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Old 13-09-2011, 08:15   #1
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Point-of-Sail and Rigging Loads

One of the first things I learned about sailing that wasn't taught in a course is that depending on ones point-of-sail (i.e. beating vs broad reaching) the same amount of sail may be too much or too little. For example, when beating into a 15kt wind with all sails out my boat is over-canvased but turning 180 degrees it "feels" under-canvased.

Now this a probably due to the fact that when beating against a wind the apparent wind speed is the actual wind plus boat speed. For example, assuming a 5kt boat speed each case, a wind speed of 15kt becomes an apparent wind speed of 20kt (15+5) where-as a broad reach would have an apparent wind speed of 10kt (15-5).

My question is if the load on the rigging is the same in both of these scenarios or does less apparent wind equate to lower loads, and visa versa? I suspect that is the case due to the 10kt difference in apparent wind speeds (using the example above).

However, one has to take care when broad reaching as it is easy to underestimate the loads on the boat (I once saw a boat lose it's mast in a 15kt wind when going down wind with a spinnaker as we were beating upwind with reduced sails). Perhaps the loads are not so clear cut.

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Old 13-09-2011, 08:32   #2
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Re: Point-of-Sail and rigging loads

Loads are all about apparent wind.

By way of example, a 3/4 oz spinnaker made of traditional materials is rated to function in winds up to 18 knots apparent. That could easily mean true winds of 25 knots running downwind. The moment you round up with that spinnaker in a 25-knot breeze, however, you could easily blow the chute apart. A lot of sailors have learned this lesson the hard way.
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Old 13-09-2011, 09:05   #3
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Re: Point-of-Sail and rigging loads

You probably have opened Pandora's box with your question. I agree that loads are all about apparent wind.

Your load on the rigging can also be affected by your rigging tension. Every mast/rigging/chainplate combination will have an optimum tension which will factor in stretch among other things. And if you use a Loos gauge to measure that tension your rigging will be much looser than you think. And if you over tighten things you will be much more prone to losing your mast in heavy, gusty winds.

As another example, the tighter your forestay, the higher you will be able to point. But if you tighten your forestay without tightening the rest of your rig, you may bend your mast in a manner that it wasn't designed for. Some masts are designed to be straight, some raked, some with a bend, etc.

Bottom line is that rigging tension and optimum loads is part science and part art. Good luck.
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Old 13-09-2011, 09:16   #4
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Re: Point-of-Sail and rigging loads

Boats sail and are loaded on apparent wind.

In cruising mode it is important to carry no more sail downwind than you can carry conservatively upwind...The reason is that if you loose a MOB then you need to be able to get back quickly without having to reef and when hit by a sudden squall you will not be dangerously over canvassed.

In cruising mode a spinnaker is a 'light wind sail' not a 'go as fast as you can sail'.

While we have a boat capable of almost 10 knots we keep her comfortable at 6-7 knots reducing loads and keeping the excitement to a minimum...We break too many things when we are in the excitement mode.

Our avitar show us sailing with 1,700 sq ft of sail in 15 knots. If my wife and I were sailing without crew, at night standing single handed watches, we would have a reef in the main and the staysail up instead of the genoa; 2 knots slower but much less excitement.

Under spinnaker you know you are pushing the limits when your wife gets thrown out of bed for the second time!

The last time we did that she used endearments such as "stupid idiotic children."
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Old 13-09-2011, 09:54   #5
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Re: Point-of-Sail and rigging loads

Quote:
Originally Posted by ADMPRTR View Post
For example, assuming a 5kt boat speed each case, a wind speed of 15kt becomes an apparent wind speed of 20kt (15+5) where-as a broad reach would have an apparent wind speed of 10kt (15-5).
This part is not entirely accurate. While it is true that a boat doing 5 kt in a 15 knot DDW course "feels" 10 knots of force, you would have to be beating exactly head to wind at 5 knots to have an apparent wind speed of 20 kt.

The "offset" of the degree to which a boat can point means that the apparent wind forward of the beam is going to be a moving target based on trim as well as percentage of the way from 1/2 (beam) to 1 (in irons with the wind roaring directly on the centerline bow to stern).

Your points about straining the rig or the sails are true, however, as is the sensible suggestion to never have so much sail up on a run that you couldn't do a 180 in a pinch without sending a sailmaker's daughter to college.

One of the more dramatic cases of turning around in dangerous conditions was during the 1996 Vendee Globe, when RaphaŽl Dinelli flipped and Pete Goss beat back into the Southern Ocean, actually found him half-dead and clinging to his keel, and resumed racing after supplying biscuits and blankets! A great story of seamanship: HOW WE MET: PETE GOSS AND RAPHAEL DINELLI - Arts & Entertainment - The Independent
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Old 13-09-2011, 10:24   #6
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Re: Point-of-Sail and rigging loads

I would say higher going to weather based purely on looking up my headstay many times on different points of sail. Lots of sag going to weather.... on a 47 footer maybe close to a foot!
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Old 13-09-2011, 10:38   #7
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Re: Point-of-Sail and Rigging Loads

How much leeward bend would you normally see in the mast,above the spreaders, when sailing under full sail to windward in 15 knots?
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Old 13-09-2011, 10:59   #8
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Re: Point-of-Sail and Rigging Loads

IMO I wouldn't want to see any side-to-side bend in the mast (fore-and-aft is a different story). The mast may lean to leeward (it usually will), but the rig should be adjusted so that the mast remains as a straight column from top to bottom. Once it gets out of column you're looking at a pretty easy failure mode.
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Old 13-09-2011, 11:03   #9
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Re: Point-of-Sail and Rigging Loads

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Originally Posted by highseas View Post
How much leeward bend would you normally see in the mast,above the spreaders, when sailing under full sail to windward in 15 knots?
Yeah.... what Desanduril said. That's one reason I do my final adjustment while sailing. It seems pretty easy to get rid of the top going to leeward. My biggest problem is usually with one reef in, keeping the top of the mast from bending forward.... especially with a roller furling setup on the foresail.
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Old 13-09-2011, 12:37   #10
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Re: Point-of-Sail and rigging loads

Quote:
Originally Posted by Ellis McKinley View Post
Your load on the rigging can also be affected by your rigging tension. Every mast/rigging/chainplate combination will have an optimum tension which will factor in stretch among other things. And if you use a Loos gauge to measure that tension your rigging will be much looser than you think. And if you over tighten things you will be much more prone to losing your mast in heavy, gusty winds.
Typically I have set my tension on what "feels" correct but I have never actually determined what is correct. Last year I borrowed a meter to help tune the rig but again it was ultimately based on feel and I used the meter to only ensure the relative tension was equal.

I would have thought that more tension was better than less as there would be less flexing in the mast.

Using a meter, how is one to determine the optimal tension?
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Old 13-09-2011, 12:55   #11
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Re: Point-of-Sail and rigging loads

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Originally Posted by S/V Alchemy View Post
This part is not entirely accurate. While it is true that a boat doing 5 kt in a 15 knot DDW course "feels" 10 knots of force, you would have to be beating exactly head to wind at 5 knots to have an apparent wind speed of 20 kt.
Yes, I agree. It was more for demonstration purposes.

Quote:
Originally Posted by S/V Alchemy View Post
Your points about straining the rig or the sails are true, however, as is the sensible suggestion to never have so much sail up on a run that you couldn't do a 180 in a pinch without sending a sailmaker's daughter to college.
That is an excellent point. However, while I try to be aware of that fact, I also try to optimize to the apparent wind conditions. It is a tough call because reducing sail increases the sailing time which would be the wrong decision if bad weather is on it's way.
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Old 13-09-2011, 15:13   #12
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Re: Point-of-Sail and Rigging Loads

Yeah, I dont like to overtension on a cruiser. Again, that's why I do the final setting under sail.... just enough to get the leeward cables a touch loose going to weather. On my heavy built 44 footer I put a big hydraulic backstay tensioner on ( my boat was right on the margin of which to order so I went next size up!) I found that I could bend the boat about 3" without an appreciable difference in the headstay sag.....!
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