Analysing Upwind Performance

[farjoe]

Marco and GggGuest,

I have yet to be convinced about your assertions on Drag and Apparent Winds at wind speeds of less than 5 knots and for Multihulls which are not beach Multihulls. The sail area/ displacement for the J80 is listed as 35m^/1300kg...That of my Strider is 31m^/900kg(estimate).

It appears to me that the wave drag component of the total drag at these speeds is pretty negligible. At least the J80 leaves hardly a ripple in these conditions.

At these wind speeds the 2 boats do practically the same speed and therefore, in my view, the resultant apparent wind angle should not be all that different.

At higher speeds I agree totally with what you say.

regards

P.S.

Marco, I am still at the old email address if you want to drop me a line.
All the best

[SeaDrive]

Originally Posted by gggGuest

...things like the 12metres and IACC boats, also slow for their length, which point like crazy.

I know this is a sidebar issue, but can you elaborate on the statement that IACC boats are slow for their length?

If you compare ACC designs to modern designs like maxZ86's they ARE slow for their overall length...
I've read that C-Class cats have the abiliy to point really high while still makng good speed. If you want to investigate this contact Steve Clark.

[mistral]

ACC are slow because their box rule keep them slow; the'y re quite heavy (IMOCA60 weight 10t, they weight 24t), very narrow, and with a short keel compared both to maxi and open; their ratio best upwind speed/best downwind speed is really poor (less than 1.4 maybe); there's one thing they do better than other boats: thay have an impressive ability to point up their nose going upwind, ACC regattas on TV showed clearly tacking angles upwind smaller than 90° in each conditions form 5 to 20 knots, maybe near to 80° more than to 90° in average wind; ACC boats cannot be compared to other classes as they're designed and imroved for a single particular kind of competition and they sail in a very narrow corner of wind/wave conditions, out of this conditions they risk to became a wreckage in a couple of minutes. Quite absurd for such expensive boats, don't you think ?????

Mistral

[brian eiland]

Dear gggGuest,
I think you got it right on a number of points, and without a lot of 'technicality'.....it's mostly to do with apparent wind and good efficiency from the sailing rig.

Obiviously if you manage to create more wind (greater apparent wind) as many lighter-weight, slender multihulls do, then your chances for greater performance are enhanced. But you must make efficient use of this increased wind. This is the job of the sailing technic (helmsman), and a function of the rig design itself.

Most competitive multihulls can create a substantial amount of apparent wind. And they generally utilize it most effeciently by sailing no higher than 45 true, tacking thru 90-100. This generally maximizes their VMG. However, I have been in numerous situations where, if we slowed down to a speed just barely above that of a real competitive monohull, we could work our way to windward of him, and boy were they surprised! (I must admit it took a moment of inattentiveness on their part to pull this off...playing the slightess wind shifts). We concentrated on getting the greatest driving efficiency from our rig at these higher pointing angles...proper slot, proper sail shape for this higher incidence angle.

When I speak to the efficiency of the rig there are so many factors to be considered. I've addressed a few of these in the past suggesting the superiorty of the headsails over the Bermudian mainsail. I'll leave this for another time.

However, I will make a note about your statement, "single-sail dinghies manage some of the highest pointing angles". Your inference here being that the uni-rig configuration might somehow be superior, or more efficient. Very likely it is in the case of dinghies! Please note that in small dinghies it is almost always a case of 'disjointed interaction' between the mainsail and a jib; ie, they are generally never 'masthead' rigged, and the booms are located substantially above the foot of the jib....three distinct, interactive areas....lots of rig drag potential in sloop rigged small boats. For some interesting discussions and illustrations visit http://www.wb-sails.fi/ and look thru their site for extensive studies on the one design 470, etc.

Is it as simple as both boats when traveling 90 degrees to the wind present the same side area and wind resistance. As the boats rotate into the wind the single hull wind area is decreasing. As the twin hull boat rotates into the wind it starts to " unmask " a second hull area which is now causing more air turbulance on the down wind bow and on the upwind stern to a lesser degree. On a single hull the wind force slides over and around the hull once. The twin is creating a series of turbulant obstacles and drags before it releases the same volume of air. The air flow in the tunnel should become most powerful at 45 degrees due to the air traveling the full length of the tunnel. More wind resistance longer, more rudder, slower speed,etc.-------- I also design competitive water wings.

[CT 249]

Sorry, Richard, are you saying that the catamaran is actually SLOWER upwind in the light when you say "more wind resistance longer, more rudder, slower speed,etc"?????

A good cat is certainly not slower upwind in light airs than a good dinghy; our 16' cat will and has, whipped the 3 time world Flyung Dutchman champ upwind in the light because our speed is so much better, but our pointing is lower. Or am i reading your post wrong?

Brian, masthead-rig dinghies have been tried, with a notable lack of success. Booms are located substantially above the foot of the jib in the fastest dinghies (49ers, 18' skiffs). I looked at dropping the boom on my Int Canoe (we tack around the back of the sail, not under it) but in all the decades of canoe sailing, no-one has found droppin the boom to be a major boost according to the asking around I've done.

Even one of the oldest major dinghy classes, the Snipe, carries its boom in the top position. The boat was originally designed to drop it low in a breeze but it has been found better to have it high.

Having said that, in windsurfers we find "closing the gap" to be very important....I'm just pointing out that the most innovative dinghy classes (those that created the modern assymetric spinnaker, the practical foiler, hiking devices etc) have found the big jib arrangement to be slow and that boom height is not a major issue.

Of course, maybe the Bethwaites, the creator of the C Class cats like Cogito, Uffa Fox, all the 18' skiff and Moth sailors and everyone else are all wrong.....but somehow I doubt it.

The original question was. Why can not my cat point as far into the wind as other boats can. My assumptions for reasoning out his statement. All single, dual and triple hulls are sized and built of same speed ,turning, tonnage, sail equipment and abilities of the crew. What is left, is who can still move foward into the wind, at what angle, and why?

[SeaDrive]

Below is a section from an essay by Richard Woods, posted on his website. It makes some of the same points as have appeared earlier in the thread.


"Self Tacking Jibs
These sound like a good idea for short handed crews. But they are only any good when actually tacking or sailing close hauled. They are no good for example when you want to sail off an anchor and need to back the jib, while when reaching you will need to barber haul the sail to control twist. If you have a roller reefing headsail then you can forget it as far as sail shape is concerned (a rolled sail becomes more baggy the more vertical the leech as the cloth rolls on top of each preceding turn).
Monohulls like the Star and Etchells have self tacking jibs. But self tackers work better on these boats because to increase VMG (speed made good to windward) a monohull has to point higher as it cannot go faster through the water as it reaches maximum hull speed in only a light wind. A multihull, on the other hand, can increase VMG by sailing faster even if it doesn't point as high, so it can usefully use a different style of rig."

[SeaDrive]

IF you had inboard jib leads, a taut jibstay, and flat-cut sails (or sails cut with exquisite precision to the sag of the jibstay), you would be able to point as high as the J-80. But your boat was designed to sail a bit more off the wind, and get the best VMG by sailing faster.

There was an article in Sailing World this year about an Etchells championship (IIRC) where the winner said he tuned his boat for max speed without worrying about pointing higher than everyone else. If you are sailing in shifty winds, sailing a little lower with the same VMG gives you more leverage in the shifts.

[brian eiland]

Those were a couple of good observations SeaDrive.

Originally Posted by SeaDrive

Below is a section from an essay by Richard Woods, posted on his website. It makes some of the same points as have appeared earlier in the thread.

"Self Tacking Jibs
These sound like a good idea for short handed crews. But they are only any good when actually tacking or sailing close hauled. They are no good for example when you want to sail off an anchor and need to back the jib, while when reaching you will need to barber haul the sail to control twist. If you have a roller reefing headsail then you can forget it as far as sail shape is concerned (a rolled sail becomes more baggy the more vertical the leech as the cloth rolls on top of each preceding turn)."

And I might add to this observation by Richard, that the 3-point staying of the most multihulls does not condone taunt forestays, and certainly not a masthead foresail.

So small jibs are the order of the day, so why not make them self-tacking. This makes sense from a handling point of view, but NOT necessarily from an efficiency point of view. Put another way, the 3-point rigging method sort of dictates big tall mainsails and small fractional jibs.

[brian eiland]

For a visual representation of headsail shape change with forestay tension one might visit North Sail's presentation at http://www.northsails.co.uk/media/fa...a/gt-step5.htm and click on the "backstay oval" at the bottom of the page.

[sjors]

Originally Posted by BOATMIK

The more combined drag from hull(s) and rig the lower the pointing angle.

Multihulls often have considerably more wetted surface than a similar sized dinghy particularly when that dinghy is heeled and the crew move forward to get the broad aft sections out of the water.

Certainly a single semi circular section has a minimum wetted surface for volume but when you have two of them it just don't work that way.

As the wind increases and the boat accelerates the multi has lower wavemaking drag so goes much faster.

MIK

You are right about the smaller wetted surface of the monohull.

When two boats are compared with identical performing rigs and daggerboards, the speedgoverning factor in light wind is the sail-area to wetted-surface ratio. I think if you look at the lift to drag ratio of the rigs of a J80 and Aclass cat, the two can be compared, as well as the daggerboard and the keel.

I sail mostly on yachts and the best light wind boat is in most cases the heaviest! When a boat gets heavier, the ability to carry sail grows faster than the wetted surface.

I am very curious if this little catamaran beats the J80 on a reach or broad reach in very light winds. The comparison shouls be made with the same sails when sailing upwind. I put my money on the J.

When the wind increases, the catamaran goes above hullspeed without dramatically increase in hulldrag. Allthoug the J80 is a sportsboat, the boat experience an increase hulldrag resulting in less speed.

If you are interested in these matters I highly recommend the book from C.A. Marchai "aero-hydrodynamics of sailing" ISBN0-7136-5073-7

[CT 249]

Sjors;

You say monos have lower wsa, but what about the figures from Martin Fischer's A Class (18' LOA) with its wsa of 24.3 or 48.6, compared to the 19' Flying Dutchman's wsa of 70ft2 or the 15' Finn's wsa of fractionally under 40ft2?

Could you point me towards some figures re the wsa of modern cats v modern multis? I wonder if changes in design have changed the relative figures. Older multis tended to have Vee hulls (large wsa, of course) whereas older monos tended to have rounder sections, so a lot has changed since Marchaj's initial work. His current editions still use older-style boats if I remember rightly.

Are you SURE the J/80 rig would be as effective as an A Class rig? In that case, why did the A Class drop the fixed stick/sloop rig they originally tried? Why didn't C Class cats stick to a "normal" rig like the J/80s? Have you sailed with an A Class rig? It really offers serious performance (at some significant hassle) whereas the J rig is largely designed for convenient moderate-pace sailing.

[farjoe]

Originally Posted by sjors

I am very curious if this little catamaran beats the J80 on a reach or broad reach in very light winds. The comparison shouls be made with the same sails when sailing upwind. I put my money on the J.

Well, with it being a Saturday and the fact that my boat is currently on the dry I went down and took some measurements. I also collected some data from the internet for the J80 for comparison.

Strider J80
sail area 31 35
LWL 6.8 6.7
WSA (sq. m) 4.1*2 ??
WSA (sq. ft) 44.7*2 ??
displacement 900 1300
area above WL 6 5.1

Most numbers are in metric units. The "area above WL" has been put in to get an idea of the hull windage and is simply the broadside area. The value for the J80 is an eye ball comparison to mine(85%). I am sure that at 45^ to the wind this ratio is a lot lower. I would be great if somebody can provide the WSA for the J80.

To answer Sjors question the J80 beats we downwind with just the main and jib in force 1. In force 3, I am faster even if he carries a spinaker.

CT 249, I think you should be careful when comparing the WSA of small cats to larger ones. This is because most small cats do not need antifouling and therefore the effect of surface roughness comes into play. Also, due to the fact that small cats can be easily righted, their sail area to displacement ratio tends to be higher than that of bigger Multihulls simply because the latter cannot afford to capsize.

regards