Changing Catamaran Beams from Wood to Composite.

Discussion in 'Multihulls' started by Mark O Hara, Aug 4, 2020.

  1. Mark O Hara
    Joined: May 2020
    Posts: 44
    Likes: 4, Points: 8
    Location: Mariveles, Bataan, Philippines

    Mark O Hara Junior Member

    Sorry! Couldn't resist! :D Joking aside yes I could, but to serve what purpose? Adding more material will just add more weight.
     
  2. Rumars
    Joined: Mar 2013
    Posts: 815
    Likes: 314, Points: 63, Legacy Rep: 39
    Location: Germany

    Rumars Senior Member

    Yes you would. Depending on the density of the lauan you have now, every beam would gain 30-50% in weight. But I don't see any other solution if you want to keep the boat original. It's a big heavy boat anyway, what's a few hundred kilos more?

    They can do all that but it is only a bandaid, it's a design problem. The flexible lashings allow movement, the result is abrasion and microcracking, then water ingress and eventually rot. The process is accelerated in the bigger, heavier designs because of the forces involved. Small boats using high strenght line or webbing under high tension have far fewer problems because they actually succeed in making the joint rigid.
     
    Mark O Hara likes this.
  3. Mark O Hara
    Joined: May 2020
    Posts: 44
    Likes: 4, Points: 8
    Location: Mariveles, Bataan, Philippines

    Mark O Hara Junior Member

    Every Wharram is different because it really depends on the builder, beams are always a problem because they move more so than other boats, movement causes wear and wear invites rot. Ironically it's what attracts a lot of people to Wharrams, they're nicknamed the "Landrovers of the Sea" because "they'll not get you there fast, but they'll get you there". Because of their flexibility they'll handle a rough sea very well but it looks as if their greatest strength in sailing is also their greatest flaw when it comes to maintenance.
     
  4. Rumars
    Joined: Mar 2013
    Posts: 815
    Likes: 314, Points: 63, Legacy Rep: 39
    Location: Germany

    Rumars Senior Member

    One can argue about the percieved advantages of flexibility (as preached by Wharram), but the disadvantages are undeniable. Also undeniable is that the best Wharrams are those that succeed in making the joints as rigid as possible (study his progression of lashing arrangements over the years). The rope or webbing size must be sized by creep, not strenght, and the lashings must be tightened regularly. Tikis that use webbing with ratchet tensioners have the fewest problems because the webbing is prestreched and oversized (minimal creep) and retensioning is easy with the ratchets, so it is done often.

    Back to your weight problem, by my estimation you have about 1 cubic meter of solid wood in the beams. If your lauan is ~600kg/cum (much variation there, +- 50kg) and yakal is 900kg/cum it would be an increase of 300kg/cum. Fabricating the ply webs from laminated yakal will add some more, maybe 50-70kg. A conservative total would be 400kg over the existing structure, eating into your 4500kg payload allowance. The benefits are wastly increased strength and superb durability (even unpainted).
    Another contender for beam wood is tambulian (Eusideroxylon zwageri, int. trade name Ulin), wich is even harder and more durable.
     
    fallguy likes this.
  5. fallguy
    Joined: Dec 2016
    Posts: 3,344
    Likes: 440, Points: 93, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    If nothing else; enjoy the comedy.

    Composite is generally about 30% lighter.

    If you build 4 beams as is at say a 20% reduced weight with composite/glass and two end beams at say a 40% increased weight, and you get the rot resistance; seems like a good option. AH said the issue was with two of the six beams if I read him right. Since the composite is generally 30% lighter; it seems you have some room; weightwise and a much larger beam won't be 40% heavier unless it is tons more glass.

    The greater issue is whether building two end beams higher is impossible on balance; other structures/remodeling. Or if the idiot in the room read him wrong and you need stiffer decks for any composite.

    I am surprised something like Coosa 26 bw can't compete better with plywood.

    Also, if you could build four beams in composite and two beams in wood?; seems like a viable solution as well. Perfect, imperfect, better?
     
  6. Ad Hoc
    Joined: Oct 2008
    Posts: 6,609
    Likes: 617, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Kind of yes.

    If you look at the main hull and its 6 beams:-
    upload_2020-8-7_8-18-19.png

    When subjected to a transverse bending moment the hulls will do this:
    upload_2020-8-7_8-18-55.png

    They rotate towards each other with the vessel's CL as the rotation.
    This means each of the 6 beams contribute to the resistance of the bending moment, as they are all working at the same time.

    But, the situation is different in a pitch connecting moment. You have this:
    upload_2020-8-7_8-22-4.png

    You have each hull rotating against the other, with the centre of rotation roughly midships, which results in this:
    upload_2020-8-7_8-23-20.png

    Which means the closer you get to the centre of rotation, the less the beam contributes to resisting this moment:

    upload_2020-8-7_8-24-31.png

    The best way to image this is if someone places their hand at the end of a door to push it open and you place your hand at the same location, but opposite side, you can hold the door in a stationary position. But the closer you move your hand to the hinged end... it gets harder to resist that push at the end....so 2 things occur... i) the push back to hold the door needs to be much greater (simple moments) which translates into a stiffer beam is required... and ii) the closer you get to the centre of rotation the less effective the beams are in resisting this torsional moment....taken to its logical conclusion, if you had your hand now at the hinge, it is no longer a push force - back - to hold it...it is a pure rotation only. i.e a shaft resisting pure torsion. And the diameter of that shaft to resist that torsion ends up as massive!

    So, in effect the only beams that are able to resist at the outer most beams, beam no. 1 and 6.

    But since those beams and with the given dimension, are insufficient to resist this pitch connecting moment, you need other ways to stiffen up the structure.

    With composites being low modulus, you can use a higher modulus materials, like aluminium. This is straight forward and easy to do. Despite what others are suggesting. This merely suggests they are unaware of the mechanisms involved and the ease at which this can be done. Every boat of any type of any construction, has failed at some point in history... thus, that is not an argument against a particular route to a solution. Understanding the mechanism of failure leads to the solution.

    So with the option of keeping what we have.. the solution is the stiffen up the existing beam/locations by other means. This means, if it is possible to connect beams 1 to 2 and beams 5 to 6 to create one much large beam at each end... since there will now be much more second moment of area, at each end, to resist the moment. Having a deck/flange from beam 1 joining beam 2... top and bottom, significant increase the stiffness. If this is a viable option, only Mark can say, then the only thing to consider is the transfer of reaction loads from the beam to the hull.
     
    fallguy likes this.
  7. Mark O Hara
    Joined: May 2020
    Posts: 44
    Likes: 4, Points: 8
    Location: Mariveles, Bataan, Philippines

    Mark O Hara Junior Member

    I've been thinking about it since you mentioned it yesterday, something along these lines of pairing the beams up to make a much stiffer ship altogether? That would be a hell of a lot of work and may change the sailing characteristics of an already well proven vessel.
     

    Attached Files:

  8. Mark O Hara
    Joined: May 2020
    Posts: 44
    Likes: 4, Points: 8
    Location: Mariveles, Bataan, Philippines

    Mark O Hara Junior Member

    I like your attitude and I totally agree with you , we can't get Coosa here but we get a lot similar from ALS Composites in Australia. I thought something along those lines would be a lot easier transition, take the ply/solid wood out, replace it with something like Coosa and no one would know the difference, except me. I just want it to perform the same.
     
  9. Ad Hoc
    Joined: Oct 2008
    Posts: 6,609
    Likes: 617, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    upload_2020-8-7_10-9-25.png

    If beams 1 and 2 and 5 and 6 are those highlighted in blue.. then it is only these that would need to be joined.
    But it does depend upon the material properties too... so material selection is also a factor to consider. Going more exotic means strength strength ... that can aid matters.

    Why do you think it would alter the sailing characteristics? All you are doing is adding a bit of extra structure, not significant amounts to affect the radius of gyration to that sense. You're not altering the hull form. It would also affect the VCG...where that is..and most likely lowered, but by a small amount, as your main VCG is just around main deck level anyway.

    But it is your boat..and I can only offer options.

    So... you can do pretty much whatever you want... but like all things in design.. it is a series of compromises. Thus which comprises (wood - composite - ally - or combo) will you feel happy taking...for your solution?
     
  10. Mark O Hara
    Joined: May 2020
    Posts: 44
    Likes: 4, Points: 8
    Location: Mariveles, Bataan, Philippines

    Mark O Hara Junior Member


    The reason why Wharrams are nicknamed the “Land rovers of the Seas” is because they flex, kind of like the independent suspension on a Land rover crawling over obstacles, the hulls adopt the same attitude crawling over waves, hence the 150mm movement on a forward beam recorded on a Pahi 63 in 3m seas. After such a sail, you just get into your dinghy and tighten up where needed. I’m not saying it’s better than a rigid hull, that’s just how they are designed based on technology the Polynesians have been using for thousands of years.
    Pahi 63 Lashings.jpeg
     
  11. Ad Hoc
    Joined: Oct 2008
    Posts: 6,609
    Likes: 617, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Indeed....as do the ends of an aircraft wing during turbulence.
    The point is.. so what?... the 'what'... is what is the purpose of the design?

    Without an objective.. what is the design "point"???
     
  12. trip the light fandango
    Joined: Apr 2018
    Posts: 419
    Likes: 75, Points: 28
    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    Have you considered laminating ply instead of solid wood, the relative weight of the original beam could be a guide and a strong box with thick walls and sections but hollow otherwise, or just solid, laminated to mimic your original shape
     
    Mark O Hara likes this.
  13. catsketcher
    Joined: Mar 2006
    Posts: 1,298
    Likes: 143, Points: 63, Legacy Rep: 790
    Location: Australia

    catsketcher Senior Member

    If this was my boat I would think of a different solution.

    The Pahi 63 has 2 deck pods between beams 2 and 3 and 4 and 5. These are big and strong items that have huge moments of inertia. They sit on the beams but seem to be parasitic in terms of structure.

    I would make new composite beams and integrate them with glass and epoxy to the pods. I would epoxy glue and glass the whole twin beam and pod structure INTO the boat, using the current beam recesses and get rid of the flexible arrangement. The result would be a watertight and monocoque structure with no issues from water ingress into the beams. A couple of gussets from the back and front of the pod would increase bonding area and reduce corner induced stress concentrations into the hulls (like chamfer panels under bridgdecks).

    The front and rear beams could remain flexibly mounted, this is the case with many stiff production cats that have pin jointed front beams. The flex would be reduced and so would any wear from lashings.

    No other cats use lashing to tie hulls together and with the judicious use of unidirectional glass, much like the chainplates I talked of earlier, the owner could use the tie downs and beam hardware already in place. The result would be a stiffer, much stronger (through the use of the deck pods in the beam structure) and less rot prone boat. My 20 year old cat has no rot because there are no places the wood can be exposed - remove the rot problem by removing the rope and movement.

    I love reading about the exploration of the Pacific with outrigger canoes and voyaging double canoes but I don't think that all of the design features in historical designs are necessary, especially when almost all other catamaran designs do not have flexible structure arrangements. Flexibility may be essential when building with logs and coconut fibre, but we have uni glass, epoxy and dressed timber now that can handle much higher loads. In fact the movement can increase load in beams as the beam ends pivot around the end and the lashings in a seaway. If the beams were glued and glassed in, the use of uni glass could feed out the stress so that the stress in the beams under the lashings and around the lashings in the hulls could be reduced.

    E glass and uni is a nice material to use. I would suggest gluing the beams in and then not including the glue shear strength in strength calculations. Then spec the uni strands to be way over strength compared to 6 wraps of the rope. Not especially hard engineering. Glass the beams in top and around the edges with double bias and double bias the pod onto the beams.

    I used to own a 31ft by 28ft trimaran that used underwires to stiffen the crossbeams. I went sailing offshore on another sailors version and his underwires were much tighter than mine. I remember how his boat seemed much more structurally sound with its propensity to operate as a single entity rather than 3 individual hulls as mine seemed to. As soon as I got back to my tri I tightened the underwires and my boat felt much nicer, all three hulls operating in a seaway as a single whole. I didn't want my hulls to move in a seaway highly differently, it felt better to have them stiffer. A wharram, with its deep vee hulls already has a sweet motion designed into it. It probably doesn't need any more flexibility built into it.

    cheers

    Phil
     
  14. Mark O Hara
    Joined: May 2020
    Posts: 44
    Likes: 4, Points: 8
    Location: Mariveles, Bataan, Philippines

    Mark O Hara Junior Member

    I did thanks, but I figured the designers chose solid because of the cell structure and long fibres spreading the load out to the ends of the beams. Also I would imagine with all ply there would be a lot of end-grain therefore a lot of capillary action.
     

  15. Mark O Hara
    Joined: May 2020
    Posts: 44
    Likes: 4, Points: 8
    Location: Mariveles, Bataan, Philippines

    Mark O Hara Junior Member

    Thank you for all your recommendations. Some good ideas in there that I may use further down the line. My only complaint about the boat is the rot on the beams other than that I’m happy enough with it. All I asked and all I wanted to change were the wooden beams for composite ones nothing else. Failing that I’ll go back to wooden ones but I’ll use a lot more rot and wear resistant Yakal graving pieces where most of the wear and rot takes place at the lashing and touch points. The rest will be built with Red Lauan as before to keep the weight down.

    One of the things I like about the Pahi 63 is that if needed to I can pull it up on the slip and strip it down with just the aid of a fork-lift, so this can be done anywhere without the expense of hiring a crane. I’d like to keep to that way.
     
    fallguy likes this.
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.