Wharram and webbing beam straps

A u-bolt with rubber blocks between the metal and the beam.

 

Ok, I will post some pictures when I can.

I did try a ratchet puller for the initial wraps but didn't find it helped. Then used a ratched puller for the frapping turns but that hasn't made them any more stable either.

I'll see if I can find the Wharram instructions also, that may make the process more clear.

 

Wharram Tiki 38 building instruction sheet for beam lashings. I won't bother posting a picture because I built to plans and mine are done this way.

 

I was hoping for a picture of the wrap job.

 

Ok, I'll take a couple and post them. They do look just like the drawings but hopefully it will tell you something useful.

And for Gonzo, you've lost me with the talk of metal, rubber blocks and U bolts. Are you suggesting replacing the lashings with a long U bolt?

 

Yes, if you are looking at ultra low stretcht rope, steel can be an option. Also, it won't creep like dyneema or polyester does.

 

Fotos of one of the lashings. (The yellow line is my dinghy painter). They are all the same though the inner set have limited access and are fiddly to feed through gaps in the deck.

 

It looks decent to the casual observer with no experience. My recall of the old video was the frapping was not as narrow, but that just means the main lashings were a bit tighter.

What do I know? Not much. Noce looking boat, wouldn’t mind the full picture if you have it n easy.

 

Where did you get that line? I'm getting cheap utility rope vibes, maybe it's not low stretch yacht braid?

 

I think the original system was way better. The lashings were a marketing plan to make them look more primitive and Polinesian. Wharram says to simply retighten the frapping and leave the lashings alone. From the photos it does not look like your lashings are lubricated though.

 

It is good polyester double braid line, not cheap utility stuff. I have the same line in different colors and dimensions on my halyards, sheets and on the rope hinges.

I have lashed some of them with several more frapping turns and it didn't make any difference to the stability of the lashings.

The lines are not lubricated from end to end. Lanolin is slippery stuff and would make it very hard to tension the lines. If you look at the Wharram instructions above, the area under the wooden strake is where the line is jammed together. That part was lubricated.

Since I dont know the loads imposed on this, I'm reluctant to use ratchet straps. A 2 inch strap would have to be rolled to fit the space under the strake, plus the MBS is 6000lb max. There is 1 inch spectra webbing rated at 40KN available but I have not been able to find buckles rated for that. I can sew a loop on one end but need a buckle to allow for adjustment and stretch on the other.

I'm going to try splicing a single loop of 8mm dyneema with hard eyes each end and a protective sleeve. Then see if multiple lashings of smaller dyneema between them will give adequate tension. I can pull the smaller line with a ratchet strap or a winch then lash it to lock it down before finishing the end with knots. At least this way, there will be more even tension and retightening will not involve redoing the entire lashing, which is time consuming.

I dont think I have any fotos of the whole boat after losing my phone recently but I can take a couple if you want to see the boat.

 

I would try different line - the stuff you have may not be as low stretch as you thought. Or maybe it needs to be pre stretched and soaked? Better to get dyneema so you can rule it out. I wouldn't mess around with the larger line with eyes, that may introduce other issues.

Frapping is where you really get the tension from, similar to the classic bird on a wire introductory physics problem. So make the frap tight!

There are other catamarans using beams that are glued on place or rigidly bolted in place and are well proven in ocean voyaging. Back in the day Piver catamarans were breaking up, which probably helped bolster Wharram's argument. But the issue with Piver wasn't rigidity, it was strength. You don't hear stories like "so and so used dyneema lashings on his wharram catamaran and that's why it broke up."

 

I would be inclined to try some line that is not jacketed.

 

Thanks for the replies, a few possible things to try. It will be a little while as I have bottom paint and a few other things to do first, but I'll follow up later with trials and results.

 

Is the line jacketed?

What is the tbick black block looking thing under the beam? Looks like an inch thick rubber mat, but I don’t know the typical construction.