Beam connection

[Raf]

Hi all. I am looking to buy a Wharram 30' to 36' and found a couple I can afford however, the design doesnt  seem correct to me, where the beams connect to the hull. Here are 2 pics of a 36' Tangaroa.

My question: is these set up, standard to the new Tangaroa design or was modified by the builder?

Thank you

[paul anderson]

No beam lashings? Are the beams bolted on. The deck, does it ''float'' on the beams?

cheers paul.

[Raf]

Hi Paul. You hit the nail right on the head, it was my question. I am too far to inspect the boat and all I can get is the owners reply to my emails. When I saw these pics I did not recognize the typical Wharram beam connection: beams overlapping the hulls and lashed. The owner replied, he build it to specs but, stretched the length from 36' to 37' so, I thought might be the design as changed. Thats why I decided to come here and ask to savvy Wharram builders. The boat was started to build 5 years ago so presumably it could be a new Tangaroa design or, the owner has taken the liberty to change the connection to make it "better" and do the typical amator F up

[Raf]

Here is another pic of the same boat. Beams aren't sticking out from the hulls to provide lashing space. Also aren't the beams supposed to be right on deck and not cut into flash to the hulls?

Thank you.

[Galway Bay]

It is a long time since I saw a Tangaroa. This looks like a beauty.

I have one sheet from Tangaroa Mk IV of general details. The beams are in troughs which are open only on the inboard gunnel, that is the outboard bulkwark is continuous. There are no pads etc. to take lashings but I do not have the sheet showing how the beams are attached. In other words my drawing is the same as this photo. I also have a photo but very small so difficult to see small details but it also seems to be like this. Both of these from approx. 1990. I am thinking the owner is right.

Lashings were I think first used on the Pahi range. These Classics are older designs.

I share your distrust of amateur changes but I do not dismiss them all. Rory uses his own system on "Cookie" and I myself am much happier since I changed my system. There are other examples also. No doubt there are many "botched jobs" out there also unfortunately.

This looks like a boat well worth viewing.

[Raf]

Ok. it seems as everything is done right. The beams are  per drawing. The owner stated there are 2 options on connection, one is lashing and the other is mounted on rubber pads. Here it is.

[Raf]

Hi Galway B.

Thanks for your input.

You right, not all personal alteration are bad. After you mentioned Rory's own system, I went to search it and could only find pics from far distance. It looks to me that he has done very similarly to Wharram's specs. See sketch, is what I think he did, probably to half the number of lashes. In all cases his system is well proven. My hat off to him for his achievements. 

In which way are you happier with your own system? How did you do? 

Cheers

Galway Bay said:

I share your distrust of amateur changes but I do not dismiss them all. Rory uses his own system on "Cookie" and I myself am much happier since I changed my system.

[Galway Bay]

I changed it to make it even more Wharram style than it was !!

  The beams pivoted on a bolt in the centre of the trough. This meant the fulcrum point when the beams came under load was too close to the lashing leading to high loadings and a system that needed constant retying in / after rough sea. I built pads at the ends of the troughs to lash the beams down to [ on rubber mat ] and used "I" section beams instead of hollow. I fitted blocks to the underneath of the beams to stop sideways movement. By reducing the number of beams to 3 [ from 4 ] I was able also to support the mast on a full crossbeam and eliminate the half beam in the centre. After 2yrs of hard use I can have confidence in this system now. 

 The fore / aft netting beams are quite strong and bolted in a short trough which sets the hulls the correct distance apart and holds them there. One through bolt at each position so the beams can move up / down but not in/out. This is the original arrangement and is a great help when assembling as the hulls are quickly aligned and held. I posted other changes in my discussion " 3x crossbeam Pahi 31"

Personally if I was building a Tangaroa I would prefer the lashing option but I would be happy to buy a used boat with the alternative system.

That discussion is in NEWS page 5 and there are pictures on my page

 Safe sailing .

[Raf]

So, you've taken the centre bolt out. This results on compression on a rubber pad and tension on a lash. this two forces are the full width of the hull apart. Doubles the length of the resisting lever and halfs the loads. It's a good job, the only concern are the locators. They might be lifted high enough to let the beam slid sideway. I thought of the same system with the difference that I would use cross lashing as locators that would allow full pivoting without side movement.

[Galway Bay]

Hi Raf - you have obviously given this some thought, thanks.

With this system I do not see visible movement of the beams in use. I can feel it if I put my hand across the beam at the end of the trough. I have no need to re-do the lashings after two years' use. The lashing system I use could not be simpler. Just round x5 times then frap with the end of the same line by hand tightening. I do this afloat which allows the hulls to take up their position naturally. The blocks are 2" [5cm] deep which  I think secure. Remember these hulls are held firmly at the correct spacing  by two strong beams bolted at bow and stern.

Trying to predict the exact position for these blocks would be difficult. Much simpler to assemble then screw them on from underneath using a thin spacer of 2mm or so.

In fact I had done a lot of daysailing before I got around to fitting the blocks and the beams do not seem to move in normal use. Needless to say I do not recommend this.

The wider, stiffer boat has a much better "punch" against headseas. It also seems faster. In fairness to JWD they did explain that the old width of 14ft was chosen to allow the boat to be taken through the extensive European canal system. A canal journey from the North sea to Med. sea would if fact be a wonderful idea.

 

[Raf]

It,s me thanking for the info kindly offered as this will greatly help me in the near future when I will own a W cat.

Point 1 gives me some to think. The lesser the movement the greater the structural stress this, in combination with a wider boat explains point 4 which it gives stronger piercing force to the bow attacking incoming waves. However, more are the times when the bow noses-in dip under water which, in turn increases stress. Looking from this perspective it can be predicted that in severe conditions, this set-up would give up earlier than leshed softly allowing more pitching degree of freedom.  

Point 3: Very possibly there is no need for locator blocks nor cross strappings as the lashings at the extremity of beam are holded from falling off the edge by a riser block and that alone should be sufficient to prevent any side slidings.

Cheers and thanks again

Galway Bay said:

1...With this system I do not see visible movement of the beams in use...

2... hulls are held firmly at the correct spacing  by two strong beams bolted at bow and stern.

3...the beams do not seem to move in normal use...

4..The wider, stiffer boat has a much better "punch" against headseas...

[Patrick John McGrath]

Perhaps I am a sceptic but I have never been entirely confident about this idea of allowing things like crossbeam ends to flex.  And the idea that the ancient cats, and even todays native Pacific cats and proas used lashing to provide flexability.  It may be the reverse----and flexing is the result of lashing---- since no other type of attachment is available to the local Asiatic natives.

Movement of the two hulls allows a twisting of the entire platform, which plays havoc with the rigging and serious  fatigue in all adjoining parts.  Even Nat Herreshoff didn't persevere with free flexing hulls after his first catamaran,  because it waggled the bowsprit and upset the set of the foresails.

The Tehini I had a sail on in Toronto had steel " i " beams bolted rigidly into the re-inforced gunnels, by 1/2" bolts thru the flanges of the beam.  The steel beams were no more heavy than the solid wood beams and the bermuda rig set well.

[Galway Bay]

Patrick -

 I mostly agree. Even if those Pacific craft were bolted together the arms made of tree branches would flex.

However the degree of accuracy needed for rigid construction is probably not appropriate to home - building. It can be done for non-demountable types where the cross structure is built in.

I have in mind an early race to The Azores And Back when a Tri. actually arrived before the committee and had to get a finish time from the local police - station. This was in the early days and was vastly amusing to multihull enthusiasts. The boat unfortunately broke up on the return. It used Alu tube beams and the bolts wore away the boltholes in a vicious circle once started - The more they wore the more they moved, the more they moved the more they wore .......

There had of course to be an enquiry and the engineering opinion was not as we might expect heavier tube or reinforcing which would only slow the process not eliminate it but that the boltholes needed to be placed and drilled to an accuracy of 1/1000 in to prevent the problem starting. Try that with a handheld Black -and Decker !!

I believe rigidity is good for all aspects of performance but a small amount of "play" or "give" at the connections is good for the amateur builder and to avoid local stress.