Correct keel fastening method

I am about to lay a false keel on a boat I am building, a flat bottom dory with about 300 mm lift at the bow. It will be epoxied and screwed down onto the keelson over the 9mm ply bottom.
The keelson is 35mm thick plus 9mm for the ply bottom = 44mm. The false keel is 90w x 19 thick. This gives me a total of 63mm.
I was going to use 2" x 10ga. screws. My query is should I put in a single row of screws down the cente or a row down each side , staggered and say 20 mm in from the edges? I would like to get this right as the keel will carry all of the boat's weight on the trailer. The I can't find and advice on this on the web. Thanks for any advice.

I can clarify

are you screwing the 19 mm thick false keel to the 35 mm thick keelson, or the other way round?

In you case I think gravity will very much be your friend and screws lighter than 10ga will be fine.

Quote:

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Originally Posted by ian

I can clarify

are you screwing the 19 mm thick false keel to the 35 mm thick keelson, or the other way round?

In you case I think gravity will very much be your friend and screws lighter than 10ga will be fine.

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Yes, the screws go through the 19 .mm false keel into the 35 mm keel

If you are epoxying the false keel down, you only need temporary screws to hold it down, while the goo cures. After which you'd remove them and plug the holes (more goo). This said, you can leave the screws if you want, if they're bronze (not brass) or stainless steel, though the epoxy will still perform all the work, a belt and suspenders approuch.

The general rule is twice the material being held down in fastener length to get a penetration length. For example, you're attaching a 19mm piece to a 35mm piece, so you'd want a 50mm screw, leaving a bit left so it doesn't poke through the 35mm piece. Ideally, if you have the room you'd have 57mm of fastener, but this runs a little long so you'd step down the next shorter length. I've had good success with the "at least as much as" rule as the minium. This means the shortest screw you'd use, is twice as long as what you're holding down, in this case a 38mm fastener. This works "okay" on hardwoods, but not so well on softwoods with highly loaded fasteners.

Lastly, when installing a bunch of screws on less than say 12" centers in solid timber, it's always best to stagger the fasteners, rather than go down the centerline. All this does is help prevent checks, splits and cracks in the solid lumber, if the fasteners try to "connect the dots" so to speak.

Thanks very much for the reply, some good info. I had intended leaving the screws in (Si bronze) and stagger them on about 10" centres. I just have to mark out all the screw positions before epoxying it down so as to avoid the 3/4" screws that hold the 3/8" ply to the keelson. I'll sort it out.
Thanks again.
Alan.

Some blokes never know when to walk away.......


https://www.youtube.com/watch?v=uqXvWIEFzHk

Could build a decent railway bridge with that timber.
Alan.

I don't know if I should start a new thread for this but it is closely related to the false keel issue so here goes. I want to put two running strakes on the bottm of the boat. I intended to have them run from flush with the back of the transom to about just forward of where the flat bottom of the boat starts its lift to the bottom of the stem. Should I use the same cross section timber for the strakes as the false keel which is 90 x 19 or can I go a bit smaller? I was thinking of something around 60 x 16 dressed. The intended purpose of these it for directional stability and also to sit inside or outside the beds on the trailer. They won't carry any weight of the boat - the keel will do that. There is nothing at all in the plans about a false keel or strakes. The boat is a Spira Seneca dory.

The bottom "rubs" are pretty common in shoal water, flat or shallow V bottom boats. For directional stability (tracking) they're best shaped with a crisp inboard edge and a well rounded over outboard edge. Also they aren't usually that big, commonly about 20 x 40. These should be treated just like the false keel, as they are sacrificial in nature as well. I usually leave mine all natural, just lightly fasten over bedding (I'm very shoal here and they get torn up fairly quickly anyway). Use fasteners just long enough to hold it in place over the bedding and as small as practical. This is because you want it to take some abuse, but if seriously hit, the small, short fasteners will just let go and the rub falls off, without taking huge chunks of the bottom with it. You can replace a rub pretty easily, but if it's fastened so well the bottom come along for the ride, it's no longer serving as a sacrificial or consumable element.