3/4" MDF for mold stations?

[Lee]

I'm thinking of using 3/4" MDF for making the mold stations (Prospector 16). CanoeCraft suggests either 3/8 or 1/2 inch stock. Will using 3/4" stock be a problem? Specifically, I'm wondering if the thicker stock will change the shape of the boat, since the strips will be bent over stations that will stick 1/4" farther forward (or aft). It seems like thicker stock would change the point of contact between the stations and the strips. I'm guessing this wouldn't be a problem amidships, but where the strips have to bend toward the stems, changing the contact point would, I'd think, affect the boat's shape.

Maybe I'm making too much of this...?

Lee

[Glen Smith]

When you install the mold blocks on the strongback you take into account the thickness of the mold material so that the reference edge of each mold lines up in the right location. The center mold is positioned so the center of the mold's thickness lands on the station line. Then you install the bow-ward molds so the leading edge (edge closest to the bow) lands on the station lines and the stern-ward molds are installed so their trailing edge (edge closest to the stern) lands on the station lines.

Does this sound clear?

[Lee]

It certainly does, Glen. You explained what I was wondering about but you did a better job of it than I could have. And now I know what to call the bending point: the reference edge!

I was thinking of using 3/4" stock because another local strip builder does, and the extra thickness allows a routed groove to be added parallel to the mold edge and about an inch in. The groove is where he puts one jaw of his spring clamps. He doesn't use staples, and needs very few C-clamps that way.

A local big-box store sells spring clamps with 2" jaw openings for $.99 each, so building with clamps really appeals to me...

Lee

[Glen Smith]

Two notes on this technique: First, route the groove on the "reference edge" of the molds.

Second, be careful with the spring clamps. Cedar is soft and some clamps can leave indentations that you will have to sand out. Do a test with a spare strip and see if you will need to pad the clamp jaws with a small block of wood, duct tape, foam or other material.

Most importantly, enjoy your project.

[Lee]

Good points. The builder I mentioned puts a shim between the clamps and the strips, and the clamps also have vinyl booties on the jaws. But I had forgotten that point until you mentioned it. He and I never talked about which side to route the groove on--makes sense now that you mention it but I'm sure I would have wondered.

I hope to get the strongback and the mold stations made this winter after the kitchen remodel is done. I have to build the boat in the garage--which is unheated--so I'll have plenty of time to lurk on this forum and toss in a question from time to time.

Lee

[ealger]

Lee, I have been using 1/2" MDF for my molds with good success.
http://s12.photobucket.com/albums/a237/ealger/Clamping/

I route the clamping groove 1/4" wide, 1/4" deep and 5/8" from the reference edge.
Remove one of plastic protectors on the spring clamp to fit into the groove. I use
pieces of cedar scrap, for a pad, with blue tape wraped around it to keep glue from sticking.
Ed...

[Lee]

That's good looking work in the photos. You keep a neat shop, too. I was pricing 3/4 MDF today and thinking it would be nice to get away with something less expensive. It looks like 1/2" can work. Did you use an edge guide on your router to make the clamping groove or is there a better way?

I see why the other builder in my area advised me to buy a large quantity of blue tape.

Lee

[ealger]

Lee, I cut the clamping grooves on my router table using a 1/4" spiral cutter and working against the fence. If you don't have a router table, you could attach a wood block to the router face and use that for a guide. If the groove is a little "wavey" it won't hurt.

Glen mentioned the "reference" edge: The reference edge can be on either side of the mold, as you may already know. It's your choice which edge to use. This edge is will be lined up on the strongback marking.

When I set my molds, I set them on the "forward" side of the mark (toward the stems) and then fair the edges of the molds to support the strips along the whole edge of the stem. The clamping groove would then be placed facing mold zero. If you choose to set the molds as usually done, without fairing, then the mold would be set so it's on the side facing away from the stems (toward mold zero) with the clamping groove facing the stems.

The reason I fair the mold edge is to avoid a mold dent on the inside of the soft cedar. The spring clamp will put a lot of pressure on the strip and if the mold isn't faired to accept the strip flat on it's edge, a mark will be made on the strip.
Ed...

PS, If you decide to set the molds, as I do, be aware that you will have to place the stem molds accordingly. Else-wise the hull will be an inch longer than you planned and you'll be wondering why the strips don't fair right as they meet the stems!

[Lee]

Thanks Ed. I hadn't thought about the mold making a dent. Though it'll be awhile until I can make the molds, I'll remember that tip. I do have a router table to use as you explained.

I assume little fairing is needed on molds 0 - 2, say, with increasing amounts needed as one progresses toward #7. Do you fair them a strip or two at a time or do you do the entire mold at once? The angle changes from sheer to keel so doing the entire mold at once seems to beg for a rule of thumb on how to predict the angle. I suppose the way to do it is to bend a strip along the molds and shave each off appropriately, then move up a row.

I must be overthinking this. I tend to build everything a dozen times in my mind before starting--which is why the last addition on the house took me 6 months! It's still standing five years later, though!

[ealger]

Lee, fairing is simple. You're right in that the fairing progresses as you go further towards the stem area. Use a strip and bend it around the molds from zero to the stem mold to gauge what is needed to remove. Use a wood rasp, sanding block, ROS, etc.

First take a magic marker and run it along the "reference" edge. As you fair, do not remove the reference edge! The MDF will sand easily and it goes quick so don't be too aggresive. It won't take more than 20 minutes or so.
Ed...

[Glen Smith]

As Elgear said, this is a fine method expecially if you plan on using the same forms to build several boats. If you intend to build only one you could round over the reference edge using a router with a small radius bit or hand-sand the reference edge with 80 grit paper. You just want to remove the sharp edge that could dent the inner surface of the strips. This technique will not alter the boat lines to a point where it is no longer true to the design.

[Lee]

Thank you Ed and Glen. I'm a lot clearer on mold making than I was. I tend to mull things over and wonder about them (even if they turn out to be non-problems). In this case it paid off in good advice from experienced builders. Thank you for being there.

Lee