Hull / Deck Joint

[michaelmrc]

i noticed on my boat that the deck joint doesnt have bolts that go right through at least from what i can see. looks like the aluminum rail is just attached to the boat via a large number of screws. if i were to take the rail off replace with some half in wood that was bolted through would this not be stronger ?

[PAR]

The image above shows the common types of joints, though the "Coffee Can" is a modified shoe box and the "Raised Rail" is an inward flange for all practical purposes.

The shoe box is a popular joint (I've never understood why) and if your aluminum extrusion has sheet meat screws on fairly close centers, this is very likely the joint you have.

Yes, you can remove your extrusion and replace it with a wooden rub rail, though insure the wooden replacement has reasonable sectional modulus, by using a fairly dense hardwood, like white or live oak. If using mahogany, then increase physical dimensions by at least 15% to compensate.

Fill the previous sheet metal fastener holes with thickened epoxy, grind the area under the new rail smooth with not more the 100 grit. This will offer some tooth, clean and smooth the surface, in prep for bonding or bedding.

The best shoe box joints are through bolted, usually with machine screws, washers and nylon insert lock nuts. The manufacture used sheet metal screws and a big 'ol aluminum extrusion to spread the load. Bed the wooden rail and I'd recommend polysulfide (3M-101 or simular) instead of polyurethane for the wet environment against wood.

If you use a 3" screw spacing you'll be able to offset the steel half oval screw holes to miss (typically pre-drilled on 6" centers). If you use a 6" screw spacing, you really should bond the deck cap to the hull shell as 6" just isn't close enough for an off shore yacht.

You'd be wise to consider a full epoxy encapsulation approach to this "upgrade" as those fasteners are going to put lots of crushing force on the wooden rail. In fact, the bullet proof technique would be to bond the fasten holes so moisture just can't find a way in.

If employing 3" fastener spacing use a S.S. #10 flathead of appropriate length. If using 6" spacing, use a #12 fastener. A flathead fastener will apply more compression under it's head then a pan or oval head. I usually just buy the longest screw I'll need in bulk, say a 2.5" and use a bolt cutter on the inside of the hull after running down each washer and nut assembly. A dot of anti seize on the threads will save a lot of frustration, especially is machine driving the nuts. The grand total is about 112 fastener/washer/nut assemblies if using the 6" spacing and 224 if using the 3" spacing. Of course you'll need some oval head wood screws to hold the half oval strips down too. This too will be about 115 screws for your 27' yacht.

[michaelmrc]

excellent info par any specific type of epoxy you would recommend. as well any beneft of backscrewing into the wood from the inside out as well.
i would most likely be using mahogany for the trim the original trim is approx 2in now i was thinking of a 1X4 so essentially a trim piece approx 3.5 wide by 1/2 thick. would this suffice?

[PAR]

The brand of epoxy isn't a big concern, so long as it's one of the "usual suspects" I'd recommend getting yours from Bateau.com - boat plans on line since 1993 as it's the cheapest going, is basically as non-blush as any A molecule can be and the 2:1 mix is easy to not screw up on. If you order by phone, tell Joel, PAR sent you.

You'll want more thickness then 1/2" on the mahogany. A typical 1x4 would be 3.5"x.75" and you should do some scale drawing to see if this looks right, because to my eye, this sounds quite wide for a rub rail on a 27' boat. To me a 1x2 (1.5"x.75") would be more in scale with this size boat, maybe a little taller, say 2" or 2.5". As far as the thickness, you should consider 4/4th's stock (1") or better 5/4th's (1.25"). This still doesn't make me very comfortable with a hard concrete dock and a too fast landing, but it should offer enough meat for the half oval and shoe box screws, for most of the indignation, it might endure.

Make sure you bond the fastener holes for the deck cap joint. These will act as miniature compression posts at each fastener, preventing you from mashing the wood under too aggressive a driver. Counter sink both sides of the fastener hole on the wooden rub rail. I know this sounds crazy, but it's an old trick. This lets the bedding form a gasket around the fastener and a much better seal as the fastener is tightened up.

For any sheer treatment like this, it's important to get both the scale and the taper right. A well done rub rail will have a taper at the ends, more at the bow then the stern. It's all about the look and some optical illusions created by prospective and curved surfaces. A well done yacht will have a "sheered" boot stripe, cove stripe (if so equipped) and tapered rub rail ends. The same is true of the wooden bulwark seen in the linked yacht. In fact on that yacht, the scale is wrong, there's isn't any taper or properly sheered bulwark or sheer strake, which to 95% of the people of the world, doesn't make a hill of beans, but to those last 5% looks novice like and amateurish. You decide which column you'd like to be in. Do some scale sketches to see what works, for what's too big or small and remember this is a lot cheaper then having to pull everything down to shave off a 1/2" or something.

I recut a new, lowered sheer on a project not long ago. It took all day, to get the batten in a place that I could live with, before I pulled out the fine line Sharpie and drew the cut line on the hull. I cut above the line on the follow morning and still made half a day's adjustments, to the sweep of the sheer, before I considered it ready for a sheer clamp and the other things it was to get, in this new lowered deck treatment. I mention this to suggest that playing with a sweeping curve, as important as the sheer (visually) takes a while to get just right. I looked at it from every possible angle, took breaks, asked for other opinions, and scrutinized all day, before marking the cut and this wasn't my first picnic in this arena.