Fibreglass help needed

Discussion in 'Fiberglass and Composite Boat Building' started by RSD1, Oct 5, 2019.

  1. RSD1
    Joined: Oct 2019
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    Location: Melbourne, Australia

    RSD1 Junior Member

    I've got a 30 foot aluminium workboat that is basically two pontoon hulls with a deck and rails etc. The boat is going to be used as a diving workboat so doesn't carry a lot of weight.

    I had a professional boat builder build it but unfortunately there has been some weight creep during construction (mainly because he made it damn near bomb proof) and I need to reduce the overall weight to keep it within road towing limits.

    The deck is 18mm marine ply which is very heavy, laid over 30 x 30mm x 3mm aluminium square tube which is laid with 200mm/8 inch centres, and so I'm looking at options to reduce the weight by possibly replacing the marine ply with a fiberglass over foam or something similar.

    What would you suggest as being the best option for foam and fiberglass layup? Once I've got some recommendations I can then compare the weight of this option to see what the weight savings will be and can then make a decision.

    Your help and input would be very much appreciated as I need to get the weight down so that I can then tow it and use it!
     
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    If your transverse square hollow section frames are laid on 8" centres, then there is only 5.5" (140 mm) between these beams which does seem to be an overkill - would it be feasible to take out some of these transverse frames such that the distance between centres of the beams is perhaps 600 mm / 24 inches?
    With 18 mm marine ply decking you could perhaps even go to 800 mm centres for the beams - this should reduce the structure weight considerably while still having reasonable transverse strength (?)
    You could still (if desired) reduce the weight further by building a composite foam sandwich deck (if cost is not an important consideration) - if the loading on the deck is going to be very low, then you could perhaps have a 3/4" / 19 mm foam core with a minimum of glass each side.
    But I think that additional info would have to be supplied first before anybody could say 'yes' to taking such drastic measures.
    Can you post some photos of the boat please?
    Is there a roof framework across the full width of the boat that could contribute towards the transverse strength / rigidity?
     
  3. RSD1
    Joined: Oct 2019
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    Location: Melbourne, Australia

    RSD1 Junior Member

    Many thanks for the reply Bajan.

    The hollow sqaure sections on 8" centres are longitudinal, the traverses underneath them are 4" x 2" x 1/8th inch box tube on 24" centres with 6" x 2" x 1/8th box tube along the outsides - he has built the whole boat bombproof which is why it has come in 750 lbs too heavy. Certainly I think that at the very least every second longitudinal square box can come out which will save about 140 lbs. No roof structure at all, the boat is designed to pass under low bridges etc.

    Unfortunately I think that I am still going to have the replace the marine ply with foam core something - the marine ply is a quarter of ton+. Loading on the deck is going to be low - crew, dive cylinders, weight belts etc. Obviously impact resistance on the deck is consideration with weight belts and dive cylinders etc.

    Some photos -
    P6.jpg IMG_20191003_141540.jpg IMG_20191003_122407.jpg
     
  4. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    I can't see how you can get rid of 750 lbs easily. Are you wanting to get the weight under 3.5 t ?
     
  5. RSD1
    Joined: Oct 2019
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    Location: Melbourne, Australia

    RSD1 Junior Member

    Weight with dive gear onboard has to be under 3.5 ton for towing legally. At the moment it is going to come in at 3.462 t with no dive gear on board and so need to shed some weight so as can get the dive gear on - some of it needs to be fitted in place - cylinder racks with cylinders, surface supplied diving umbilicals etc. If I can get the weight down as much as I reasonably can then I can work out how much dive gear can go on and make alternative arrangements etc. Total pain in the backside but not much else I can do.
     
  6. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Could the red side panels be replaced with cloth or lighter still netting?
     
  7. RSD1
    Joined: Oct 2019
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    Location: Melbourne, Australia

    RSD1 Junior Member

    The side panels don't weigh much - they are 2mm composite material, and I think that it would be much better to have solid sides.
     
  8. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Your project highlights the importance and usefulness of doing a detailed weight estimate before starting construction.
    You have a nice deck in place already - it would be a shame to rip it all up now in order to replace it with a foam sandwich deck, which is probably not going to save an awful lot of weight.
    Do you have a built in fuel tank? If yes, it might be easier to use portable fuel tanks instead, that can be carried in the towing truck when you are towing the boat?
    I can see the need for fixed cylinder racks, but the cylinders could also be stowed in the truck fairly easily, and then loaded on board the boat when you arrive at your destination? The odds are that you would have to take them off anyway to re-fill them?
    I like the twin anchor set up! Does each anchor have it's own windlass? If yes, is that really necessary on a boat this size? You could save a few lbs by replacing the anchors with (eg) Fortress aluminium anchors (?).
     
  9. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    You can contact a company like Fibertech, they make custom panels for applications like this.

    They can engineer the panel to meet your needs and tell you what it will weigh.

    Their costs are frequently lower than what it may cost you to do it yourself.
     
  10. RSD1
    Joined: Oct 2019
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    Location: Melbourne, Australia

    RSD1 Junior Member

    The boat builder did some sort of calculations before building and thought that it would weigh no more than 3000kg - but has missed that mark by quite a lot unfortunately :-(

    Being that it is a workboat we are more concerned about the weight than the look of the deck - weight belts etc will mark it up pretty quickly any way.

    Unfortunately moving things from the boat to the truck doesn't solve the problem - we would still be over the gross combination mass for truck + trailer unless we get rid of some weight somehow. The fuel tank is below the tank and unfortunately with the console and everything else that is aft there is no room for portable tanks on the deck in that area.

    Each anchor has its own windlass, unfortunately we do need to set two anchors quite often on fast flowing rivers - it gets pretty ugly sometimes.

    The marine ply is about 10 kilograms per square metre and we have about 26 square metres of it so if we were to go to a foam core with glass on it then that should hopefully come in around 3-4 kilograms per square metre so we would be saving 150-180 kilograms roughly (330 - 400 lbs) - I think that is the obvious place to save a lot of weight.
     
  11. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Come here and ask for help and then refuting it isn't gonna help.

    You need to do a full weight calculation of everything above the structural hull.

    And that means everything.

    You are not going to come by 750 easily.

    Take everything off above the deck. It is all too heavy.

    I calculate your deck to weigh 2.1 pounds per square foot above the crossmembers. No glass assumed.

    A glassed both sides 12mm foam core panel hand laid with 17oz biax each side is 0.7 pounds same.

    Vac bagged is 0.6 pounds.

    The panels need fastening which adds some, but on a boat with say 200 sqft of deck; you save about 300 pounds. Not enough eh. And the deck is nowhere near as strong..but you are past that..

    You can build pseudo structural i beams with foam. I have not weighed them. The ones I made carried a span of 30" well. They were made with 1-1.5" glassed core ripped down, bonded as I beams, filleted and glassed. They build easy. They are very light.

    If you use vac bagged panels and the foam i beams would probably get you close to your 750 with some weight reductions above like lighter rails, etc. The picture below is a 30" section of foam i beam. You can easily jump up and down on it. It moves, but doesn't fail. I can calc the weight if you want it.

    5DDA6396-5E3C-4BFD-8E22-C5670C353898.jpeg
     
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  12. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    This attitude you need to revisit.

    It might not be the panels, but the rails supporting and the fastenings all totalled that are too much versus other means.

    Every element must be considered for weight reduction. The thing with the yellow vests? Look heavily built as well.

    The rail system appears to have a double top rail as well. That would be adding a lot, too.
     
  13. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    What about a bigger truck?
     
  14. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Sorry, I dodn't realise that it is the combined total weight of truck and trailer that is the limiting factor.
    Re Ondarvr's suggestion for a bigger truck, I guess this then means that your combined weight limit should also increase significantly - enough to cope with the extra weight of the boat?
    Thinking a bit laterally, I presume that you will have a crew member or two, who will probably have their own vehicles - would it be feasible to have a separate vehicle just for carrying 'loose cargo' ?
    Rather than having a built in fuel tank under the deck, I still think it would be easier if you could get away with using portable tanks (that could be carried in the 'cargo truck' if this is possible) - it would also be safer in terms of petrol fumes in the bilge.
    Do you have one or two outboard motors for propulsion? Would it be possible to use smaller motors?
    Is it absolutely necessary to have two windlasses for the anchors? Do they both have all chain rodes? If you have Fortress anchors, no windlasses (motor up to the anchors when retrieving them), and the minimum amount of chain that you think you can get away with, then that could save some weight?
    Fallguy mentioned that extra tubular rail on top of the side bulwarks - you could probably lose that, and the rails would still be effective.
    Be brutal in your quest for weight reduction - lots of little amounts saved add up to quite a lot in the end.
     
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  15. RSD1
    Joined: Oct 2019
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    Location: Melbourne, Australia

    RSD1 Junior Member

    Agreed - but in fairness I think that you are the first person who has answered what I asked in my opening post.

    Certainly not going to come by 750 easily, but I'm hoping that by achieving some weight savings with the floor and seeing what that brings us down to and then looking to see what equipment we can do without we might get the weight sorted out without a major rework of the aluminium - plan A

    That will be the next step if plan A doesn't achieve the weight savings

    I think that numbr is about right

    Would you suggest using PVC or a Corecell M foam?

    OK vac bagging isn't a problem - we do some carbon fiber work ourselves but we have no experience with working out what fibreglass layups to use and have never specced something like a deck before.

    There is about 280 sqft of deck so that should save 420 pounds, so a substantial part of what is needed, and then hopefully plan A... The current 18mm of marine ply is probably total overkill for our needs, what you have proposed sounds like something better matched to our needs

    That might be part of plan B if needed.

    If you could that would be great - I can then compare it to the weight savings that can be achieved by removing every second row of the longitudinal box tube.

    View attachment 150577
     
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