sheeting and halyard loads better than harken

Hello everybody,
I have spent about three days now, with googling and reading my university handouts finding a good approach for estimating sail loads.
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It is quite easy to get answers for sails as such being in condition xyz develop a force of xyz.
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But if I want to get good answers for halyard or sheeting loads it gets silent....apart from some simple formulations from harken which proof to be useless for downwind sails or halyard loads.
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Anyone a good idea?? Perhaps some practical solutions with or from FEA or CFD??
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There are a several issues that make it hard to come up with sheet and halyard loads from first principles. First, there are high internal loads. For example, say you raise a mainsail with just enough tension to get to full hoist, like you'd do for light winds. You can estimate the halyard tension from the weight of the sail and the friction with the mast. Now crank on the haylard or Cunningham to tension the luff and flatten the sail for higher winds. All that luff tension is an internal load that adds to the halyard tension, but isn't very apparent from calculating the static loads on the mainsail. If you did a free-body diagram of the rig, the luff tension wouldn't show up. But you have to include it in the halyard specs.

The second problem is dynamic loading. When a gust hits or the rig is accelerated abruptly causing inertial loads, the transient loads can be very high. But it's hard to estimate the expected range of these dynamic loads.

Finally, and this is somewhat related to the internal loads, the tension is a very nonlinear function of the shape of the sail. As a simple example, consider the tension in a cable carrying a uniformly distributed normal load. The shape of the cable will be a parabola. If we call the slack in the cable the difference between the straight-line distance between the ends of the cable and the actual arc-length of the parabola, the tension in the cable does not change much when varying the length of the cable if the slack is large. But as the slack becomes less than, say, 1% of the chord (meaning the actual length is 101% of the straight-line distance), the tension starts to climb very rapidly as additional slack is pulled out of the cable. A change in the applied load also means a large change in the tension when the cable is taut.

When you consider the camber shapes of sails and the sag in the luff of a staysail, the sail is operating in this nonlinear regime of high tension. So the sheet and halyard loads are high.

This is why rules of thumb, like the Harken formulae, have been developed. Think of them as statistical design criteria - even though the mean load may be much less, if you want to include a respectable amount of the high-end tail of the distribution, you have to design for much higher loads. The experience that's gone into the Harken formulae capture the statistics of raceboat operational spectra.

 

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I just purchased an older boat and need to do some minor repairs to the hole. I haven't ever worked on a boat before and need to learn how to repair gel coating and fiberglass repair. Can anyone help or recomend a web site that might give me step by step instructions on how to do this?

Any help would be greatly appreicated

 

thank you for that answer....
So you are saying that there is almost no answer to estimating halyard loads??
The Harken formulas are moreover just capabel of predicting sheeting loads.
I really can`t imagine that for a maxi yacht or something like that, people are not looking for better answers......
And for flying sails for instance when thinking of close reaching downwind?

There has to be an answer out there for getting an idea of what deck gear is to chose when you want to cope with a 600m2 gennaker. That doesn`t throw a good light on a yacht designer when he is doing just guesstimations.

 

Halyard loads etc.

The Harken formula's are in Brian Toss' book :"The Complete Riggers Apprentice". He also includes graphs by Lewmar for mainsheet, jibsheet runner and halyard loads on page 372.
Once you know the sheet or halyard loads, calculating the loads on halyard sheaves, blocks and winches is handled well in Wallace Ross' Sail Power under Chapter 14: " Hardware: Mechanics, Materials and Properties".

 

What I'm saying is if you want a better answer, it's going to be very expensive. You're going to need to instrument a full-sized boat for measuring the sheet and halyard loads, and record what's happening in a wide range of conditions, most of them at the extreme end. After a long time, you can build up an estimate of the probability density function for the loads you seek. Then, when you determine the confidence level at which you want to design the rigging and taking into account the deterioration in material properties over the design lifetime, you can set the design loads for selecting the hardware size.

Sophisticated yachts often do have load cells that could be used to provide some of this information. For example, I've seen a racing yacht with a load cell on the forestay that was used to trim the rig under different conditions. a really large yacht is likely to have load cells that are used by the crew to ensure they don't exceed the design loads.

If it's deck hardware you're trying to design, you can take a reverse approach. How strong are your sheets and halyards? You want the line to fail before the deck hardware does.

A wise man told me, "You calculate the loads you know, and design for the loads you don't know."

As for spinnaker sheet and halyard loads, the steady load will be less than for a staysail of comparable size, or producing the same lift (limited by hull stability), because the spinnaker is a much more rounded shape (more slack).

 

Josch,
Call a couple of spar builders/riggers, and ask them how they calculate things. Chances are, they look in the Harken book as well.
If you are at college in Soton, then maybe that should become a course project (to come up with a better formula)
Steve

 

Josch,
Which university in Southampton? (I'm assuming UK) sheet loads are an interesting question. Mainsheets are fairly easy to calculate. But jibs/genoas are slightly harder. I would suggest that you took this on as a 3rd year project. You can go into it in as much depth as you like and it would make a good research-based project.

Tim B.

 

I would agree with earlier comments that the dynamics make actual loads very tough to predict, so rule of thumb methods and experience that are in place now are pretty good. Crashing off a 20 foot wave in the gulf stream and slamming to a stop in 35 knots of wind is a tough load case to quantify, but could be the governing one. So how to design for this? Whatever does not break is a good place to start...