Fat-headed mainsail

[catsketcher]

Gday Geoff

This may be part of the reason but I would predict that the accommodations place the mast in a particular place and the designer works from there. You need the mast to be on a massive bulkhead (although some nice boats get around this).

I once read an interesting article about rig design for the AC mono boats - from 1990 to about 2006. In it the designers did a heap of work calculating how much time was spent doing different angles of sail and weighting each aspect. For them the start was really important as was height to windward. The first boat to the first mark won the race about 70 percent of the time so getting to the first mark first was vital for the rig design.

For our boats I would suggest that light air upwind performance is of little interest to most users, most of us motor in very light conditions. Our rigs have to do well off wind and this is where my pet bug bear about most rigs comes to light. A small jib and big main will tend to make the boat want to round up, especially if you can't ease the main because the designer didn't draw a wide traveller and you want to stop the main from chafing on the shrouds. You really need a big foresail to help pull the nose away downwind - which is why a screecher is so good for performance and handling.

So to work out the proper rig for a cat we need to do a proper analysis of how we will use the boat and then design the rig. My thoughts are that you want one that does well in tradewind conditions upwind and downwind first and foremost. For me this is a smaller main than often drawn with a cutter jib and a screecher.

A little story to show this. One Whitbread mono around the world race had a change in course from previous years. Careful analysis of the winds showed more reaching would be involved. Peter Blake had Farr design him a large ketch rig that was much faster on close and broad reaching courses. Steinlager won the race and they had very close racing with another ketch boat - Fischer and Paykel. When the course was changed back the ketches were relegated to history.

There is not one perfect rig - square top or otherwise.

cheers

Phil

[Kim Klaka]

As the naval architect in the originally quoted article that started this thread, I thought I'd better explain my heavily self-attenuated statement: " 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."


Let us keep things simple by ignoring any roll or yaw moment effects. I will also make things even simpler by defining forces in terms of thrust and sideforce rather than lift and drag i.e. vectors in-line with and perpendicular to the yacht centreline, instead of in-line with and perpendicular to the fluid flow.
For sailing in a quasi-steady state (i.e. not accelerating or decelerating), the aerodynamic thrust from the sails must be equal and opposite to the hydrodynamic resistance of the underwater bits (Newton's laws). Similarly, the aerodynamic sideforce from the sails will be equal and opposite to the hydrodynamic sideforce from the hull. If we then make a small jump in terminology and call the ratio of thrust to sideforce the "lift:drag ratio" (not exactly correct, maybe this is what caused the confusion), then that ratio must be exactly the same for the aerodynamic forces as it is for the hydrodynamic forces - otherwise the forces are out of balance and the boat accelerates or decelerates until they are back in balance. It follows that, if the two lift:drag ratios must be the same, then they will only ever get as high as the lower of the two.
So if you put a high efficiency rig on a low efficiency hull, you are limited by the best the hull can do, regardless of the rig efficiency.
Clearly I have oversimplified things somewhat, but you have to start with the basics before you add the nuances.
I consider the fat-head mainsail and tiny headsail rig as high efficiency because it approaches an elliptic loading on the main, and the jib is very high aspect ratio. Happy to be challenged on that though.
Yes, there are other characteristics of a fat-headed main worth considering. One that has been mentioned in this thread is the ability to twist off in the gusts. This can also be achieved by having a laterally tapered topmast or, more simply, using less vang and a low mainsheet tension (with the traveller up to compensate).
The discussion about fat top main area distribution in relation to triangular mainsails slightly misses my point: why not put the fat-top sail area into the tiny high aspect ratio jib? it makes the jib a lot easier to set (they twist off horribly when you ease the sheet), and easier to control - furling a jib is usually easier than reefing a main.
A comment was made that there is more than just windward sailing. Yes indeed, and there are pros and cons in having a fat top mainsail downwind. In my experience the cons slightly outweigh the pros, but others will doubtless disagree.
In summary, I believe the flat top main/high aspect jib is yet another racing feature transferred to cruising yachts more for marketing than effectiveness in most instances. (and don't get me started on asymmetrics and prodders on displacement cruisers!). Now, just give me a couple of minutes to put my flak jacket on.... :-)

[Dockhead]

Originally Posted by Kim Klaka

As the naval architect in the originally quoted article that started this thread, I thought I'd better explain my heavily self-attenuated statement: " 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."


Let us keep things simple by ignoring any roll or yaw moment effects. I will also make things even simpler by defining forces in terms of thrust and sideforce rather than lift and drag i.e. vectors in-line with and perpendicular to the yacht centreline, instead of in-line with and perpendicular to the fluid flow.
For sailing in a quasi-steady state (i.e. not accelerating or decelerating), the aerodynamic thrust from the sails must be equal and opposite to the hydrodynamic resistance of the underwater bits (Newton's laws). Similarly, the aerodynamic sideforce from the sails will be equal and opposite to the hydrodynamic sideforce from the hull. If we then make a small jump in terminology and call the ratio of thrust to sideforce the "lift:drag ratio" (not exactly correct, maybe this is what caused the confusion), then that ratio must be exactly the same for the aerodynamic forces as it is for the hydrodynamic forces - otherwise the forces are out of balance and the boat accelerates or decelerates until they are back in balance. It follows that, if the two lift:drag ratios must be the same, then they will only ever get as high as the lower of the two.
So if you put a high efficiency rig on a low efficiency hull, you are limited by the best the hull can do, regardless of the rig efficiency.
Clearly I have oversimplified things somewhat, but you have to start with the basics before you add the nuances.
I consider the fat-head mainsail and tiny headsail rig as high efficiency because it approaches an elliptic loading on the main, and the jib is very high aspect ratio. Happy to be challenged on that though.
Yes, there are other characteristics of a fat-headed main worth considering. One that has been mentioned in this thread is the ability to twist off in the gusts. This can also be achieved by having a laterally tapered topmast or, more simply, using less vang and a low mainsheet tension (with the traveller up to compensate).
The discussion about fat top main area distribution in relation to triangular mainsails slightly misses my point: why not put the fat-top sail area into the tiny high aspect ratio jib? it makes the jib a lot easier to set (they twist off horribly when you ease the sheet), and easier to control - furling a jib is usually easier than reefing a main.
A comment was made that there is more than just windward sailing. Yes indeed, and there are pros and cons in having a fat top mainsail downwind. In my experience the cons slightly outweigh the pros, but others will doubtless disagree.
In summary, I believe the flat top main/high aspect jib is yet another racing feature transferred to cruising yachts more for marketing than effectiveness in most instances. (and don't get me started on asymmetrics and prodders on displacement cruisers!). Now, just give me a couple of minutes to put my flak jacket on.... :-)

I am not an aero engineer, but I think that the difference between lift and drag is not just thrust vs sideforce. This strikes me as a simplification too far, way too far. Drag acts on a close-hauled boat not sideways; nor does lift act exactly forward, and this is quite essential, it seems to me. For example, the force of lift on a sail of a close-hauled boat also adds to leeway, just like drag does. The difference is that this force vector also produces thrust, which can be turned into lift by the keel which counteracts leeway. Whereas drag counteracts thrust and reduces lift of the keel.

[Factor]

Originally Posted by Kim Klaka

more simply, using less vang

No need for a flack jacket, but I do note that maybe one in 100 cats I have sailed have a vang?

[SVNeko]

Originally Posted by Kim Klaka

So if you put a high efficiency rig on a low efficiency hull, you are limited by the best the hull can do, regardless of the rig efficiency.

This is essentially what Catsketcher said.

[mikereed100]

Originally Posted by Kim Klaka

If we then make a small jump in terminology and call the ratio of thrust to sideforce the "lift:drag ratio" (not exactly correct, maybe this is what caused the confusion), then that ratio must be exactly the same for the aerodynamic forces as it is for the hydrodynamic forces - otherwise the forces are out of balance and the boat accelerates or decelerates until they are back in balance. It follows that, if the two lift:drag ratios must be the same, then they will only ever get as high as the lower of the two.

So if you put a high efficiency rig on a low efficiency hull, you are limited by the best the hull can do, regardless of the rig efficiency.

The second statement seems to contradict the first. According to your first paragraph above, the forces will seek balance, thereby increasing speed. Not as much as a more efficient hull, but an increase nonetheless. Don't slower boats need all the help they can get?

The discussion about fat top main area distribution in relation to triangular mainsails slightly misses my point: why not put the fat-top sail area into the tiny high aspect ratio jib? it makes the jib a lot easier to set (they twist off horribly when you ease the sheet), and easier to control - furling a jib is usually easier than reefing a main.

Unless you have added battens to it in order to create a 'fat-top', then roller furling can no longer be used.

Now that you've got your flak jacket on....

[geoff326]

In summary, I believe the flat top main/high aspect jib is yet another racing feature transferred to cruising yachts more for marketing than effectiveness in most instances


Kim,

How about the Lagoon 39 or the Lagoon 42, where a square top main apparently is standard-are Lagoon simply doing this for marketing reasons?

[django37]

Aspect ratio and areas of minikeels differ quite a lot on different cruising cats. The Lagoon 380 has some of the shortest and deepest minikeels. But mainly the heavy cruising cats lacks sailarea, therefor a squaretop mainsail is an easy way to ad sailarea.

[Kim Klaka]

Thank you all for giving this topic your time and thought, I am seem to have come off relatively unscathed so far. My replies:
To Dockhead, re using thrust/sideforce is a poor surrogate for lift:drag. Really it's the other way round. A quite good way of defining efficiency is the useful force generated, divided by the unwanted force generated. This equates exactly with thrust/sideforce. Lift/drag is only an approximation to it because each component contains both useful and unwanted force as a consequence of the direction along which the vectors are defined.
I am not concluding that you don't need to reduce drag on a rig, there's a whole bunch of other algebra and physics to look into there; I am pointing out that, for a given sailing condition, the rig cannot operate at a higher efficiency than the hull (assuming the hull is the lower efficiency) It is simply impossible for the hull to operate at a higher efficiency than its maximum achievable value, regardless of the maximum efficiency of the rig.
to Factor re vangs. Yes, very few cats have vangs. Partly because they are wide enough to have effective travellers
To Geoff 326 re are Lagoon 39 and 42 supplied with fat head mains just for marketing. I have to be careful what I say here, as both boats are designed by two of my former students ;-) . Marketing isn't the only reason, but I think it contributes a large part, playing to the notion that a cat is a sporty beast if it looks sporty (well, the rig anyway). Cruising cats are not fast, as shown by the analysis of full scale voyage data in my article that kicked off this thread ( Cruising Helmsman June 2015). You can read a variant of it at https://www.fsc.com.au/wp-content/up...20monohull.pdf

[Stumble]

Originally Posted by Kim Klaka

Thank you all for giving this topic your time and thought, I am seem to have come off relatively unscathed so far. My replies:
To Dockhead, re using thrust/sideforce is a poor surrogate for lift:drag. Really it's the other way round. A quite good way of defining efficiency is the useful force generated, divided by the unwanted force generated. This equates exactly with thrust/sideforce. Lift/drag is only an approximation to it because each component contains both useful and unwanted force as a consequence of the direction along which the vectors are defined.
I am not concluding that you don't need to reduce drag on a rig, there's a whole bunch of other algebra and physics to look into there; I am pointing out that, for a given sailing condition, the rig cannot operate at a higher efficiency than the hull (assuming the hull is the lower efficiency) It is simply impossible for the hull to operate at a higher efficiency than its maximum achievable value, regardless of the maximum efficiency of the rig.
to Factor re vangs. Yes, very few cats have vangs. Partly because they are wide enough to have effective travellers
To Geoff 326 re are Lagoon 39 and 42 supplied with fat head mains just for marketing. I have to be careful what I say here, as both boats are designed by two of my former students ;-) . Marketing isn't the only reason, but I think it contributes a large part, playing to the notion that a cat is a sporty beast if it looks sporty (well, the rig anyway). Cruising cats are not fast, as shown by the analysis of full scale voyage data in my article that kicked off this thread ( Cruising Helmsman June 2015). You can read a variant of it at https://www.fsc.com.au/wp-content/up...20monohull.pdf

Kim,

I assume you are trying to simplify for the forum, and I appreciate that. But I have to object to the idea that the rig cantbe any more efficient than the hull. While it's absolutly true that a boat as a system is limited by the worst component it doesn't therefore follow than ever subsystem can't be more efficient than another system.

So for our rig/hull, a rig that develops less drag, from whatever design change, results in less drag to the entire system. It maybe true that the boat won't speed up much, or point much higher, but that's a quantitative issue. If it were possible to design a dragless rig I think would have to effect the sailing characteristics of the boat.

For a normal cruising cat we may be able to state that in above 12kn of breeze the difference is meaningless in practice because the hulls are generating so much drag on their own the rig drag is overpowered. But that isn't the same thing as saying the rig is limited by the hull.


Since all the research I have seen indicates a substantial drag reduction from moving to a square headed main, I find it difficulty to accept that adding one to a cruising cat has no effect. I might be willing to agree that relative Tom the boats performance it doesn't matter much, but not that it has no effect.

Perhaps a more clear cut example would be switching to a rotating mast. Drag goes down, laminare flow improves, and while I haven't seen any VPP plots for a cruising catwith a rotating rig, my guess is that's you would see a reasonable improvement in race times.

[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. 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). 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. So the efficiency of the system is governed by the least efficient force.
So how come reducing aerodynamic drag is good? Again, lets not talk about lift or drag, let's consider a reduction in windage (holding the boat back) as being an increase in available thrust (pushing the boat forward). If thrust increases then the boat accelerates until the hydrodynamic resistance increases and once again matches the opposing thrust. In the process, the hydro sideforce will change and the aero sideforce will change to balance it. The net result is that the boat is now operating at a different sailing condition - it is going faster - but the efficiency is still constrained by the hull efficiency. There is an iteration here, as the maximum efficiency of both the hull and the rig will change slightly as boat speed changes (also as leeway, app wind angle, windspeed etc. change). However, these are second order effects, and are most unlikely to change the pecking order of efficiency (in our example, number 1 is the rig, number 2 is the hull).
I don't think I am wrong, but if I am then please identify the flaw in this explanation.

[Stumble]

Kim,

I like to argue by analogy, so here goes.

If you are correct that reducing drag from the hull will have no effect on the system, then adding drag to it should have no effect either. At least within some reason we should be able to intentionally degrade the rig and it wouldn't matter, at least until such a time that we have degraded it past the efficiency of the hull. But this is clearly absurd. Just like dragging a bucket thru the water will slow you down dragging an air brake thru the air will slow you down.

So then the question is, in an absolute sense does switching to a more elliptical shaped sail versus a pin head reduce drag? And the wind tunnel tests (and real world tests) all indicate that even though going to a square headed sail increases skin friction, the net effect is a reduction in total drag due to minimizing vortex creation. At the same time a square head sail adds sail area.

In other words we decrease drag and increase the force vector at the same time. About the only negative effect is increased weight aloft, which given modern materials is going to have a minimal effect on a cruising boat.

It deals with keels and rudders not sails but as you pointed out they are all the same thing, just with different working fluids. https://www.ericwsponberg.com/wp-con...der-design.pdf

[geoff326]

Re: Fat-headed mainsail
Quote:
Originally Posted by Kim Klaka
Thank you all for giving this topic your time and thought, I am seem to have come off relatively unscathed so far. My replies:
To Dockhead, re using thrust/sideforce is a poor surrogate for lift:drag. Really it's the other way round. A quite good way of defining efficiency is the useful force generated, divided by the unwanted force generated. This equates exactly with thrust/sideforce. Lift/drag is only an approximation to it because each component contains both useful and unwanted force as a consequence of the direction along which the vectors are defined.
I am not concluding that you don't need to reduce drag on a rig, there's a whole bunch of other algebra and physics to look into there; I am pointing out that, for a given sailing condition, the rig cannot operate at a higher efficiency than the hull (assuming the hull is the lower efficiency) It is simply impossible for the hull to operate at a higher efficiency than its maximum achievable value, regardless of the maximum efficiency of the rig.
to Factor re vangs. Yes, very few cats have vangs. Partly because they are wide enough to have effective travellers
To Geoff 326 re are Lagoon 39 and 42 supplied with fat head mains just for marketing. I have to be careful what I say here, as both boats are designed by two of my former students ;-) . Marketing isn't the only reason, but I think it contributes a large part, playing to the notion that a cat is a sporty beast if it looks

Kim,

I kind of see where you're coming from whereby the aerodynamic forces have to balance the hydrodynamic ones. However, in the case of the Lagoons 39 & 42, I'm mystified. They both have short booms, so in order to get a reasonable sized main they have little choice but to go 'square topped.'

[Kim Klaka]

Originally Posted by geoff326

Re: Fat-headed mainsail
Quote:
Originally Posted by Kim Klaka

Kim,

I kind of see where you're coming from whereby the aerodynamic forces have to balance the hydrodynamic ones. However, in the case of the Lagoons 39 & 42, I'm mystified. They both have short booms, so in order to get a reasonable sized main they have little choice but to go 'square topped.'

Well yes, if the designer elects to limit mast height and boom length, and wants to keep the jib self-tacking, then putting a fat head on the main is probably the only option available for getting enough sail area. As an aside, I thought i'd try putting some numbers to those dimensions but none of the brochures etc. seem to have a sail plan. Says something about their target market??

[Kim Klaka]

Originally Posted by Stumble

Kim,

I like to argue by analogy, so here goes.

If you are correct that reducing drag from the hull will have no effect on the system, then adding drag to it should have no effect either.

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.