Square top Mainsails

[rnlock]

Unless you taper the mast severely, which probably wouldn't work with stays, the "airfoil" near the top of a triangular sail will be awful. A really big lump up front, and a short, thin section in its wake. Furthermore, if you try to get an elliptical lift distribution, either the rest of the sail will be loafing or that skinny area up top will be stalled. On a triangular wing, the pointy tips will experience an upwash, increasing the angle of attack and making them stall. I suppose a triangular sail that's set right will have a fair amount of twist and may be able to evade this particular problem. A square topped sail wouldn't have to twist as much, and could have something like an elliptical lift distribution. Plus, the mast could be a little shorter and have less windage. I admit I don't know just how they rig those square tops. Presumably not quite like a gaff rig.

Seems to me that if a cruiser wants to make things simpler, a free standing rig makes a lot of sense, particularly when we can make one out of carbon fiber. You could say that would be more expensive, but is it more expensive than all the hardware and stainless wire that aren't needed any more? If there's some gadget to grab the halyard and hold it, then the mast can be skinnier still, because it doesn't have to withstand both halyard tension and the downhaul force on the sail. I understand that there ARE such gadgets, I just don't know how they work.

 

[Paul Scott]

There was a two boat sail test between same model Saphire sailboats, one with a squarehead, and the other with a pinhead, if you can find it- I can’t, but it’s been some time. Anyway, IIRR, it was very specific about the differences between the different sails, as far as performance and sailing technique, among other things. One was not totally superior to the other, it was more about sailing style.

 

[skaraborgcraft]

There are halyard locks. But excuse my ignorance, if tension is put into the sail via a halyard and a down haul, i see little difference in the compression load on the mast even if it was fixed at the head, to gain the same luff tension.

Short headed gaffs as used on many Dutch boats and some wharram designs are a good compromise, i think, between a long gaff or none at all. There is always more breeze at the masthead for any given height, hence topsails.

 

[wet feet]

There are halyard locks. But excuse my ignorance, if tension is put into the sail via a halyard and a down haul, i see little difference in the compression load on the mast even if it was fixed at the head, to gain the same luff tension.

As I understand it,the justification for halyard locks is to reduce the compression by taking away the mechanical advantage given by the sheave at the masthead leading to the length of halyard coming down the mast.Put simply if there is a load of T in the halyard and it goes from the head of the sail to the sheave and then down,you have a force of 2T applied to the sheave pin.By using the lock half of the load is eliminated.This stands whether a downhaul applies force at the tack or not.

 

[skaraborgcraft]

For sure i understand the tension on the sheave pin, but that would not neccesarily mean the mast needs to be bigger, or can be reduced?

Given the same luff tension on the sail, either by combination of halyard/downhaul or locked and downhaul, surely the compression loading on the mast is the same ? I make the assumption that though the load will be higher on a top block, if the halyard is a non stretch line, what difference if it is made fast to the head compared to a winch at the base? Either way, luff tension is winched on puts the spar into compression......as i understand it.

 

[wet feet]

As you understand that the sheave pin sees more load,where does that load get transmitted to,if not the mast?The 2:1 purchase if you don't use a lock does more compressing unless the lock is fitted at a location after the halyard passes over the sheave.In which case it makes no difference.

 

[skaraborgcraft]

Way i see it, if the halyard comes down to a cleat at the base of the mast, and the luff is tensioned at the tack, if a load cell was placed before the cleat on the halyard, then that is showing load " X" for a certain luff tension.

To achieve the same luff tension via a tack downhaul, with the head fixed at the mast, i would have assumed same compression on the mast column.

Whether the head is on a halyard or a hook, i do not grok why , for the same luff tension ,would give different mast loads. I do not see a single masthead sheath being a 2:1 advantage......maybe that is where we differ on load? Sorry if im really dense.

 

[wet feet]

Does it help to consider that the load X applied to the lock at the masthead imposes a compressive load of X on the mast and that if the same load is applied to a halyard passing over a sheave at the masthead also imposes a load of X on the halyard that runs down the mast?then if we add together the two forces on the sheave we arrive at 2X.

 

[skaraborgcraft]

Thanks for perservering, im not quite there yet. For simplicity sake:
Sail luff is "hooked" to the masthead, and the luff is tensioned to say 100kg. Mast is in compression, understood.
Sail is hoisted over a single sheath and cleated at mast base, then tensioned to same 100kg. You suggest the forces are doubled.
Lifting 100kg via a single block, is still lifting 100kg, im using the 100kg lift as luff tension.
Therefore, the load on the sheath pin or lock, (or at the cleat) in my mind, will be the same 100kg load, and so same mast compression.
How does having a halyard cleated at the mast base via a single sheave change that?

Should i be thinking off the luff as a bow string on a archers bow, in that forces on the halyard/luff, is the same forces on each end of the bow/mast?

 

[Blueknarr]

Understanding how loads are multiplied and divided by blocks and purchases systems can be tricky.

As the sail is raised the tension on the halyard and luff is equal. Eventually 100kg on halyard and 100kg on luff. Since both loads are on the same axis of the mast they are added equally as compression to the mast.
If the sail is locked at the masthead and the halyard released then the compression load of the halyard is zero. Leaving the mast under just the 100kg compression of the luff.

Your archery example is very complicated because the loads are at odd angles to each other. Trigonometric calculations need to be used to determine how the loading changes as the angle between arrow and string change

 

[skaraborgcraft]

. Eventually 100kg on halyard and 100kg on luff. Since both loads are on the same axis of the mast they are added equally as compression to the mast.
If the sail is locked at the masthead and the halyard released then the compression load of the halyard is zero. Leaving the mast under just the 100kg compression of the luff.

That makes more sense. The release of the halyard once the head is hooked, twigged. I realised after i posted that the bow string analogy was not apt for this situation.

There is a reason i use excepted mast plans for the spars i have built, and not designed from scratch. Lotta stuff going on in a mast depending how its set and rigged.

Thanks for the explanation.

 

[Blueknarr]

I'm glad I don't have to try to explain the trig formula. My math is way too rusty.

Wet feet.
The purpose of masthead sail locks is not to reduce mast compression loads. Halyard loads are typically insignificant to the mast. The compression from the stays is usually ten or more times what a halyard would be. Even an unstated mast of sufficient stiffness will easily handle halyard loads.

The first generation of synthetic fiber rope was stretchy. When a puff hit a racing boat all the lines would stretch before the boat would accelerate. Wire rope with fiber tales became popular in the 1960s. This solved the stretch but was heavy aloft, but the wire chuffed the mast witch then chuffed the dacron. Halyard lifespans where short. Masthead sail locks developed in the70s eleminated the need for thewire/dacron halyard. Now there's low stretch fiber that weight less than the locks.

 

[wet feet]

I completely agree about the compression loads from the rigging being much greater than the force the halyard can impose-if the boat has standing rigging.The only boat I have owned with a halyard lock was a Finn and the mast in a Finn is unstayed,hence no force from that which is not there.In the case of other dinghies,the mast taper usually begins a little above the height of the hounds and this thinner section only sees compression loads from the halyard.The induced bend from halyard tension can be quite considerable and will be affected by shroud tension and the amount of spreader deflection the rig is intended to sail with.In boats with keels and much more substantial mast sections there is almost no discernible effect by comparison.For the record,my own main halyard is cleated at deck level and is made of dyneema.Releasing it is quick and effective and I wouldn't have the same faith in a halyard lock.We have strayed a long way from square top sails by now.

 

[skaraborgcraft]

.We have strayed a long way from square top sails by now.

My bad, sorry for the thread drift. Heres my fat-head mizzen, utilizing an in-pocket-gaff.

It gives more area for a lower mast height, a lower CoE was also a goal.

 

[The Q]

Interesting discussions, especially about square tops moving the C of E upwards, I'd say it also moves the C of E forwards.
It's something I've spent some time studying... Because....

In my boats case, the hull designed is deliberately narrow, originally it had a gaff rig that was quite heavy. Then I got given a aluminium mast, boom and sail from another boat (Kestrel), roughly the right sail area.

After experiments and a seasons sailing, the mast ended up raked a long way aft.

So with a narrow hull as well she needed to have the C of E moved down and aft.
The kestrel sail shape while not square top, is an old full top batten type, so most of the way there.

Then I was informed the original Kestrel sail was used on another class (Wayfarer) as a reduced main.
On research it was discovered the Wayfarer boom was 10 ft not 8ft 6inches, and their main pin top, not near square top.
So a sail from that class has been bought, now just fishing around trying to find a second hand metal boom from a wayfarer. If one isn't found soon I'll have buy new due to lead times for next season..

Anyway I agree a square top sail is more efficient for a certain mast height, but there are times when it's just not suitable.