Running downwind with two foresails - excessive mast compression?

R

Ric

Well-Known Member
Joined
8 Dec 2003
Messages
1,723
Visit site
I have a slutter rig on a 10m boat. The Genoa is 130%, and the staysail is just around 10% smaller than the main.

I have been experimenting with the best downwind rig - goose winged or twin poled out headsails. The twin poled out headsails has slightly less area, but I like it better because it the boat rolls less due to centre of pressure being further forward (I think!).

However, should I be worried about mast compression? The Genoa is just on a single halyard and furler, but the staysail is on a continuous line furler with a dyneema halyard, passing through a block on the head, then back up to the top of the mast, to tension up the luff line. The tension in the dyneema is quite substantial. I thus have five lines compressing the mast instead of the two that the structure was designed for. I don't have backstays, but the staysail is mounted very high on the mast.

I am supposing that the mast has enough structural redundancy to cope with the additional loads but I am not a rigger.
 
I'm curious. I thought a slutter rig was just a DIY cutter with a removable inner forestay. What happened to the backstay? Do you have runners?
 
My impression was that a slutter had a large genoa and a staysail rather than a cutter which has a jib and a staysail.

I am probably wrong, but someone very experienced described our previous boat with a 135% genoa and a staysail as a slutter. When we deep sixed the big genoa for a 100% high cut yankee he told me he was pleased I had made it into a cutter!

He might have been wrong too............................
 
Might you need to add running backstays to support the mast at the inner forestay? Here is a quote I found here:
http://www.wavetrain.net/boats-a-gear/680-cruising-sailboat-rigs-converting-a-sloop-to-a-slutter

Some riggers I consulted when I was converting Sophie's rig suggested running backs are unnecessary in any event. They argued that back-loading on the mast from the mainsail offsets the front-loading from a staysail. This makes sense, I suppose, but if you want to be able to douse the main and run off under a staysail alone, which is a desirable option to have in rugged conditions, it is best to somehow back up the inner forestay.

Personally, I would think that you are adding much less mast pressure than would be added by a large spinnaker. Have fun.
 
If you have the staysail forestay withing 600mm of the main forestay I would have thought that there was no problem on a boat of your size. If the attachment is any further from the main forestay then you would likely benefit from runners. On my boat we have the same setup but the inner forestay is 2m from the top of the mast on a 17.5m mast. We fitted runners and the mast pumps a lot less when going up wind in tough conditions. I doubt there would be much benefit from runners if you were going down wind unless you don't have a backstay in which case runners would add some piece of mind if nothing else.
 
davidej said:
I still can't work out how the OP's arrangement increases the number of lines compressing the mast from two to five.
Click to expand...

OK, I was ignoring the mainsail halyard, but I leave this untensioned.

The Genoa halyard runs up the mast two a block, then back to the head of the genoa (2 lines compressng mast)

The staysail halyard (dyneema) runs up the mast to a block (just below the genoa block), back down to a block on the staysail head, then back up to the top of the mast where it is fixed. The purpose is to be able to be adequately tension the staysail luff. This makes thee lines pulling down on the head of the mast.

Hence I have 5 lines compressing the mast instead of two. The staysail halyard is generally much higher tensionthan genoa halyard.
 
Ric said:
OK, I was ignoring the mainsail halyard, but I leave this untensioned.
....

Hence I have 5 lines compressing the mast instead of two. The staysail halyard is generally much higher tensionthan genoa halyard.
Click to expand...

But if you were goose-winged under main and genoa then you would have the main halyard tight, making 4 in this scenario?

I would not be worried about compression loads (unless you have a boat specific reason to be), but rather about pulling the mast out of column. If your yacht has a stiff, masthead rig with the inner forestay close to the top of the mast then this is reasonably standard, though I am unclear about why the staysail halyard needs to be so tight.

If you have a fractional rig, or the stay was retrofitted, then someone must have done some calculations to make sure it is alright? Or am I being optimistic?
 
Ric said:
OK, I was ignoring the mainsail halyard, but I leave this untensioned.

The Genoa halyard runs up the mast two a block, then back to the head of the genoa (2 lines compressng mast)

The staysail halyard (dyneema) runs up the mast to a block (just below the genoa block), back down to a block on the staysail head, then back up to the top of the mast where it is fixed. The purpose is to be able to be adequately tension the staysail luff. This makes thee lines pulling down on the head of the mast.

Hence I have 5 lines compressing the mast instead of two. The staysail halyard is generally much higher tensionthan genoa halyard.
Click to expand...
You count the genoa halyard as two lines. You should think of it as one point where there is force applied to the mast. It doesn't matter how many lines feed into that point or out of that point. It is one point where force is applied.

Unless you have a specific reason to worry about this particular mast, I would not even give it a second thought. Frankly it would not have occurred to me to even give it a first thought.

The halyards or turning blocks are likely to break long before the mast suffers a compression failure.
 
bbg said:
You count the genoa halyard as two lines. You should think of it as one point where there is force applied to the mast. It doesn't matter how many lines feed into that point or out of that point. It is one point where force is applied.

.
Click to expand...
Err..... no.
A foresail halyard running over a turning block near the top of the mast will cause the compression forces in the mast, resulting from the foresail, to be nearly double what they would be if the same sail was simply tied to the top of the mast, for a given luff tension. (nearly double, because one lead of the halyard is angled away from the mast at usually about 25 degrees).
For a given luff tension the OP's inner foresail arrangement actually only puts about 1.5 times the compression force on the mast, than if that sail were simply tied to a point on the mast. Here, the forces of the two leads of the halyard going to the head of the sail sum ( each taking half the luff tension), so that the mast compression forces are only 1.5 times (3 x 0.5 luff tension).

All that said, masts are very strong in compression. Masts normally fail by buckling, which occurs when a mast under compression is deflected out of column by an unbalanced lateral load. The OP has a potentially unbalanced lateral load on the mast where the inner staysail's halyard attaches near the top of the mast. He is right to be concerned by this, and the earlier suggestion to use runners is reasonable.
 
Last edited:
scrambledegg said:
Err..... no.
A foresail halyard running over a turning block near the top of the mast will cause the compression forces in the mast, resulting from the foresail, to be nearly double what they would be if the same sail was simply tied to the top of the mast, for a given luff tension. (nearly double, because one lead of the halyard is angled away from the mast at usually about 25 degrees).
Click to expand...
Where did I say otherwise? A halyard lock is almost unheard of in cruising boats so the idea of counting a single halyard as "two lines" is somewhat confusing.

I don't read the OP as saying that there is much, if any, lateral force on the masthead. He hasn't described his rig so maybe there is a concern about whether anything should be mounted at the masthead. If there is a design for a masthead spinnaker, I think he has nothing to worry about.
 
Hmm, no backstay?
So I assume your spreaders are well raked aft.

There should be very little compression from a foresail on a furler as the halyard is just there to hold the sail up, it should not be under any significant tension.
So the only additional compression is from the the inner stay which again should only be tensioned enough to stop any significant sag when sailing upwind.
Off the wind it can sag a little as this can be beneficial.
In which case there would be little additional compression.

However, if you really don't have backstays then the loading on the cap shrouds is at least 15% swl static, load up the fore=triangle and the loading on the caps will increase.
My worry would be therefore not the mast but the cap shroud chain plates.

If you google your boat type, rig and look in images you might find a photo we could have a look at which would help tremendously.
 
You must log in or register to reply here.

Share this page

Top