Spreader question

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kirielad

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I have searched the forums but haven't seen an answer to this;

How do you calculate the correct angle for aft swept spreaders? Should this be the same angle that the chain plates 'makes' relative to the base of the mast?

I have cap shroud plates that are aft of the mast looking athwartships, yet the mast is fitted with inline spreaders!? /forums/images/graemlins/confused.gif

I have just compared notes with a boat of the same age / type / chain plate positions - the difference is this boat had aft swept spreaders, and a 'nice' bend to the mast - which mine has always lacked!

I know it's a question I shall ultimately have to put to Z-Spars or Selden but I would appreciate peoples thoughts and some background knowledge.

Rig is single spreader, single cap, 7/8ths fractional, single forward and aft lowers.
 
I'm sure you've answered your own question in your second sentence and the crosstree should be in line with the shroud and therefore at the same angle as the chainplate but look forward to seeing someone with detailed knowledge reply. Personally, I have the unusual combination of a fractional rig with straight spreaders.
 
A lot depends on the cut of the sail .. Same principals apply as dingy sailing .. If you put bend in the mast then you will take some of the fulness out of the sail .. The original sailmaker would have cut his sail for either a straight mast or one with some bend .. Bend is measured by using the halyard from the top of the mast down to the goose neck and finding the distance this is from the mast at spreader level .. So the more you pull the spreaders back the more bend you put in the mast .. Our Beneteau 21.7 had about 3" of mast bend .. If you moved the spreaders forward the main became very full .. Not a pretty sight .. How does your sail look and does it do what it says on the tin .. Does the boat point ok .. If so you may well have it right .. /forums/images/graemlins/grin.gif
 
Think I may have had a 'light bulb' moment about this, and why I can never seem to get the mast to set with any appreciable bend.

From the top of the cap shroud as far down as the spreaders (which are straight) there is obviously no forward or aft angle involved, just downward force. However, below the spreaders there is an angle - backwards - to run down to the chainplate aft of the mast.

This means (I reason) that the more I tension the caps, the more it's trying to pull the mast backwards via the spreaders, effectively trying to invert the mast, as well as causing sag to the forestay. Or have I missed something here?
 
Thanks to all for replying. The boat is a Feeling 720, from 1984. The caps are attached via tangs more or less in centre of the mast.

The lowers have 'T' shaped ends which fit into dedicated sockets below the spreaders. The spreaders themselves are held by 2 clevis pins at their inboard end, and so to all intents and purposes don't have any freedom of movement.

Below decks the cap chain plates are bolted onto an athwartships bulkhead, and the aft lower is also connected via a steel rod to a strong web bonded into the hull. The forward lowers are just bolted through the deck with no other support!

The more I think about this the more I think the whole rig is just not setup properly.
 
How much rake do you have, if I picture the rig properly, a raked mast would line up the spreader tips with the cap shrouds going from hull to mast head /forums/images/graemlins/ooo.gif
 
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How much rake do you have, if I picture the rig properly, a raked mast would line up the spreader tips with the cap shrouds going from hull to mast head /forums/images/graemlins/ooo.gif

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The mast rake I have is negligible - if any at all (forestay doesn't have a bottlescrew fitted so can't be adjusted 'easily').

Its another good point you raise though; if the mast was raked backward and the cap shrouds were more or less above their respective chain plates, then presumably the mast would be more inclined to bend forward via the spreaders - be they straight or swept, and tension the forestay to boot.
 
I had to replace the mast (fractional) on a previous boat. Of the various manufacturers, I recall that Selden were the most particular about spreader angle. In my case the angle was just about on their upper limit of angle. If you are designing/building from scratch be careful to get rigging proposals and quotes before fixing items such as chainplate location.
 
I tend to disagree with much that has been said. Much depends on what the designer had in mind.However...

The aft sweep of the spreaders is to transfer a forward push to the middle of the mast. This is usually balanced by aft setting of the chain plates attaching lower stays which hence pull the middle aft to couteract the push forward from the spreaders.
This arrangement means you don't need an inner forestay to pull the middle forward. It does rely on the spreaders being robust in holding their aft swept angle. If the spreaders merely meet the cap shroud then there is no push forward only a resistance to push aft. But if the spreader is swept well aft beyond the straight line of the stay it will present a forward push. I think this is how it should be.
The mast should be straight with little backstay tension and not sailing.Back stay tension will bend the mast middle forward to take out some of the camber. The sailmaker will put in an appropriate amount of camber which can be pulled out by bending the mast in strong winds.

Now that is the typical set up for a modern fractional rig.

However OP says he has an inner forestay. That alone should provide forward pull to the middle of the mast against the aft pull of the inner side stays. Which makes one wonder if the cap shrouds should not be nearer to abeam the mast with square spreaders and intermediates only attached aft of abeam the mast.
This would be typical mast head rig set up. 7/8 fractional is so close to masthead in terms of mast bend that it doesn't make much difference.ie compared to 3/4 rig where there is alot of mast unsupported above the stays.

The disadvantage of aft swept spreaders and shrouds is that the mainsail will chafe on the spreaders and wire, when running square with main right out.

So if i read your post correctly you can either move the ap shrouds to a chainplate further forward. Expecting that the spreader tips will essentially follow. This will alleviate chafe on the mainsail.

Or go radical and remove the inner forestay. (A huge advantage when tacking frequently.) This would require that the spreaders be made robust in the sweep aft and in fact sweep further aft than the cap shrouds straight line. Or you might like to keep the inner forestay disconnected but able to be connected (high field lever works well here) for strong winds.

Just to give you some ideas on my 21fter with 26ft mast the top 1. 3 metres is unsupported. ie above the forestay.
The chain plates for both cap and intermediate are about 40cms aft of abeam the mast. The spreader tips are are about 25 degrees aft of abeam ie aft of the straight line of the stay.

An adjustable backstay can give me about 10 cms of middle forward mast bend. I hope this helps in describing the classic fractional rig.

You may have a hybrid which depends on strength of the mast in fore and aft bend and the inner forestay. good luck olewilll.

PS keep asking around for more ideas.
 
I agree with much of what you write, except:

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The mast should be straight with little backstay tension and not sailing.

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Fractional rigs are frequently set up prebend in the mast put there by swept back spreaders, high tension in the shrouds and the amount of pre-bend set/limited by the aft lowers. The backstay tension adds more mast bend for upwind/stronger wind sailing.

For those who have said that it is unusual to have in line shrouds with fractional rigs, I would point to all the racing boats who have just that rig. Such boats need runners though to give forestay tension. I was offered a delivery job once on a boat in which the mast would fall down if you didn't get the timings right for the two sets of runners! (There was no backstay at all.)
 
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