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Old 28-12-2016, 23:07   #1
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Fat-headed mainsail

While perusing a back issue of Cruising helmsman (June, 2015) I came across the following comment:-

" I have never understood why so many cruising multihulls have adopted the fat-headed mainsail used in high performance yachts.There is no advantage in a high lift-drag ratio rig in a low lift-drag ratio hull of a multihull with stub keels(dagger boards change the equation). So a cruiser ends up with a tiny headsail and a giant mainsail with no benefit in windward performance."
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Dr Kim Klaka,(naval architect), Ralph Newton.

It would be interesting to hear the feedback from owners of such rigs with stub keels.
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Old 29-12-2016, 22:21   #2
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Re: Fat-headed mainsail

There are more points of sail than just to windward. A supposed naval architect ought to know that.


Square tops give more sail area. They're very forgiving, as pressure increases the leach up high opens readily. Very easy to trim, easy to avoid closed leach in light air too.


I've found that even though our square top main is bigger, we have less weather helm than with the round top.
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Old 29-12-2016, 22:42   #3
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Re: Fat-headed mainsail

Quote:
Originally Posted by 44'cruisingcat View Post
There are more points of sail than just to windward. A supposed naval architect ought to know that.


Square tops give more sail area. They're very forgiving, as pressure increases the leach up high opens readily. Very easy to trim, easy to avoid closed leach in light air too.


I've found that even though our square top main is bigger, we have less weather helm than with the round top.
A square top is sometimes described as the first reef as it does open up in gusts or as the wind increases. Since many cats have a larger traveler it is also possible to move the CE not only in light air but going to windward as well.

But I am not sure the term "fat head" means the same thing as "square top". But let's assume it does. The advantage a square top has over a conventional pin head in gusty weather seems like a big OK. If one is worried about a "tiny head sail and giant mainsail" the obvious solution is to reef. It is also common for cats to have a bow sprit with a big screecher and a smaller working jib.

But as 44 points out a square top is very easy to trim. My experience has been the biggest improvement in performance is the result of well trimmed sails what every type they are. Not to mention the blurb in the OP's post did not say there was any disadvantage to a square top only that it did not have an advantage to windard. Seems like it has advantages at every other point of sail as well as in gusts and being easier to trim.
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Old 30-12-2016, 00:35   #4
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Re: Fat-headed mainsail

It seems to me that any kind of extra added drag is bad, with extra lift being just the opposite. With both factors directly impacting boat performance, & seaworthyness (which are tightly tied together). So I can't say as I understand the gent in question's comment. Part of my reasoning on this was quantified long ago by another NA, John Shuttleworth. But it's something that any performance oriented sailor would swear to as well.
Here's Shuttleworth's article on same, & other factors that affect multihull seaworthyness Considerations for Seaworthiness
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Old 30-12-2016, 03:41   #5
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Re: Fat-headed mainsail

A square top mainsail, besides the advantages already named, regarding a traditional main with the same area (that need a bigger mast), would have a higher sail center of efforts, raising also slightly the CG and in a catamaran that is nor desirable for safety reasons.
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Old 30-12-2016, 05:23   #6
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Re: Fat-headed mainsail

The one that originally wrote this :

" I have never understood why so many cruising multihulls have adopted the fat-headed mainsail used in high performance yachts.There is no advantage in a high lift-drag ratio rig in a low lift-drag ratio hull of a multihull with stub keels(dagger boards change the equation). So a cruiser ends up with a tiny headsail and a giant mainsail with no benefit in windward performance."

got pretty much everything wrong.

1) There is an advantage of having a high lift-drag ratio rig in a low lift-drag ratio hull of a multihull with stub keels. Any decrease in aerodynamic drag always increases performance, if lift is kept the same.
2) A fat-headed mainsail (or a square top main) does not imply any better lift-drag ratio, that is a false assumption. L / Dinduced is mostly dependent on span loading, lift force / (mast height) squared, but also dynamic pressure. Taller mast or higher windspeed makes span loading better under full sails. The part of mast above reefed main makes it worse. L / Dtotal depends also on windage on bridgedeck structure of a cruising cat, hulls, mast & rigging, etc.
3) If mast height and boom length are kept the same, a square top allows for a bigger mainsail, improving performance in light air. IF done like that, the full main has higher center of effort than a triangular main, and thus greater heeling moment in same apparent wind. But if the square top main is reefed to the same area as the triangular reefed main, it is the square top which has less heeling moment due to having lower center of area. Therefore the square top is better in both light air and heavier air if reefing points are done correctly by the sailmaker.
4) It is typical in condocatamarans with squaretop mains of having a short stubby mast, as a result the main is not giant at all, but of the same size of the triangular main it replaces. It also means the mast is shorter, lighter and cost less, while also having a lower center of gravity. In such case there is no benefit in windward performance at least in light air. In heavy air the shorter lighter mast helps a little bit.
5) The benefits of twisting the top automatically are already well brought up in this thread and correctly too.
6) A squaretop main has compression on the top battens, which require batten cars to work properly while sliders on a triangular main of the same size might still work well enough and cost less than the track & cars.
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Old 30-12-2016, 09:01   #7
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Re: Fat-headed mainsail

3) ........IF done like that, the full main has higher center of effort than a triangular main, and thus greater heeling moment in same apparent wind.

How about when the sail twists off? Wouldn't the CE automatically move down?
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Old 30-12-2016, 14:09   #8
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Re: Fat-headed mainsail

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Originally Posted by geoff326 View Post
3) ........IF done like that, the full main has higher center of effort than a triangular main, and thus greater heeling moment in same apparent wind.

How about when the sail twists off? Wouldn't the CE automatically move down?
Yes, see 5).
Case 3) was about light air when you keep max power.
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Old 30-12-2016, 14:48   #9
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Re: Fat-headed mainsail

The point I was trying to make was that presumably when a pin head sail is replaced by a flat head sail on the same mast, the increased sail area and subsequent higher CE shouldn't be a problem because the CE will move lower as the wind increases.
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Old 31-12-2016, 03:30   #10
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Re: Fat-headed mainsail

Changed from 47 sqm standard main to a 56 sqm squaretop and love it.
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Old 31-12-2016, 04:14   #11
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Re: Fat-headed mainsail

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Originally Posted by geoff326 View Post
The point I was trying to make was that presumably when a pin head sail is replaced by a flat head sail on the same mast, the increased sail area and subsequent higher CE shouldn't be a problem because the CE will move lower as the wind increases.
You are correct that it isn't a problem and that the CofE does move down.
However it does move down with a pinhead main as well, just not as much and not so automatically. It is still a fact that in the case referenced, a squaretop main has a higher CofE, but it is not a problem because main can be reefed or twisted even more if needed.

It only means that when designing reefing points, the squaretop main should be reducing more sailarea out in each reef than the pinhead it replaced, so that when all reefs are taken, the squaretop should not have more area than the pinhead, but just the same area left up. It is not mandatory to do so, but it is the smart thing to do in a cruiser, because it allows more safety while keeping all the advantages.
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Old 31-12-2016, 23:04   #12
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Re: Fat-headed mainsail

How does the square top perform when heavily reefed ?
Does it still twist off to de power as readily ?
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Old 01-01-2017, 06:58   #13
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Re: Fat-headed mainsail

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How does the square top perform when heavily reefed ?
Does it still twist off to de power as readily ?
At worse it can twist off almost at the same apparent windspeed as a full sail does, which in practice results always being fully twisted off when reefed.

More sheet tension and battens that are stiff enough can delay this effect into higher windspeeds. How much so depends on saildesign which can vary.
Lower aspect ratio sails shift their twist off wind range the most when reefed and keep the automatic twist off fully working in higher windspeeds with reefed sail.

In any case properly designed and built squaretop main is no worse than a pinhead in any windspeed when reefed.
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Old 01-01-2017, 14:42   #14
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Re: Fat-headed mainsail

The naval architect quoted here talked about fat headed mains - probably any cruising cat main built today , elliptical or square top would qualify.

Looking into what the naval architect is saying is interesting. Our boats usually, unless they are accelerating, have the rig forces equally and oppositely opposed by the hull forces. If this didn't happen we would fly away with the wind like a balloon.

Heavy catamarans with minikeels have an underwater configuration that is high drag. They cannot operate at high lift to drag ratios. There is a lot of tip losses, skin friction and the keel may not be the ideal NACA-like section. For a 3 metre long (chord) minikeel a 0009 NACA section (which is about as thin as most designers go) would be 270mm wide. I have never seen a keel this wide so the designers go to thinner, probably too thin to be efficient, keel sections.

This means that the underwater parts of a minkeel cat are in high drag mode. The sections cannot be then asked to operate at low angles of attack. The lift to drag co-efficient will be low.

The rig will produce forces that get exactly opposed by the hull and appendages. If we put on a rig that is very efficient and can operate at very narrow angles of attack you may think that you could point very high into the wind, like an Etchell or Laser. The problem is that rest of the boat has very high drag - the cabin, davits, high freeboard, lots of weight, immersed transoms etc. This means that the drag of the entire boat has been only marginally reduced but the lift of the rig (think power) has been reduced to get a rig that can sail at lower angles of attack.

Now because the underwater sections are inefficient you cannot sail high into the wind, even with efficient sails, without the underwater sections getting into a high drag configuration. You have to bear away and "get the boat going" or it just stops. A good cat designer will understand that the boat will be in "get going mode" most of the time and design a rig that suits. Not a tiny jib/large main rig that is seen in AC cats but one that has a larger jib. One that has a planform that is efficient at a high power (high lift) or low lift/drag configuration. This is what the naval architect was probably talking about.

If I design a 40ft racing cat with no accommodations the rig planform, sail shape and said design would be very different from a 4-5 times heavier minikeel cruiser. The racing cat will be operating at low angles of attack and at a high lift/drag co-efficient both above and below water. It will rarely be in "get going" mode but instead be in "pointing"mode. The cruiser will be operating at a totally different performance zone and its rig should be designed as such.

Of course minimising aerodynamic drag will be helpful for the minikeel cat, so efficient sails, maybe with an elliptical planform to reduce induced drag will help. But the rig should have a larger jib than the racer and the sails cut with deeper draft for higher lift (and more drag).

Whether a square top is best for cats is problematic as we don't have a definition of a square top and it is rather expensive to do the testing. In Australia we have a development class called the NS14 that has no restrictions on the rig other than sail area - it is 100 square foot and you could have it all in the main if you wanted. The attached photo shows the start of last years nationals titles. Close observation will show that there is a range of head designs - no predominance of square tops over combination square /elliptical. These boats operate at much high lift/drag ratios that most cruising cats and so rig drag reduction would have large effects in performance and if large square tops were faster then everyone would have one. I haven't used the example of A class or other cats as they operate at very different performance areas than our boats.

The rig design of the Seawind 1160 is probably something similar to what the naval architect was talking about with small jibs/ large mains. In this photo the ratio of jib size is smaller in the Seawind than the NS14, even though the NS 14 operates at higher lift to drag ratios which favour small jibs. Of course a small jib is nice to tack and our boats have to do a broad range of jobs whereas a racer like the NS14 has to do only one thing - win races.

So the naval architect is pretty much in line with current thinking. Still is is always nice to reduce drag any way you can.

cheers

Phil
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Old 01-01-2017, 16:42   #15
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Re: Fat-headed mainsail

Thanks for such an informative response.
I take it then that such boats as the Lagoon 39 or Lagoon 42, where the mast is located further aft with a subsequent decrease in main sail size while allowing for an increase in foresail size is more in line with his thinking?
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