shroud tension - hull stresses?

allweather

Member
392
76
baltic
Got into a discussion after a round the cans club race about if I am not worried about sailing with such tight shrouds.
Specifically someone wondered if my shroud's are failing(replacements in the trunk, just need to drop the rig and switch) because of the high tension and that it may be dangerous in general. Also mentioned hull stresses and so on.

Point is that I didn't have any facts to quote to refute them and would like to ask you guys about it in general.
Considering the numbers I sail with on the loose gauge are after the guidelines from the class association and have been used by dozens of boats for years and years I am not worried too much. Highest setting is also just shy of 25% breaking strength for the 4mm wires.

We're talking about a H-boat that is using the following shroud tension(wind ranges are lowballed due to lacking hiking crew):









conditions


forestay


main shrouds


lower shrouds




LW lightwind


16


28


20




MW-I medium 3bft+


20


34


24


 




MW-II medium 2 4bft+


23


36


26




HW-1 highwind 1 5bft+


25


37


31




HW-2 highwind 2 6bft+


27


40


33






Are there any particular concerns I should be aware of? I thought I was well within limits of everything.

In particular since it is my understanding that the majority of tension on the shrouds comes from when the boat is actually heeling over? Supported by my observation that only with HW-2 the leewards shrouds aren't coming lose anymore when knocked flat in a particularly strong and too late reacted to gust.

With which I mean that having the tension itself should not be dangerous at all? I would not let the boat sit like that for weeks if not sailed purely out of habit but can't say I have ever read that it would be a safety risk. Only the opposite in one article that mentioned that leward shrouds going slack causes high dynamic forces that are more of a danger.

 

slug zitski

Super Anarchist
4,976
774
worldwide
Got into a discussion after a round the cans club race about if I am not worried about sailing with such tight shrouds.
Specifically someone wondered if my shroud's are failing(replacements in the trunk, just need to drop the rig and switch) because of the high tension and that it may be dangerous in general. Also mentioned hull stresses and so on.

Point is that I didn't have any facts to quote to refute them and would like to ask you guys about it in general.
Considering the numbers I sail with on the loose gauge are after the guidelines from the class association and have been used by dozens of boats for years and years I am not worried too much. Highest setting is also just shy of 25% breaking strength for the 4mm wires.

We're talking about a H-boat that is using the following shroud tension(wind ranges are lowballed due to lacking hiking crew):









conditions


forestay


main shrouds


lower shrouds




LW lightwind


16


28


20




MW-I medium 3bft+


20


34


24


 




MW-II medium 2 4bft+


23


36


26




HW-1 highwind 1 5bft+


25


37


31




HW-2 highwind 2 6bft+


27


40


33






Are there any particular concerns I should be aware of? I thought I was well within limits of everything.

In particular since it is my understanding that the majority of tension on the shrouds comes from when the boat is actually heeling over? Supported by my observation that only with HW-2 the leewards shrouds aren't coming lose anymore when knocked flat in a particularly strong and too late reacted to gust.

With which I mean that having the tension itself should not be dangerous at all? I would not let the boat sit like that for weeks if not sailed purely out of habit but can't say I have ever read that it would be a safety risk. Only the opposite in one article that mentioned that leward shrouds going slack causes high dynamic forces that are more of a danger.
If you have aft swept spreaders, fractional rig ,  you are also developing headstay tension with the shrouds 

hence you will be sailing with substantial tension 

 

El Borracho

Sam’s friend
6,234
2,326
Pacific Rim
With straight spreaders the pre-tension does not add to the sailing tension unless the leeward shroud never slackens even at crazy overpowered heel angles. With swept spreaders the head stay load voids this rule. But at least headstay loads are shared by both lee and weather shrouds. 
 

Warping the hull would be my concern. 

 

allweather

Member
392
76
baltic
Yes, it is a fractional rig with swept spreaders where most of the headstay tension comes from the shrouds.(do pull quite hard on the aftstay to flatten the main, 28LG, and that does have an impact on headstay tension, but not the main contributor)

With swept spreaders the head stay load voids this rule.
In what way? Do you mean that if I were to measure shroud tension at a given angle of heel(15degrees for example) with highly pretensioned as well as less tensioned shrouds the latter should show significantly less units on the loose gauge?

Also about warping the hull, are we talking simply about the bend induced by tension or something else? Because in the former case I'd have to figure out how to measure it and have never heard it to be a problem in the race class.

 

Overbored

Anarchist
711
59
So. Cal
the tension on the wires is far greater when sailing then not sailing so how is it dangerous to pretension them to any tension that is less then sailing loads. spent back spreaders on a fractional rig keep a higher tension on the rig to prevent the forestay curve from changing every time rig load changes when sailing. its a way to prevent mast pumping which keeps changing sail shape. makes a faster boat

 

Foolish

Super Anarchist
1,686
363
Victoria, BC
The Olson 30 is famous because most owners have added a jockstrap and beam of destiny.  The jockstrap is specifically to counteract shroud tension and the beam holds the hull in place in the waves.

Olson 30.jpg

 

allweather

Member
392
76
baltic
added a jockstrap and beam of destiny
Comfy. :D

Never seen anything like that in h-boats, but I feel like I should mail my class about this entire thing. They do have a vast treasure trove of experience to fall back on, though most have far newer boats nowadays.(kind of like how most dragons being raced aren't wooden hulls)

 

neuronz

Anarchist
875
71
europe
Pretension in the range of 20% break load is nothing unusual. As a measure of thumb the leeward shrouds should only become slack at your maximum sailing heel angle.

In general the hull should not deform permanently. If you give it some rest during the offseason it will go back to its original shape. If you keep the tension for several years or if the boat is weakened otherwise it is a different story.

 

Bull City

Bull City
6,848
2,501
North Carolina
We're talking about a H-boat that is using the following shroud tension(wind ranges are lowballed due to lacking hiking crew):









conditions


forestay


main shrouds


lower shrouds




LW lightwind


16


28


20




MW-I medium 3bft+


20


34


24


 




MW-II medium 2 4bft+


23


36


26




HW-1 highwind 1 5bft+


25


37


31




HW-2 highwind 2 6bft+


27


40


33
@allweather What is bft+ , and are the numbers Loos gauge or % of breaking strength?

 

allweather

Member
392
76
baltic
bft - Beaufort scale -> Rough estimates for where I want the trim to be before adjusting

Numbers are Loose Gauge otherwise.

 
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