# Mast compression

Discussion in 'Multihulls' started by waynemarlow, Jun 10, 2016.

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### waynemarlowSenior Member

In my innocence and nievity I think that the mast compression created from purely the sail will be same total compression force whether you hook the top of the sail AKA beach cat and then downhaul using the downhaul combined with the tension on the leach from the mainsheet or you run an internal halyard over a top pulley and lock it off to the lower part of the mast.

Equally if the base of the sail was locked off and you "downhaul" ( effectively pulling the top of the sail upwards ) the sail by tensioning the halyard.

If the lock off was to the boat and not the mast that would be different I think.

Any verdicts from you maths boys out there.

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Why not use a masthead halyard lock to reduce compression?

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### waynemarlowSenior Member

Thats not the answer I was looking for.

Let me ask another way, if it is the case, why does a masthead halyard lock reduce compression over a halyard simply looped over a mast head pulley and attached to the base of the mast ?

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### StumbleSenior Member

Halyard locks are 1:1, every pound of force applied to the luff of the sail adds one pound of compression.

The halyard sheave is the fixed point of a 180 degree bend. The halyard tail pulls down at 1:1, the sail pulls down at 1:1. So the compression is 2 times the luff tension. It's the same reason a foot block with a 180 degree bend needs to be twice the strength of the load.

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### gonzoSenior Member

If the mast was absolutely rigid, the compression would be almost the same with an outside or inside halyard. The small arm created by the mast radius can be ignored. However, mast flex (get out of column) so the halyard is not pulling along the neutral axis of the mast, but along the cord of the curve. That tension increases the bend in the mast. Also, as the mast bends the cord becomes smaller, so the tension in the luff decreases. The decrease in tension makes the sail fuller which is the opposite of what you want as the wind increases.

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### patzefranpatzefran

You omit the most important point. Fullness in the sail is mainly created by round in the luff, so when you tighten the halyard (or cunningham) you bend the mast fore and aft and you absorb the excess length of the sail with respect to the chord. This diminish the draft and flatten the sail. Every beach catamaran or dinghy racer know this ! Mainsheet tension also tighten the leech wich pull the mast aft and increase fore and aft bending. Together with mast rotation, these are the most important way to flatten the sail.
Cheers

Patrick

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### waynemarlowSenior Member

OK I'm starting to get the picture, but therein lies another thought. Excuse my lack of understanding but for the top pulley to act as a 2:1 pulley, it has to move downward to shorten the mast. So then for every 1mm of mast shorten it will use up 2mm of rope to do so ( 2:1 ) But the mast doesn't shorten ( well Ok it does but only after the forces creating the stretch of the sail overcomes the resisitance to collapse of the mast, which will be far higher than the sail resisitance to stretch ).

If we ignore the minimal mast compression distance and assume the distance between the bottom of the mast and centre of the top pulley remains the same, then the top pulley only acts as a directional change of force, for every 1mm movement up the mast, then the halyard only moves down 1mm ( 1:1 ) For the halyard to become a genuine 1:2 force increase you would need another pulley on the halyard inside the mast with the halyard looping around the pulley and back up to the mast top, with a seperate line from the pulley block down to the base ( for every 1mm moved of the block down, 2mm of the halyard will be moved ) and taking the strain. Only then would you get a doubling of the forces.

So in my view as long as we assume that the mast doesn't shorten ( which in reality will be very minimal as Aluminium is pretty hard to compress, its just the bend in the mast shortening the mast which will be mm only ) then if we fix the halyard at the base of the mast then the forces are simply 1:1 which is the same as a halyard lock at the top

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### patzefranpatzefran

The figures are :
1 ) mainsail hooked at the top of the mast :
compression on the mast = luff tension
2) halyhard 1/1 ( thru a pulley at the top of the mast to the head of the sail, attached down the mast) compression = twice luff tension, that is pull of the luff + pull of the halyhard)
3) halyhard 2 /1, ( attached down side the mast, thru a pulley at the top side, down to a pulley at the headsail, up to a fixed point at the other top side) compression = 1.5 time the luff tension, that is luff tension + one half luff tension on the descending line
It is a simple matter of geometry and addition of the tensions on the luff / lines !

3) is the best option if you need to reef the sail, as it lower the compression wrt single halyhard (but still higher compression than hooked)

Cheers

Patrick

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### SamSamSenior Member

My theoretical guess is there might be two types of compression. One is compression of the mast itself and the other is compression of the mast and the hull. If the halyard is cleated to the mast, all that compressive force is confined to the mast. That compressive force is x, no matter how many pulleys are used to get the tension in the sail. It doesn't multiply compressive force 2x or 3x depending on the pulleys, it just makes it 2 or 3 times "easier" to get the required tension on the sail and thereby compression on the mast.

With a stayed mast, the windward stay will form the third leg of the mast, hull, stay triangle. The stay will be in tension, the mast in compression and the hull just suffers quietly. If the halyard is cleated to the mast, all that halyard compressive force will be confined to the mast itself. If the halyard is cleated to the hull, all that force will be added to the mast, hull, stay triangle.

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### StumbleSenior Member

Nope.

Let's say we have 1000lbs of tension on the main halyard. So that side of the line is pulling down with 1000lbs of tension. The Luff of the sail is also carrying 1000lbs of tension pulling down. The turning block, thus has to carry the load of both ends pulling down at 1000lbs, or a net load of 2,000lbs.

It has nothing to do with the amount of purchase the line is working with, that's an entirely different concern.

This is the exact same reason why you can jump a halyard by pulling it sideways, but could never pull it strait. Bending a line even under tremendous load is easy, the greater the angle of the bend the higher the loads are. If a line were perfectly strait under a nearly infinite load, the first minuscule angle take an infinitely small amount of force, with the amount necessary to add each additional degree going up.

Eventually at a 180 degree bend the load is doubled.

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### waynemarlowSenior Member

Ok I think I now understand, thanks for the replies.

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### gonzoSenior Member

If the halyard goes to the bottom of the mast, the compression is 2T. There is 1T from each run of the halyard.

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### SamSamSenior Member

My understanding is fixed pulleys only change direction of force and give no mechanical advantage. To lift 100#, one foot, requires you to pull the rope down one foot with 100# of force.

So while running the halyard over a fixed pulley and cleated at the bottom of the mast changes the direction of applied force, it does not give any mechanical advantage but doubles the compressive forces on the mast.

So, and this is theoretical and not practical, say there was a fixed 2 wheel pulley at the top of the mast and a fixed 1 wheel pulley and a cleat at the bottom of the mast. All wheels the same diameter. The sail luff needs 1T of tension. The halyard attached to the sail head runs through the top pulley, down the mast and through the bottom pulley, back up through the top pulley and then down to the cleat. Does this give no mechanical advantage, but while putting 1T of tension on the luff, does it end up putting 4T compression on the mast?

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### patzefranpatzefran

Yes, 4 T and no mechanical advantage you get only a powerful mean to bend the mast.

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### gonzoSenior Member

If the halyard goes from the deck to the masthead and has one unit of tension. Then goes from the masthead to the deck and has one unit of tension. The total tension is : 1(deck to masthead)+1(masthead to deck)=2 units of tension.

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