Monday, June 5, 2017

Three Brothers - a 3D Virtual Assembly



As part of my attempt to learn how to use a 3D CAD program, I have been using my Three Brothers powerboat design as an exercise. Although the design is still under development, I thought that some people may be interested in seeing a stage-by-stage stitch-and-glue assembly sequence.

This assembly sequence is only one of several approaches to stitch-and-glue construction, but most have a common theme of not requiring a strongback or mould set-up. The shape of the boat is determined by the accurate design, marking-out, and cutting-out of the primary parts - if everything is done correctly, the boat assumes the correct three-dimensional shape without a strongback and set of station moulds, greatly increasing the speed of construction.

Three of the six main panels cut from plywood and laid on the floor. The accuracy of the design and cutting of such panels is the key to a successful build.

Two bottom panels stitched together along the stem (bow) and centreline of the bottom. Stitching is carried out with the two panels laying on each other, and when complete, the panels are opened up 'book fashion'. The assembly will sit on the floor as shown, but it is sensible to have some cradles for a project of this size (details will be provided in the plans).
Pre-fabricated bulkheads, frames, and transom are positioned on station marks and loosely sewn into location using cable ties.
Topside panels stitched into position. By this stage the glass-taping of joints will be taking place.

Cabin sides, including coamings, stitched into place.
Longitudinal webs glued and taped into position. Ventilation holes are suggestive only, and may be changed depending on style of emergency flotation employed
Outboard motor splash-well structure added.
Floorboards and other horizontal panels introduced.
Longitudinal deck-beams and roof structure in place.
Fore-deck and aft-deck panels attached.
Plywood cabin roof and front panel of cabin attached.
Gunwales, outer stem, and structural trim around cabin and coaming finish the basic job.
This is a very quick illustration of the basic assembly method. For more detail, I suggest reading Sam Devlin's wonderful book on the subject, "Devlin's Boatbuilding.

Sunday, June 4, 2017

Getting 16 feet Planks from 8 foot Panels...

In 2002/2003 I was becoming increasingly aware of my lack of bench space in the Wynnum workshop.

At one stage we had six boats under construction at one time, and when commencing the planking of a glued-lapstrake (clinker) sailing dinghy, I realised that I simply did not have the bench space to scarph together 8ft x 4ft sheets of plywood into 16ft x 4ft sheets, which had been my standard procedure.

The space being taken up by 16 foot-long panels had been on my mind, but on this occasion the chickens had come home to roost! Necessity being the mother of invention, I experimented with an alternative approach, and the result was highly successful. A recent call for help from a builder in America has brought the subject back to mind, and I thought the method may interest others.

Lapstrake plank patterns laid out on a 16ft x 4ft panel in my first rented workshop - that is a lot of space taken up!
In order to produce accurate planks from separate 8 foot-long sheets of plywood, I decided to capitalise on one of the by-products of my favourite method of spiling (i.e. method of determining plank shapes when laid out on the flat), and that is the production of lattice style spiling battens. You can find this method of spiling in a number of text books, but it is also shown in illustrated form in the instructions which accompany my plans. Here is a brief explanation of the method I use to produce accurate 16 foot-long planks from 8 foot-long sheets of plywood. Not only does this method save bench space in the workshop, but it also allows for more efficient utilisation of the plywood - i.e. less wastage. (click on images to see full detail)

Here is a plank pattern laid on a single 8ft x 4ft sheet of plywood in a manner which produces minimum wastage. The pattern shown uses diagonal truss-type bracing, but I often use rectangles of thin MDF hot-melt glued to achieve the same result more quickly.  See earlier photo above. Trace accurately around the pattern with a sharp pencil or a fine ball-point pen.
Where the pattern crosses the edge of the 8ft x 4ft plywood sheet, carefully and accurately mark the pattern as shown above in red.

Next, mark a line eight times the thickness of the plywood from the edge of the next 8ft x 4ft sheet. This line is drawn to allow for the amount of plywood which will be taken up by the chamferred  edge of the ply to allow for a scarph joint.  In this case, the plywood is 6mm thick, so I have shown the line 48mm from the edge (if using butt-straps or a "Payson Glass Joint", just make sure the marks are clear of the edge of the sheet).  Lay the remaining portion of the plank template on this sheet with the red marks which were added in the previous step inside the line. 
Adjacent to the marks on the pattern, and inside the line marked eight times the distance of the thickness of the ply from the edge (or from the edge itself if using a butt joint), mark the plywood accurately as shown by the black arrow-heads in this drawing. Then trace accurately around the rest of the pattern.
Remove the pattern to leave the tracing on the plywood and mark a line eight times the thickness of the ply beyond the arrow heads. In this case, with 6mm ply, the distance is 48mm (see drawing above). If the joint is a butt, disregard the last mark.
Cut the half planks from each of the 8ft x 4ft panels, and plane an 8:1 scarph on each half of the plank (disregard for butt joints).
Glue the plank halves together using the scarph or a butt-joint and before the epoxy has cured, lay the plank template over the assembly to ensure that the two halves line-up accurately with the full template (ensure that you place waxed paper or plastic sheet between the glued panels and the template to prevent gluing them together).
This procedure is more difficult to illustrate and explain that it is to carry out. I can assure you that with the application of common sense, it is a simple and practical process which will save you time, space, and plywood.

Monday, May 29, 2017

Increasing Sail Area

One of the subjects I deal with frequently in email correspondence is the question of increasing sail-area of that shown on plans. The answer, of course, is that sail-area can be increased as much as you like, but is it advisable?

Here is part of the text of a recent email exchange on this subject which may be of interest:-


I have plans for the First Mate , bought a couple of years back before we bought and started to renovate our workers cottage here in Germany. I am nearing completion of the of the major work on the house and will have time to build the boat for myself and the rapidly growing 2 kids we now have.
We live in a land locked position with several large lakes to sail (10 suare km not to Somerset standards). However the winds are usually very light as a general rule. We can drive to the Netherlands and sail the inland waterways  again this involves making a weekend of the journey. ( 6 hour turnaround). 
The other use will be some trips down several of the main rivers that flow into the north sea. The planned trips back to Denmark and Sweden.
I have looked at and bought several sets of plans but have always come back to the Phoenix III/ First Mate as it will do everything without a fuss. 
Sorry about the long winded explanation.... Could I squeeze  up to 10 square feet more in a lug design? I'm looking for a rig that we can drop very quickly when we sail in the river and for when the kids get older, for them to sail on there own.

The author of this message is a knowledgeable and sensible fellow who was asking about a quite modest increase in sail-area (about 15%), but there are more things to consider than just the increase in area. Below is my reply:-

I can definitely draw you a larger Balance Lug rig, to any increment within reason. The issues are that as the rig area increases, the yard and boom lengths increase as well, and therefore they increase in cross-sectional area and weight, meaning that storage issues become more of a problem and the boat has more weight that needs to be driven by the sails - and so on in a cascading (well, slightly rippling..) manner. In addition, space under the boom becomes an issue, and you need to look at increasing the length and diameter of the mast.

What I put your way for consideration is the standard Balance Lug rig, with the standard spars, but with the heel of the boom pulled aft (I can show you the details of the rigging) almost to the mast, turning the Balance Lug into a Standing Lug. The aft end of the boom cocks up high, helping with room underneath for the helmsperson, and a nice consequence is that as the longer aft part of the boom swings out when eased, it is higher above the water. This helps to prevent the boom end dragging in the water, and sailing the boat over no matter how much the sheet is eased.

Not only does the boom move aft and up, but the centre-of-area also moves aft relative to the centre-of-lateral resistance. This allows one to set a jib to bring the centres back to the correct location, and provides much more boost and windward performance than a larger lug would have done. This is all achieved with virtually zero increase in rig weight and length of spars, as the jib is set flying i.e. not hanked onto a fore-stay. A jib set this way can be flashed out and in an a matter of seconds, and does not require silly roller furlers and such like. You still have the original Balance Lug available in an instant, simply by loosening the boom parrel. If required, you then move to reefing the Balance Lug normally, and there are few rigs better adapted to reefing than a Balance Lug - the Chinese Lug (or Junk) rig being a notable exception.

Let me know your thoughts - I'm happy to draw the bigger lug if that is your preference. I've attached a PDF to show you the idea.