Square Top Main vs Roachy Main

[Bash]

Originally Posted by 44'cruisingcat

Originally Posted by Cheechako
it seems with the sq top main you could get rid of much of the roach and related battens and issues on a cruising boat with about the same sail area.... I've always wondered what possible good that top 3 ft of sail with the poor shape hidden behind the mast could possibly be good for.....

A square top main lets you go without battens? Sorry, I've never seen that - every square top main I've seen has full length battens, and in fact usually one of them needs to be removed to pack the sail away.

Have I missed something?

You've misrepresented the original statement by Cheechako. Please be careful when editing quotes to which you're responding.

[44'cruisingcat]

Misinterpreted? Here's what was said, VERBATIM, not edited:

Originally Posted by Cheechako

it seems with the sq top main you could get rid of much of the roach and related battens and issues on a cruising boat with about the same sail area.... I've always wondered what possible good that top 3 ft of sail with the poor shape hidden behind the mast could possibly be good for.....

To me that reads that the poster believes a square top mainsail doesn't need battens. To which you answered:

Originally Posted by Bash

Exactly. Think in terms of weight aloft vs laminar flow. A fat-head main, especially if it's been designed into the rig from the start, allows one to improve the center of effort while reducing weight aloft.

What's not to love about that? (Excepting, of course, those cruisers who actually like going slow.)

Which to me reads that you agree - square top mains don't need battens.

Again, what have I missed here?

[Lagoon4us]

Originally Posted by S/V Illusion

Interesting reading. Particularly the guy who cited some real world experience sailing a J and his analysis of the comparison which was so easily dismissed by the internet experts, one of whom clearly has a self-esteem problem far exceeding his expertise and likes to 'bash' others.

I actually learned something here despite the petulant child behavior makes me discount his contributions.

Agree there people seem to love to push their point, it's interesting Lagoon have converted to these Square Heads on the 450 etc. I haven't had a 450 pass us, quite the contrary.

We are pricing a new main as ours is a bit lightly built but in good condition, i've asked the sailmaker to consider a Square Head on it however as our main furls inside the boom (Pro-furl) we may have issues with the diagonal batten wrapping around the furled sail,mmmmm?

Their newer boats have the mast moved way aft akin to an America's Cup cat.

Watching with interest here as well.

[Barra]

Well thanks to all that have contributed so far (except bash of course). Not sure that we have found any solid conclusions yet however -

1. From the limited anecdotal posts it seems there has been some improved performance noted from changeovers although they were not from already highly roached mains which is the point of the post. CC44 makes a good point in that there probably arent that many that would bother with a change from highly roached unless they are already due for replacement so a direct comparison would be difficult anyway

2. differentiation between upwind and off wind impacts is a good point with assertions that downwind its sail area that counts

3. Yet to have anyone come in on the actual increase to sail area (though thats going to have to come from a real world changeover or a sailmaker)

4.The improved aerodynamics are an interesting one. Seems to be consensus that the square top theoretically adds in this area at the top of the sail plan and also potentially in overall shape. Much of the discussion is still coming from a comparison to a pinhead top not high roached main though.

5. there are some good points made on practicalties on a cruising cat vs racing boat with need to remove top batten for stacking, additional wear points etc.

Found the same topic on another forum and one anecdotal post which was pretty interesting was from a charter operator running a few FP Oranas - 2 with charter version mid range roach mains and 1 with a squaretop. He claimed 10 to 15% better performance which is pretty exciting although there wasnt alot of detail on what conditions etc...

thanks

[Stumble]

Barra,

Without getting into scientific detail, and trying to just deal with it at a consumer level...

1) a square top main will increase performance significantly compared to even a large roach main. Assuming you don't downsize the square footage to significantly.

2) the effect is true both up and down, though for different reasons. Except for DDW sails still act as lifting bodies, and so the more efficient and larger shape will always win out over a smaller less efficient one.

3) the change in sail area really depends on the two sails in question. There isn't a direct conversion. For instance it is possible to envision a square top where the top is 90% of the foot, but the foot has been made much shorter, us the same or equal sail area as even a pin head with a longer foot.

This is exactly what is happening in a lot of OD classes that have sail area restrictions, but not shape restrictions. The mains are becoming more and more square, with shorter foot length.

4) the gains are not theoretical, they are measurable and quite large. The numbers indicate that about 70% of the entire wind drag of a sailboat is induced drag from tip vortices, not friction drag as many expect. Since the strength of a tip vortex is determined in part by the aspect ratio (the higher the ratio the smaller the vortex) it is pretty clear that significant drag improvement can be gained by going to a higher aspect sail. How much you will gain is just dependent on the shape of the original, versus the shape of the new one.

Since large roach mains are really just an intermediary between pin head and square top, the performance gain is likewise in between the two. But trying to quantify it requires a lot more information.

Basically think of it like this... I am planning on installing a new engine (sail) in my car. If I increase the horsepower by 50hp, how much faster will it be? Well from the available information it is impossible to tell. If I have a car with 750hp already the gain might be measurable, but not huge. On the other hand if I only have 250hp right now a 50hp jump would be massive.

Same thing here. Without knowing what the current sail design is asking "what's the performance gain" doesn't have enough information. It could be determined scientifically by fluid analysis, a good sail maker might have a good guess, but only if they know exacally what is being replaced and what is replacing it.

5) the top batton usually can be left in place if the sail is being stored on a boom. Either with a roller furler (just don't roll it all the way up), or if flaked.

[Barra]

Thanks Greg well explained

[Southern Star]

I suggest that care must be taken in assessing some of the potential advantages to a flat-top main as they are not necessarily complimentary:

1. While Bash is correct that a flat-top can be used to lower the Ce of the sailplan (and lower is better for transverse stability on a cat), that is only the case if the sail area is not increased over the original big roached main - i.e., if the flat-top has the same length of luff, the Ce of the new sail will actually be higher, thereby reducing transverse stability.

2. Only if the sail area is increased as a result of the flat-top design (and the SA is correspondingly increased), will there be any real improvement in peformance off the wind.

3. Against this backdrop one must also consider the following:
- a replacement flat-top main is apt to be more expensive
- a flat-top main will be more difficult to stow and will likely require a new (and larger) boom bag
- reefing a flat-top main tends to keep the Ce of the sailplan higher unless the reef points are also adjusted downwards. While the sailmaker should take this into account in the design of the sail, it will also require a change in the attachment points on the boom for the reefed clew.

Even bearing in mind that the potential performance gains will be largely upwind, one must understand that to optimize these gains one will also be moving the Ce of the sailplan higher than the original designer intended. At the very least, this will require additional vigilence with respect to reefing.

Brad

[Dave]

Originally Posted by Barra

Well thanks to all that have contributed so far (except bash of course). Not sure that we have found any solid conclusions yet however -

1. From the limited anecdotal posts it seems there has been some improved performance noted from changeovers although they were not from already highly roached mains which is the point of the post. CC44 makes a good point in that there probably arent that many that would bother with a change from highly roached unless they are already due for replacement so a direct comparison would be difficult anyway

2. differentiation between upwind and off wind impacts is a good point with assertions that downwind its sail area that counts

3. Yet to have anyone come in on the actual increase to sail area (though thats going to have to come from a real world changeover or a sailmaker)

4.The improved aerodynamics are an interesting one. Seems to be consensus that the square top theoretically adds in this area at the top of the sail plan and also potentially in overall shape. Much of the discussion is still coming from a comparison to a pinhead top not high roached main though.

5. there are some good points made on practicalties on a cruising cat vs racing boat with need to remove top batten for stacking, additional wear points etc.

Found the same topic on another forum and one anecdotal post which was pretty interesting was from a charter operator running a few FP Oranas - 2 with charter version mid range roach mains and 1 with a squaretop. He claimed 10 to 15% better performance which is pretty exciting although there wasnt alot of detail on what conditions etc...

thanks

I know this is an old thread but it seems to me an important piece has been missed.

Roachy mains are about increasing sail area. Square tops are about increasing dynamic twist. Hugh difference !

The square top sail is designed to twist off by itself as the load increases. Higher wind speeds have higher gradients so the added twist results in maintained trim and lower CE.

Racers like them because they can carry the same sail area into higher winds. More horsepower, not per square foot, but in a greater usable range with less tweeking to maintain trim.

Cruising boats, and cats in particular, benefit from a more forgiving sail particularly in gusty conditions. The twist dumps wind high up where the release is needed most. It can be argued that a square top main is safer on a cat due to its more dynamic shape.

Dave L38 #38

[Lagoon4us]

That's absolutely correct, the apparent wind is totally different at that height, just not sure cruisers are enjoying the benefits.

Frank

[davecalvert]

We have been experimenting with square top mains sense the early 90s.These are now standard on any race boat that allows them. Main advantages besides the obvious extra sail area is, improved flow at the top, better twist in the upper leech, moving the power lower in the puffs, as the top twists off, and increased performance on all points of sail in the light to moderate winds.Even with the increase in sail area, these mains can often be used in as much, or more, winds before reefing than the pin head mains.I do not think these are good on cruising boats though. We design "elliptical" head mains with the same sail area but, without the diagonal, "gaff" batten. This allows the sail to be raised and lowered without the need to remove the head of the sail from the head slide assembly.The performance is about the same, at least for cruising boats.Using cross cut dacron for large square top or wide top mains is not a very good way to go. Best to use more performance materials in a triradial cut or load path constructions, for multihulls in the 40 ft and larger range.There is now a warp dacron in styles up to 12 oz. This cloth, in a triradial cut, makes a great performance cruising sail with the durability of dacron.

[foolishsailor]

Square top mains arent really about twist. They are, as Bash tried to explain, about Tip Vortex.

Tip Vortex is not just about lost of efficiency due to disturbed air from turbulence above the sail. The tip vortex affects airflow over almost the whole main AND head sail.

The low pressure created at the end of a triangle airfoil pulls the laminated flow across the surface of the foil towards the tip. On a sail this can mean that the air flowing over a head sail and main near their heads can be over 30degrees above horizontal.

This not only decreases efficiency but also changes the angle of attack further exacerbating the issue of wind shear at altitude above the surface of the water which is why twist is so important in sails. In addition to the natural altitude lift induced by surface friction and altitude.

By maximizing the edge of an airfoil you not only decrease energy loss due to turbulence associated with the tip vortex but you also dramatically reduce the altered airflow across the foil that feed into the low pressure of the vortex and increase the edficiency of the entire mainsail and head sail.

In terms of stowage for cruising boats:

There are detachable track car systems that allow you to lower your main to the headboard and detach the last car of the headboard to lay it in a lazy bag.

[foolishsailor]

Also,

Just to emphasize the power of just increasing the head size...

One of the single largest IRC racing handicap for sailplan changes comes from changing the size of the head of the main...

...for good reason

[yeloya]

I've sailed extensively with both sails on catamarans. Actually I even sailed the same cat with this two different type of sails in all kind of weather in every angle to the wind. There absolutely is NO difference ...
I am not interested in what the theory suggests but this is my conclusion after 20.000 nm of sailing on cat, both coastal and offshore.

There are a lot of difference beetween a good sail and not so good sail but a main sail is not necessarily better because the cut of its head is this or the other way..

The square head is more difficult to hoist. One needs to go to the head of the mast to hold and pull the head of the sail to attach it to the head of the sail. In most of the cases you need two people to do that in big main sails.

Durability is about the same (app. 6-7 years in charter, 20-25 weeks of charter per year)

Cheers

Yeloya

[Dave]

" Main advantages besides the obvious extra sail area is, improved flow at the top, better twist in the upper leech, moving the power lower in the puffs, as the top twists off, and increased performance on all points of sail in the light to moderate winds. Even with the increase in sail area, these mains can often be used in as much, or more, winds before reefing than the pin head mains."

Why do you think these benefits are not good on a cruising boat ?

"We design "elliptical" head mains with the same sail area but, without the diagonal, "gaff" batten. " , "The performance is about the same, at least for cruising boats."

I'm not sure I understand. Do the "elliptical" head sails you mention have the same benefits you outline at the start ? or are you saying those benefits are less important on a cruising boat. ?

Dave L38 #38

[monte]

re: stowage, our lagoon 380 has a system that allows it to stack flat into the lazy bag, no extra effort required than a standard sail.
re: comparison to standard sails, my only experience is when we sailed along the coast of spain in around 15k true on a broad reach and an older 380 with standard main was sailing next to us. Boat speed was about the same for an hour or so (I couldn't shake them, I WAS trying!) till I cheated and hoisted the Gennaker. Boat speed went from 7 to 8 knots. I don't know if we were more heavily loaded or less and can't comment on different points of sail.