Light air performance priority...which one?

It is generally well known that for a sailboat to perform well, certain ratios
have to be within certain bands.
I guess the most common are, Displ/LWL, SA/Displ, SA/wetted surface,
and perhaps Ballast Ratio.

If you were designing a boat around 38 ft overall, and would want her to have v.good light air, upwind performance (up to 10 knts) which of the
above ratios, or any other ratio would be your top priority to get it right? Why?
And how would you rank the others in order of importance.
And if you think this is a valid argument, you can state your optimal figures to be achieved.

As a note,the boat is to be used/raced (IRC) mostly in protected coastal waters, minimum creature comforts, no exotic materials or building techniques.

 

light air: SA/wetted surface, and length/beam are what I would focus on. Of course if your target is around 10 knots, you want to make sure you have enough righting moment to balance the sail plan.

 

SAIL AREA. Just pile it on. You can't get enough.

Look at the race boats on the lakes in central Europe and you'll get the idea.

Right -- its really SA/wetted surface, but you can't do much about wetted surface, you can trivially double or quadruple the sail plan. There is no optimal SA/WS ratio -- more is simply always faster in light winds.

Specifically, go with an 80 foot carbon stick (weight aloft is VERY important on tall rigs), with gigantic roach, and a stem-to-stern genoa.

Second, you also need stability. Lots and lots of it. Check out this site:
http://www.classe-libera.de/

These are really fun boats to sail in light air!!

There is no rating rule that will fairly handicap a cloud of sail. In light air you will win, in heavy air you will not.

But there is nothing fun about sailing a undercanvased boat in light air. In the Med and Middle East, its light all the time except 3 to 5 hours per day.

 

Omeron, I've always liked light air performance (upwind) in boats that had a ratio equaling or exceeding 3:1 sail area:wetted surface. My cruising sled Amati is just above 3:1 (40' boat, 10 ft Beam, 8 ft draft, 96 D/L) SA:WS. High AR, though, about 3.4ish. Our U20 (D/L 36) was just over 3:1. Both boats are crisp in the light stuff, say 1-4 knots. We start to depower at about 6-8 K on Amati, and start thinking about reefing in about 12-14K

In light stuff downwind 6:1 goes crisply, and I don't feel underpowered.

Heavier boats are a bit different, as Ted Hood showed over and over again. His boats were around D/L 250-300, went really well in the light stuff. I remember a Famous Yacht Designer telling me to study Hoods boats as far as light air performance was concerned. If I remember right, IRC boats are in the 150 ish D/L range, so I'd think weight begins to be an issue.

Touchy number too, is Prismatic, depending on your belief system. .53-.54? The TP52 guys won't even publish theirs, esp concerning Med conditions, both for light air and chop.

I suppose more SA is better, but you've got to be able handle it easily.

Paul

 

Thanks all.
Paul, D/L of 96 is unbeliavable for a 40' boat. And 36 is even more unbeliavable. Are you sure?
What you are all saying is correct ofcourse, and if you want to keep moving
in light air, you need minimum skin friction, where the wave making resistance
is not an issue.
But at the same time we know that IRC loves weight and stability. And penalises sail area. So my original question is now,to what SA/WS one can go
sensibly, before IRC bites you back and takes all your advantage away.
I know that IRC is a secret formula, and nobody can calculate an exact number, but this is in response to above replies, stating the more the better.
I have a feeling that probably a very high ratio would trigger a disaster in terms of IRC rating.
What do you think?

 

Omeron- Oops! 36 is the SA/WS for the upwind SA on the U20. The D/L, without crew, if I got the WL right is more lke 84. Sorry.

Amati is not a carbon or kevlar boat- she is a C.O.V.E. boat built by Steve Rander at Schooner Creek, in Oregon, USA, using Port Orford Cedar, epoxy, unidirectional e glass, and klegecell, layup is glass, wood, glass, klegecell, glass, wood, glass. I don't think of her as particularly light.

Hood really had heavy boats dialed in- seriously, take a look at his stuff. As long as you give up going beyond hull speed very often, why not go really heavy? You can package heavy/WS a lot more efficiently than you can light/WS in a stable monohull. Are centerboards allowed in IRC? hee hee.

Paul

 

One of the things about a heavier boat in the light stuff is not only can you have a good SA/WS ratio, but the mast is typically taller (because you have more stability and often also need the SA). The taller mast is often in better wind and the weight of the boat can carry it through the lulls as long as you're not collapsing the sails.

I have a lot of miles on Bob Perry's Nght Runner, and that boat has a DL about 220 and displaces about 24,000 pounds. But she's a witch in the light stuff, we've won Swiftsure overall once and have firsts in class a few times as well - all in the light. We create our own apparent wind and have had to defend ourselves at the bar against claims that we were motoring (one time we were doing 6.5 knots with no ripples on the water due to wind). I'm convinced that light air perfromance is not only the domain of the ultralights.

 

I've always admired Nightrunner. Esp. the bright hull. Sand sand, brush brush. Amati's one of Bob's boats too...

 

This, i think, is very very interesting.And opens a new discussion.

Lately, we got accustomed to think that, in order for a boat to perform well,
it has to be super light, able to plane easily,and has to have DL of very low
values.

Whereas, in my waters (Med.) we get to race mostly in light winds and small chop.

I was almost convinced that a boat with a DL ratio of anything over 150 would have no chance in light winds.

So i am very excited to hear about Nightrunner. (would love to see pic.s)

So, should we all go back to the drawingboard and start looking at designs
with high SA/WS and not be overly concerned about D/L?

This is exactly what i am looking for. A moderately high displacement boat,(DL of around 180-200)
docile in her movements, slippery in light winds, and not heavily penalised by IRC.

Do you know of any other boats as such?

Also would like to hear from others who think this is not quite possible.

 

Look at Bob Perry's site, I think there is a sailplan drawing of Nightrunner there at least. Look in Sailing Magazine archive of Bob's designs, or buy one of his books of his own designs. (You could commision him to do an IRC boat....?) I think he did a review of her when Nightrunner was launched. I think there was a sister ship (or two?) also. IOR went really heavy for a while- check out some of that design philosophy- plank on edge too?

That said, I have quietly held theory (until now) that it boils down to the weight supported by each square inch of WS is that matters- kind of like Wing Loading on an airplane, more like WS loading(?). Which would explain why really narrow hulls have some major advantages. It's one of the reasons that I insisted on going narrow on Amati. Even though light, the WS is efficient, in terms of Displacement Loading (tm!). She still planes though. Didn't work for the Farr VO70's though, did it? I'll have to chase Nightrunner around next time I see her in ligh airs. Will report.

Paul

 

Omeron,

You are right that light displacement is not a factor in light air performance: light displacement reduces wave drag, and wave drag is irrelevant until you hit about the square root of the WL. As you go faster, and especially as you exceed 1.34 x sqrt(WL), light displacement makes a bigger and bigger difference.

Rating rules that try to give more-or-less one number to a boat's performance implicitly assume linearity in drag compared to competition. If you are dramatically lighter than the competition, then in heavy air downwind (where speed can be high but stability does not matter much) then you will win in any rating rule. If yu have dramatically more sail area than the competition, you will win in very light air regarless of rule, and almost regardless of displacement. One reason is the ability of a greatly "over canvassed" boat to generate apparent wind when other boats see 1-2 knots.

The libera boats I linked to earlier are like that: if there is absolutely no wind, so cigarette smoke goes straight up, the boat stops. As soon as the smoke drifts even slightly away from vertical -- long before the surface shows any ripples -- the boat will start moving, and will run at 3, 4, 5 times wind speed. No rating system will keep up with that speed advantage.

As mentioned, light displacement is not needed for max light air performance in the Libera classe: the boat I raced aboard in the mid 80's (Garfield) was by far the heaviest, with a hull form similar to a 35 foot Soling. But we had the most sail area, and we would simply dominate in the light stuff. At about 6-8 knots of breeze, a very light boat could get by us perhaps. At about 10 knots of breeze, a double trap Tornado catamaran could just about keep pace with us upwind.

Garfield was 38 feet with sugar scoop transom, about 7 feet wide, about 7-8 feet deep stainless fin with some lead shot in the bottom, and a 65 foot keep stepped aluminum rig out of an Admiral's Cup yacht. The main boom traveller was the top of the transom, and was nearly two-blocked when fully sheeted in: about 20 feet on the foot, 60 feet on the hoist, with a decently fat roach. The number one genoa was stem to stern and masthead, so about 35 feet on the foot and 60 foot hoist. So upwind sail area about 60x35/2 + 60x20/1.75 = 1700 sq ft. Displacement was about 4000 lbs, crew was 2200 lbs, so 6200 lbs all up. SA/Displ = 80!!

Downwind, we had masthead chutes that, when fully drawing, would still be at the water's surface, so our downwind SA/DISPL was close to 200.

Sure, we would reef, and use smaller blade jibs upwind and fractional chutes downwind when the wind came up. No problem.

 

Here's the sail plan of Night Runner, with her big foretriangle:

http://www.perryboat.com/largeview/crhper-kbpic-nsailplan-slge.jpg?Nightrunner: 41-11 Cutter

And I've attached a couple photos - she's a pretty boat.

So let's compare her design ratios. Using simple triangles (I,J,P,E) she has a sail area to wetted surface ratio of 2.52. If you include the 150% overlapping genoa and main roach, her upwind SA/WS ratio is 3.49, which compares pretty favorably with a modern lightweight racer like a Farr 40 (http://www.farr40.org/content/view/21/26/). And considering the large foretriangle, her spinnakers are pretty good sized to so her SA/WS ratio downwind is around 6.7. I have very fond memories of overtaking a Cookson 12 meter (very simiar to the Farr 40) during Swiftsure one year, sailing under half ounce and making about a knot to a knot and half more in the light stuff.

One of the things about the heavier boats is they add volume below the water but not a lot more wetted surface. The lightweight skimming dishes (say the farr40) I would estimate have a hull wetted area (minus foils) of around 250 square feet, but Night Runner is a little over 300 (Night Runner is 2.4 times heavier).

The disadvantage of a boat like Night Runner is she is hard to work compared to the lightweights. The sails are heavier, the loads are bigger, and the sail inventory is larger. So it's not all peaches and cream but all in all I don't think you could find a much better ocean going vessel. To keep Night Runner going in the light stuff we make copious use of the drifter, #1, and half ounce chutes, and the foredeck can be a pretty busy place switching between all of them. Our motto is "A busy foredeck is a happy foredeck." Night Runner has plenty of righting moment, what with 11,000 pounds af ballast at the bottom of an 8' fin, so we don't really depower until the low teens (flattener in the main) and switch to the #2 genoa about 14kts apparent. Cruising is done with roller furling and about a 110% overlap, good enough for most sailing days.

Paul, I really like your Amati (my brother is a cellist too). It's pretty close to the ideal coastal cruiser as far as I'm concerned, plus it's got that Perry style. You can tell that some boats have a lot of Bob's heart in them, yours is certainly one of them.

 

Thank you all.

Paul, i like your terminology of WS loading, or Displ.Loading.
In fact, if Displ. is divided by the WS, then dont we get a dimensionless
ratio just like D/L or SA/D.
How come this has not become a parameter in boat design, or performance
criterion? Or is it? But i have never heard of it before.

I think it would be very interesting to analyse a number of boats across a wide spectrum and see what sort of results one can get out of it.

Mind you, in this day and age where there are no stones unturned, i am sure this is also a well studied subject.

If there are any published papers on this, i would love to find out.

 

Hi,

Disp/WSA gives an idication of hull form and as such is sort of covered already by Cb, Cp and Cm (in a smuch as a round bilge hull will have a low WS compared to a triangular section, for a constant displacement).

 

I am not knowledgeable enough to make a distinction between Cp,Cb etc and the Displ/WSA. It is probably already covered.

But to me, somehow, Displ/WSA is a clean, simple parameter to look at, and
is easier to visualise what it means.

If we take two boats to compare, one with a rounded bilge heavy displ, and
the other a flat bottomed, lightweight planing hull, could it be that they
have close volumetric coefficients, whereas they have far apart
Displ/WSA ratios?

Because whereas actual displacements are making a big difference to begin with, the greater WSA of the second boat only goes to widen the difference.

If we assume a 6000 kg hull with a WSA of lets say 25 sqm, the ratio is 240
If lets say boat B has 3000Kg hull with 30sqm the ratio is 100. Big difference.

I do not know whether volumetric coefficients would bring out such a difference?