making new bottom boards

[Anonymous]

One of my winter projects is to make some new bottom boards. Just starting to think about it and came up with the following queries:

1. Is standard exterior ply ok or does it need to be marine ply?
2. What finish / treatment is recommended? (presumably with non slip additive).
3. Would it be an advantage (e.g. if you had to lift them up whilst on the water) or a disadvantage to have each one in two 4′ long pieces (with a support piece under the join)?
4. Are there any other little design features to consider adding in when making new ones from scratch?

Any thoughts would be very much appreciated.

Jonathan
W8866
Dumdum

[Swiebertje]

@Jonathan Jenkins wrote:

1. Is standard exterior ply ok or does it need to be marine ply?

Yes, The floorboards usually have to be replaced due to wear. Using the far more expensive marine ply does not increase their lifespan, assuming they are maintained properly at least once every year. Without maintenance marine ply may last a little longer.
@Jonathan Jenkins wrote:

2. What finish / treatment is recommended? (presumably with non slip additive).

Roll on one layer of two pot paint and while still wet cover it with sand. Not just any sand, that may be too sharp. Beach sand is less sharp or use the special anti-slip sand you can buy at your chandler. That sand has been drum rolled to take the sharpest edges off. The next day, after the paint has set, carefully knock off the excess sand and roll another layer of two put paint over the sand. This creates the perfect anti slip coating and it is exactly how Porter’s does it. (Collect the knocked off sand for re-use on the other floorboard).

Treat the underside the same way (without the sand obviously). Two layers of two pot paint should sufficiently waterproof it. While painting, put the boards horizontal. That way you can put a thick layer on without runners. The disadvantage is of course, you can’t paint both sides in one stint.

Pay special attention to the sides, that is where water most often creeps in. Building ply, in contrast to marine ply, sometimes has cavities where two parts of a layer are joined. Fill these cavities prior to painting, preferably with a strong two pot putty. Make sure no water can get in between the layers of the board from the sides. Double check that the sides are very well covered with paint.

What I do is paint the sides together with the top and paint them again when the bottom is done. That way the sides receive a four layer coating.
@Jonathan Jenkins wrote:

3. Would it be an advantage (e.g. if you had to lift them up whilst on the water) or a disadvantage to have each one in two 4′ long pieces (with a support piece under the join)?

The short answer: No
A two part board isn’t stable enough and you will regret it after a while. There is so much going on while sailing that retrieving anything from under the boards should wait until you get back ashore. And in most cases the bench supports, rigging and equipment is in the way anyway.
@Jonathan Jenkins wrote:

4. Are there any other little design features to consider adding in when making new ones from scratch?

I have some fiberglass added to the bottom of the board on those spots that wear most. You can use special laminating resin for the fiberglass but it can also be done with the two pot paint. Two pot paint is often polyester resin too and if the glass is thin it doesn’t matter how you glue it, in the case of floorboards. Don’t use this method on any hull part though.

If you are a heavy person (14+ stone), consider adding some stiffenings that run from starboard to port across the bottom of the floorboards. A real stiff (e.g. 18mm ply) board would be too heavy and would not be class legal. Also I think it would not fit in the grooves near the hog. 9 mm ply is the standard AFAIK and is a bit bendy. Transversal stiffenings don’t need to be big, something like a half inch square piece of wood should do fine. Important is to use a very strong glue to ensure the wood doesn’t come off. The bending stress can be quite high. Obviously the stiffenings should not interfere with the floorboard supports on the hull.

[Gordon Davies]

delighted to know that 14 stone is considered heavyweight…at 130kg (what’s that in non-european measures) I must be a giant..
Gordon

[Anonymous]

Many thanks for the very detailed and interesting reply. I think I now have everything I need to know so just need to get on with it!

Thanks
Jonathan

W8866
Dumdum

[Swiebertje]

@Gordon Davies wrote:

delighted to know that 14 stone is considered heavyweight…at 130kg (what’s that in non-european measures) I must be a giant..
Gordon

I was refering to the pre Gregorian (Julian) Rhinelandian stone

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