A Photo & Discussion Forum for Wharram Design Enthusiasts
Hi everyone! I am sure many of you have some good ideas about this - how tight should the lashings for the standing rigging be? I have a Pahi 42 with conventional rig (ie bermudan cutter with boom). I am using dynema rope for all the lashings from shrouds and stays to chainplates etc. What I am puzzled by is that dynema doesn't stretch (or so I am told..) but I can't get my rigging to stay taught. I am thinking that possibly the problem IS the dynema - that because it doesn't stretch it is difficult to get it really tight in the first place. Anyone else using dynema, or any suggestions?
Thanks - Andy
Yes Andy, given one hull perfectly horizontal, the bow and stern of the other hull, should be free of moving up and down about 1'. For this very reason, personally I do not consider Dynema, Spectra, and other non stretch cordage, suitable for lashing the beams to the hulls, as the all purpose of lashing, instead of bolting, is that of having a flexible join in order to absorb stresses caused by the opposing pitching of the 2 hulls. This degree of independent pitching is what makes Wharram catamarans so brilliant and unique, however, this property, as desirable as it might be, poses a big challenge with the rigging. So much so that most cat designers give up the system in favor of rigid heavier structures.
Hi Raf, yes I agree with you regarding the hull to beam lashings. I use regular braided rope, but getting it tight, even with the frappings, is quite a job. As you say, when adding the rig onto this flexible platform, it is virtually impossible to produce a stiff setup. I can see why some stretch in the shroud lashings is a good thing - some elasticity to take up the slack when the rig is wobbling about in a big sea. It's not a problem when you're sailing constantly on one tack with the lee shrouds slack, but when we then hit some ferry wash it is like the whole boat is thrown around for some minutes, twisting this way and that! All the stress points get highly tested...! So maybe instead of dynema etc for the rig lashings, something equally strong but with some elasticity would be better? Is that then the prestretched polyester as per plans, and if so, does anyone have a brand name for me?
I use "Liros PreStretch". I think Dacron is also a brand for prestretched PES...
polyester double braid is what we use for the lashings. Then we frap with a mid grade of spectra. There is plenty of strech left in the lashing assembly this way even after using a truck to pull the frapping tails. Imo the frappings are designed to absorb shock loads. The beam to hull connections should not allow the rig to flop around. This will highly stress your rig and is not very effective either.
Hi - thanks for your replies. Using spectra/dynema for frapping a polyester lashing shounds like a good solution. But I'm not sure about your last two sentences - even though the movement of the hulls/beams has effect on the rig, the two lashing "systems" are seperate - all my shrouds and stays are led to the hulls, apart from the forestay bridle on the bow beam. I totally agree that the rig should NOT flop about, that's what I am trying to solve - so thanks for the tips! As the rigging lashings are narrow there is little effect from any frapping when tying off. Should I be trying to get the lashings tighter in the first place, for example using a winch to tighten them? Up til now I'm using a wooden handle at the end of the line as a good hand hold. If you use a winch (or truck!) how do you then release it to tie off without loosing the tension?
when using a winch , i use the gaff halyard so the lashing goes as vertical as i can , then i tie a secondary small rope to the lashing bundle right next to the most convenient turning point , so i can release the lashing rope without losing tension and then tie the bitter end. Other way that works fine is by holding the lashing bundle with a wise grip (can jump into the water though) next to a turning point. it works fine but might damage the lashing rope if used without any protection (a piece o fabric).
but as i said in another previous comment, i am trying to find the point where the lashings are in the delicate 'balance tension' between being lose and too tight. my goal is to get the fore stay as straight as acceptable (i will never get a straight tensioned fore stay without bending the wooden mast) and a straight mast. I would really love to have JW opinion about all this topic. I guess that the correct tension for the gaff rigged boats, as per JW design , would be the point where you put the common sense. our boats are not racing boats, and avoiding material damages and stress will be always the main goal, without to compromise speed too much.
We should also meditate how the tension of the rigging will perform under extreme stress in the event of a storm. Will a over tensioned rigging stand the added stress of a storm? will a poor tensioned rigg compromise the mast structure?
In my believes so far , the bottom line would be to tension the mast till it starts to bend. a fine rope stretched from the top to the foot and against the mast will help to visualize when this tension point is reached.