Forums: The Deben 5-Tonner: Cold Moulded thoughts New Topic Post Reply
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GregH
Posted: Nov 1st, 2011 at 23:47   |   Subject: Cold Moulded thoughts
I am thinking a little ahead of myself here, but maybe that is not suchna bad thing! ;)

I am wanting to do the cold-moulded wood/epoxy version on laminated frames. Will the combination of strip wood and ply strips be 1"? I am guessing 3/4" cove and bead strips with 2 overlays of 1/8"th ply.

Doing this method, would it be practical to make a mould frame for each station? That way I could laminate the frames right on the mould, and then move on to the strip planking.

On another off-beat thought, has anyone done strip planking and then using 3-4mm ply lapstrake overtop?

cheers
Greg
debenriver
Posted: Nov 2nd, 2011 at 12:26   |   Subject: Cold moulded thoughts
There will be some basic differences in the backbone structure for a cold moulded hull that we need to attack first.

The backbone structure consists of the stem, wood keel and sternpost – and depending on the method of construction, there may be an apron inside the stem and a stern knee.

On a traditional carvel hull, the hull planking fits into rebates in the stem, wood keel and sternpost. You can see these on the appropriate drawings in the 5-Tonner portfolio.

On a cold moulded hull, the hull skin generally runs right over the stem, wood keel and sternpost – the ends/edges of the skin being cleaned off flush with the outer faces of the stem, wood keel and sternpost. Often the whole backbone (or at least the stem and wood keel) is a single laminated structure.

The reason for this is that on a cold moulded hull we have far fewer transverses (frames). On the carvel construction, we have steam-bent timbers at about 150 centres. On a cold moulded version we would have laminated frames at something like 800mm to 1000mm centres. So we need the hull skin, the frames and the backbone structure to form one whole bonded monocoque construction.

As an aside, this is why we generally design a cold moulded boat to inside of hull skin and a traditional carvel boat to outside of planking.

The new shapes of the backbone components for a cold moulded hull can reasonable easily be derived from the laying off floor. As a starter, you will need to set off the thickness of the hull skin inside the sections on the body plan on the loft floor. Then you can derive new waterlines and the new shape of anything you want.

It is important to be clear that the hull skin thickness has to be set off at 90º to the line of the sections on the body plan – so they are true parallel lines inside the sections. Then you construct new waterlines etc. by measurement to to new inside-of-skin sections.

Another option is to assume that the existing lines are to inside of skin. You will then simply get a boat that is a little beamier (by twice the hull skin thickness). The only change you would need to make would be to lower the top of ballast keel line, because the ballast keel is going to get wider as well, but we don't want the weight to increase – or if it does, only by a very little amount.

You have to do this by trial and error – lowering the top line and recalculating the volume of the keel (and thus its weight) until you get to a weight that is much the same as the existing ballast keel.

Building the boat outside of the existing lines plan will give a boat with a little more displacement. The new ballast weight can remain at the same proportion of this new displacement as the old ballast keel was of the original displacement (about 34% - 35%).

To calculate areas and volumes of curved shapes we use a numerical calculus called Simpsons Multipliers. I can outline how to do this if you want. It is very straightforward, just a bit tedious.

OK – now to your basic questions!

The thickness of the laminated hull skin depends somewhat on the spacing of the frames, but 10mm or 12mm strip planks plus two diagonally opposed layers of 3mm ply should be fine for 750mm frame spacings. If you went to 1000mm frame spacings, you would probably need to increase the hull thickness to 12mm or 16mm strips plus 2 off 3mm diagonal layers. I don't think you need to go as thick as 3/4" unless you wish to.

There are a couple of ways to make the frames once you have chosen the positions.

Laminated: Lay off the body shape of each frame to inside of skin. Then screw blocks to the floor at regular spacings around the frame line. Then clamp the frame laminates around inside the blocks. Take a look at the new Design 119W and you can see plenty of photos of this process – and if you follow the the link to Achim's site, there are lots more.

Frame scantlings of about 25mm sided by 45mm moulded at about 750mm centres should be about right. If you increase to 1000mm spacings, then I would make the frames about 35mm sided x 60mm moulded. 1000mm spacings is about the biggest distance you can go and still have the strip planking stay nice and fair.

Remember that to get the edge bevels you will need to make the frame to the larger size (generally the aft face for the forward frames and the fwd face for the aft frames). and then bevel them off to the smaller face. You can get the bevels by laying off both faces of the frame on the loft floor.

Ply sandwich: Each frame can be made from three layers of ply. This is a bit more tedious to lay off, but much simpler to actually make the frame. Frame sided thickness needs to be adjusted to suit ply thicknesses (3 off 9mm or 3 off 12mm).

3 off 9mm layers gives you a 27mm sided frame and 3 off 12mm gives you a 36mm sided frame. You can also make 2 off 9mm + 1 off 12mm to get 30mm sided, and 2 off 12mm + 1 off 9mm to arrive at 33mm sided.

You would lay off each frame in three layers, 9mm apart for a 27mm sided frame, or 12mm apart for a 36mm frame. Draw the inside of the frame (45mm in for a 27mm frame or 60mm in for a 36mm frame). The inside of the frame is the same for all three layers. The outside of the frame is a little different for each layer. In this way you get a stepped bevel on the outside edge.

You can then bond the frame layers together like a sandwich. You can get all the bits from a standard ply sheet, just staggering the butt joins by about 200mm or more from one layer to the next. The grain direction in the face veneers of the ply layers is not important. If you mark and drill locating holes in the sections (say 8mm diameter) you can locate all the bits accurately using a short length of dowel rod (which stays bonded in place).

Frame spacings don't have to be exactly equal. You can move them apart or closer together to suit the accommodation if you wish. Particularly with the ply sandwich method you can incorporate bulkheads, shroudplate knees etc. in the ply sandwich structure. If you are going to do that, then go for a frame makeup that includes at least one 12mm layer as this generally makes a more stable bulkhead.

The frame in way of the mast should be a bit larger than the standard frames. If you are going to keel step the mast (a preferred option for a gaff rig) then the mast frame will be interrupted by the mast hole and mast partner.

The frames will have a deeper floor section and additionally you will need an intermediate floor between each frame throughout most of the boat.

I don't think there is much to be gained by laying lapstrake planking on a strip planked inner skin. We really need the diagonal layers to give the hull torsional strength and the lapstrake skin won't achieve this. And there will be a tendency to leave gaps on the inside, between the lapstrake planks and the inner skin. So basically I don't fancy the idea at all!

You could build the boat with a total lapstrake skin if you wished (about 12mm ply planks, lapped 40mm) but she is not a very easy shape for this sort of construction and I really don't recommend it. A canoe bodied hull is much more appropriate.

On a cold moulded hull you don't need the bilge stringer, nor the shroudplate clamps. You don't actually need the shelf, but I usually include a (smaller) shelf, bonded directly to the skin (thus notched through the frames), as this makes the hull/deck join much easier to accomplish.

I would be happy to draw a typical frame and pop on the site, so that you could get the general idea of proportion and the longitudinal components that will run through.

If there are things here that I am saying, or words I am using that are not clear – please just ask again!

I hope this helps. One thing that is important is to make all these decisions before starting off. This includes a fairly detailed interior layout so that you know pretty much where everything is going. Otherwise you will be almost certainly making a lot of extra work for yourself later on.

George



GregH
Posted: Nov 4th, 2011 at 02:10   |   Subject: Cold moulded thoughts
You have given me much to mull over!

I was askig about the lapstrake just as a curiosity and was thinking the same as you did. But I had to ask! LOL

I would be interested in the Simpson rules. I did some research on it, but am interested in hearing how you use them.

If it works I think using the lofted lines as the iside of the hull material would be just fine. I understand about having to take everything down lower and adjust the ballast keel shape to compensate. It would need to be almost 50mm wider where it overlaps the hull skin and not as high to compensate. As you mentioned, the actual numbers coming from the lofting.

Getting the mould angles from the plan view makes total sense. When you were talking about the 3 layers of ply I got a bit confused about the offetting of each layer piece by 9mm. Did you mean that the inside edge would all be the same (at the 45mm mark); the first layer would be 45mm wide, then the leck layer would be 36mm wide, and the third 27mm?

I'll go back and reread that section again. :)

Greg