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2flit

Help with Headsail Sheet Loads

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I have a 40' trimaran with a 108,000 ft-pound righting moment; a 63 sq-meter laminate headsail up to a 25 knot apparent wind speed... What would you figure sheet loads to be?

(I am adding tweaker line hardware to the boat and figure that will be about 30% of the sheet load)

IMG_20150910_155208050 (2).jpg

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Look up the breaking strength for your sheets, 1/3 of that?

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Have you considered talking to the sailmaker?  I'm sure any sail loft would have software to answer your query. You could no doubt get a sanity check from rigging suppliers like Harken or others

 

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Is that headsail really 63 m2? 

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40 minutes ago, Training Wheels said:

Is that headsail really 63 m2? 

That's the jib in the picture. These are loads for a new screacher. Sailmaker said to use the Harken calculator but that is for a monohull (it comes up with 815kg) and says to contact Harken for a multi. I did that last week, but no reply yet.

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Nice boat..... I especially love the tea bag hanging from starboard ama

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The highest load scenario would be the maximum righting moment with only the screacher deployed.  The healing moment is created between the center of effort of the sail and the center of lateral resistance of the hull and daggerboard. The vertical distance between these two is the healing arm. You can roughly (and conservatively for this calculation) approximate the center of effort of the sail by by saying 1/3 of the hoist of the sail. You can roughly approximate the center of lateral resistance by using 1/3 of the draft of the daggerboard.

Now the maximum force on your screacher is the maximum righting moment divided by the healing arm. That total force is divided between the tack, head, and clew. A rough approximation would be 1/3 of the total force is taken by the clew. This would give you a ballpark number. As mentioned above, your sailmaker could give you a much better number.

You would really need to look at the specific geometery of your tweeker setup to figure the load on it. 30% of the sheet load may be low, given the horendous tweeker angles I've personally witnessed for a code zero on a monohull. 

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On 6/1/2020 at 5:08 PM, Shu said:

A rough approximation would be 1/3 of the total force is taken by the clew

I expect this to be way off as it ignores the tension in the sail: this would be true for a "loose" sail where the sheet points directly downwind... It should be many times that, just like the load in the forestay is a lot more than the force you need to capsize the boat by pulling on the halyard. The screecher sheet loads can be really high when you pull the foot of sail tight!

That said, I would expect the tweaker load to be significantly less than the max sheet load.

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roughly need about 2 ton.....should do it.

use 10mm dynema type rope with good case.

You could use smaller rope but it will be a nightmare to handle.

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We have decided to work with 150% of the on-line Harken Calculator's sheet load. Which will be 1,242 Kg. and have tweaker hardware at 600 kg. Then use a X 1.2  safety factor.

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I think that's a reasonable approach, given that you probably won't be sailing with your screecher close-hauled in 25 knots of wind, but should be well off the wind in those conditions.

I've sailed extensively on a Cheetah 30 monohull, and am half-owner of an F-82R trimaran - both lightweight performance-oriented boats which are very similar in length, weight, and sail plan. My un-scientific assessment is that the sheet loads on the trimaran are definitely higher under normal sailing conditions, but no more than 1.5-2 times higher. When hit by a gust the Cheetah heels and accelerates, while the F-82R simply accelerates - both of these ease the pressure on the sheets. The problem would be if the F-82R couldn't accelerate anymore, like if the bows were stuffed, in which case the loads would accelerate exponentially, but then you've got other problems to deal with.

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I just had another thought - I also own a 12 metre monohull that has #1 Genoas about the same size as the OP's screecher. 

The heavy #1 carbon definitely loads up the sheets per the Harken calculator when sailing close-hauled in 20-25 knots apparent, but the light #1 which is built much more like a typical screecher would have exploded long before that.

We only fly the light #1 close-hauled up to about 7-8 knots apparent before switching to the heavy #1, mostly to save the sail.

In any case, I doubt your screecher could survive even the 100% loads from the Harken calculator.

    

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15 hours ago, Airwick said:

I expect this to be way off as it ignores the tension in the sail: this would be true for a "loose" sail where the sheet points directly downwind... It should be many times that, just like the load in the forestay is a lot more than the force you need to capsize the boat by pulling on the halyard. The screecher sheet loads can be really high when you pull the foot of sail tight!

That said, I would expect the tweaker load to be significantly less than the max sheet load.

Yes, the loads on the head and tack are much higher because of the tension. But that tension is largely the two pulling against each other.  The wind force on the sail would be roughly divided between the three attachment points. The clew load will get high too if you really squeeze the stuffing out of the sail.  Tweaker load is dependent on the amount of deflection it is applying to the sheet.

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our 40 foot cat  5 tonne 12m - 48 sqm screacher is on 10mm sheets - the twitchers are 8mm as that was what we had......  could probably go 6mm 

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