structural changes

Discussion in 'Multihulls' started by taezow, Aug 28, 2010.

  1. taezow
    Joined: Jul 2009
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    taezow Junior Member

    Hi all,

    I'm wanting to make some structual changes to the rear bulkhead, on my Searunner 25.

    This is a rough drawing of existing bulkhead. It is out of 1.25 ply

    existing-bulkhead.jpg

    This is the porposed change.

    proposed-change01.jpg

    I think that this change should be ok, but not sure what strucual members will be required. I would guess that a laminated 3.5" x 4.25" should be plenty strong for the vertical, with the top piece a 2x4 with 1x4 on top to make a T beam with main sheet traveler on top of that.

    The piece I'm cutting out is 2" x 1.25" ply wood.

    I build houses not boats, so I'm guessing at this.

    Thanks for any advice and or imput
     
  2. ThomD
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    ThomD Senior Member

    Seems kinda overbuilt, you might think of contacting John Marples. He handles those plans and is an engineer also. Helpful person. He is a little easier to get by phone, but you could try email first since you have the drawings. A lot of it depends on how you bond the bits together.
     
  3. catsketcher
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    catsketcher Senior Member

    Stop

    I wonder why you are wanting to change the cockpit of the Searunner design. It seems an awful amount of work for little reason.

    The weight of the cockpit will be in a less ideal position and even if you get more room you will have little payload to do anything with it. Be very careful about making big changes without drastically affecting the boat's ability to sail and be seaworthy.

    cheers

    Phil Thompson
     
  4. hoytedow
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    hoytedow Carbon Based Life Form

    The arch over the pass through opening contributes a lot of strength to the structure. If you eliminate the arch you could end up wish-boning the bulkhead, basically turning an O into a V. Guess which one is easier to deform.
     
  5. catsketcher
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    catsketcher Senior Member

    not the way to reinforce

    If the idea is to reinforce the bulkhead then this is a very bad idea. The 2x4s will bend and the boat will deform. If(and I think it is very silly) the structure should be changed it would be better to go box beams under the cut out. This is the normal way.

    I see very bad things ahead for this build if this is the standard of engineering and design. Boats aren't houses for a reason and the Searunner has an awfully good reputation for a reason.

    cheers

    Phil
     
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  6. taezow
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    taezow Junior Member

    I realize that to cut the top out of the "O" will eliminate the structural integrity of the bulkhead. What I’m trying to do is move the compressions part of the "O" 18" higher.

    The 18” legs will take stress that will try to bend them. An 18” 4x4 will takes the weight of 2 of my boats before it noticeably bends let alone breaks. The top piece will be in compression and this loading is now held by a 2x1.25 piece of plywood. The main sheet loading on the top will generate a bending force of maybe 2000lb. A 4’ T beam built from 2 laminated 2x4s will easily carry this loading.

    The weak link in this design will be the 4 conection points. I will epoxy them and use several bolts.

    Is laminating and glassing 3 clear#1 2x4s, weaker than making the same size box beam?

    Should I use 1x4 or plywood to laminate the beams instead of 2x4?

    Thanks
     
  7. ThomD
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    ThomD Senior Member

    "Is laminating and glassing 3 clear#1 2x4s, weaker than making the same size box beam?"

    Assuming perfect wood fiber, and similar species solid will be stronger than ply for several reasons: 1) It has more material, and weight in it 2) it is all linear fiber so for beam type functions it has more oriented strands in it than the ply. So in your terms a header made out of plywood would be weaker than a header made out of glue lams. However, a section with 20 percent walls would have roughly 80 percent strength, and strength goes up by the 2nd power when you adjust dimensions in line with the load, so minor changes in beam dimensions would give a similar strength beam with less weight. However, you would have to deal with the fact that the plywood will be significantly weaker due to fiber orientation and quality control factors. So your optimized choice would be some linear wood fiber in the load faces, and the plywood in the sides of the box that are like the web in an I-beam. See also construction issue below.

    "Should I use 1x4 or plywood to laminate the beams instead of 2x4?"

    You should never use 2x4 for two reasons. One, in a marine structure that you are presumably going to glass and epoxy, you should never use a section thicker than 3/4". If you do the internal stresses can outpace the ability of the envelope to accommodate stability. Two, while 2x4 can be good source material for wood fiber, you need to start to think of wood differently when building boats. The fact that wood for housing is graded, at high speed on an assembly line is of no use to us. One can find near perfect sticks at my local HD from time to time. But I go on the hunt with a wood fiber mentality. There are places in NA where the local construction grade lumber is boat grade, or it was last I visited parts of Oregon, etc... but one has to look at it like you are judging a beauty contest, and with a table of weights, approximate strengths, and workability/capabilities in your mind.

    "The weak link in this design will be the 4 connection points. I will epoxy them and use several bolts."

    That is quite possibly structurally adequate, but will not hold up as well as some alternatives might. For instance hardware bonding some all thread in there, or working to flow fiber around the corner.
     
  8. cavalier mk2
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    cavalier mk2 Senior Member

    The published cruising payload of a Searunner 25 after normal sailing gear and crew (2, I think) is 400 lbs.
     
  9. Petros
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    Petros Senior Member

    It appears you want to enlarge the companion way as well as moving the mainsheet track high out of the way. I am sure this will improve the comfort of the boat quite a bit, but you dont want to do this at the expence of safety. I am a structural engineer who works with wood daily, what you are proposing is possible and will likely work out fine, but there is a big risk that if just one link in your new structure fails you could not only ilreparably damage the hull but also put your life, and crews life, at risk.

    I suggest you either contact the designer and have him work out the details of the proposed change (if he is willing), or find a local marine engineer familiar with this type of construction and pay him to develop the details. IT is well worth protecting the value of the boat, and you and your familiy's life.

    The bulk head you attach the vertical strust to might not take the new load distribution well and buckle contributing to possible failure, the joints at the ends of the horizontal member appear too little contact area for adaquate stength. I would consider putting in plywood gussests here to distirbutre the load and stiffen the members. You also run the risk of buckling the members under high compression loads, the shell structure of the orignal hull is not as subject to buckling as a free unsupported straight member (consider the horizontal member will be under large compression load the the main sheet will also be pulling up on it at the same time!).

    Another thing to consider it the finished appearance, if it looks awkward and unattractive, no matter how functional, it will detract from the resale value of the boat.

    I could do the design for you if I had more information about the boat, and help you with all these issues, but I think it might be to your benefit to have someone local to work with.

    good luck.
     
  10. Waterat
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    Waterat Junior Member

    Hi, I think the design of what you propose is 'ugly' for what you are trying to do. Also you are creating big windage on your main hull; on a reach you are
    going to bury your leeward hull much more with a resulting loss in speed. I'd
    give the designer a bell. My two cents worth, no offence meant, J.
     
  11. alan white
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    alan white Senior Member

    This is a very simple fix but without any design knowledge nor any experience in boat construction, this could turn out to be a real boat-messer-upper.
    In short, you need someone to draw a flattish bell-shaped curve that is high enough to allow height at the hatch, has enough visibility over the bulkhead to each side of the hatch, and is properly structured to carry the desired loads while appearing to have been designed that way from the start.
    Forget Home Depot materials. You need a sheet of marine plywood and some epoxy and a qualified boat carpenter to assist you. Otherwise you will at the very least destroy any chance to sell the boat one day, let alone you might comprimise the boat's strength.
     
  12. taezow
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    taezow Junior Member

    I'm committed now. I have cut the rear cabin off, cut out the top of the bulkhead, ruffed in rear cockpit and started on the over head brace.

    Tri construction_08.jpg

    Tri construction_07.jpg

    I moved the uprights out for better visibility and more pleasing and structural angle. The top height is perfect, sitting I look under it and standing over it.

    The cock pit railing on the motor side will be low so I can get to the motor easily. The back will be low to give clearance for the back stay and tiller. They will be 2’ 4” above the water line.
    Will this be high enough?

    The top piece and legs of the over head will be 4 clear doug fir 1x4s laminated together, no more 2x4.

    I play to use ss all thread to reinforce the joints. What I’m thinking for the lower connection is a vertical piece going into the plywood bulkhead and up vertically into the legs. For the top connection one that goes through diagonally in the same plane at the structure. I’m not sure that this would be any better than imbedding them perpendicular like you would with bolts?

    I’m most concerned about the overhead piece buckling, with the compression of the amas and the load of the main sheet. It is now 5’ long. My thinking is to imbed a 5/8 ss all thread into the center of the beam. Perhaps 4 smaller ones around the perimeter would be stronger?

    Your input is greatly valued thanks
     
  13. catsketcher
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    catsketcher Senior Member

    Please STOP

    This is the last time I will say this but you obviously have not looked at how multihull beams are put together. Go and look at a few. You won't see single bolts on joints such as these. We don't use bolts and timber beams, we use ply gussets, glass and timber. You won't see a compression member such as yours. The system would probably take the mainsheet load but the crossbeam load too - No way. You are going into totally untried territory.

    This is unsafe and I fear for anyone who will go sailing with you. Please do not take anyone out with you until you have thrashed the boat.

    DON'T TAKE ANYONE WHO CAN'T SWIM WELL WITH YOU
     
  14. hoytedow
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    hoytedow Carbon Based Life Form

    Well, others have been committed for less.:D
     

  15. Petros
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    Petros Senior Member

    Taezow,

    What you are doing is extremely risky, you must stop. You came on this forum asking for advise, and than you appeared to ignore it all. Many people have written for your benefit at no cost to you, but you proceeded anyway.

    No one is telling you not to modify your boat. What you need is someone who is familiar with that design, and who is familiar with ALL the forces involved, and who can work out a loads analysis so it will be built strong and safe. From your pictures it appears your alternations are inadequate to carry loads.

    This is not a trivial matter, the part of the hull you are altering is one of the most highly loaded parts of the structure. IF it fails you will lose the whole boat.

    If you are going to ignore well intentioned and knowledgeable people who are trying to help you, please do not bother to ask for help, do not waste your time on this forum. There is genuine concern for your safety by strangers who only wish you well.
     
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