What a fool believes

[Rattlebolt]

What a test of a riggers skill1

I've just got a boat that some madman altered radically, and I mean that in a very respectful way. The previous owner actually stayed up nights dreaming of sailing a 19 foot boat to Hawaii. He took the hull of a day-sailer and put a cabin on it and a tiny little cockpit. And then he put double back stays, double (cutter) forestays, 7/8 and 3/8 shrouds, and spreaders, all with double sized wire for the original craft. What was an open boat day-sailer, is now an off shore surf board with a cabin. And the workmanship is all very good.

But! The thundering question here. How do I tension this rig?

The boat was originally speced to carry 1/8th inch wire. Now it has 1/4 inch wire on it. How much tension can I safely put on this forestay? there's about 16 inches of rake on the mast as it's standing now and the fore stay is very tight. The fore stay to the pointed end of the boat and the back stays to the corners of the transom form a little triangle. And all the shrouds barely tight at all. Beneath the deck step is a well built glass deck supported by a curved 2x4 deck beam resting on top of a bulk head made of 1/2 inch ply. Not quite a keel stepped mast, but not purely deck stepped.

I'm looking for suggestions here. Any one out there fancy themselves a good rigger?

 

[Salty John]

Typically the tension on the 3mm (1/8") wire would be 145 kg (319lbs) for the forestay and 110 kg (242lbs) for the shrouds. The equivalents for 6mm (1/4") wire are 590 kg (1298lbs) and 390 kg (858lbs). So the logical answer is to set the 1/4" wire at the tension of the 1/8" figures to give the original design tension for the boat. I would say you can go up on these figures because the design would have allowed you to go as high as 25% of the breaking strain of the 1/8" wire - about 500lbs. I'd use 500lbs as my maximum.
There are some notes on rig tensioning with a Loos gauge on my website.

 

[Gordonmc]

I would be more concerned about the strength of the chainplate fixings ... or even the integrety of the hull, than the load on the wire itself.
I write with the experience of replacing rigging on a wooden hull, but the same would apply, I guess, for a hull not initially designed for such loadings.
You might just end up with a banana shaped boat!

 

[Clyde_Wanderer]

You could tilt her on end and use her as a bow using the mast as an arrow. /forums/images/graemlins/laugh.gif
All kidding aside you will need to be wary of tensioning the rigging too tight as the boat design will not be able to handle excessive loadings on the hull which would cause, at the least stress cracking on hull and deck, in other words the boat could start bowing in the midships.
Sorry to butt in with a similar question,
I have a Hummingbird 30, which I have increased fore and back stays from 6mm to 7mm, but dont know what the tension should be for either size, or the shrouds either.
There been no info available on this class, how/where canI find out how much to tension the rigging? and would it be safe enough to leave the mast stepped while boat is on her cradle this winter?
Cheers, C_W.

 

[Salty John]

Typically you would use 12 - 15% of the wire breaking strength on shrouds and up to 20% on stays. If you increase the wire size it would be prudent to drop the % tension so that the load remains the same as the design wire size. Thus you will be applying no more stress to the hull than would be the case with the original wire size.

 

[Sans Bateau]

All that unnecessary additional weight aloft. What has that done for the stability of the boat? /forums/images/graemlins/confused.gif

 

[jj1000]

Echoing Gordon's comment above, watch out for the load on the entire system - I recall a number of years ago a friend overtensioned the rig on her 470 and ripped the chainplate out of the hull. That was the end of the boat.

Any chance of a photo of your beefy boat?

Cheers John

 

[colvic987]

and bonfire night will be here in a couple of weeks, so it should make a good fire.... /forums/images/graemlins/laugh.gif

 

[William_H]

The concept of deciding what static tension you put on rigging from the size of the wire is a flawed logic. It is excellent to minimise fatigue damage to the wire by providing tension on the wire. However wire on a yacht does not fail from fatigue. It does fail from corrosion where salt water settles into the bottom swages. So I don't believe we bneed concern ourselves with fatigue failure of the wire.

However by using this high level of static tension we put the whole of the load circuit under large stress. ie the mast the chain plates and especially the hull itself. The hull particularly can be distorted with large load over a long time.

So I reckon that static rig tension should only be enough to give the whole rig the correct shape and bend. When sailing loads will be large but if the tension of the stays is correct in relation to one another then the rig will maintain its shape straightness etc.
ie only enough tension to keep things tidy not sloppy. Yes the lee shrouds will always go loose when sailing.

Back to our poster. I would suggest that for the sake of stability that some of the duplicated stays be removed and or the wire size be reduced so that the rig is adequate to support the mast and no more.
from there the remaining wires should be tensioned to give a straight mast correctly supported under load. olewil

 

[ytd]

Those shrouds will break the boat before they get to 1% of their breaking strain. Just go sailing in a blow and loosen the leeward shrouds off until just before they start to flop about.

 

[Salty John]

1/4" (6mm) wire has a breaking strength of 3700 kg. If the boat won't take a 37kg load at a chain plate I wouldn't be going out on it.
If the boat was designed for 1/8" wire, then a tension on any size wire that doesn't exceed that load will do no more harm to the boat than the original spec. Whether the original spec is right is another question.
My 19 ft boat has 4mm wire tensioned to 240kg on the forestay and 160 kg on the shrouds.