching. If additional sail area in jib vs. spinnaker result in the same percentage speed increase, then it would be better to increase the jib.

However, I don't agree with assumptions 2 and 3. When sailing upwind, there's a diminishing return on additional jib area. As long as a 70% self-tacking jib attaches flow to the back of the mainsail, a 130% overlapping jib won't add a correspondingly large speed increase to the boat. The Isotope jib is large for better downwind speed, not upwind speed. It isn't as effective as a spinnaker, though. Also, a sloop-rig won't sail the same course downwind as a spinnaker boat. A spin-boat can sail deeper angles, resulting in a shorter total distance. Furthermore, because a spinnaker lifts the bows, you can push a spin-rig boat harder downwind with less fear of pitchpoling.

But mainly, adding 10% to a sloop's total sail area in the jib doesn't provide nearly as much benefit as increasing the total sail area by over 90% in a spinnaker. At least I don't think so. Do you?

I agree with most of your points and cannot speak for the myriad of math used for the SCHRS formula.

It would make sense that adding a huge spinnaker at the loss of a small amount of jib area could make a boat faster.

Perhaps they are limiting their calculations to displacement modes rather than planing modes?

How about this potential scenario (totally need to check my trigonometry on this):

- light air limiting boat to hull speed upwind/down (without spin)
- 1 NM w/l course
- 30 TWA up
- 180 TWA (DDW) down with no spin
- 120 TWA with spin

So the two boats sail:

NO SPIN boat
Distance sailed = 2.366 NM
Upwind speed = 5.36 kts (hull speed)
Downwind speed = 5.36 kts (hull speed)

Time upwind = 15.29 min (1.366 NM zig-zag)
Time downwind = 11.19 min (1 NM strainght downwind)
Time on course = 26.48 minutes

SPIN BOAT
Distance sailed = 2.73 NM (because you're not sailing DDW)
Upwind speed (reduced jib area) = 4 kts (25% reduction in speed)
Downwind speed = 7 kts (30% increase in speed)

Time upwind = 20.49 minutes (1.366 NM zig-zag)
Time downwind = 11.71 minutes (1.366 NM zig-zag)
Time on course = 32.2 minutes


Furthermore (also suspect to math error):

To match the non-spin boat's 26.48 minutes on the course, the spin boat (sailing the longer downwind course and having a slower upwind speed) would have to sail almost 13.5 kts speed downwind to catch the non-spin boat.

So if cutting the jib down drops the upwind speed by 25%, the spin has to almost triple the downwind speed (from hull speed) to sail the longer course in the same time as the non-spin boat...

You have downwind backwards. A spinnaker catamaran sails both FASTER and a SHORTER DISTANCE downwind than a sloop catamaran. In order to sail the best VMG, a sloop has to sail a hotter angle than a spin-boat.

If you're going to have the sloop sail dead downwind, you'll have to slow it down by more than a factor of 1.4.

You're also postulating that a 10% sail area increase in the jib results in a 34% increase in upwind boat speed, which I don't believe is reasonable. Certainly not when you postulate that a 90% increase in sail area only increases downwind boat speed by 30%.

Think of it this way, if you could sail dead downwind without a spinnaker at speed X, then if by adding a spinnaker, you could sail at speed 1.3X, but you'd have to sail 1.4 times the distance, your VMG would be 7% slower. Why in the world would you ever put a spinnaker on the boat in the first place?

That obviously is not how catamarans sail in the real world.

yes, I was reaching deep in imagination to find some parameters which would favor the non-spin boat sailing the shorter course (like DDW) as faster than a spin boat.

Only by reducing upwind speed significantly (like removing jib altogether) would there be enough time delta for the spin boat to not be able to make up the time/distance downwind.

In my anecdotal experience, only time DDW works is when it's so light that the spin boat never reaches hull speed. As in less than 5 kt breeze.

Also noteworthy is most boats add a spin to the sailplan without reducing other sail area(s), unlike your example of reducing upwind area and adding downwind area. But that may be worth thinking again, since your discussion shows a little tweak in upwind sail area could develop quite a ratings beater...

Eric,
I run the SCHRS technical committee, and I can see that there is some confusion about SCHRS measurements. Please send me an e-mail address and I would be happy to help you on a one to one basis. My e-mail is william@sunnucks.co.uk.
William

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