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Old 18-02-2010, 20:38   #16
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Originally Posted by Jaywalker View Post
Thanks. I didn't know hulls took on different shapes under stress - I assumed they were essentially built to be rigid and any flex was something to be designed out (or around), as for instance with cored hulls.

Do builders recommend changing standing rigging tension on the hard or in a blow, or is this something we assume makes sense? Wind force at any given level on an unmoving boat will be constant, and as SailFastTri suggests, by changing tension we simply choose where to have it act - on a small area at the base of the mast or a larger area deforming/changing the shape of the hull.
It was an exaggeration to say the hulls change shape (in a strong boat) and would have been more accurate to say they distort. Generally not enough to do damage; although I do know someone who owned a Hallberg Rassey that was improperly blocked for winter and a combination of frost heaves and mud sinking caused major hull damage that affected structure of keel, cockpit, deck and hull-deck joints. Dry-sailed racing boats are either lightweight trailer-sailors or if they are keel boats they almost always have custom-fitted stands that conform to hull shape to support the boats evenly at structural strong points.
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Old 19-02-2010, 00:32   #17
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Quote:
Originally Posted by Jaywalker View Post
Thanks. I didn't know hulls took on different shapes under stress - I assumed they were essentially built to be rigid and any flex was something to be designed out (or around), as for instance with cored hulls.

Do builders recommend changing standing rigging tension on the hard or in a blow, or is this something we assume makes sense? Wind force at any given level on an unmoving boat will be constant, and as SailFastTri suggests, by changing tension we simply choose where to have it act - on a small area at the base of the mast or a larger area deforming/changing the shape of the hull.
Most mono's, particularly GRP mono's and especially lightweight GRP mono's will distort when the rigging is tuned. This is not some design flaw, just normal material behaviour. If this was to be designed out, a significant amount of additional material would be required - adding weight and cost for no particular benefit.

So do we detune the rig when the boat is on the hard? I'd say that it's probably not necessary for the vast majority of cruising boats, but that it's probably beneficial for lighter boats - to make sure the elastic deformation doesn't become plastic deformation. And most small dry sailed race boats do just this.

If we are talking about rig tuning, the best place to look is a race boat - they will have several settings based on (typically) actual windspeed on the day. This is not because they think the rig will come down, but to improve sail shape (typically the lighter the wind, the looser the rigging). Indeed, many will adjust the rigging on the way around the course (eg. backstay & babystay tension).

We cruisers however, typically set and forget, and I'd hazard a guess that most of our rigs are overtuned most of the time - simply because we don't want to go around tightening things up every time the wind picks up.

But all this is put into perspective when we consider slamming off a wave. The shock load induced will increase the load on the rig by about a factor of 2 - so again, its not the wind you need to worry about, its the waves.
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