# Forces on mast by sails yet again

Discussion in 'Motorsailers' started by Fanie, Feb 13, 2008.

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### FanieFanie

Hi guys,

Sorry to yet again start another one of these, I have however not found any practical answers on some of the old(er) threads regarding this, or I have missed it.

I don't want to bore you to death by trying to find formulas and so on either.

What I want to know is the force one can expect per square meter on a sail at a given wind speed.

So lets say the sail area is 50m^2 and the windspeed is 20m/s what force would be pushing against the sail ?

Has anyone measured or attempted to measure this before ?

Considering if I'm using an aft mast setup and the main sail is as a jib it would be safe to say the force on the sail would be about the force down on the mast, and plus the weight of the mast it would be the beam load.

If for example the wind load per square meter of sail is 10kg at 20m/s on a 50m^2 sail, then the total sail force is about 500kg plus the weight of the mast say 200kg then the beam should be able to carry 700kg plus a safety factor - at that wind speed.

I don't know if a 20m/s wind will load a square meter of sail with 10kg - which is what I hope to find out from you guys !

If I can find the wind force per square meter, I can design the mast, the beam and the auto tacking strip for it amongst other things.

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### FanieFanie

Come to think of it, the force per square meter should be known factor. Times the wind speed should give you the force.

Any idea ?

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### deepsixSenior Member

Im no expert, but as I understand it there are fairly standard sail force coefficients used by NA's, Im not sure if there is something suitable for your sail plan. Look on amazon.com for C. A. Marchaj - maybe sail performance or aero-hydrodynamics of sailing, maybe someone will recomend one.

As I understand it the sail force is not that important in designing the mast and rigging, these loads are all related to the righting moment. The wind speed, sail area and sail force coefficient merely determine when the maximum righting moment is reached and you must shorten sail.

You can get a resonable idea of the loads involved by just using basic trig, or you can use freeship to produce control curves and then calculate stability curves and righting moments.

Have you tried this? http://www.wb-sails.fi/news/SailPowerCalc/SailPowerCalc.htm

Last edited: Feb 13, 2008
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### FanieFanie

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### MikeJohnsSenior Member

Fanie
If you want a very basic approach the important consideration is the righting moment. The heeling moment from the sail (acting at the center of area) cannot exceed the force required to reach the maximum righting moment of the vessel. Work out the division of forces amongst the sails and find your maximum rigging loads accordingly. You will have to make a call on which sails will blow out and which will heel the boat given a 10 second hurricane.

You should to consider these 'bullets' hitting the vessel in all sail combinations.

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### FanieFanie

I have found some information as well as a graph that implies the force per sqare feet at so many miles per hour wind - not a linear graph.

You would have to decide at which wind speed you want to switch to a storm sail so you won't cause damages to the rig.

Now to find a conversion for these old units - wish the US would come to the table with their units

The righting movement of the boat as well as the forces of the sails will be taken into consideration, thanks Mike. Amazing how quicly numbers add up when you start calculating these forces. Currently exceeding by quite a bit to what I expected initially.

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### deepsixSenior Member

It sounds like you are describing this

Lift(N) = ½ ( D * Area(M2) * Coeff of lift * Speed2(M/s) )

Where D is the density of the fluid, about 1.2 in air. Coefficient of lift(Cl) is highly variable dependant on sail planform and section shapes ect, around 1.0-1.5 is the general ballpark, but it can be very different.

This shows that all things equal the forces produced by a sail increases with the square of wind speed. So when the wind speed doubles, the force produced quadruples.

Calculating the forces produced by the sail is quite complicated because it varies according to many factors, aspect, section shape, angle of attack, windspeed ect. These factors can change depending on the course you are sailing and how your sails are trimmed, so one Cl is only valid for a small set of circumstances. The amount of force that is translated into driving and heeling moments is also variable.

You must design to the maximum righting moment of your boat, this effectivley limits the maximum force that the sail exert on the rig, because the heeling moment cannot exceed the maximum righting moment or else you will capsize.

It might me easiest to design to one of the scantlings rules, here is part of a draft of ISO 12215 for small craft, Im not sure if there may me something more applicable, maybe someone will suggest something else.

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### FanieFanie

Hi Deepsix,

That formula is to determine the lift, also used on aeroplane wings.

In this case I want the head on full wind force pressing against the sail. Just the pure unprocessed brutal force.

It seems there can be quite a bit of force from the sails. I'm surprized nobody in the 'old' days used these sails as catapults before. You could throw a bomb further with it than you could by hand.

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### FanieFanie

The local guy here who can make carbon fibre or fiberglass masts is away for a week. I must get an idea what stiffness and load rating one should expect for different sizes as well as the weight for the materials.

My rough calculations indicates the mast could be around 160 to 180kg ! and the beam carrying the mast should be able to carry up to 1500kg to 2000kg !

Can this be ? Did I move a decimal point somewhere ?

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### FanieFanie

Anyone have any idea ? This forum is supposed to be packed with experts.

11. ### Guest625101138Previous Member

Mast compression load will be slightly less than the total displacement unless you overtension the stays. This is based on chainplates mounted on the outside of each hull. If you bring them inboard then compression load will increase.

Mast section will depend on the number of stays (fewer the better from sail handling point of view). Usually two sets are enough. Surprise me if the mast weighs more than 10kg per meter if in aluminium so you would expect much lighter with CF.

Rick W.

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### FanieFanie

Hi Rick,

Thanks, youve always been very helpfull all along.

There will be only three stays - two aft and one foreward which is also the stay where the sail furls onto.

If the mast is going to be around 10kg/m I'll be very happy - it will make some other mechanical issues a lot less complicated. I have calculated it slightly heavier, but it is still within what I expected.

If I use the total displacement to determine the mast compression - which is 2600kg's - per hull (gulp) - it comes to quite a bit. This is 5 tonnes !

This will require some super structure to support Have I misunderstood you on this ?

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### deepsixSenior Member

That might be a little simplistic, how far from the stern will your mast be?

14. ### Guest625101138Previous Member

Fanie
I made an assumption that you will have side stays. Eliminating these may increase the compression load. It will certainly increase the mast bending if you do not have lower stays. Your original weight estimate may be close to the mark.

The compression is very nearly the total displacement if you have side stays so if that is 5t then that is the compression load you need to resolve in the cross beam.

The torsional load through the cross beam is also substantial and needs to be resolved.

Rick W.

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