Fat-headed mainsail

[Stumble]

Originally Posted by Kim Klaka

Hmm, I haven't explained myself well enough. I am not saying that reducing drag will have no effect on the system; quite the opposite, it will. As I said, the boat will indeed go faster. But the efficiency hasn't changed, the hull force vector can't swing any further forward so the aero force vector can't either. The rig will be operating below the peak of its efficiency curve. If you have a marine engineering background, it is rather like matching a prop to an engine - if they are not matched correctly the prop will not work at its best, regardless of whether it has a high maximum efficiency. That comparison/analogy breaks down quite quickly, so don't pursue it too far.

In that case I am not sure what you mean by efficiency.

For the same mast height square headed mains have lower drag and more power. When I use efficiency I mean the ability of the rig to covert kinetic force in the wind to propulsive power. Square headed sails convert more of the available kinetic power into propulsion for the same mast height or sail area than a pin headed sail does.

So to me they are more efficient. But if you measure it differently then under some situations I guess you could design one where the efficiency works out the same.


Continuing on with your prop analogy, by switching to a square headed sail you get a higher maximum speed and better mpg. So even if it has more slippage than the original it I should still a net gain.

[geoff326]

To cut to the chase in this argument isn't it about the most powerful sail that that will produce the best performance? Accordingly, if one argues that replacing a pinhead main with a square top is a wasted effort then isn't one making the assumption that the original main is already the most powerful sail for the best performance?

[Barra]

Originally Posted by geoff326

To cut to the chase in this argument isn't it about the most powerful sail that that will produce the best performance? Accordingly, if one argues that replacing a pinhead main with a square top is a wasted effort then isn't one making the assumption that the original main is already the most powerful sail for the best performance?

Interesting discussion this has turned into. I dont think anyone is arguing that replacing a pin head with a fat head is wasted rather that -

The limiting factor for efficiency on most cruising cats is the hydrodynamic drag from the hulls so while you may see some gains from a fat head main they will be limited by the hulls. Less performance designed hulls = less gain

Seems kind of obvious right?

[44'cruisingcat]

Originally Posted by Barra

Interesting discussion this has turned into. I dont think anyone is arguing that replacing a pin head with a fat head is wasted rather that -

The limiting factor for efficiency on most cruising cats is the hydrodynamic drag from the hulls so while you may see some gains from a fat head main they will be limited by the hulls. Less performance designed hulls = less gain

Seems kind of obvious right?

But given that most cat's aren't limited to hull speed, it could still produce worthwhile gains.

Anyway, I've found the square headed main to be much easier and more forgiving to use, so even if there were no performance benefit, (there is) there are other considerations.

[Barra]

Originally Posted by 44'cruisingcat

But given that most cat's aren't limited to hull speed, it could still produce worthwhile gains.

Anyway, I've found the square headed main to be much easier and more forgiving to use, so even if there were no performance benefit, (there is) there are other considerations.

Yes we have both seen better performance on our square tops but we have more performance oriented designs with more efficient hulls DBs etc. so the gain on these boats vs the lagoon 39 for example will be much greater - thats what Kim was getting at as I read it.

[Red Horse]

Would a square top main be of any benefit to my 38 foot Cross tri ?

[paxfish]

I only have half a season on mine, but I like how it attenuates the gusts.

General rule of thumb puts highest gusts at 1.3 to 1.5 times the average on inland water. That means a comfortable 14 knot windspeed will often have gusts in the 17 to 20 knot range. Which means I am overpowered.

During the gust, the top two battens fall away and the fathead attenuates the shock of the puff. As the gust subsides, the power comes back on. It does not flutter under pressure though. Just falls away and then comes back on.

I don't have a shot of it bending away, but I have seen it . Here is the profile of the sail on it's previous boat (center):

[jdazey]

Originally Posted by Red Horse

Would a square top main be of any benefit to my 38 foot Cross tri ?

From this thread, it would appear so, but would it be worth the cost and effort? Your Cross has a main hull l/b ratio of around 8:1, same as many condo cats. The amas are finer, and as the main hull starts to lift and the weight gets transferred, things get better. There are other mods that will make a bigger difference in performance.

Perhaps a thread on performance tuning of cruising tris would be in order.

Cheers,

[Red Horse]

My Cross was rigged as a ketch, took off the mizzen as it was always in the way.
Main mast still in original location. I found a drawing that shows the boom is too short by about 3 feet. Got a replacement boom from Minnies. So since I am going to replace the boom I need a new main. I have taken a bit of weight out. got rid of all the cabinet doors, took off all the 60's era formica. Shortened the keel and gave it a foil shape. A discussion on performance improvements would be helpful. Which is why I was wondering if the square top would be useful. Crossing from Cal to Hawaii I was able to do about 15 knots surfing. But boat suffers in light air. Plan to head to Tahiti and Fiji next year.
Rich

[Cavalier MK2]

Originally Posted by Red Horse

My Cross was rigged as a ketch, took off the mizzen as it was always in the way.
Main mast still in original location. I found a drawing that shows the boom is too short by about 3 feet. Got a replacement boom from Minnies. So since I am going to replace the boom I need a new main. I have taken a bit of weight out. got rid of all the cabinet doors, took off all the 60's era formica. Shortened the keel and gave it a foil shape. A discussion on performance improvements would be helpful. Which is why I was wondering if the square top would be useful. Crossing from Cal to Hawaii I was able to do about 15 knots surfing. But boat suffers in light air. Plan to head to Tahiti and Fiji next year.
Rich

The mast could come aft if it was ketch rigged. Some were rigged as a cutter with a compression post to the keel since there isn't a bulkhead to support the mast there. This lets you use bigger jibs without driving in the ama bows.

A fat head main with your current mast location and a longer boom would bring back area you lost with the mizzen and move the center of effort back and should help.

The biggest performance improvements to a cruising multihull are reducing weight, reducing windage and improving leeway resistance.

For weight look at what is stowed and start throwing things overboard unless they are really needed. The cabinetry is a big culprit, especially on home builds. Make sure it isn't heavier than spec and don't use things like cupboard doors or heavy teak trim. Bins and shelves with nets for the front in storms are both lighter and better at keeping your provisions in place, especially with a capsize.

A remodel while thinking along the lines of a airplane instead of decorating a house is a great place to start. Aloft replacing wire halyards with amsteel/dyneema as well as life lines will take weight out.

For reducing windage reduce deck clutter, solar panels on stilts are terrible, put them on deck. For shade something that can be removed is better, even at anchor it helps to have less pull in a storm. Dinghies on davits add weight in the wrong place and windage. etc...

For leeway improvement on a Cross I'd add either a daggerboard in the main hull or one in each ama. There is enough beef there to handle it and it is straightforward to add more if you need it. Then you could cut down the cruising keel and leave a grounding skeg. This will reduce the wetted surface and let you run shallower as well as dry out flatter. Of course then you'll need a kick up rudder.

My advice is to take it slow, lighten ship first and always make sure the bottom is clean as well as using a folding prop.

[tomfl]

Have to point out that a lot of posts seem to be limited to how a well designed square top performs in what I will call moderate to heavy winds (15kn+). While we all love to sail in say 15-20 knots a lot of the time I find I am sailing in 5-10knots (or even less). Just my two cents but since I am doing a lot of sailing in less than 10 knots my experience has been that a square top really excels there, as well as at higher wind speeds.

When I am at wind speeds that require reefing ease of reefing is more important than an extra knot or two of top end speed; in fact sometimes I would rather be going slower than hitting the cutting edge. But when I am doing four knots with a square top and the boats around me are doing three knots with a pin head I really have a big smile on my face.

[Cavalier MK2]

Back to Red Horse, a disadvantage of the big head main is losing a regular backstay. These masthead rigs are really driven by the jib and jibs like forestay tension. I have some Cross pdfs that show the cutter rig location on a couple of the bigger designs if you are interested. Some work though, compression post, new chainplate locations and new forestays and cut down back stay.

A mizzen in the light air adds area if you sail a bit free and the ability to add a mizzen staysail for more offwind area. Arthur Piver started the cruising ketch idea on tris, Rudy Choy on cats. Art's contention was that the main was a light air sail dropped first while you went on with the jib and mizzen. This allowed a full cut like a mono for more power. The mizzen needs to be flat as close hauled it is always in the closer angle coming off the main.

In light air you can add more area forward with a bigger full cut liight genoa for a cheap code 0 or a real one using a sprit. This is very effective but you need a wire luff on the jib to point.

Buying used while you try things out is a good way to go.

[geoff326]

Originally Posted by Barra

Interesting discussion this has turned into. I dont think anyone is arguing that replacing a pin head with a fat head is wasted rather that -

The limiting factor for efficiency on most cruising cats is the hydrodynamic drag from the hulls so while you may see some gains from a fat head main they will be limited by the hulls. Less performance designed hulls = less gain

Seems kind of obvious right?

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."
-Dr Kim Klaka,(naval architect), Ralph Newton.


On the contrary, I think it is you that has missed the obvious point.

From the above, Dr Klaka is indeed arguing that replacing a pinhead main with a flat-headed main is a wasted effort- the whole point of my original posting. Even when pressed re the situation with the Lagoon 39 or 42, where the flat-headed main is standard,he regards this as mostly a marketing exercise.

I suggest you read all his posts thoroughly, before commenting.

[Barra]

Originally Posted by geoff326

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."
-Dr Kim Klaka,(naval architect), Ralph Newton.


On the contrary, I think it is you that has missed the obvious point.

From the above, Dr Klaka is indeed arguing that replacing a pinhead main with a flat-headed main is a wasted effort- the whole point of my original posting. Even when pressed re the situation with the Lagoon 39 or 42, where the flat-headed main is standard,he regards this as mostly a marketing exercise.

I suggest you read all his posts thoroughly, before commenting.

No need to be rude geoff. i have read all the posts - thats why i said it was interesting. Kim very clearly elaborated on his view though subsequent posts so until you just chimed in it was interesting.

Now it seems you just want to convince yourself to buy a lagoon 42, go for it sport

[Just Another Sa]

Originally Posted by Kim Klaka

It's a bit difficult to explain without animated diagrams, but I'll have a go. Let's stop using the term drag as it is unhelpful in this context. The situation we have is that the aerodynamic force must exactly match the hydrodynamic force in both magnitude and direction.

In order to allow a sensible discussion, lets assume the former is done.
The statement in bold is an absolute fact, and thus needs no assumptions.

Originally Posted by Kim Klaka

The angle each vector makes to the yacht centreline is a direct measure of efficiency- the further forward it points, the more efficient it is (the thrust/sideforce ratio is the cotan of the angle).

This is the source of your troubles, you have only defined efficiency for aero part, and not done the same for hydro part, not in your text above and not even for yourself in thinking part.
For a generally accepted point of view, the more efficient hydro part would mean more sideforce for less hydro drag, ie more efficient hydro part means the total force points more to the side further away from the centerline of the boat.
Now if the aero part gets more efficient and aero and hydro part keep pointing to the opposite direction, the hydro part must point more towards back of the boat, meaning reduced hydro efficiency.
This is how efficient racing boats work.

The worst condomarans on the other hand have huge windage on platform and small inefficient sails in comparison. As a result the total aero force points more sideways and less forward than thos of a racing boat. I hope we agree it then has less efficient aero part. But the hydro force also points to the opposite direction, and hence more to the side than aft compared to the racing boat. That means that in contrary what you seem to think, the condomaran has much more efficient hydro part than the racing boat if using your definitions as I understood them.

For the more scientifically oriented that use lift and drag as the useful consepts, it should be obvious that the end result would be different. The reason for that is that the direction of apparent wind and therefore airflow related to the boat is in different direction than water flow related to the boat. Also those directions for fluid flows are different for the different boats, and you miss that out completely by your definitions of efficiency not related to flow direction at all.

Originally Posted by Kim Klaka

We can now ignore any talk about aero- and hydro-dynamics, or even yachts, as they are irrelevant; the problem is reduced to applied maths. We have two forces that must balance, and each one can only swing forward to a certain angle, representing its maximum efficiency. The one the can swing forward least (i.e. the lowest efficiency one) is what limits the angle of the other one. It can not swing further forward, because the forces would then go out of balance.

But the total hydro force is never pointing forward in any boat going to windward, it's pointing sideways and aft!!!
Are you now claiming the hydro efficiency is increased with pointing closer to centerline and therefore closer to directly aft?!?
A hovercraft without touching water and without anything producing lift has then the best efficiency, directly aft force and hence closest to centerline.
That seems totally absurd definition for me.

Originally Posted by Kim Klaka

So the efficiency of the system is governed by the least efficient force.
I don't think I am wrong, but if I am then please identify the flaw in this explanation.

I disagree with the first statement.
I believe the explanation you requested is above.