New aluminium masts

A new year and time for new junk-rig masts

Goodbye to 350kg of steel

Goodbye to 350kg of steel

Although I replaced my broken Red pine mainmast with a steel pole back in 2013, I have never been very satisfied with it. The 350 kg steel tube was within acceptable limits for the boat, not making it too tender (prone to excessive heeling) – but it had a few drawbacks.

One drawback being its excessive diameter below, taking up too much living space (approx 9″ at the partners increasing to 10.5″ diameter at the step).

Another drawback was its weight – reducing the amount of extra weight that would be reasonable to load on for long distance cruising.

mainmast partner hole

Then there was the nagging worry that so much steel wagging about in big seas would be too much strain on the structural integrity of the hull and deck. This may not have been a strictly rational concern – but it was an emotive one.

And we sail to enjoy, not worry, right?

Another factor in prompting me to install new masts was the fact that I sourced a supplier of aluminium poles only an hours walk over the mountain from where I live. The company, ALC, import aluminium poles from the Netherlands mainly for street lighting uses but were willing to both source and do some custom welding on the poles for me before delivering them to Lawrenny Yacht Station – where the boat is currently moored.

look at all those potential masts at ALC!

The main mast pole weighs in at 87kg with an 8 inch  (199mm) diameter and a wall thickness of 3.5mm. You can view the blueprint for this pole here.

What’s interesting about this pole is that the 8 inch diameter remains constant for the first 18 feet and only then tapers to 76mm at the top.  This means that it provides significant strength and resistance when reefed down in heavy weather. It also has the benefit of not unnecessarily increasing in size between partner and step leaving it easier to accommodate down below.

The same is true for the 6 inch diameter foremast pole. It only begins to taper after approximately 9 feet down to a top diameter of 76mm. Wall thickness 3mm. You can view the blueprint if the foremast pole here.

foremast crane

For the masthead cranes I modified an idea forwarded to me via the Junk Rig Association forum. I fabricated two circular discs of 50mm thickness.

The bottom plate had a hole cut out of the center approximately the size of the pole’s top diameter – to allow for wires to be passed up through.

The top plate had a small hole just to let the navigation light wires (mainmast) and VHF cable (foremast) to exit.

The two plates were then bolted together with stainless steel U-bolts from which the masthead lines and blocks were attached.

For extra strength, 6  triangular (10mm thickness) buttresses were welded to the pole and plate. If that sounds all confusing take a look at the picture above of the foremast crane. (I was planning to take more detailed pictures of the design but got too involved in doing it to remember to get the camera.)

mainmast wooden plug being loaded for transport to Lawrenny (Wales)

old, new and wooded plugs

Having ordered these poles I was a little concerned that the bare poles by themselves may not provide sufficient strength in extreme conditions – conditions one has to face occasionally when sailing long distance. To assuage this worry I designed and built solid wooden plugs that could be inserted into the poles to extend well above the partners.

The design is based on the fact that given enough force the place where standing masts will buckle (assuming no internal defect in the pole) is at the place of maximum resistance – at the partners where the mast exits the coach roof.

pulling the mainmast plug in with the aid of block and tackle

The plugs themselves completely fill the lower internal cavity of the of the poles and then are increasingly reduced in bulk for a further few feet until it is only a 2 inch thick cross contacting the internal pole walls before coming to an end.

The reduction in bulk is to allow a degree of bending of the plugs themselves and mitigate against buckling of the pole at the place of maximum resistance – which would be at the top of the wooden plug – if they remained solid and came to an abrupt stop some 8 feet above the partners.

This design is meant to produce free-standing masts that are a reasonable in size and weight while ensuring sufficient strength and flexibility in the most demanding conditions. Should work ;).

The mainmast now weighs approximately 300lbs (140kg) and the foremast 150lbs (70kg) – including weight of wooden plugs. A reduction of over 450lbs (200kg) of mast weight. The waterline has dropped an inch or more.

main mast being lifted – notice the wooden insert has been squared off to fit neatly into the remade main mast step

fitting in the main mast (with a little help from a crane)

The sails and rigging are now refitted with only a few remaining tasks to be completed before sailing off for the summer.

We were originally planning to sail to Lofoten, Norway but time is slipping by and the weather has turned a bit more unfriendly for a spell – so we’ll just have to see how it goes …

Watch this space.


6 responses to “New aluminium masts

  1. Very interesting, what sort of money was involved? Was the transport an issue? Tony and Sally s/v Ron Glas

    • Just noticed this comment now. I’m a bit lax online! Both poles came out to approximately £1500 (including VAT at 20%!). There was no extra charge for welding the masthead crane on. (I fabricated the parts myself from a 5mm aluminum sheet they gave me free of charge). Transport was not hard to arrange but cost me extra of course. Around £100 to Lawrenny I think. Can’t remember the exact cost.

      • Thanks for that, sounds interesting if and when we come back!!, used to keep our junk schooner…… SALOTEAN.. IN Garrons Pill. In winter and Dale in summer. Since we have been cruising Ron Glas the Med has been home but with Brexit we are looking at the Atlantic islands, West coast of Ireland and Scotland. …. Maybe our paths will cross?!. TONYANDSALLY s/v Ron Glas

  2. Charles A. Dunn III

    loved your site. Great to see some original, out side the box thinking on rigs. I’m installing some used carbon sticks in my plastic presto boat. I’m thinking of using rubber spartite donuts to relieve hard tohjard at the partners. Any thoughts?

  3. Sorry to follow up on quite an old post, but I wanted to ask about your mast strength.

    I’m thinking of doing a similar conversion on a Hartley 39 and thinking about unstayed (of course) aluminium masts. In my case, I’m looking at mast extending 10.7m above the deck supporting a 9.5m high sail (I’m going for a relatively low aspect ratio, 2, and 90 sqm total sail for SA/D of 20 and equal-sized sails; I haven’t looked into exactly where this would put the centre of effort yet, but it’s what I’m working with for the moment).

    I’m designing for winds up to force 7 (33 kt), assuming that if any gust higher than that is headed my way, I’ll have reefed before it gets there. The calculations are slightly back-of-fag-packet, but I’m working out a load of ~8kN at that wind speed on a 45 square meter sail. Using a mast that is 200mm across with 3mm walls and applying that over the length of the sail, the mast tip deflection is 3m, or just over 15 degrees, which seems worryingly large to me. Of course, there are no safety factors in that calculation and your masts are also tapered, so the deflection will be higher. I’ve assumed 6061 aluminium (Young’s modulus of 69GPa); other alloys will be stronger.

    It’s quite possible that I’m overdesigning – I don’t have any good way of estimating the righting moment of the hull, so it’s quite possible that a 33kt gust square to the sail would just capsize the boat rather than bend the mast.

    How are your masts working out? Can you comment on the rough calculation above and give any advice?

    • As a follow-up to my “overdesign” comment, applying the load as a distributed load in force 6 gives a deflection of ~1.5m and in force 5, ~900mm. So maybe I just need to pay more attention to the shipping forecast and reef more conservatively…

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s