How long is a piece of string?

Varies enormously with wind and waves and point of sail.  Example:

Cherub v 49er z 14 v 12' Skiff Weymouth, light wind.  49er/14 disappear (waterline length) Cherub same speed as 12 upwind (less drag from rig as non-wiring conditions, better pointing, hull shape and ability to keep crew forward better for light conditions), 12 dissapears from Cherub downwind (sail area and angles) but still not as fast as 14/49er round the course.

Cherub v 49er Weston Skiff event, medium breeze, solent shop.  9er much faster upwind and through tacks (Cherub stops dead!), however Cherub nearly as fast sometimes faster downwind in the gusts.  LWL irrelevant as very little of Cherub in the water.

12' skiff v 14 yesterday, Draycote, windy. 14 much faster uphill (more rag and more LWL than 12) until wind pipes up a notch, 14 overpowered,12 sails out from underneath (*puts tin hat on and waits for the forum fun to start!*).  Downwind 12 is faster due to angles and kite area and weight.

Just too many variables I think!

We're teasing somthing out of this though.  I'm sure we'd all agree length is an advantage in sub planning conditions and naturally hull shape.  My recent experience is between 800s, 4000s and a 400.  The 800 has the length but very little rocker and so once the wind drops to very low the 800 stops, the 4000 can creep up to them but the 400 sails right past.

So in a force 2 I'd say the PY for a 400 is about 950, a 4000 about 980 and a 800 about 1100!  Force 3 and its 400 - 950, 4000 - 950 and 800 - 900.

Having sailed a Cherub many years ago I'd say in force 3 its dire but about to get interesting.

Any thoughts out there?

If I put it in my simple terms, straight line performance is about drag versus power.  However, as the ad says, power is nothing without control.  If you want to perform, then surely the foiling moths show that what is important is low drag in a controllable package (without the auto-wand on the Moth the controllable bit isn't there and you don't go very far).

So, if you colour the picture on all the things listed above with the ability to maintain control, then you get a pretty good picture of each.

Length - reduces drag at displacement speeds and improves controllability at higher speeds.

Weight - need to differentiate here between moveable ballast and fixed ballast - lighter fixed weight is generally faster but too light and the lack of momentum starts to hurt.  More moveable ballast is generally better and the more it can be moved the better (in general) - but then you get back to the limits of control.

Righting Moment - more is generally better but there are limits as you still have to control the beast (18' skiffs on Waterloo Bay versus Sydney Harbour is the classic example - smooth wind in Waterloo bay made control easier so the boats got wider and faster).

Sail Area - more is better up to the limit of control / power etc etc

etc etc.

Would have been interesting to have had a bit of a race with the 12 on sunday to see how the different advatages would have worked out over the course. From our perspective (in the 14) we certainly pointed higher up wind at a similar boat speed until the wind piped up - then needed more rake!

The bit I thought was the most interesting was how high you were sailing down wind even with the bigger kite. We just could not wind the 14 up that high without the apparent wind pushing us way off to leward? An example of a more efficient kite shape?? I don't know.

The thing that makes the biggest difference to our speed is our T foil - let it off up wind and the boat feels like its stopped! Pull it on down wind and again you can really feel the boat lower it nose and accelerate, or alternativly, as we were doing sunday afternoon leave it off for the more impressive ride (or lazyness - afterall we weren't racing). 

If we were racing a windward leward somewhere small, I'd certainly favour the chute option over the bags... plus easier when swimming 

Hmmm, I actually thought we (12) was sailing lower than the 14 downwind but I'll stand corrected.  You were certainly outpointing us uphill!

Thinking about it we were probably sailing artificially high to try and get the reach down into Toft Bay and also give us a few options if a big gust came in.  We were on the knots with the main well out for a lot of it, there was the anchored committee boat that seemed to be in the way most of the time too (as a 20 foot boat would be on 700 acres...) 

I think if we had been VMGing it then we would have poked it a bit lower and also sailed a bit flatter.  Don't suppose you had a GPS on board did you? Be interesting to compare tracks...

Hang on, this is turning into a skiff-only thread!

The reason the 400 is relatively good in the light airs is that it has an "old-style" hull-plan - one that compromises between displacement and planing sailing. In the light stuff it's hull is sufficiently hydrodynamic to cut the water better than a 4000. However, when the wind gets up a bit, its rounded aft section doesn't pop onto the plane as quickly as a skiff with a ultra-wide chined stern. Now all things are relative and the 400 has a more of a planing hull than, say, an Albacore.

The amount of sail you carry is relatve to the righting moment you can produce. The most efficient sails are those with a high aspect ratio as they minise drag against lift. However, they also the least forgiving of inappropriate sheeting angle. The same is true for foils. Long skinny boards and rudders are the most efficient but you have to be moving fast for them to work. And they stall more easily than chubby centre-boards.

Once you get your weight out-board on the wire, you are loading up the foils, forcing them to work harder and produce more lift. At that point a lighter craft will be more responsive. When you are in-board however, all-up weight of crew plus boat is what matters and a carbon tiller can be offset by a full bladder!

Catamaran's break most of the rules above as they are generally displacement-only craft. They go very fast for their length due to the narrow section of their hull which allows a higher hull-speed. However, their designs are now starting to look towards some element of a planing hull in their designs - I know this is true of the F18 Capricorn and there must be others. But, for the geeks amongst us, it's well worth looking at how the SCHRS cat handicap formula works. It's fairly reliable for all cats and shows where the emphasis of weight vs sail area vs hull length goes. Although SCHRS doesn't cover monohulls the laws of physics are the same so you should be able to get some idea of the importance of the various factors.

The other good source for anyone wanting to bluff their way in marine architecture is "Sailpower - The Science of Speed" by Laurie Smith and Andrew Preece.

Foiling boats like the Moth occupy the same sort of lack of compromise as the 12'. Dogs in displacement mode but lighting fast once up to the speed at which they are designed to sail. The problem lies in getting them to that speed and then keeping them there.

Appologies if I have got anything wrong but I think it's generally correct.

Someone mention SCHRS

As I think I alluded Jim, "old-style" is a relative term. As an assymetric boat with a fair amount of canvas th 400 is going to have to plane well to make the angles pay. However, its design is not as uncompromising as that of the skiffs. I only mentioned it because Redback had made the comparison between 400, 800 and 4000.

I'm with you on Laurie Smith. Fortunately Andrew Preece seems to have done pretty much all the writing.

You are right to say that cats are unrepresentative but then so are skiffs to that majority of boats that occupy the middle ground. Hull speed and the associated wave drag are dealt with differently, that's all. The skiff does it by overtaking the wave (planing) while the cat has such a high K-factor that wave drag is minimised. Cats have some advantage here in that they do not need the grunt to get onto the plane and so all their sails can be extremely high aspect. Once on the plane, however, the skiff is having to overcome a lot less drag to maintain its speed and sail efficiency rather than size is what matters.

A what?????

Bluffer's Guide? Seems to have worked though!

K-factor is somethin you get when you divide the waterline length by the max width of the hull and then possibly square-root it or something. For most monohull it is in the 1.3 - 1.4 range. For catamarans it is around the 5 mark. This determines the maximum hull speed, ie. the max speed a boat can go before she planes. K*Sqrt[LWL] is the max hull speed.

You've taken me out of my comfort zone and rerquired me to elaborate. I have thus panicked and started to bulls**t. Sorry about that

Ian, GPS is on our wish list at the moment, unfortunately maintaining the 14 has drained most of our funds - Well thats unfair as my mrs and daughter drain mine, and the boat drains Chars.  I know what you mean about the commitee boat we just always seem to cross it, which proved messy at one point!! 

IC and AC ?

1.34 X square root of the waterline length (in feet) gives you the theoretical maximum speed of a displacement hull (in knots). This is more relevant to more traditional keelboats and non planing powerboats, where however big an engine you put on you will go no faster. Once the hull gets narrow enough not to be stopped by it's own bowwave (cats are the obvious example) or planes like a dinghy, other factors come into account, making the above fairly meaningless. A cherub will pop onto the plane before it reaches it's theoretical max hull speed of 4.6 knots. 

As someone who finds this thread interesting but has no idea half the time what your all talking about… what makes some boats perform apparently punch above their weight. For example to my uneducated eyes boats like the 400 and 29er always seem to perform much better than a cursory glance at their size and shape would suggest.

Ian

FD is 6.06m LOA and has a PY of 880, or matbe 878 these days. The extra length certainly helps with control in big wind and waves, which is where the boat really comes into it's own. Fact of life though, length costs weight, and "weight is only of use in road rollers".

Personally, I and I suspect most other FD sailors, am quite happy to put up with the extra weight in return for the sensation of blasting upwind in 25 knots and big waves.

On the subject of longer boats. Uffa Fox designed and built a double handed canoe in 1935 called Brynhild. She was 20' long 4' wide with 2 sliding seats and weighs all up 190 lbs. She had "long and easy buttocks".......dream on boys!

Mr Fox thought that as he had achieved 16.3 knots in a 16' canoe Brynhild should be good for 20 knots. Not so, it only managed 15 but this is over a measured half mile, none of your peak speed nonsense. Remember this is a wooden boat with a wooden mast and cotton sails, oh, and no kicker.

What is perhaps more remarkable is that in June 1936 Uffa and his chum set off from the IOW and sailed to Cherbourg for a bit of a holliday. They carried camping gear and a typewriter. On the crossing they encountered everything from flat calm and fog to 25 mph winds but still achieved the journey in 12 hours. Had it not been for the flat calm off Cherbourg they would have averaged 10 knots but as it was they only managed it in 5. Remember, no radar, no sat nav, no radio. Oilskins wooly jumpers and, if they had them at all, cork lifejackets.

It is not mentioned in the book but I bet they ate hardboiled eggs and jam sandwiches wrapped in greasproof paper. Spirits would have been kept up by whistling the tunes of Henry Hall.

I notice the V3000 is considerably faster than the Laser3000 even though the hull is the same shape and the sails about the same area.  Is this increase proportional to the decrease in weight?  Or is this, as I suspect, the L3000 was a bit heavy for the hull shape and is now "released" to perform as was originally intended?

Or is it that weight is under estimated as a performance factor?

You could write a thesis on this question!  Ian Hannay has written an excellent AYRS publication called Rig theory which goes into this in some detail.  In  a nutshell:  length, weight and stability are the three primary parameters that affect boat performance.  Rig height, foil depth and sail area are the secondary parameters and detailed design such as hull lines and sail profile are the tertiorary factors.   All other things being equal (which they never are) long, light boats will be quickest across a range of conditions and courses.  Stability is obviously vital to harness power - an extra trapeze will allow you to carry bigger sails.  More sail area lets you reach your top speed in less wind, but doesn't actually give you a higher top speed.  Rig height and foil depth are key to efficiency, minimising induced drag, and allowing you point higher.  So, for any boat of given length, weight, stability, rig height, foil depth and sail area there is a theoretical maximum perofrmance that can be achieved.  Compromises in hull shape, sail planforms, rudder sections, mast sections etc prevent this maximum being achieved.

Compare the Dart 18 and the Cherub.  The Cherub has better foil sections, (arguably) a more sophisticated rig, better attention to detail, deeper foils, more sail area and is lighter.  And yet, becasue the Dart wins on 2 out of 3 primary factors (longer and more stable), it is quicker (on average, according to PY).  Its longer length and more stable platform give it  higher potential speed.  The excess weight and simple design mean it probably only reaches 60% of its potential, compared to the Cherub reaching maybe 80% of its. but the longer, more stable boat starts with more potential.  Cherubs of course sail extremely well when planing, but simple physics prevents it sailing fast when not planing.  A 12ft boat can never be fast in displacement mode, no matter how well designed.

As another example, my NS14 is the same length as a Merlin Rocket, and both are hiking boats with similar stability (the MR is a little wider, but in broad terms they are similar).  The Merlin has bigger sails and a spinnaker, so you might think it was faster.  However, because the NS14 is 5 stone lighter and has less drag it is probably quicker.  I look forward to finding out over the next few weeks!

Interesting, but I am not sure how much I care about the relative performance of a class to another ... what I care about is:

1) is the boat fun to sail

2) is it good for class racing

3) are the people nice to hang out with ....

I would argue that a national 12 (a 12ft boat) is fast in displacement mode especially light wind sailing. They are well known for there speed in light winds although i don't think this is because of weight because they are relatively heavy for a 12ft boat and the sails arn't particularly large.  

True enough, but that's not really answering the original question about weight, length, sail area etc.  Anyway, many of us are unwilling or unable to travel the country to get some fleet racing.  So you may as well sail whatever tickles your fancy in the local handicap fleet.

Not sure how to do a 'double quote', but in response to Smight's comments:

N12s are light by most standards - 78kg all up.  Okay, that's more than a Cherub, but still...  Anyway, to clarify my point, a 12ft boat can't have a high maximum displacement speed (some times called hull speed).  Max displacement speed is physically related to boat length, so a 12ft boat can not be as fast a 14ft boat of equally good design.  However, light air performance is also determined by wetted surface area, and 12s are pretty good from that point.

I guess my point is that whilst these issues are of interest the most fun comes from quality fleet racing.

BTW the N12 is 78kgs for the hull, mast & c/b ... I'd guess a sailing weight of about 85kgs.