Designing the Junk rig for a Hunter Liberty.
I am possibly at a critical point of decision making about WABI”’ and seriously thinking in terms of either setting her up for some longer distance cruising or getting her on the market and swapping to something with a tiny bit more sailpower and living space. I am working on a whole series of posts about a potential offshore and ocean passage with WABI”’ and my primary concern right now is all about the strength of the existing rig which i think just isn’t strong enough for long distance sailing. The 2 rig options i am working on for WABI”’ are first, refitting the conventional rig with better sails and a better sail handling set-up, and secondly an ocean-going junk rig along Roger Taylor’s lines. The principal problem with the Liberty rig layout being that weight forward is a really bad idea so the critical foremast needs to be as light and strong as i can make it. Right now i am actually refitting the existing rig with better hardwear and deck organisation for my spring trip back to the Tamar.
In my first post about the WABI”’ refit project i talked briefly about my intention to build a junk rig as an alternative to the existing cat-ketch, in this post i am going to take things into the planning stage for the masts and sails. Once again, the rig i am basing the project on is a similar Hunter Liberty 22 called ‘Alouette’ which i guess is going to be appearing frequently in this series. In this video you can get a good idea of what her rig looks like with the best clip at around 5 minutes in.
So far, what i know about the rig is that the masts aren’t the original Proctor tubes but were built specifically for a German Liberty called ‘Golden Wind’ which is a one-off Liberty with higher topsides and originally the rig seen above. I believe that she now has a single mast stepped all the way forward a bit like a true cat-boat and somehow it doesn’t look quite right. The schooner rig as above looks better to me although it almost certainly is less closer winded than the sloop BUT…having all of that weight and sail area forward seems unbalanced. One thing i am very conscious of with the Liberty is not loading lots of weight forward which is why the new masts (plus booms and battens) will have to be as light as i can get them and in offshore conditions i will really have to think about bringing my anchor and chain aft.
The masts on Alouette are around 21.5 feet compared to about 26 for my front rig and about 21.5 for my existing mizzen. Sail area though is higher, the standard Liberty rig is only about 180 square feet and Alouette is about 220. It might even be worth pushing that up a bit by going to slightly taller high aspect sails. From memory Roger Taylor’s junk rigged Achilles is 280 sq.ft. and the junk sloop liberty 260.
For a first look at potential masts i have been trying to source tapered alloy tube or tapered carbon tube and there was a hint of both being produced for flagpoles but so far i have had no luck with either. The industry approach as i mentioned is that there is nothing ‘off the shelf’ and for a mast-maker to design and then make the tooling would be prohibitively expensive. That throws me back on a backyard build which really means timber masts of either hollow quarter-stave or hollow birdsmouth construction as below.
The flagpole/street-light idea hasn’t produced anything of approximately the right size so far although that is exactly what Roger Taylor used for his Achilles 24 rebuild.
Birdsmouth construction hollow timber.
Working from the proposed sail area, displacement and height of rig i have used the Hasler book to work out the mast diameter. In his book he gives approximate values in a graph form but my boat is right at the bottom of the graph so the numbers don’t extrapolate clearly. What i have thought though is that both existing freestanding sticks are the same diameter at base and pivot, also the existing foremast is taller at 26 feet and carry’s the same sail area as one of the proposed new masts but does so at a greater height and i guess centre of effort. I may not be far wrong to build the new masts at the maximum diameter currently allowed by the pivot and the ‘bury’ slot in the foredeck well. I should though be able to go up a little bit in diameter which i feel would be better if i don’t use the existing pivot system which is a narrow alloy casting…i think that would have too much point load and crushing force on a timber spar. Ultimately i think i can get a 120mm base and pivot diameter and then work from there : the alloy masts are said to be 3 1/2 (87 mm) inches and i should be starting at just under 5 inches (120 mm).
Now going to the birdsmouth calculator i enter the values of inside and outside diameter which also gives the wall thickness of 20% of spar diameter, and the number of staves, this then calculates the dimensions L and H which is the width and depth of each stave. The birdsmouth angle with an 8 sided spar is obviously 90 degrees in the centre of the mouth which is the easiest one to work with and gives a potentially flat bearing surface at the base fore and aft for the bolt and at the pivot point.
**For my mast L = 38.7 mm and H = 22.8 mm. (call that 38 and 22 + some glass on the outside surface) Back-engineering with those values gives me an OD of 98mm and an Od of 58mm which seems ok given that the whole thing will be a bit fatter with the epoxy glue and some external glass cloth and resin.
Having worked out the specification for the staves i can now request a quote for the timber, although more expensive it might work out best to have the strakes cut to size and have the birdsmouth all machined by the timber supplier which at this stage is likley to be Robbins timber. At first look i will actually need 368 running feet of strake timber but that will need to go up depending on what length the strakes are cut to and i guess that will be determined by the initial board length. Above that i should allow for wastage so i may need as much as 500 running feet of stock to work with. At that length i suspect that it would be worthwhile them setting up the machinery to cut the birdsmouth groove. Further down the line i will then have to work out the best use of the stock that i do get so that my scarphs aren’t all at the same point and i know that one of the harder jobs will be to scarph and then taper 16 strakes.
Just to mention that i am trying to reduce the total number of actual tasks that i have to do so getting the stock all machined beforehand is one thought. I will still have to build some scarphing jigs, a mast table, scarph and shape the strakes, build the butt , pivot and cap plugs (hey a wooden butt-plug….oooh-er)……just to build the actual sticks before shaping them.
According to the Hasler book the spar should be barrel tapered down to just over 40% of maximum diameter at the tip which sounds too much of a taper so i am working on adjusting that one up to about 60% which gives a healthier spar dimension higher up. The tapering is then partially done by shaping the other narrow edge of each stave and ultimately by also shaving the spar thickness a bit near the tip. Today’s technical job is to work out the lofted dimensions and then back-engineer the required stave widths at each lofting point to see if the whole thing still works out as build-able,
I don’t think the existing pivot system will work with a wooden spar because of the point and crushing load at such a narrow point, remember that nearly all the heeling and bending forces will go through that point. When i am next at the boat i will try and get some better pictures to illustrate the problem. Because my wooden sticks are likely to be thicker at that point anyway i can see a way of making external cheek plates through-bolted and using the existing casting on the front of the coachroof.
Same castings : mizzen arrangement.
Spar, booms, lines arrangement.
I’m not far enough ahead to start working out the yards, battens, booms etc but what i must do is work out the spar arrangements so that the lines can lead properly. According to the manual all lines come down externally and the halyard particularly has to come down to the deck opposite the yard and outside the parrels. Apparently the normal way would be to set the foremast up with it’s yard to port and halyard to stbd so that heaving-to for any purpose would put the boat on the right tack for right-of-way. That won’t work on the Liberty as the useful place to bring lines aft is to port so the yard will have to go to stbd. Usefully that means that the mizzen will be set to port which should keep the sail bundle to the left of the stick (and hatch) in the above picture.
So : next stages.
Read more of the technical stuff in Hasler’s book.
Work up the quote request from Robbins timber. As i write i have had 2 quotes from Robbins to provide timber for either a 4 stave ‘hollowed out’ stick or an 8 stave birdsmouth stick plus machining for the latter. Cost of timber for 2 sticks in birdsmouth works out at around £1300 so that is my reference point for anything else. Carbon just doesn’t seem available in this country at all.
**originally calculated for a 100mm dia spar.
Organise an inside building space for a couple of weeks in summer.
Build a full scale sample of the bottom section of one mast complete with the plugs and pivots all worked out.
Talk to a sailmaker who knows about Junk rig.
Birdsmouth calculator. http://www.duckworksmagazine.com/10/howto/birdsmouth/index.htm
Postscript. There is a tantalising possibility of acquiring a complete 2 mast junk rig already built for a Liberty but slighty taller. I am chasing that one down before going any further with my plans to build one.