How much "flex" is okay in a GRP hull?

Discussion in 'Sailboats' started by souljour2000, Dec 30, 2009.

  1. souljour2000
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    souljour2000 Senior Member

    Okay...this is a difficult question to ascertain or answer but I have taken a sawzall to my Hunter 20 and am in process of adding
    a new hard coachroof and have widened out the cockpit seat coamings. The new rear cabin wall ( companionway wall) will be perpendicular instead of angles forward and a few inches longer to allow the person sitting at the starboard dinette near the companionway real room to sit comfortably. The widened cockpit seat coamings will allow me to lay down and take a nap in the cockpit and make room for a small tent maybe...
    I realize that the initial design limits are all changed now and all bets are off now... I'm okay with the added weight and so forth ...There are places where I have glassed in gaps between the underside of the cockpit sole and the flanking quarterberths inside to make watertight compartments.. Then there are places where I have or am contemplating adding symetric glassed rib-like 2x4 bulkheads or 3/8 plywood on the sides of the hull at about even to the companionway to facilitate cross members of the new cabin wall and then some more a bit further astern to shore up the widened cockpit seats . These mods will stiffen the boat in these areas... regions that would seem to have been prone to flexing alot before but within the design limits ... similar to other late 70's and 80's smaller grp's I suppose...still If I can reduce that "el cheapo effect" of the design and it's resultant flexing a bit as a by-product of the mods I'm doing ( the added rib/bulkheads and closed gaps in the quarterberth/sole areas )then it seems okay.
    The crux of my question is that given this type of design..I am trying to get a general idea of how much flexing should be retained and where I want to make sure I retain it. She's a 20-foot Hunter 1983 hull/liner/topside cap GRP boat ...
    I'm thinking a bit of stiffness added by these fairly localized mods... is a good thing...as long as it's isolated and it's not competing with the overall "springy plastic soapdish" idea of the designers. If you get my drift at all with this thread and if anyone's with me still...you may fire when ready...
     
  2. wet feet
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    wet feet Senior Member

    I'm not familiar with the boat or the waters you intend to sail in and thus not ideally suited to comment.My initial reaction to the question about flex is to answer it with another question-"how much flex is OK in a cars driveshaft ?".The similarity is that the structure of your boat is the element that has to transmit the drive generated by the rig and in order to do so it needs to be adequately stiff.Will the changes you propose alter the way in which the backstay loads are fed into the rest of the boat?Failing to deal with such matters makes it more likely that doors and hatches will jam when subjected to sailing loads.The portion of the boat not subject to rig loads needs to be strong enough to walk around on and to keep the water out;it helps if it can deal with the occasional knock as well.
     
  3. capt vimes
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    capt vimes Senior Member

    as an interested boater (sailing there is) one gets around on lot on different boats - looking for a ship to buy...

    there is one guidline when inspecting an offered boat and doing the usual testsail:

    if the doors got jamed for instance once heeled over with some windforce in the sails - run away and do not ever think of buying it...
    because this means that once the rig is loaded the hull starts flexing... no good at all and a common problem especially of older GRP boats!
     
  4. Fanie
    Joined: Oct 2007
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    Fanie Fanie

    Souljour,

    I have never heard of there should be flex in a boat. You want a boat as stiff and as rigit as possible.

    Am I misunderstanding your question ?
     
  5. fg1inc

    fg1inc Guest

    Most likely the flexural situation is not so much a concern as is the attachment of your new frames/bulkheads. All new structural members should be positioned about a quarter of an inch off the hull with the fiberglass tabbing radiused from the frame to the hull. NO ninety degree angles anywhere. Otherwise, the frame will form a hard spot and with normal flexing the hull will start to fracture along the line of the new frame. A proper radius would be about the same as a quarter (25 cents US).
     
  6. Fanie
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    Fanie Fanie

    With a little bit of the grey matter working it is not that difficult to replace the old structure while making space for the new features.

    You have to add structure, any large unsupported area on a hull could flex severely especially in rougher water and at speed.

    Such flexing will tent to try and break loose from where the then structure starts.

    Best is always to up a picture or a drawing of your intentions.
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Actually, there will be a surprising amount of flexing, depending on load, but this isn't the real set of issues. Load absorbsion and transmission in the hull is very complicated.

    It's difficult to suggest how you can proceed without a good look at what you're doing. I have a fair idea, but it's not clear.

    The era boat you have should be fairly stiff, with a thick hull laminate. The liner really doesn't offer much, but the deck cap sure does.

    I've made some radical modifications to boats like yours. I butchered a Catalina 22 look a like, by removing it's liner and deck cap and now is an open boat with a yawl rig. I also have converted another sailboat to a harbor launch, again removing it's liner and deck cap and making a new interior. Both came out slightly lighter then the originals and I feel this is key.

    Weigh the material you remove. This will give you an idea of how much you've to put back and when it starts becoming a burden for the boat.

    Use good building techniques, with sufficient tabbing and encapsulated wood elements. A couple of bullheads under the cockpit will restore the athwartship stiffness to the hull, you probably only need one actually, if most of the cockpit cap is still intact. Again, a lot depends on how much you've cut away, so post some pictures.
     
  8. souljour2000
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    souljour2000 Senior Member

    Okay..thanks for all the replies so far...I get now that flexing in general is not good on boats...I'm an old aviation nut so when I think about flexing I think of airplane wings like the 25 feet range of travel wingtip flex designed into a 747's wing....boats and brittle reinforced resins are way different from aluminum 80-foot spans...anyways..you guys are due for some pics...pics are coming soon...got a lot of 'em...they're all high-res ...mega-pixel right now and I just need to photo-shop them down to a size that I can post on here.Then it will be easier to understand what I've done...I'll try to post some tonite...
     
  9. gonzo
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    gonzo Senior Member

    2X4's? That will add more weight than strength. Use lumber with a higher ratio of width to thickness.
     
  10. Fanie
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    Fanie Fanie

    Holding my breath in the meantime...
     
  11. souljour2000
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    souljour2000 Senior Member

    finally some pics

    well..here's some pics...you can see the one of the area under the cockpit sole where there is sag from glassing the gaps on upper sides of the pic...that area created in factory mold from the quarterberths and cockpit sole...at top you can see the bolts from the fiddle block that holds the main sheet in the cockpit.
    I was not going to use any of the old companionway but not having to frame out a new v-shape saved weight and a lot of work so I incorporated the still usable remnant. Now the companionway and rear cabin bulkhead are perpendicular instead of at a forward slant like before.....will post some more in a few minutes that show some recent progress with new permanent coachroof...stay tuned...
     

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  12. souljour2000
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    souljour2000 Senior Member

    here's some more...you can see the original "pop-top' roof and hatch The new companionway/rear bulkhead is 3/8 ply..it's not marine ply but it's going to have a lot of glass over the outside by the time I am done blending in the old companionway remnant which was hollow(no balsa..no foam).I'll fill those hollow areas with 1/4 " ply and then build up the area around it with more 3/8 in a big composite soup covered with decent layers of cloth. Then there is halved 2x4's in the cabin side to stiffen the panel and be a frame for the finished interior wall. Clumsy and a bit heavy but I'll try to live with it.
    I saved some fine old teak from the landfill over the last month..all from warehouse park dumpsters...some was from a cabinet shop then the other I think had been an old gazebo...covered in old paint but had many nice curved 1x4's..'360 degrees of them and three slats to every 90 degrees of bench...they're heavy but very strong and good cond. so am using two of those as trusses of a sort...they will add a few inches of headroom and a hopefully a hint of open-ness to the cabin space...the stringers are just cheap light 'whitewood" to keep the weight down but transfer load forward from the rear cabin wall and flanks of the companionway...all are tied in at each joint with aluminum angle.....screws are wood-glued ...outside of this frame will probably be covered with 1/4" luan ply and a heavy and then a light layer of cloth and polyester resin....the aluminum angle joints on the inside of roof will be faired out with epoxy and light cloth to hide them a bit...maybe you can see the cockpit seat widening...that required a bulkhead or two below to support it...where as before it was the mold form that supported it pretty much....the cabin mods give real room around the dinette..some headroom.. an upright rear wall for mounting electronics/shelving and let me get into those lazarettes in the quarterberths easier now too...
     

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  13. PAR
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    PAR Yacht Designer/Builder

    Pop tops are heavy and a contrivance that don't work that well, but you're way over building this thing, likely adding several more times the weight you've removed, which is a fairly critical thing in a boat this size.

    1/4" plywood (marine) is all you really need. All those beams, reinforcements and braces are just weight you don't need. Since everything will be covered in 'glass anyway, let the laminate do the work and the thin plywood (5 ply 1/4" marine) serve as a form and load bearing core (you only need a few sheets at most).

    The new coach roof could be a 1/4" over foam sandwich, which has no beams is only a couple of inches thick and smooth on both sides. You could walk on it and hide wires inside it too, plus it has insulating qualities built right in (a really good thing in Florida, trust me). The same with the aft cabin bulkhead. Of course you'll want a reasonable sheathing schedule over all this too.

    I wouldn't even bother with the solid lumber, unless it was to be finished bright (if it's flat grain, it'll look "novice" built).

    A single piece of 1x4 well tabbed down the center of the cockpit sole would solve the sagging issue, ditto the cockpit seat tops. Foam would also work the same duty, but it would need more 'glass work.

    Don't over engineer this thing. You can see how little the structure is, as it came from the factory. It's 30 year life span shows it wasn't all that flimsy after all. Keep it light, forget about beams and braces, just some gussets and lots of fabric will do.

    Damn, I see what you mean about the cabin side ports. I'd be taking a jig saw to them too, but that's me. At least that's a relatively flat area where you could make some new, rectangular ports.

    Keep up the good work. She's going to look a whole bunch different, I hope you'll have some before and after pictures to prove to yourself how much you've actually done when it's finished.
     
  14. souljour2000
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    souljour2000 Senior Member

    Yeah your right again PAR...it is probably going to be 3-4 times as heavy as the old pop-top shell the way I'm making the coachroof...I am going with 1/4" really thin luan ply they sell at the big box stores..whatever that is...that stuff has gotten much better at HD lately...then maybe only one layer of decent weight bi-axial and poly resin faired with epoxy and painstakingly sanded ..then sanded some more...the whitewood stringers are real light and there are only 2 1x4 arched teak trusses but your right.I weigh 225 lbs maybe a bit more so I wanted it solid...it will be..maybe too solid...I can put the battery down lower ...probably in the amidships lazarette next to the keel bolts instead of the port quarterberth or in the stern port cockpit locker...will have to study the battery out-gassing thing with that though....probably will be a single big golfcart battery...I do need to re-examine some things like the bulkheads..I am over-engineering...I'll try to stop if it's not too late though I'm in deep now...I can do some stability tests when I get the battery in place and re-rig her...I could shorten the mast...it's damn long at 29.5 from the water for a boat her size. That might compensate for the weight up top but that extra sq footage of sail up high is the only thing that keeps her faster than your average retired beachcomber...I intend for her to be a fair-weather bermuda high...offshore wind from the east (I live on the west coast of florida)inshore coastal cruiser at times but mostly will stay in the bay with her...with the boats I tend to sail we like to learn all we can about meteorology...
     

  15. PAR
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    PAR Yacht Designer/Builder

    Why would you use polyester, knowing how poorly it performs over wood? For this type of work epoxy is always the choice, especially for the novice.

    The 1x4's you've used are flat grain and will expand like crazy unless encapsulated in epoxy.

    Placing the battery down low and cutting the top of your rig off seems a really half assed way of dealing with the excess weight. If you've glued things down, drag out the saws-all and have at it, you've done it before. It wouldn't be the first time someone had to rip something out and do it again.
     
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