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  1. #61
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  3. #62
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    Apr 2008
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    Connecticut, USA
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    Default Glassing the bottom?

    Hey, all. My kids and I are planning to build a GIS this summer (that's Northern Hemisphere summer, starting in a couple of months). I bought the plans from Duckworks last week and I've been studying them carefully. I am extremely pleased with what I see.

    One of the main reasons (other than the fact that it is beautiful) that I chose the GIS over other designs was the light hull weight. We plan on building a dolley and pulling the boat by hand back up to our house after each use. The standard racer class around here is the 420 -- it's what I raced as a kid and what my own kids are learning on -- so we originally thought about buying a used one. But when we thought about its 100kg hull weight (for 0.5m less LOA!), we decided to look for something lighter.

    But now I'm torn. The plans mention the option of glassing the whole bottom if the boat will be used on a rocky shore. Our beach is sandy but has rocks poking through. And there are many sharp rocks just offshore which lurk just below the surface at low tide. I don't want to become the crazed psycho-dad screaming at the kids to be careful of scratches. I want them to enjoy the boat. So I'm thinking about glassing the bottom for abrasion protection.

    So, my questions are: About how much weight would this add to the hull? (5kg? 10? 15?) And would it add enough protection to be worth it?

    Thanks in advance.

  4. #63
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    Jul 2005
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    'Delaide, Australia
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    Hi Paulie,

    A few rocks for people who are used to handling boats are usually not much of a problem. So if you are used to launching a racing 420 from the same place without being too concerned, then the GIS will be fine too.

    The glass does add quite a lot of weight relative to the plywood - simply because plywood is so light in comparison. Glass laminates are is more than 3 times the weight of wood for the same thickness. If you start to look at lightweight timber like the gaboon/okoume it is possibly getting up to 5 times the weight.

    Suddenly the 100kg of the shorter 420 starts to look less heavy! How did they make it so light using a heavy material like fibreglass!? (the reason is that they accept a lot more flexibility in the hull panels which make the boat slower than a wooden version).

    However, if you sleep on it a few times and think you still want to glass I would suggest only do the bottom panel of the hull. And use the lightest weight glass possible.

    Instead of using the 6oz glass (200gsm) that most people use some 2oz (75gsm) and the weight gain will be much less. You could use lighter. You can mail order it as the quantity you need would be quite postable.

    Glass gives protection largely because it is much harder than wood. And if the surface is hard it probably doesn't matter how thick the glass is from a protection point of view.

    So if, on reflection, you think it is necessary then just use a very light woven fibreglass cloth.

    I've had lots of plywood boats and I have had little damage other than paint scratches from rocks - however if your local beach is really bad then it might be worth considering a light layer of glass.

    Remember too that lightweight in a wooden boat is because of the general materials - so if you try to select lightweight materials it removes a lot of weight.

    As you are in the USA I would strongly recommend Western Red Cedar for all the internal framing as it is nice and light, not too expensive and readily available (?).

    You can use it for the gunwales too as they are capped with a piece of something harder on the outside. It is great for centreboards and rudder blades - but make the front and rear pieces of something slightly stronger (fir/spruce something like that).

    That actually is a great weight saving strategy - make the body of a part out of a lightweight timber, but protect it where it might get bumped or dragged with a thin lamination of something tougher (but not toooooo heavy)

    The mast could be made of spruce. The square hollow mast is the best choice for weight saving.

    If you and the kids have a background in 420s you will find the GIS a very rewarding boat to sail. It might even be fun to enter it in some open regattas - send the organisers a photo - and they will give you a very favourable handicap - "lug rigs are slow" "oh - and it has a flat bottom - who would design a boat like that".

    I have done that a number of times with my BETH sailing canoe (same perceptions - plus "oh, it is surprisingly pretty but ketch rigs are not efficient").

    It is a lot of fun when I can match an average Laser for speed! Ensures good regatta results - the first time anyhow.

    The right plywood makes a huge difference in weightsaving too.

    Best wishes
    Michael

  5. #64
    Join Date
    Apr 2008
    Location
    Connecticut, USA
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    Default

    Michael,

    Thank you for the quick, thoughtful, and complete reply.

    Did I mention that I think the GIS is beautiful and that I'm very impressed with the plans? I looked at a lot of designs before making my decision. I choose the GIS in large part for its beautiful lines.

    Based on your reply, I think that we will build the boat as spec'd and forget about glassing the bottom. It sounds like a lot of extra weight for marginal gain.

    Scratches in the paint don't bother me -- any well-used boat will earn a few scars. My real concern is scratching through the epoxy and allowing salt water to soak into the plywood underneath. We'll just have to be careful when launching and beaching and repair anything that looks too deep. And I'll try my best to relax about it and not ruin the kids' fun with my worries!

    Thanks also for all the info about materials. I'm just starting to make phone calls now to suppliers. I hope to have all the materials in hand before the kids are out of school so that we have no delays once we start.

  6. #65
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    May 2003
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    Quote Originally Posted by paulie View Post
    Scratches in the paint don't bother me -- any well-used boat will earn a few scars. My real concern is scratching through the epoxy and allowing salt water to soak into the plywood underneath.
    When Redback first hit the water, I was touching up scratches everytime I used her, just a matter of slopping some paint on after giving her the after float wash down - easy to do since she lives upside down. After some months, the enamel finally hardened and it's become less of an issue. I wouldn't worry about it mate, just be ready with the paint pot until you finally realise you're wasting your time.

    Richard

  7. #66
    Join Date
    Oct 2007
    Location
    Sydney
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    Default

    I had thought about glassing the front bottom of my (future) GIS as well since the beaches around Slovenia and Croatia mostly look like pictures below - yes it is beautiful, clear blue sea but nasty rocks on the beach.

    This is obviously a question for the future when the boat is being built, but if the coast is like this (i.e. no sand at all), doesn't it make sense to glass even though a little weight is added?

  8. #67
    Join Date
    Mar 2008
    Location
    Philippines
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    30

    Default Metal covered skids??

    Hi Theodore,
    Not sure how it would work but if you put skids on the bottom about 1 inch deep by 2 inches wide and then put flattented copper tubing on top of that the copper would take most of the punishment. Stailess or aluminum should work too as long as the metal is tough, faily cheap and non rusting. Not sure how that would affect the sailing characteristics of a GIS though. Probably add weight and drag. Perhaps sheeting with kevlar??? Sounds expensive though. Good luckl

    Your local beaches there look like they'd raise hell with most boat bottoms. Not as bad if the rocks are smooth and rounded but far from ideal.
    Conditions may vary but sailing still remains the best pastime there is. (BTS)


    Enjoy,
    CN
    Onward through the fog.

  9. #68
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    May 2003
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    Redback's got two 1cm square strips of hardwood on her bottom (plus a hardwood facing on her keel). The paint still gets knocked around but the worst of the damage is taken by the hardwood rather than the ply. Worth a thought?

    Richard

  10. #69
    Join Date
    Jul 2005
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    'Delaide, Australia
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    Howdy Theodore,

    For a beach made of pebbles I would recommend glassing even with the additional weight.

    You could use brass strip or put hardwood on the skids - still do mostly of light softwood but put a 6mm thick layer of a heavy, strong hardwood on the bottom edge.

    I would not suggest the copper tube - it might be cheap and available but it would be quite ugly and the goat is such a pretty boat.

    Michael

  11. #70
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    Oct 2007
    Location
    Sydney
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    A hardwood strip on the skids makes sense .. nice idea. I will plan for glassing the bottom as well.

    Since Koala is keen to make some PDRacers first, we'll experiment on those first.


    Cheers,
    Theodor (no final 'e' and I have no reason for that)

  12. #71
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    Sorry about the "e" Theodor!!!!

    The hardwood only needs to be quite thin - 6mm is enough

    Michal

  13. #72
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    Oct 2007
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    Quote Originally Posted by Boatmik View Post
    Sorry about the "e" Theodor!!!!

    The hardwood only needs to be quite thin - 6mm is enough

    Michal

    I wouldn't worry about trying to get an internet alias correct (especially when you can't spell your own name ). I'm not entirely sure why I'm using that name!

  14. #73
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    Some craziness is a good thing!

    It is not too good when the world becomes a completely rational place!

    MIK

  15. #74
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    May 2003
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    Quote Originally Posted by Boatmik View Post
    Some craziness is a good thing!
    You mean it's optional?

    Richard

  16. #75
    Join Date
    May 2008
    Location
    Edmonton
    Posts
    1

    Default GIS 1/2 in ply bottom?

    Ok, great boat. Bought the plans, and now found this Forum!

    I am looking at building the GIS with the bottom out of ½ in rather than ¼ in, for two reasons:
    1. Lots of mass in those that will be in the boat, and don’t want to put a knee or elbow through it easily, and;
    2. I have two sheets of 4 x 8 ½ in marine ply already.

    I am also looking at cutting the transom out of one of the ply, thus the bow for the bottom will need to be two triangles cut from other ‘scrap.’

    Anyone see any problems with this? (Beside the weight issue)

    Glen

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