Weight difference between 10m plywood and FG sandwich hull

Discussion in 'Boat Design' started by pironiero, May 24, 2022.

  1. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Hummmm.... To be fair with the GRP. With a fiber mass content of 0.3, 3000g of mat makes a composite with a number that is more like 5.1 k/m². If the fiber mass content is 0.1, this correspond to 900g of dry fibers. In terms of strenght, this is not the same.
     
  2. linda.vrdoljak
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    linda.vrdoljak Junior Member

    I tought complience of 1.4 on top of the iso fos=2.0, so it's ~ 2.8x over the snaping point, a good strucure...

    let's go to the basics;
    roving soaks less than 1.1x of resin, and CSM soaks up to 2.33x (or your own sample or just trust the iso table)
    the thicknes in 7mm, 7layers of 450mm mat is 3.1 kg, and if the laminate is csm mat you need to multiply with 2.33x (amout of soaking ) so only csm is 3.1kg, and only the resin for that about is 3.1x2.33=7.2kg only the resin,

    and the total weight is;
    3.1kg for CSM, and 7.2 kg for for basic resin, your plate totals in 10.3kg

    other toughts;
    1. below 7mm is ludicrous on 10m boat bottom, and you already have a narrow 250mm panel
    2. the plates it will be heavier than the calculations, you'll always hunt for thant 10-15% where did your drum of resin went
    3. you can cut the soaking of CSM down to 2.0 it will give you a dry laminate, and hard to put it together
    4. this CSM-only laminate is for weight test, for a real laminate you need some stronger fabric

    will it be lighter in stronger fabric?
    1 mm thickess of CSM laminate is 450+(450x2.3)=1485 grams 1m^2
    1 mm thickness of Roving laminate is 800+(800x1.1)=1680 grams 1m^2
    no

    when you add gelcoat and some better fabric it will be at least 11kg 1m^2, prove me if I'm wrong, and yes you could go thinner with your calculations but not feasible in the real world
     
  3. Russell Brown
    Joined: Jul 2012
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    Russell Brown Senior Member

    Seems like the question should really be " Weight difference between a plywood and a FG sandwich boat".
    There are some insanely light wood/epoxy boats that just keep going and going.
     
  4. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    the resin you refer to matters

    a biax with mat like 600/225 easily wets at 1.0x

    we were comparing plywood to foam sandwich, no?

    so how does solid frp come into play here or frp with polyester or any ester

    and you cannot even get 2.3x epoxy into 600/225-it is simply impossible, so there is some disconnect here

    no disrespect here, there is something just amiss
     
  5. linda.vrdoljak
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    linda.vrdoljak Junior Member

    1. It looks like your're rediscovering the boiled water that composites are ligher and stronger structure at the premium cost, but that's not the whole yacht..
    2. but what's the cost of your 10m hull with fancy cloth and epoxy? probable 10x more over common frp and a missed investment, once the whole boat is done ( properly done; bavaria/beneteau/ salona) it's not any lighter it will be close to a 5 tonner either way, with the ballast, unless you prefer a jail like open frame interior at 1 ton less..
    3. 2.3x epoxy into 600/225 are your words, my example was purely on csm with poly resin just for the mass idea at minium of 10kgm^2 as basis
    4. you're comparing strawberries vs potatos (epoxy biax vs plywood) even worse epoxy sandwich vs wood, materials and knowledge 500 years apart...
    5. your laminate of 600BX+225CSM can pass 250x500mm panel at 9knots 10m sail, in 5.75mm, the panel weight would be 1125 x 2.3 (csm)+ 2400 x 1.1 (biax) = 2587 + 2640= 5 227 g ~ 5.3kg
    6. the numbers look very good, but where is your skin coat? you'll get the print through, your plating is less than a screwdriver bit (6.35mm) for a 10m hull whole structure will be very flexibile due to thin framing for 9knot hull, I would love to see pulling that 10m piece out of the mold if ~ 5mm thick even framed, I had some flexing on 60 + kn framed hull under HSC, and twice the material than 10m sail
    on 5.75mm how yould you do your frames? so you don't distrub the skin, if you go up to 0.5x the skin thickens how high can you frames be? 2.5mm sides? etc you have watch the estethics as well and once you prep and paint the body will you get a new car glass finish or a 10 EUR garbage can look

    7. drum of epoxy is 10x the price of iso poly, 10k vs 1k EUR, and find me a sold 100 kg/m3 pvc foam in 15-20mm, even thats thin for some Class Soc. it's a ghost chase...

    8. There's a sandwich vacuum bagged 16.5m yacht at 20 tons dry (has the heavy mdf interir) 20knot yacht, and there a sold biax on vinyl well built at 18.5 tons 40knots loyyd yacht so what is the point, (Tridend 550, and Mondial 54 to be exact) so whats the point of a light plating if the whole project is a hipopotamus at 10x the cost? calculate the whole yacht and than substract your 25% lighter hull and add 750% more cost, and make shure you remove the 50% of overweight interor before you substract any shell mass, the planing mass by itself means nothing, nada, study the bavaria and salona sails first
     
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  6. philSweet
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    philSweet Senior Member

    The driver for weight tends to be what one considers acceptable puncture resistance and ball peen hammer resistance. If a 200 pound guy can damage a 10 meter hull with a 2.5 pound ball peen hammer, it isn't strong enough. That's just an over zealous blister test. Same with forklift trucks - poking at the hull with forks should either slide the boat over gravel (small boat) or stop the forklift dead (small lift truck).

    On a 10 m hull, I expect to be able to mount one of these absolutely anywhere without a care for panel strength.

    upload_2022-6-9_19-38-17.png

    If you do the math based on puncture resistance and pull-out strength, you don't end up with 80 density PVC cores, except maybe for the front panel of the silverware drawer, and okoume is marginal, it requires a good glass skin and best practices all around. Even 200 density pvc has at best 1/10th the compression and shear strength of okoume. Even with best practices, epoxy annuluses, etc, that padeye will rip the pvc-cored laminate to pieces. At 200 density core, you'd need a fronting and backing plate about 8 inches in diameter.

    Okoume makes a decent core for a 10m boat. But if you just want to put a thin veil on it for weatherproofing, it's a bit weak IMO. I used to be able to get 13 ply 9mm Okoume ply that was damn good and strong, but I don't see it anymore. It was 100% seamless even in the crossgrain direction. I've had 5/16 bolts shear at the surface without so much as egging the hole. After a 30' Contender tried to give me a tow, I pulled my tow line back aboard and found his cleat and a couple sqft of his transom. He hammered the throttles on a pair of Yamaha 300s while towing my swamped 600 pound, 16' plywood sailing dinghy. I had 20,000 pound hardware on the bow - idiot proof, girlfriend proof, Florida proof for 25 years. Same boat got rear-ended twice, totaling both cars. It took a teaspoon of epoxy to fix the boat. It took new axles, springs, wheels and a porta-power to fix the trailer. If you can find that kind of okoume ply, you'll be fine.

    So figure out what all the performance requirements are for the panels. Using the old "divide length by 1000" scantling rule, a 10 mm solid glass hull of 2412 biax stitchmat vs a 9 mm okoume core with 3 mm glass inside and out would be pretty similar. Either can grind most modern opponents to dust and take on works of civil engineering for hours and hours and not come to any harm. This sort of strength doesn't add to costs of a one-off. It only costs more if you are making hundreds hulls at a highly skilled and efficient factory. And the weight difference of the entire boat is far less than most would suspect. Unless you have access to expert analysis tools for cored composite systems with structural grids and bonded liners, lighter hull plates usually just mean more framing that takes up more of the boat's interior space.
     
  7. fallguy
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    fallguy Senior Member

    Wow, did this thread get spun up now...

    OP asked for sandwich or plywood and he got a polyester csm solid layup.

    I have never in my life heard of a 2.5# ball peen hammer test.
     
  8. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    About 687 pounds, depending...

    While we're at it: "How long is a piece of string?"
    (Quote from the late PAR, a former member here for many years.)
     
  9. philSweet
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    philSweet Senior Member

    Later posts suggested this was more of a pub conversation than a serious engineering quest. Purely a hypothetical question. The problem with designing to meet ISO specs is that they aren't intended for you to do that. You are supposed to engineer for the job, then check to see that the specs are met. For any practical hull - no worries - they will be. But the fact that there isn't a ball peen hammer test is a real problem for me. I need that sort of strength.

    I compared two reasonably similar-strength hull structures - one with ply and one without. To me, the fact that you can construct a hull panel that meets a particular ISO standard, but that you probably can't walk on with bare feet without causing damage, doesn't seem like a good way to settle the dispute. Particularly if there's only a handful of places on the planet that can produce a working vessel using it.

    Comparing materials that differ by a factor of 25 in strength (80 density pvc vs okoume plywood) is also a bit odd. You need to explain how they compare in other tests when designed to one test's minimums. The panels will score very differently in other measures.

    Suppose a 30' boat gets thrown ashore in a hurricane. It ends up in the median of a highway. I want to push it half a mile back into the water with a bulldozer. Up out of the median, across the highway, down an asphalt road, between a couple buildings, then down to the water. Design the panels for that, and see which is lighter. It's a far more reasonable scenario than designing to a minimum ISO spec. It's also perfectly doable.
     
  10. philSweet
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    philSweet Senior Member

    Here is a very successful 32' plywood boat. Bottom is 1.5 inch douglas fir ply, sides are 1.0 inch dugfir. Now reverse engineer equivalent cored panels that will do everything this one will do - including beaching it on a stony shore because you like the look of the place. This boat was designed by the owner for the owner. It later was replaced by a bigger version with 2" thick bottom. The op didn't ask which was the lightest way to make any boat, he wanted a comparison like for like. Hogfish Lips is as reasonable a point of departure for that comparison as any.

    Chris Morejohn: Building Hogfish and Hogfish Maximus. http://chrismorejohn.blogspot.com/2014/10/building-hogfish-and-hogfish-maximus.html

    [​IMG]

    For a second attempt, lets choose the E-Scow. Melges gives us a nice cored panel production version. Reverse engineer an equivalent plywood hull that can do everything the Melges boat can do structurally. Getting the shape might be tricky, lets allow 5 flat panels per side with chines. How do the weights compare when this all pans out?
    [​IMG]
     

  11. Russell Brown
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    Russell Brown Senior Member

    How about Adagio? 35 feet long, 2600 pounds, 52 years old. All plywood/epoxy. Until recently the fastest rated boat under 40 feet on the Great Lakes.

    DSC_5913.jpeg
     
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