GREAT AUK Mast

There are enough nice photos of GREAT AUK’s mast going up and down that the subject really warrants its own post. And it really has been a bit of a project, overall. Photo credit for all the above: Suzanne Jean

One of the design goals for this boat has been that the mast should be easy to raise and lower, without drama, or strain, and without having to rustle up extra-muscly help. This approach makes the mast consistent with the design goals for the boat overall: ease of use, ease of maintenance, and general comfort (not necessarily in that order). Thus we have that high tabernacle, which allows the mast to lie comfortably on the top of the cabin, with enough of the lower mast below the pivot to form a substantial lever that provides for simple winching, both up and down. (We had a couple of complications that led to that upper pin being so close to the edge; bronze reinforcing straps can be seen in the third photo below.) Photo credit: Christopher Lariviere Photo credit: West Cove Boat Yard shop cam
Photo credit: West Cove Boat Yard shop cam

Passage under bridges, generally, and under the bridge on the north side of Mount Desert Island, specifically, have been particular inspirations for this setup. Ease of trailer launching and retrieval has also been high on the list. I’m happy to say that testing the system has been quite successful. There will be some complexity when it comes to managing the junk sail bundle during this operation, which we’ll figure out in the spring, but all is looking very hopeful.

Originally, this boat was going to have an aluminum flagpole mainmast. I had tried once in the past to find a way to get a carbon mast for AUKLET, but was unsuccessful – the spar company that I approached said they could do it, but after a lengthy two months of repeated phone calls and emails to check on progress toward an actual work order, they very apologetically said that they could not do it after all. The boat was too small, and they did not have an appropriate mandrel for building a round, tapered spar that would suit the situation. Time was pressing by that point, and AUKLET acquired an aluminum flagpole. This worked out, but stepping and unstepping were not entirely smooth.

This time, at the ideal serendipitous moment in our building process I ran across a notice about somebody who had just gotten a carbon mast for their relatively small junk rig cruising boat. The company that had built it was quite happy about this, and featured the project on their website. How perfect! They were not only familiar with the overall concept (different in various respects from western rig masts), but happy about it! I was on the phone with them within days, and we started working out the design questions. Thanks so much to Nate Williams at GMT Composites in Rhode Island. Their website is here: http://www.gmtcomposites.com (Nope, not receiving anything for posting this, or for any clicks – just eternally grateful!)

If I had known just how well the tabernacle and winch arrangement was going to work, I might have gone with the aluminum flagpole approach for GREAT AUK, in spite of the weight. The tabernacle structure and the worm gear could easily handle it. But at the time of decision-making, all of our struggles with the weight of AUKLET’s mainmast were high in mind. That mast is hard to put up and down, as well as being hard to move on and off the boat even after the mast is horizontal. Far too many times I have looked on with trepidation, as two strong people have dealt with the awkwardness of anything to do with moving that pole. It’s not even that heavy, as these things go, at about 80 pounds. But still, the whole process has not inspired confidence.

The mast for GREAT AUK would be 4 feet taller, and heavier, in aluminum. It’s also high on the boat, adding weight where you don’t want it on a flat-bottomed barge hull, with no keel to bring it back up if it goes over. Carbon masts don’t come cheap, but the trade-off in ease and peace of mind was substantial. And there was that serendipitous availability, at just the right moment.

GMT Composites was quite thorough in their design process, and it gave me the opportunity to learn more about mast design and boat stability tables, which involve a fairly technical calculation process from which I had previously shied away. AUKLET’s mainmast was originally wood, built as designed by Phil Bolger for that boat’s original gaff rig. But then before we launched AUKLET for the first time, we had a delamination nightmare. This led to swiping the aluminum mast from the Peep Hen, because it was available in the garage. The sail area for the Paradox rig that went onto AUKLET was smaller than that of the Peep Hen, so it seemed reasonable to do this. But that Peep Hen mast never inspired confidence. It flexed quite a bit, and just made you think it could break, looking at it under trying conditions. So when we did the junk rig, and needed a taller mast anyway, I got a flagpole that was 1 inch larger in diameter than the one from the Peep. It looked sturdy, and in use was never frightening, including in some quite demanding situations. I was confident in copying that for this new boat, especially since the plan was to use the very same junk mainsail from AUKLET. (Technically that was not quite comparable, or proper – AUKLET heels easily, reducing strain on the mast, while a big flat barge will be quite stiff, increasing mast strain for the same wind and the same sail area.)

The dimensions of that 4″ flagpole are what I gave to Nate at GMT. He then asked if we had the “righting moment” for this hull. Now that’s a can of worms! Triloboats designer Dave Zeiger was gone sailing, and not reachable at that time. It turns out that righting moment is actually not one figure, but properly a table of figures, calculated from possible different angles of heel (leaning to the side) for the boat.

As it happens, the owner of West Cove Boat Yard, Christopher Lariviere, is both a person with a mechanical engineering degree and CAD skills. He’s the one who has done the nice CAD drawings of the plans for GREAT AUK, for our build there at WCBY, and he used work that he had already put together to calculate this new set of figures.

Christopher wrote this wonderful explanation of these calculations, at the time we were working through this:

The righting moment is not a single number but is a function of heel. Just so you know this is how it is done:

1) First you figure out where the center of mass of the boat is (mostly where it is vertically). You do this by figuring out the mass of all the individual parts of the boat and their individual centers of mass. Then you calculate the mass weighted average height for sum of the components which is the vertical center of mass. My calculation (on a spreadsheet) shows a total weight of components of 4900 lbs and a center of mass of 24.4″ above the very bottom of the boat.

2) You then place the hull at a variety of angles of heel in the cad drawing and let it figure out how much of the hull will be submerged and where the center of buoyancy is located. The horizontal distance between the center of buoyancy and the center of mass is your righting arm. The righting arm length times the mass of the boat gives you the righting moment.

For example, let’s say the boat heels to port 10 degrees. This causes the hull on port to submerge into the water a bit and the hull on starboard to come up out of the water a bit. So the center of buoyancy moves to port. The center of buoyancy is pushing up. The center of mass is pushing down. The result is the the boat tries to right itself.

Now if the hull continues to heel to port further and further, eventually you reach a point where the center of mass moves past the center of buoyancy. When this happens the righting moment changes sign and the boat flips over. So as long as your righting moment is positive you are ok.

I set the hull at different angles of heel and found the numbers:

As you can see even with 55 degrees of heel, the boat is still stable. However you can also see that the righting moment is dropping quite quickly at 55 deg of heel so you don’t want to go much further!

Attached [shown below] is a cad image of the hull sitting at 55 degree of heel and the resulting center of bouyancy. The starboard side of the hull is sitting way out of the water! Scary but still stable.

~ Christopher Lariviere, from May, 2019 email (shared here with his permission)

Drawing: Christopher Lariviere

Scary indeed! Makes my stomach do flips, just looking at that…

Meanwhile, I had sent that righting moment table off to Nate at GMT Composites. His conclusion was that we needed a 4 inch diameter carbon mast, at minimum. A 5 inch diameter carbon mast would be truly stout, but a lot more costly; the 4 inch version just squeaked in, for satisfying the design numbers. Interestingly, it turned out that the 4 inch diameter aluminum mast would have been seriously below the proper specs.

Since then I have had the occasion to really go through the materials at the JRA (Junk Rig Association) website for calculating junk rig mast dimensions (links at bottom of post). I did this for another project with which I am helping, and it was illuminating to finally get a little more understanding of those figures. In fact, going by the JRA-sourced calculations (thank you Arne Kverneland), an aluminum mast for GREAT AUK would indeed be more appropriately 6 inches in diameter with a 1/8 inch wall thickness.

Sometimes serendipity comes in many forms. Not driven hard, I think that the aluminum mast I was originally considering might have been okay. But I’m very happy to have a mast that is actually sized according to some appropriate math. I’m also very happy to have a carbon mast that, at an amazing 22 pounds (!), is truly manageable for people with a wide range of strengths. Feeling sheepish about the extravagance, I did get it painted an innocuous color, in hopes of very few people noticing that it’s not more basic aluminum… Photo credit: Nate Williams

Installing this nice carbon mast came with its own challenges, mainly to do with the holes for the 1/2 inch steel rods that form the upper pivot and the lower locking pin. Both of these horizontal holes in the mast need to be lined up with matching holes on either side of the tabernacle, which is not so easy to work out.

With an aluminum mast, you could use an extra long bit, start with the hole in the tabernacle (being very careful that everything is square), and then drill through into the aluminum, out the other side of the aluminum, and through the other side of the tabernacle. Easy peasy, the path for the pin would be all lined up. But with carbon, the holes for those pins need to be specially reinforced, and they are built at the same time as the mast. Photo credit: Nate Williams

Drilling through the tabernacle and hoping for the best, as far as hitting those existing holes, was a scary thought, and much pondering and a bit of postponing was going on at the boatyard, in the face of this task. I had no good solution either – Theo, in Holyoke, had done some very clever alignment by sight when we changed masts on AUKLET, and had to match existing holes in the aluminum. But it was chancy, even though it worked, and she has an extraordinary eye.

Fortunately, one of the crew at West Cove came upon that drilling conversation and had a much better approach (also saying, “no, that line up and hope for the best is never going to work!”) But he had a tremendous low-stress way to take care of this, which I’m explaining in detail for anybody who might be presented with the same problem:

First you take two pieces of “G 10″ tubing, with 1/4″ thick walls. G 10 is a fancy fiber-reinforced plastic that takes well to epoxy and is very, very strong (it can be found at places like McMaster-Carr). You cut sections of that tubing that are the length of the thickness of each side of the tabernacle (about 3 inches in this case). The outside diameter of that G 10 tubing is 1 inch. Then you drill a hole in each side of the tabernacle that is noticeably bigger than 1 inch – at least 1-1/8”. This way, you can position the mast, put the (well-waxed) pin through the hole in the mast, put the G 10 tubes on either end of the pin, slide them into the oversized holes in the tabernacle sides, and then glue those G 10 tubes into position inside the tabernacle sides, filling the gaps with thickened epoxy. Brilliant! This way the G 10 pieces can set themselves at exactly the correct alignment for the pin, and then become a permanent part of the tabernacle. Once the hinge pin and tabernacle/G 10 holes are in position, with epoxy hardened, then the mast is raised and the same procedure happens for the locking pin. You might notice in the photo that epoxy was injected through smaller holes perpendicular to the G 10 tubes… Photo credit: Christopher Lariviere

This is a puzzle that has followed me for so many years, and I am delighted to know this new approach. Thank you Durwood (aka Keith Fage)!!!

As described in more detail in the previous post, the mast raises and lowers with a worm gear winch. It’s working like a charm, and the whole thing looks so sharp, all raised. Photo credit: Suzanne Jean

Over the winter we’ll be working out a cap for the top of the mast, and a set of Dyneema webbing loops for a masthead fitting, to hold the various rigging lines and blocks. This will be somewhat similar to the masthead webbing on MARIGOLD, the Portland Pudgy dinghy seen in previous posts. Annie Hill and Arne Kverneland, both of the JRA, but on opposite sides of the world, have successfully used webbing masthead fittings for boats of around 24 feet, which gives me confidence in the approach. It’s easier than getting a custom metal fitting fabricated, and also adds much less weight to the top of the mast.

Originally I was puzzling about how to fit a bracket for a tricolor navigation light at the top of the mast, in a way that would not conflict with the webbing loops. There was also the question of a bracket for a VHF antenna, looking toward the possibility of AIS (the electronics that helps you not get run over in fog), like on AUKLET. I’m still curious as to how Annie made that work on her cruising boat, without chafe issues between the webbing and the light bracket – if she used one.

As the pondering was going on, Suzanne helpfully pointed out that this boat really isn’t intended for big open water, where lights mounted on the cabin or deck can be obscured by large waves. This point about the intended use is quite true, so that took care of the lights question, and bypassed some significant effort and complication that would be involved in running those wires up the mast, at the same time as allowing for its raising and lowering. Navigation lights will be mounted on the cabin.

Then there’s the question of AIS, but this also has has a simpler answer. Powerboats, without masts, also use AIS. The antenna is different: tall, skinny, and flexible, intended to be mounted on the top of the cabin, with a hinge so it can lie flat when not needed, or for trailering. This setup will be easy, and with the cabin top already about 7 feet above the water it should be high enough to work just fine. So the mast can be completely uninvolved in wiring, which is a great relief. All wires from lights and possible AIS can run directly into the cabin, right near their eventual destination. It’s nice to get to cross a complication off the list before it even starts!

So that’s the full story on GREAT AUK’s mast. When spring comes we’ll put on the sail and have some fun seeing how it goes. In the meantime, we get to keep tinkering with the boat, which is being a pretty good time in itself.

Thanks to everybody who is involved in this project! What a great group.

Photo credit: Suzanne Jean

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For more information:

Junk Rig Association
http://www.junkrigassociation.org

Junk Rig Association: Arne Kverneland’s book – see chapter 6b for mast size calculations
https://junkrigassociation.org/arne

West Cove Boat Yard
Sorrento, Maine
http://westcoveboatyard.com

GMT Composites
Bristol, Rhode Island
https://gmtcomposites.com

GREAT AUK construction photos (click on picture to go to link)
Photo: Jon Mickel

Triloboats design info
http://www.triloboats.com

(I am not receiving anything for any of these references; they are included for readers’ convenience.)

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