deck on an old aluminum boat what to use? plywood+epoxy, ceder?

Discussion in 'Materials' started by leaky, Jan 16, 2014.

  1. leaky
    Joined: Sep 2008
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    leaky Senior Member

    Hi,

    I've got a 1973 16 foot Starcraft that I gutted and customized 10 years ago, made it into a nice little aluminum CC for the flats. Was the 2nd similar hull I did actually (other same year starcraft actually, just was a 21 foot).

    At the time I was broke so went for 3/4 inch primed/painted plywood deck; I raised the deck using a pressure treated "stringer" system underneath (not touching actual aluminum hull or parts, understand that is bad). This allowed accommodating a fuel tank and batteries underneath.

    The deck did what you'd expect, 5 years later or so started taking up water, rotted. Sub-deck rodents tore all the foam into little shreds and made nests (thinking I will make large foam blocks and add a layer of fiberglass around them this time). Transom and sub-deck pressure treated remains in perfect condition. I've cleaned the thing out, ready to do it better this time.

    Anyway - my initial idea was cut back on some weight, use 1/2 plywood this time, encapsulate with thin cloth and 2 part epoxy..

    The only thing that doesn't work so well is it's not like a glass boat, basically I will end up cutting panels, removing and glassing panels, then fitting them back in, screwing them down, having problems with the fact they are slightly larger when finished, then top-coating once complete. Totally do-able just alot of steps and shuffling parts around.

    Second idea I've considered was maybe using planks this time, possibly ceder as it's available reasonably priced in a 5/4 inch and is lighter than other materials.. Probably the same cost as cheap 1/2 inch plywood after I add in cost of epoxy. It would look nice with some sikkens and should last I think?

    Similarly considered something like poplar or hardwoods that can be had in 3/4 planks at a reasonable cost (did not do all weight calculations but suspect poplar in 3/4 similar or lighter than ceder 1.25).

    Figuring weight looks Ceder is about the same weight as 3/4 plywood by square footage.. Or in other words I will be putting the ~150 lbs back into the boat it had in it before (~70 square feet of 3/4). May even be able to plane it down and shave some weight too, as I don't think I need the whole 1.25 inch.

    Is the ceder thing a bad idea? Anything else lighter (and fairly cost effective) out there for what I'm doing?

    Looked into marine composites that come in sheets and the price gets outrageous, looks to be around $1000 in material as the shapes I need to make create alot of waste (hopefully some day these things find a use in home building like PVC board as the price will drop).

    Thanks for any advice in advance!

    Jon
     
  2. PAR
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    PAR Yacht Designer/Builder

    Cedar the same thickness as the plywood will be considerably lighter. This would be a good choice and it's readily available. Forget about the PVC planks, they're not structural. Poplar is a poor wood choice as it will rot if you just talk about moisture. The hardwoods available at the big box stores will be red oak, which is much better than poplar, but still doesn't like moisture, plus it's heavy.

    1/2" plywood will likely be the cheapest and more then stiff enough to hold you up, if properly supported. There's two types of exterior plywood; one says "Exposure 1 - Exterior", while the other says only "Exterior". You want the panel that says just Exterior. The Exposure 1 panels are water resistant, not waterproof glued. You could also use 1/2" MDO, which is waterproof too.

    Monsterliner is an option that works, but only if the the whole structure is coated, meaning if using plywood (for example) all sides and edges need to be painted with this goo. Other wise, moisture will get into the wood from the uncoated side and work between the coating and the wood, eventually ruining the bond.

    Aluminum is another, not so cheap but practical option. Maybe some diamond plate with a few well selected bends, so it's stiff and fits well. You don't have to paint it, it's not going to react with the boat, like other materials, it's waterproof and light weight. Pick up some junk plate at a scrap yard, hit it with a buffer to pretty it up if you like and go for it.
     
  3. leaky
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    leaky Senior Member

    Thanks - maybe ceder then!

    Looking at Lowes I see I can get it in at least three sizes locally, 1.25, 1, and 3/4 inch. By my math I think 3/4 plywood ends up closest to the weight of the 1.25 inch.

    The most expensive option in that list ends up running me $72 for the equivalent of a 4X8 sheet of plywood. The cheaper option is more like $40 (ends up being they sell ceder fencing material in a 3/4 X 5.5 inch X 6 foot).

    I'm going to pickup some samples today and see how strong the stuff is. This might work out really nice.

    Jon
     
  4. PAR
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    PAR Yacht Designer/Builder

    You don't need 3/4" plywood anywhere on that boat except the transom. 1/2" plywood will be fine. Fence stock usually is of pretty poor quality, unless you get posts and mill them down on a table or band saw.
     
  5. leaky
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    leaky Senior Member

    Thanks - I will see what the stuff looks like; the 3/4 they sell is that fence post, the 1 inch is made to be decking.

    I just mention 3/4 plywood because it's what I used before. The way the deck supports run the biggest gap is around 20 inches.

    I'm sure I could have done it with something thinner, and if I go plywood this time I certainly will, just at the time it took less consideration.

    I did vet out the aluminum idea; it's practical weight-wise, would be "final", and aside from a coat of bedliner, it would cut much of the process out. 1/8 inch would weigh and otherwise be about the same as 1/2 inch plywood with thin epoxy/fiberglass encapsulation.

    Cost wise it's a little pricey but not cost prohibitive, in scrap about $120 for a 4X8 sheet (or $400 to do the boat). There is a place locally I've been in that I think has some sheets that can be had.

    What makes me want to plug my ears though when someone mentions aluminum is the idea of making long cuts with table & circular saws. I've done alot of that and it just raises my blood pressure (noise, the way the tools work on it, I dunno just not my favorite thing to work with!).

    Secondly though vetting it out I'm not sure if it's what I want to be standing and kneeling around on when the boat is finished and it creates alot of sharp edges mitigate. Still considering aluminum as an option though, I may also take a peek in the scrap yard tomorrow AM.

    Jon
     
  6. leaky
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    leaky Senior Member

    I picked up some ceder samples; ends up I can get actual sizes of 3/4 and 1 inch in Ceder locally. The 3/4 are finish boards with a really smooth and a rough side, the 1 inch is decking with both sides semi-smooth.

    Both work out to the equivalent of a 4X8 sheet of plywood at $70, definitely cost effective.

    I weighed the planks out, 5.5 inch X 1 inch X 8 feet is 7 lbs; 7 inch X 3/4 inch X 8 feet is 6.5 lbs.

    Total deck weight works out to ~135 lbs with the 1 inch and ~100 lbs with the 3/4.

    The 3/4 plywood I removed would have been ~165 lbs.. Replacing with 1/2 plywood would end up ~110 lbs before epoxy/glass.

    Anyway so splitting hairs worry about weight, the ceder is in the ballpark. Any recommendations on the best way to fasten and finish?

    My initial thought was screw them in with a little bit of dark colored caulking in between the boards (use something that would not be softened once dry by oils), then run a sander over the top to knock off any imperfections, then finish with Sikkens.. Maybe pre-coat the bottoms before fastening with that also.

    Thanks,

    Jon
     
  7. PAR
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    PAR Yacht Designer/Builder

    I wouldn't caulk the seams on cedar, unless it was quite rift and well laid. If it was me, I'd soak my planking stock (after cutting to size) in a tub of water for a day or two. Remove when fully soaked and install with the edges pushed moderately tightly closed, then fasten. Let this dry and the boards will shrink up a bit, leaving some gaps. Unless you sink your boat, the planks will never get this wet again, so there will always be a slight gap at each plank edge, which will let water drain off and the boards can breath normally.

    As to treating the wood, well there's a few ways to go, Dutch oil will be the simplest and cheapest, but you'll need to apply more a couple of times a year. Paint is an option and I'd paint all of the planking, not just the tops. Clear finishes are very difficult to maintain, unless a straight oil job, so pick you poison wisely.

    Use stainless (sheet metal) screws only on cedar and counter sunk clearance holes, so the planks don't split.
     
  8. Petros
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    Petros Senior Member

    western red cedar is a good choice, however it is will see a lot of wear if it is a surface for walking on. Cedar is soft in cross grain compression and will dent and wear away.

    A better choice for that would be Alaska Yellow Cedar. It is not as light but it is rot resistant and very tough (and attractive if finished bright). You will have to go to a specialty cedar yard to find it, and it will cost more, but it will hold up much better if you expect a lot of wear on the surface.

    to make it look really nice you might counter sink the screws and install wood plugs over the top of each screw. You buy a plug cutting tool (I bought mine at Lowes, most hardware stores have them) and make the plugs from scarp stock. it only adds a few min. to installing each screws but it looks great and better prevents moisture intrusion around the screws. I use Tightbond III wood glue on the plugs, sanded and than finished over it with polyurethane floor finish.

    Good luck.
     
  9. leaky
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    leaky Senior Member

    Hi,

    Thanks for the input!

    A question on spacing of the planks. I'd actually rather have them be more water-tight, so the water escapes where I want it to. That's why I was thinking of using caulking, something that can expand/contract some.

    My current plan is to put a couple self bailing holes in the transom that can be plugged when I don't want them, ie for washing fish related mess out or if it gets a little rough and there is a possibility of taking water over the side. Additionally there will be a couple similar drains in the deck (ie remove plugs from transom, put in deck or vice versa).

    Now, no matter what I do short of actually welding a deck on the deck is going to leak in the boat because of the way it fits on and the dissimilar materials, just I want 95% of the water to go where I want.

    How would I best install Ceder planks to be fairly water-tight?

    Thanks,

    Jon
     
  10. PAR
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    PAR Yacht Designer/Builder

    Petros, he's looking to use Lowe's/Depot stock, so good species like Spanish or Yellow, just will not be available.

    The problem with cedar is it moves a lot (expands) with moisture content. Caulked seams will not work unless the cedar planks are fairly thick and most importantly really rift sawn stock. Getting rift or vertical grain stock from the big box stores is all but imposable. This stuff will be flat sawn, which will be the worst stuff to hope for dimensional stability. Simply put, it'll spit out any caulked edges pretty quickly.

    If you want to make cedar act like a solid surface, you'll have to edge glue it together, with epoxy, and encapsulate the whole thing, so the moisture content can be locked down. This will add quite a bit of effort and cost to this project. You'll pay as much in epoxy as you do the wood.

    Plywood is looking better and better isn't it?
     
  11. leaky
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    leaky Senior Member

    Haha yes it might be. Will make a decision on that tomorrow and either start cutting out the plywood or compromise to remove the self bailing deck.

    For sure not going to bother with trying to seal Ceder in 2-part epoxy. The idea there is lay it down easily and paint something on it, let it weather/wear, or don't bother at all.

    So more questions about plywood follow then ;).. Any big reason to go with marine plywood if I'm going to 2-part epoxy? I can get it locally in 1/2 inch, of course at about 3X the cost of exterior.

    If I go the plywood route, you guys have any experience with the cheaper epoxies on the market (like duckworks or raka?).. Wondering how they fare curing in cold temperatures and how the blushing is (only care about that because I'd like to gel coat for a finish, and an epoxy that blushes excessively is more likely to interfere with the polyester curing).

    I've done work with System III, System III Silvertip, and some with Progressive epoxy.. The Silvertip viscosity and tendency not to develop crystals I like, plus comes in a fast hardener that will go off in seemingly any temperature.. but the only way to buy it reasonably is 7.5 gallons at a time.
    Progressive or regular system III is OK (a little thick and both tend to crystalize), just is not sold in the size kit I need. I think for the quantity I need the 3 gallon kits raka or duckworks sells would be economical while still providing headroom for waste.

    Thanks,

    Jon
     
  12. PAR
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    PAR Yacht Designer/Builder

    Jon, epoxy brands are all about the same in physical qualities. You really can't gelcoat over epoxy (well you can, but it's not cheap or even gelcoat), so you can skip this idea.

    The reason you use good plywood is to get a WBP glue. Only "Exterior" or "MDO" have this adhesive (unless you get marine grade). "Exposure 1 - Exterior" doesn't and regardless of what you put over it (epoxy) it'll delaminate in time.

    If you use plywood, you don't absolutely need epoxy, though most recommend it to help waterproof it. A good primer and paint will last years, if well cared for and it's cheaper too. Of course, you'll have to touch up the paint from time to time and repaint at some point, but this is normal on painted surfaces.

    Blush is a fact of life and not a big deal with epoxy. In the environment you'll be doing this in (my assumption) the epoxy will need to be cleaned and toothed up anyway, making a non-blush formulation moot. Simply put, you'll have to sand previously coated surfaces anyway, so . . .

    The Marinepoxy from Bateau.com (also at DuckWorks) is available in 3 gallon kits and will rival any other price, within a couple of bucks. It's a two to one mix and has the same physical attributes as regular System Three.

    Personally, I'd just cut a hunk of plywood, paint it and drop it on whatever frame work you've devised. I'd epoxy bond the hold down fasteners so these don't cause rot and keep an eye on it, so that scratches and dings don't get out of hand. I'd just replace this every decade, for less then 50 bucks.
     

  13. leaky
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    leaky Senior Member

    OK I'll take that advice on the plywood. Looking at the box stores online I do see some plywood listed as "Exposure 1" and some where I can't differentiate what the rating is (is exterior for sheathing but no official rating shown).

    It's not much more to buy plywood at one of the actual lumber yards around here and a couple of them sell marine grade 1/2 inch. If I can't confirm a real "Exterior" plywood I will just go marine grade.

    The prime and paint thing worked OK I'd say for 5 years and that was with good maintenance of the topcoat and storage of the boat. At the time I used some marine and some unknown exterior plywood; even the marine went bad.
    I'd like to do something more permanent as this is the 2nd time.

    The epoxy stuff doesn't scare me, I layed up 20 gallons of recently, just was hoping there was an easier way for good results and sounds like there just isn't.

    What's the stuff that works like gelcoat on epoxy?

    I've actually been successful coating epoxy with gelcoat - some say you can and others you can't, admittedly it's "against the grain" due to the chemical issues but it can work. Last spring I pulled my hair out with it but am confident I could do it without issue now.

    The key thing is basically sand and then wash the epoxy very well with water to start, then create perfect conditions for the gel coat to cure otherwise. Needs to get going and go off before it has a chance to become contaminated. One thing that helps is to use regular polyester as a tie coat; still key thing is to make all other conditions perfect. The process is similar to if you had to use gel coat in 50 degree temperatures, can be done but wouldn't work well if hardener ratio not right or you tried to thin it etc.. etc..

    Jon
     
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