Pro's/cons - solid vs lashed on beams?

Pros/cons - solid vs lashed on beams?

Pro's/cons.For 9m cruising cat? Whether demountable or not? Rigidity vs a bit of give..... what are the comparative stresses to beams and hull joins? Could allow an easy method for variable beam width. Either aluminium or wood? Round or squared? Is cost an issue?

 

Any of these methods (rigid or flexible, wood or aluminum, round or square) work as long as they are properly engineered. Emphasis on the "properly engineered" part.

Lashed beams may seem a bit primitive, but as thousands of Wharram boats have demonstrated, it can work quite well if the stresses on the connections are accurately calculated and the system designed accordingly.

For a high-speed boat, where the fine details of rig tuning are critical, the advantage would likely go to the more rigid platform. In a cruiser, I think it's more a matter of personal preference- and being able to accurately predict the stresses on the connections. Any of these methods will fail in a spectacularly expensive manner if over-stressed, and all of them can serve well for years if they are well designed.

 

"accurately calculated"

I think that is unlikely... With Wharram it is more a mater of copying pre-existing forms.

Give vs. no give. Obviously there is no need for give. The vast majority of multihulls do not have give, so it is clearly non-essential. Maybe a certain design will rely on it for some structural reason, though I would find it hard to see that as an advantage. Wharram-wise they have been commercially built with bridge decks or even aluminum tubes. The biggest bad thing about the Wharram approach is the how wasteful it is. Modern Wharrams often hit the water with deck pods as well as beams. Integrating these elements into one (possibly demountable) piece would increase structural capability many many fold. Moving from something of a section of 8 inches to something with a section based on, say, 30 inches would be a 60 fold increase. So even if the 8 inch beam needed to be babied the larger section would not. This would be possible by using the habitable form as the basis for main beam dimensions, etc...

The Gougeon Adrenalin trimaran, and several earlier version of the same thing for the little Americas cup etc... made use of suspension in the ama connectives. This allowed significant ama rotation to contour to waves etc... This design has been a winner, though to what extent has been frustrated by the tendency of those facing it to ban it... But this is far more comprehensive than a little give.

 

I don't think the Gougeon's flexible tris were banned anywhere. The tris weren't allowed in the normal C Class races, but it seems that's because the C Class and its rules were specifically designed for cats. And some say that the tris were only competitive in light winds, anyway.

The F40 Adrenalin did well, but I think it broke and retired from the series before the big-ama tris came along and started to dominate.

 

Wharram's lashed beams actually work very well. The idea is that there is a minor amount of shock absorbtion in the beam to hull connection. If you look at other similar catamaran types (demountable) you will see that very many of them have problems with failures at these connections. The shock loading does get intense here. With the lashings you have the ability to absorb this shock load with a lightweight solution as opposed to a heavy rigid connection stout enough to handle these loads. Obviously if you are building a bridge cabin catamaran there is ample mounting area to dissipate these loads through a much larger surface area. Demountability would then be another issue. Our 36' Wharram style catamaran will fit into a single 40' container or transport on one standard load truck for transport. Demountable can be very cool. Obviously you can't do this with a condomaran. David

 

Allow me to quote from page 20 of the "The Wharram Design Book":
"...shipping authorities in Australia have been requiring that commercial catamarans should be strong enough to have one hull clamped to a wall without the bridge deck and other hull falling off (i.e. cantilevered).
We have used this safety standard, plus a safety factor of minimum 3, since 1957, plus other methods of stress calculation based on our many years of sea observation."
Wharram's "Classic" designs used rigid mountings on the Maui, Hinemoa, and Tanenui. The larger "Classic" boats used angle brackets with vertical bolts and rubber compression blocks. The "Pahi" series pinned the cross-beams with a single large steel bolt in a pivot point on each hull, with lashings being used as shock absorbers. The "Tiki" series (1982 on) were the first Wharram designs to rely solely on lashings (rope on all save the 30, which was originally designed for straps). Regarding lashings "The Wharram Design Book" states: "...We consider both the Pahi and Coastal Trek lashing methods safe only with the use of modern high tech synthetic ropes."
Wharram's designs were inspired by, not copied from existing ethnic forms. They are evolving toward a minimalist modern expression of those forms.

 

Matt: I'm not familiar with the Wharram design, but with a pre-tensioned retainer such as a lashing holding a beam against a hard point, you should have a rigid structure that does not flex until the tension of the lashing is overcome, after which it should return to its former position. That assumes the flexibility is in the lashing not the beam. Properly designed the fine-tuning of the rig should not be impacted IMHO.

 

To answer the origional question:

demountability can be nice but it may cost you money, time, convenience or weight. On a 30' cat you have to assess if your going to build in bits and assemble at the waterside ? Once on the water are you going to take it apart again ? If either answer is yes you need demountability.

Rigid vs flexible lashings: Provided the boat is designed properly it won't break. The hull shape and the water you sail in matters a lot more than flexible lashings. Then there is personal taste. Some people like graingers work, others wharram. Both boats will sail, one will usually do something better than another. Your path is determined by your destination.

Variable beam ? Your taking about folding or demounting a 30' cat for trailering ? many have tried to build that boat. IMO the closest to a convenient trailer cat over 20' is the wizard/sango of Richard Woods. Others will no doubt disagree. A 30' sango with nesting coach roofs is something I've had in mind for some time. Doable, the question is would it be affordable/convenient/useful ? I don't know.