Square Top Main vs Roachy Main

[44'cruisingcat]

I think a real comparison would be hard to find. Even if someone did replace a normal full roach cat sail with a square top, it's unlikely they replaced a NEW full roach main with a square top. It's more likely on old flogged-out full roach main was replaced, so there would be a noticable performance improvement.

[drunknsailor]

A while ago I stumbled on a review of the same boat(a nautitech I believe) one with a square top one without. I believe the square bought .5kt in 10kts of wind. Unfortunately, I couldn't find it to provide a link.

[Bash]

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.....

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.)

[Bash]

(adding to the above, an analogy for those who can't get their brains around the aerodynamics.) Think of the fat-head main in terms of what a transom does for the boat's stern. Without a transom, the boat would come to a point, but all that weight behind the boat's missing transom would just be dead weight in a place where you least want hull weight. The pointy stern wouldn't add to waterline or buoyancy, although it might look nice. Same with the top several feet of a conventional main. It adds nothing to the effort of the sail. It just adds weight aloft in terms of an extra few feet of mast in the precise spot where you don't want extra weight.

[Bash]

(one last thought for troglodytes who are against all things modern just because they're modern) There's not much difference between the modern fat-head sail and the gaff rigs of old. A gaff rig squared off the mainsail, eliminating the inefficient tip of the sail. Unfortunately, the gaff itself involved a great deal of weight aloft as well as additional windage aloft from the additional tackle.

The fat-head main gives you all the advantages of the gaff rig of old, while eliminating the major disadvantages (weight aloft) as well as an entire extra level of rigging complexity.

[Bash]

(another last thought since I'm on a roll) Something that any skilled sail trimmer will notice after making the transition to a fat-head main is that the design isn't just about what's happening at the sail's head. Indeed, the entire shape of the sail is easier to control because of how the luff and leech separate at the head. Inducing or eliminating twist is far easier, which means the sail can adapt more to light or stiff breezes without having to flatten or reef the sail. This is especially valuable on a boat with a rigid vang.

In light air, the difference one is able to make by controlling twist could be better than the 10% increase being bandied about on this thread.

[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.....

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.)

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?

[Cheechako]

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.....

Just thinkin out loud. if you have no roach, battens become less necessary... I imagine for performance full battens would be better yet, but a lot of weight and trouble compared with none!

[waikikin]

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?

No nothing missed, that diagonal batten up top might now be known as the gaff & the fabric area forward of it as the gaff topsail......

[jonno]

Being the ex owner of a Seawind 1160 (as in the photo), and spending many hours looking up at that big roachy mainsail, I think most of the comments in this thread miss the point. Cruising cats rarely sail hard upwind, and when sailing off the breeze, whether its a square top or big roach, if the sail area is the same in each case you would get the same result. Into the wind you have a thick mast, wind indicator and in my case a television disk aerial throwing up all sorts of drag and turbulence, so you can forget about improvements to sail efficiency. The whole idea of these fat tops is to get more sail girth for more of the sail on a shorter mast, and doing this with either design makes perfect sense. In the 1160 photo the owner is driving the boat with max area and subsequent driving force but that force is as low as possible. A beautiful thing.

On another matter, my 1160 mainsail battens on my boat very quickly started to S-bend. As the the enormous strains on a catamaran mainsail load up the leach, and leach stretches, even marginally, the load lines move to the shortest distance route - a straight line, hence the mainsail started s-bending, with the leach just falling away. I am convinced this would happen with a square top as well. It would not stop me getting a roachy main or square top in my next cat, the benefits are too great, but just cautious in things like design and sail cloth material and weight.

[shomewo]

Beware assuming that a square top is fast cos' the racers use it. Open 60's now have a rig height limit, and as their main races are reaching ones, they will always want to cram as much SA as possible on. Same with Class 40's.

[thinwater]

The trouble with battens seems to include some misconceptions. I've had sails with no battens, leach battens, and full battens.

1. Wear on pocks and battens flying out is mostly confined to short battens. When the sail flogs the forces are large ant the inside end and bad things can happen. Full batten sails don't flog.

2. Full battens require batten cars. Only if the sail is over about 50' on the luff. Below that, slugs work just fine (I've had 2 full batten sails like that that lasted over 15 years).

3. Battens keep the sail shape fixed. No, not really. The battens can be changed in stiffness and taper (soft near luff, stiffer from midpoint to leach) to fit the usage and sail. Stiff battens with taper help with S-bending, but don't stop it. The leach can still stretch, the pocket can still creep aft. In light winds, they do restrict sail shape, but as it picks up, the cut determines the shape.
http://sail-delmarva.blogspot.com/20...and-slack.html

4. Yes, there is some wear at the inside and where the shrouds bear. However, these points are fixed and can easily be protected. Good inside caps and hard wearing cloth at a few key spots solves that. This is simply a part of good sail design; if it chafes, the sail maker goofed.

I'm interested in hearing more about the issue of increased cloth stress near the head. Two sail makers have mentioned ti to me, but I gather both of them were traditional and may not actually have expereince in this. I'm looking for a new main and would like a bit more kick. I also like the idea of moving the center of effort forward rather than more roach aft.

[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.

[Factor]

Originally Posted by 44'cruisingcat

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?

Nope - well not in this part of the world, have not seen a square top main without battens on a multi ever, and I think I have seen a good proportion of the larger Multis in Australia

[cwyckham]

To clear up one item, induced drag from tip vortices increases with lower speeds, not higher. It is proportional to the square of the coefficient of lift.

Also, the most efficient is an elliptical lift distribution, not an elliptical planform. There are many ways to achieve the former without the latter. A tapered lift distribution with square tips is only marginally less efficient than an elliptical distribution anyways.