Do you have an inclination to build a hollow spar? Then read on.

In September 1999 I started building a 6 metre yawl designed for me by lain Oughtred, very much like a Caledonian Yawl but with seven strakes rather than four and a slightly different underwater form. A short time before, I had a piece of luck in coming across an article in the American edition of Wooden Boat magazine and as a result I made five wooden hollow spars (including a 4.5 metre unstayed mizzen mast) using a method apparently as old as the hills but largely forgotten. I can highly recommend the method and for those interested it is described below based on my own experience The full article appears in the July/August 1999 Edition #149. I adapted the actual construction method to suit my own equipment.

General Method

The spar is made up of 8 lengths of lath (batten] with a cross section as shown. The geometry is such that when fitted together they form a cylinder with minimal work being required to make it completely round. The 90 degree angle is important.

Calculations

The x dimension is 0.4 of the finished diameter of the spar eg. a 60mm spar requires a 24mm x measurement [try scaling this drawing].

The y dimension can vary depending on the strength required, wood used etc. The suggestion is that for small boats a figure of 0.1 could be used although the engineering formula provides for 0.2. I followed the suggestion of 0.15 to give a wall thickness of the main mast of 9mm. Consider what fixings you intend to use. It should be possible to insert solid plugs during assembly but I had no need other than at the ends.

The finished spar length should be increased by 30-40mm at this stage to allow for easier assembly.

Machining

I had already purchased timber for the spars intending to make them in an entirely different way so I had to work hard to make full use of it. Starting from scratch it should be a doddle if you have a thicknesser, table saw and router. If not, as the machining is so straightforward I am sure that it would be easy to get the wood prepared at little cost by a joinery /carpenter shop. Or a firm like Robbins Timber in Bristol would provide what you need by mail order; ask for a catalogue (Marine) 01179-633136. In other words do not dismiss this method just because you do not have all the equipment. Routers seem to be getting really cheap so this could be just the excuse you need....

You have a workshop! Very good…..then plane a wide board to thickness x. The board(s) need to be sufficiently wide to be able to get your 8 laths plus perhaps 2 spare. Remember the saw thickness and the margin for planing to thickness (both sides will need planing). It is very important that all laths are planed to the exact same thickness [y] in order to make the routing of the v shaped groove easier. So put them through the planer thicknesser at the same final setting.

The following sketch shows my router set up to make the groove. Because the laths are so thin and the machining needs to be consistent I preferred to keep the router stationary and feed the wood to it in a close fitting channel. The depth of the router bit needs to be such that the top edges of the groove are not razor sharp (and thus fragile). I used mdf to build up the channel but I found I needed apply candle wax to reduce the friction [although the channel is only about 150mm long] as the channel needs to be quite tight. Melamine would be better. The other requirement is two strips screwed to the top of the channel one in front of the route, the other behind, to keep the long lath from tipping as it passes under the router bit. The end result is more of a tunnel than a channel.

Tapering

You can taper one end, both ends, or even the middle. Imagine - you could be the only boat on the water with a mast that resembles a west cardinal .Enough. The y dimension remains the same. The x dimension is altered to suit the required mast diameter eg. Your mast has maximum diameter of 60mm reducing to 40mm. All your work up to now will concerned with making 8 laths for a straight mast of 60mm. Assuming a factor of 0.4 R the x dimension is 24mm. The x dimension at the 40mm end is 16mm so assuming a straight taper, turn a lath on its side and draw a taper line from one end to the other and then using a sloping base board if you have a thicknesser, a bandsaw or plane remove the surplus If you want a non straight taper you simply have a few more calculations and measurements along the laths. Remember to remove wood from the square edge not the grooved edge.

Gluing up

A moments thought will convince you that handling 8 long thin lathes covered in glue requires prior preparation in order not to have one left over after clamping . Ideally two people are needed but as I was on my own I was again fortunate in being able to make use of someone else’s idea and I had no problem gluing up single handed. The secret is to screw one end of each lathe to an octagonal block of chipboard so that the free end of the lathes can be fitted together (very easy) for securing with hose clips or whatever (see below). This is the reason for the bit extra on the length on each lath. I hope the sketch makes sense. The octagonal block has to be about one and half times the cross section of the spar. Screws are better than small nails particularly if you have a power screw driver.

For easy application of glue clamp the lathes together grooves up .This restricts the glue mainly to the gluing surface (tip. If you think you have dropped a blob of glue take your glove off and you will put your finger straight on it). What is the best way of clamping? I used hose clips and I needed quite a lot both in quantity and different sizes. The lathes have to be clamped at sufficient intervals to ensure a good bond the full length of the spar. Even with a power screw driver it was fiddly and although using gloves as protection against the epoxy resin it seemed impossible to keep it from the tool. Next time I would try old inner tubes from cycles and wrap them around as tight as possible. Epoxy does not require much pressure: polyurethane (Balcotan) rather more so. Plastic ties might also be a solution. Finishing

One advantage of this method is that you now only have to remove the 8 small corners and a little bit extra to have a round spar. I used a hand plane and in no time at all it was finished. Both ends can be plugged but I wonder whether it is wise to seal the inside .1 plugged the ends where it was necessary for fixings but left the other end open. My family say it is merely another example of my inability to finish anything 100% but if through old screw holes etc water gets in will it do more damage than if left open? My latest thought is to fill with something like deks oljey. Leave for a few days and then reclaim what has not soaked in to the (dry) wood. For the outside treatment I offer no comment other than to say that I am having doubts about the wisdom of applying impervious coatings to wood. I would be interested to know what members think. If I have made this all sound complicated and not worth the bother it will be a shame because although I had many moments of pleasure building my boat I think that spar making was the most fun.