strung out

Discussion in 'Boat Design' started by renovatio, Jan 5, 2009.

  1. renovatio
    Joined: Jun 2008
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    renovatio Junior Member

    So it has been a while since I have posted any thing about this project that I started last spring. We had a cold start to the summer and somehow it was turning winter before the project was even close to finished. So here's the latest update. I decide to use SAS thermo-lite board to replace most of the wood in the boat. The final decision came to the fact that the SAS board is 50% lighter than wood, waterproof and is available a 5X12 sheet, that meant that could rebuild my stringers in one piece as they where originally. So I purchased 2 5X12X.75 sheets and 3 5x10X.75 reinforced sheets to use for the stringers, transom, and floor. Three of the stringers are set in place. I have already laminated one of my outside stringers and am now just waiting to have the use of another shop. It was far too difficult to maintain the right temperature in my garage. So the original layup for the stringers was 1 sheet of woven roving and a chopper gun to laminate the visible areas. I was going to use this layup again but after working with 1708 biaxial I think that I have changed my mind. My only hang up with the biaxial is that I have only seen it 12" widths and that is not wide enough to cover the center stringers in one piece. This might not be a problem but I thought I would see what you all say about it. I also wanted to see what you all thought about my through bolted engine mount design (see photo). Using lag bolts was not recommended with the SAS board. Anyway I am looking forward to some ideas and some criticism.
     

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  2. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    Hi renovatio,

    This is that old Sunsetter you were talking about in June, right? ( http://www.boatdesign.net/forums/fi...ding/malibu-stringer-reconstrution-22799.html )

    I'm not familiar with SAS board. It sounds like you think it's worth looking at?

    Chopper gun goo really isn't that great a structural material. It makes an OK filler but I don't trust it for highly loaded parts like stringers. A single layer of roving with CSM doesn't sound like nearly enough for stringers in a boat like that... typical production builders.... Your selection of biaxial glass sounds much better, even if you do only have it in 12" widths. You can use multiple layers of this stuff to build up the stringers and tab them out into the hull bottom, thus spreading the loads over a larger area. (A stringer that's just glued or choppergunned in place creates a hard spot that can lead to cracking or other damage.) If I recall correctly, the boat was only 17 years old when the original stringer grid gave up- that should be a good clue that it wasn't adequate the first time and that you'll need to use more glass, more strategically placed, than the original builder did.

    Through-bolting a beefy L-angle to the engine bearing stringers, then bolting the engine mounts to this, is a good idea. I hate it when I see engine mounts lagged into the tops of stringers- this invites water intrusion and rot. Make sure to seal the edges of the through-bolt holes well with extra resin before you actually bolt the L-angles on. Every precaution you can take to keep water from entering the stringers is a good one.
     
  3. renovatio
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    renovatio Junior Member

    Yes I did start this project in June. Sadly it has been on the back burner this long. I am sure that many of you will not approve but I did glue the stringers into place. I chamfered the bottom of the stringer and I used sikaflex 252 which has great elasticity and should allow the bottom of the hull to flex as it needs to. Also the bracket for my engine mount is not an L bracket it is a 2" X 6" X 1/4" piece of 304 s.s. with 316 1/2" nuts welded to it. I built a jig to cut out the pattern for the bracket. I think that this is going to better than any lag bolt setup. I am also planning on building some bulkheads. One thing that I have not decided yet is when to put the foam in place. The original foam was injected through holes in the floor. I was thinking about trying to devise a way to do this the same way. I was hoping to get some ideas for this. Anyhow I should be starting up on this again in a couple weeks I'll try to keep my progress posted.
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The only problem with these panels (for those unfamiliar, it's a closed cell polyurethane board with a mat sheath on each side) is they (much unlike solid lumber or plywood) aren't self supporting panels and require an engineered sandwich laminate to absorb anticipated loading.

    As a direct replacement for wood, you'd have to engineer the panel laminates to the required stiffness, compression, tension and elongation of the wooden elements it was replacing. This isn't as easy a feat as it might seem, especially if you wan the structure to be as light and strong as the original wooden structure. As a rule you'll find it quite difficult to gain the physical qualities of wood or plywood with, mat poly resin and foam cored panels. In other words, without careful consideration of a laminate schedule (the chopper gun suggest there is none) you'll be heavier and likely weaker, at least in most regards.

    Well tabbed with biax, rather then chopper is the ticket for strength and durability, as you've already figured out. I agree with Matt in that you should increase the sided dimensions of the stringers and tab well back into the hull shell. I also agree the beds should be handled in a way that precludes moisture ingress. I prefer solid beds or well bonded wooden ones. Taper your core down before and after the engine mount areas, replacing the lost thickness with solid laminate. Use an epoxy bonded fastener technique to hold the angle stock for your mounts.

    Looking forward to your progress reports. Keep up the innovative efforts.
     
  5. renovatio
    Joined: Jun 2008
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    renovatio Junior Member

    Stringer progress

    At this point I should consider my self lucky as the shop that is available to me is perfect for this kind of work. The outside temp here was 30 Deg. and the floor in the shop bay that I was in was 105. This gave me the opportunity to control the working temperature. If I ever build a garage It will defiantly have in floor heat! With the surface temperature at 60 deg. I seemed to have enough time to work the mat onto the stringer. I purchased a mason pan at home depot and used this to wet out the 1708 mat then I brushed the core with resin and set it into place and began working out the bubbles with the help of my fiancé, we had plenty of time to work out the bubbles. I let the first mat set up for about 1 hour so that it would not move around as the second layer was set on it. After working with woven roving I have to say that 1708 is like a dream to work with. For the two outside stringers I have used 2 layers of us composites 1708x12" with the laminate mat sewn to it and the B440 resin it seemed to take about 2 1/2 quarts of resin to wet out an 11' length of 1708. I Was planning on using 4 layers for the center stringers and sandwiching 6' of woven roving centered where the engine bolts to the stringer. I hope to hear some feed back on this idea. I could go the entire length with the roving but it seems to soak a lot up of resin and is a pain to work with. The 1708 is most likely enough but I read some where that woven roving sandwiched with laminate has a good amount of strength to it. I want to be sure that I have enough strength in the layup to support the structure as was mentioned. I am somewhat concerned with my decision to use a composite material. I based this decision on these facts.
    1.) Malibu's new fibecs stringer system has no core material at all, It appears they have molded the stringer separately and then glued it into place.
    2.) Most of what I have read here is that the layup of the stringers should be enough to support the structure without the wood. Some boats are built this way new.
    3.) The compression and density of the composite board is comparable to wood.
    4.) Composite board will never rot or absorb water. Since water somehow seems to find it's way no mater what you do.
    5.) Composite board is 50% lighter then wood that means I can use more layup and put the weight where it is needed.
    6.) This is the second boat that I have purchased that needed major repairs due to rotten wood.
    7.) I have a hard time accepting the norm.

    I look forward to some feed back.
     

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  6. renovatio
    Joined: Jun 2008
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    renovatio Junior Member

    Project update

    Just thought I would post some pics of my progress. Saying that this is a lot of work would be a gross understatement. I'm pretty happy with the turn out so far. I was planning on adding some bulkheads in some places where extra support might be needed. Thought that I might see what you think. I have just cut the pieces I have not glassed them in yet. I don't really understand the purpose of running bulkheads all the way to the side of the boat. I was thinking of just adding them near the engine mounts between the side stringers and at the front of bow between the two center stringers. What is the main purpose of bulkheads anyway is it to add rigidity to the overall structure, support the floor or to support the hull. Or all of the above.
     

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  7. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    Hi again renovatio :)

    Lotsa work, indeed! (Just curious, how many angle grinders have you managed to burn up so far? ;) ) But it's starting to look good.... it looks like you've tabbed the stringers into the hull bottom pretty well. Interesting to see the lapped stringer cores in the engine bed area.

    In a relatively open boat like this, you will have a hard time getting full bulkheads in. But partial bulkheads are a good idea.

    If I recall correctly, a Sunsetter is around six metres or so LOA? I'm just checking against Dave Gerr's scantling rule (Elements of Boat Strength- a handy $30 reference, if you don't already have a copy) which would seem to suggest four bulkheads; I'd space them roughly equally. The same board you used for the stringers ought to work, tabbed in well to the hull (thick plywood, maybe 3/4", would be a more typical choice). Yes, bulkheads or ring frames support the hull sides as well as the sole; they are a key element in the overall rigidity of the structure. Don't forget to allow for bilge water to drain.
     
  8. renovatio
    Joined: Jun 2008
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    renovatio Junior Member

    The angle grinders have held up pretty good I've been using a small air angle grinder with a 24 grit disc to touch up spots before I lay the glass down. Although it takes more time, this seems to make less mess and deeper scratches than the 4" Dewalt with the flapper disc. No matter what you do its not a lot of fun and it itches. I have a lot of respect for those of you who do this everyday.
    That overlap in the stringer is the original design and that double width is necessary for the engine bed. This is probably not the best design for a stringer because it puts a break in it at the highest stress point. Good thing its all wrapped in frp. The frp is hopefully the source of all my strength. Its a little late now but I think that I could have ran the outside part of the inner core all the way to the front of the boat. That extra width would not have hurt a thing and would not have added much weight. The total weight of the inner inner stringer including the two S.S. nut plates is just 27 lbs (pic).
    The dimensions you are asking about are (in feet) LOA 20', Beam 7.5', DOH 3'. The doh does not include the windshield. Not much of a boat for choppy waters, with it's flat bottom, almost 300 hp and a 12X13 prop it's propose is for one thing, maintaining 36 mph on smooth water for skiing. Top speed should never be more than 45 mph.
    I do not have that book I wish I had known about before I got this far into the project already. I did take a sneak peak of it on goggle books preview. I will be eager to get it to see how to do my next project. This site has been a great reference there is a lot of good info here.
    I'm not sure if want to make those holes for drainage. The shape of hull puts the low point of the boat about in the center of the boat. There are two drain plugs in the back of the boat but all they do is drain a small collection area. Originally the boat appears to have had no drainage holes. The cavity was completely sealed and filled with foam. What I suspect is that the wood used to build the floor rotted and allowed the water in. The difference now is that I will be using materials that are completely waterproof using sikaflex 252 to seal and bond it all together and then using baix tape over all the joints. At any rate it should be as good or better then the original. I can always add holes later if I suspect I have taken on water. One thing I have not figured out is how and when to put the flotation foam in. I wanted to inject it after the floor was in but I don't think that this is going to work very well with the pore in foam that I have. Any Ideas?
    Thanks for looking that up about the bulkheads. I will try and see if I can fit four in there. The center of the boat is tricky to place a bulkhead as the center channel of the boat is the source of fresh air for the engine. Any how I've rambled enough.
     

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    Last edited: Mar 16, 2009
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    If you seal in the sole, creating chambers below, you have built in floatation compartments, with or without the foam. If you don't fill them with foam, you can come back and make one or more into live wells, storage areas, etc. quite easily. With foam, you have the fun of removing it, which you're well acquainted.
     
  10. renovatio
    Joined: Jun 2008
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    renovatio Junior Member

    Ah yes removing the foam what a joy that was. The original foam was injected into the cavities to the point that it looked like it pushed the the floor up. That's a good point that you make about making a storage compartment. However I am wondering about the effects of leaving the cavities open. Do you think that the foam was providing some support for the hull and what about sound dampening. The front part of that cavity did not have anything supporting it at all. You would think that this section would take the most abuse. I extended the outside stringer 16" because it did not make sense to me to have the stringer run short of the length of the hull. I think that this part will have to have foam but I think now that you mentioned it I will consider putting in some extra storage. You can never have enough of that on a boat.
     
  11. marshmat
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    marshmat Senior Member

    That pour-in foam provides virtually no structural support. Its only purpose is to provide emergency flotation if the boat is swamped. As far as sound deadening, in a boat like this the engine noise will be one or two orders of magnitude louder than any noise from water on the hull, so extra sound deadening in the bilges would have very little effect on what you actually hear from the cockpit.
     
  12. renovatio
    Joined: Jun 2008
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    renovatio Junior Member

    2 steps forward and 3 back

    So I have made some progress with the structure. And have move on to the next p.i.t.a. The gel coat. The previous owner had painted the the exterior of the boat with an automotive paint. That may have lasted a little while had he used some type of primer. So I thought that now would be a good time to put a fresh coat of gel coat on it. I follow the recommended procedure as well as possible. I sanded her down removing auto paint. Cleaned throughly with acetone. Applied first coat with no wax added. then applied second coat with wax. My surface temp. was 78 deg f. and it turned out o.k. accept for one little detail ever where that I sanded down to the fiberglass, the gel coat fish eyed. Since then I have applied body filler sanded again and repainted. This time was a success. Thinking that it was a fluke I tried to apply gel coat to top portion of the boat this time I mixed a very small amount to test the water and bingo it did it again. I am using a dump gun and gel coat that is said to be made for external application. What the heck am I missing.

    The way I see it My options are as follows.

    1.) You spill the acetone. You strike a match. Now your cooking fish eyes

    2.) Use resin to seal open glassed areas and feather sand. I did not think this necessary because I thought that gel coat was made to be sprayed onto fiberglass.

    3.) Spray entire area with un-waxed resin using dump gun and apply gel coat while resign is still uncured. Not sure about the stability of resin in thin applications.

    4.) Apply marine glazing putty to areas feather sand spray gel coat.

    5.) Use gel coat primer on surface ,not sure if it exists.

    6.) Spray white gel coat fill. in fish eyes, color sand, repaint.

    7.) Bite the bullet and take project to boat repair facility

    Please help as I am fast approaching the end of my rope. I was told that it would be possible to spray a coat of unwaxed white gel, then a coat of unwaxed metal flake and clear and finally a coat of waxed clear. I am currently wondering if this is possible on an entire exterior surface.

    Seeking wisdom, encouragement and advise

    Ren
     

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    Last edited: Jun 21, 2009
  13. Tiny Turnip
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    Tiny Turnip Senior Member

    Sorry Can't offer any expertise- only encouragement, and as a fulltime job holding single parent of two pre-teens, and an inveterate enthusiastic project starter, 1) have a day or a week or a month off and just have the odd look and think about the project, the odd beer, and chill. no pressure. 2) if you end up letting it go, or throwing money at it and getting some one to sort it for $, thats totally ok- you had a go, you know the problem, use your expertise in whatever field it is to pay for someone who's expert field is glassing. that's what humans do and why we're the most successful species on the planet!
    good luck and strength.
    pie
     

  14. renovatio
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    renovatio Junior Member

    thanks for the advice

    My body right now is forcing me to take a break as I feel I have a touch of the flu. So I think that will take some time off and research the best way to handle this. The diffcult part about that is that the summers here are not too long and really want to get this thing in the water! Thanks for the advise and good luck with the teens I can't imagine how much work that is.

    Cheers mate

    ren
     
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