Lofting from full size templates

Hello all, and seasons greetings.

Has anyone had any experience of lofting out a hull from full size templates?

I have a full set of plans, as supplied, but no offsets. There are seven full size templates and the transom, rim pieces and keel/deadwood. Designed for double skin ply or double diagonal timber.

I think the best way will be to grid the full size templates and generate offsets for each frame.

I have contacted the plans supplier (no offsets avaliable) and the designer recently passed away.

Why?

1. I would like to have a go at lofting and I like this boat. (and have the plans)

2. Investigating the possibility of building in cedar or similar strip, so need to generate more moulds at closer spacing (original spacing is on 3 foot centers)

3. ability to loft and cut bulkheads to fit, rather than the tedious spilling normally needed.

Any thoughts from old hands?

Thanks.

 

Welcome to the forum.

If you have templates (likely station molds), your lofting is nearly complete.

Simply put, you'll trace your station molds on their reference line or the LWL. This will give you the body plan, from which you can extract and extrapolate the plan and profile of the hull. Of course, it's important to know if the molds are to the inside or outside of the planking. Usually they're to the inside, so you'll have to add the planking thickness back to the molds, as you draw them up.

This is fairly simple if you understand the lofting process, a real head scratcher if this is your first time. Naturally, the drawing is being made full size, which is a little inconvenient, but there are several books that cover the process, which you might want to brush up on.

 

The information you have from your templates is pretty much the same as what you would get when trying to generate a table of offsets from an existing hull. Your plan of gridding off the sections sounds good.

 

Now why didn't I see that! All the lofting instructions I have seen start with the profiles, (Chapelle is my constant companion)
Starting with the body plans would get the same result, from a different starting point...duh!

Is there any convention regarding the water line heights? Shoot for 6" spacing?

This thing is about 8m long, which is also the same depth of my barn. Tight fit.

Photo shows a completed version in plywood.



(I do not know the owner of this boat, or the photographer, so all rights belong to them)

 

If your 'templates' are paper patterns be aware that paper can 'change' with the weather (humidity). If you are a desert you are probably okay, on the coast, be prepared to 'adjust' things.

 

They are paper, another reason for wanting to loft this out.

 

Years ago I and a builder were in a similar situation and (if I remember correctly) essentially laid down the grid, set the patterns where the 'body plan' would normally go, projected the lines to make the profile & plan, removed the patterns, re-drew the body plan. If you have never lofted, try to find someone who has to help you. There are a few tricks that can really avoid frustration. Comments:
-One common oversight is not using end moment on the battens. If ignored you can almost guarantee an unfair hull.
- If a long skinny hull - fore-shortening can make life a lot easier since it exaggerates slight curves and make fairing easier.
-Lay the profile over the plan (use same grid). Sounds messy but can safe a lot of time and improve accuracy.
-Use waterlines & buttock lines but don't forget the diagonals... they can really speed things up.
-"measure twice, cut once". Lofting on the floor is much easier that re-fairing a crooked hull.

 

Thanks, all good advice.

I havent done one on this scale, so I had better find some help. Better clear the floor...

 

Why would you want such tight mold spacing (6")? I can't think of any reason for this.

 

Sorry, I was referring to the waterlines on the lofted profile plan.

Most of the lines plans I have show them at 6 inch spacing.

Mold spacing is currently 3 feet, but I might need to close up a bit if I go strip construction, possible 2' depending on strip thickness.

 

Instead of drawing up a new set of molds, you could just spring diagonal battens, "let" into the current mold spacing, to offer more bearing area for strips. At first glance, 36" mold spacing on a hull like that would work fine. The only place I think you might want additional molds would be in the forefoot and at most you have a single additional mold on either side of the first mold. Strips for this hull would likely be in the 3/4" thickness range, which will easily span a 36" mold spacing, without sagging. What are the strip dimensions? Judging be her general shape and size, I'd say you could use 1x2's as strips pretty easily, saving some of the tedium of the method.

 

Hey, thanks for that. I was wondering if the standard spacing would work. I was going to knock up a test piece, using 3/4 and 1" strips. It would certainly save some time and effort.

 

Although this is a dinghy in the pics below, the same principles of setting up the moulds applies. Shot one shows the frames (in this case at 610mm spacing) and the second shot shows a Datum string which goes through each frame. This very tight string allows the vertical setting of the frames to be accurate. there is also a base/floor level string to allow the base of the frames to be provisionally set. Centrelines on the frames allow alignment fore/aft and vertical after setting with a spirit level. Note the cross of pencil lines on the frame by the hole, by centreing the string correct position is arrived at.

In the case of this set of frames, I used bolts at each side to 'push' agains the floor. When each frame was correct extra pieces of timber were screwed in to maintain that position during build. Also to hold vertical in the other plane. Note spacers between frames to help integrity of the whole lot. The bow area requires slightly different treatment but keep your centreline. This build was for double chine in 6 and 5mm ply with bow tank and transom as part of the build. They still required offset frames to hold them on station. This design also has two internal stringers each side, I laminated them in situ because it removed all stress. In fact only had 5 screws holding entire shell to frames during build. Rock solid when she came off too.

If you can, build the bulkheads in as part of the hull at this stage. In fact I put in the side tanks before planking as you get a perfect joint that way.

Ought to be very similar for strip plank build as there is enough stiffness in the strips to keep integrity.

The drawings were CAD but dimensioned to the correct offsets in terms of inside the intersection of each chine and nominal keel intersect. Each frame was carefully measured, marked and modified (as they were an existing set) as required. As the frames were set level ie waterline is horizontal lots of references back to the CAD data could be made to double check accuracy. Later the waterlines were lightly pencilled on the hull panels to verify position and allow for the rather tricky forefoot area to be built. Full size paper can be used (as cutting guide on frames) but only if fresh and tacked onto the frame material with a no shrink adhesive. Tracing paper moves lots, but even CAD plots on 80 gsm paper will too, and old dyeline prints will be well out. Remember to allow for the frame thickness ie one side of the frame will need bevelling or building up depending on the fore aft position of the frame from its nominal position. Not sure you need to do much more than this type of set up, you can put a load of references or try and loft on the floor but it should not be necessary. This one came out pretty accurately. I don't have a perfect flat floor big enough so this mode was perfectly acceptable. A lot of pros use a Dexion set up to change to other frame sets.

 

That looks like a great build. Here is the one I have just finished. Just a small knock-about for the kids (Ok me too)

Similar setup, ply on frame.


And finished.

 

Do you have any idea if the drawings are inside or outside of planking.
Most times this note would have been added close to the offsets.
Older plans were usually laid up to the outside of planking.
Just something to watch out for.