Compression Struts for Main mast beam T38

[Dave Vinni]

Whenever I am not busy at work my mind wanders to boat things.

My latest musings are to do with the T38 mast beam. As this beam rests on the inboard edges of the hulls and not in a full width trough as do the others, it puts a tremendous downward pressure on the hulls which is taken up by the aft beam inner lashings. These lashings creak and stretch more than the others as a result.

To overcome this I thought it would be simple enough to put a "dolphin striker" at the bottom of the mast beam quite close to each hull. Compression struts would have to be put between beam #2, the mast beam and beam#4.

This would be a light and simple means of resolving these forces.

Or am I smoking the wrong stuff again?

Cheers,
Dave

[Dave Vinni]

Hi Ron

You are right about beams 2 and 4 being pulled toward each other. To overcome this I would put a compression strut between the two. In practise it would not be possible as the motorwells are in the way.

This is more of thought than a serious plan!

I have used three types of lashing material, 8mm prestretch, "spectra" (which after a cruise to Mocambique and back proved it wasnt spectra at all) and now a 10mm line using 8 turns instead of 6.

I use a 4:1 block and tackle to tension the frapping turns and this has got all the lashings very tight.

Its more the creaking noise which is a nuisance as there is very little movement in the beams.

Cheers,
Dave

[The Ethnic Catamaran Company]

Umm, define 'wrong stuff'?

[Pete Rigby]

Dave,

Regarding the mast beam on your T38 - I must say I had misgivings about the detailing of this on my Tangaroa. As on your boat, each mast beam end sits on the inner side deck of the cabins.

Whether a boat has lashed beams, [yours], or fixed beams, [mine], I think that too large a point load is placed on the deck edge framing.

Your dolphin striker idea is interesting. A difficulty might lie in the fact that the wire rope anchorages on the strikers would have to be quite beefy jobs, as you would be aiming for zero flex. Certainly with the box beams I have made up, I would have to anchor the wire ropes to some sort of strap which went right around the beam.

The solution I have adopted is much cruder. Under the deck and gunwale upon which the mast beam sits, I have fixed an Iroko pad 500mm long. This is jointed to a strut, [laminated ply 300mm wide and 30mm thick],
which is bonded to each stringer on its way to the keel, to which it is fixed. Below the cabin sole, I widened this strut to 400mm.

A bit crude I agree, but so is the whole boat, and that is one thing I really like about Wharrams.

I've just read the line above. It doesn't sound very nice, so I will say that MY boat is a bit crude. In truth, the joinery shown in the pictures on this website is great stuff - very impressive

[Dave Vinni]

Hi Ron

To be frank I dont know what the constituents are of the lashing line I am using. I have replaced all the lashings after 4 years and they were all tight as steel and showed no sign of chafe or stress. I replaced them with the same stuff.

It is only the inner lashing of the aft beam that seems to be an issue. Not that is a major one, More of an observation than anything else, its not going to fall apart but does creak sometimes whereas the other lashings dont.

I will look around for some polyester and when I replace those inners will give that a try.

We have had weeks of lousy weather and yesterday started out bad. I had committed myself to a sail with Chris who helped me build the boat. It turned into a peach of a day. Flat sea, mild breeze. We beat up the coast against a onemaran with a deep keel and ballast tanks. The wind vane steered us upwind. We overtook the onemaran as its sails were badly set and they misread the current. When we sailed for home the kite went up - the vane doing all the steering and we left all tne other boats in our wake.

I need days like that!