Wednesday, January 4, 2017

Mast Design With Counter Weight

It has been a while since I've made a post with any real meat in it in terms of design ideas but because of the weather these days I have been able to put some time in.

I just got back from checking on the boat by the way, and she's frozen in!  So here are a few pics to start of with of the harbour and river:

So I have been working on my mast and tabernacle design.  I have decided to make a hybrid mast with the lower 20 feet constructed of 6 inch Schedule 40 aluminum pipe.  This pipe is made from 6061-T6 alloy, and the O.D is 6.625 inches (168.275 mm) and has a wall thickness of 0.28 inches (7.112 mm).  A 20 foot length weighs 131.28 lbs.

The upper section will be made with fir, laminated up from the pile of leftover 2 by 4's I have left over from the build.  This section will insert into the top of the pipe by around 18 inches, and will be bonded in with epoxy.  I'll taper it down to 4.5 inches at the top, and epoxy fiberglass it.  This section will weigh around 50 lbs at 12 feet long including the inserted end.

The whole mast will weigh around 185 lbs with fittings and will be 30 to 32 feet long depending on my final sail design.  The pivot in the tabernacle will be about 5 feet up from the bottom, leaving 25 to 27 feet above the partner (tabernacle pivot).

The bottom of the mast will have attached a lead counterweight weighing in the vicinity of 300 lbs, that I will cast from a pile of tire weights I have already.

I worked out the counter balance as follows:  The weight of the aluminum mast above the pivot point will be 98 lbs.  The center of balance of the pipe will be 7.5 feet up from the pivot so its moment times the arm will exert 738 ft/lbs with the mast horizontal.  The wood section's center of balance will be 19.5 feet up from the pivot, and with the wood section weighing about 50lbs, will exert 975 ft/lbs.

Add the two - 738 ft/lbs plus 975 ft/lbs and we get a total of 1713 ft/lbs to counter balance.

If we take the weight of the mast below the pivot point (32.82 lbs) and put it's center of balance at 2.5 feet down the mast when horizontal, we get 82.05 ft/lbs. That still leaves us with 1713 - 82 = 1631 ft/lbs to counterweight.  But if I can get a lead counterweight at the bottom of the mast that weighs around 300 lbs, then I can generate 300lbs times 5 feet or 1500ft/lbs.  That leaves around 131 ft/lbs to overcome manually to raise the mast.  Pushing down on the weighted end will only require about 26 lbs of pressure to raise the mast!

I can make a simple mold from plywood to cast the counterweight.  All I need is a honking big propane burner and a galvanized bucket to melt the lead tire weights in and a means of safely handling the whole works.

As for the tabernacle itself, I'm thinking to get it welded up from plate steel.  3/8" should do and if I paint it with epoxy it should stand up well.  I'll put a base plate that sits on, and is bolted to the deck.  It will also be bolted through the cabin bulkhead to additional steel structure inside the cabin, that carries the load all the way down to the floor.

I can likely carry a steel channel down to the bottom stringers and bolt it all the way down the bulkhead.  That bolt you see in the floor by the way is attached to the copper grounding plate on the bottom - so we have good lightning protection.

Fore and aft stresses have me a bit concerned if I allow the roof structure only to bear them, so I may carry another brace from the top diagonally down to the bulkhead aft.  Still thinking on that...

Anyway, here's a few more pics of the inside.  It's starting to clutter a bit but I'll do a good cleanup once the weather breaks.


  1. here's an idea, make up a steel box the right dimensions to hold your lead, made with flanges on it to make it easy to bolt to the mast and hold down bolts to keep the mast upright. Melt the lead in the box, once full allow to cool down, paint and bolt to the mast. Also, add loops to lift the weight with block and tackle to get it in place. I am looking at putting captive bolts on either side near the bottom of the tabernacle that would fit into slotted flanges welded on the sides of the weight. Then to lower the mast, loosen the nuts just enough to slip out of the slot in the flange, allowing the mast to lower. I have a picture in my head on how to do this, I hope my description makes sense.

    1. It does Dennis, and thanks! I especially like the welded box.

  2. Hi Alan,

    Bad news is I think your leverage analysis is off. Good news is its better than you think. But remember that you can't multiply load times distance (as in 300lbs x 5ft)... you multiply times advantage (assuming 1:5 (dis)advantage, 300lbs at the heel would lift 60lbs at the masthead).

    The lower to upper mast proportion is a round 5:25, or 1:5. So far so good, but that's the tip ratios... mast heel to head. You'll be exerting force near the heel, but not anywhere near the head. The weight, as you've observed, centers well down the mast, and that's the point from which to calculate lever arm.

    The two centers of effort (CEs) for the aluminum and fir are round 100lbs@7.5ft above, and 50lbs@19.5ft above. Their combined CE will be the same distance proportion between them as their weight proportion (same as calculating combined sail CEs, though in this case the upper mast is a Center of Gravity CG).

    The weight proportion is 2.aluminum to 1.fir, so the CE will move 2/3rds the total distance between them toward aluminum. 19.5ft - 7.5ft = 12ft, 2/3 x 12ft = 8ft, 8ft from 19.5ft (from fir toward aluminum) is 11.5ft.

    So, we have 150lbs @ ll.5ft above vs ??? at... let's say 4.5ft below as the lower CE. That's some better than 1:3, so you only need to counter some less than 450lbs near the heel. Or, looking up, your 300lbs can lift better than 100lbs at the combined upper CE.

    However, to complicate all this, don't forget that the mast will be rigged. 8( That's where real naval architects earns their salt! Still, if you remove the bundle (complicated), fittings, blocks and line don't stack up to much.

    Good luck!

    Dave Z

  3. Love that galley sink and counter! DZ

  4. Thanks Dave,and Good Lord Dave my head is spinning! :-) What I am going to do is build the mast at home here, including the counterweight. We'll balance her out by adding material on the weighty end until it doesn't feel too heavy for my strength extrapolated out to my eighty year old self! I have 300 lbs of tire weights now, but the scrap dealer is down near the harbour where I can get more, and Autarkia can handle the weight whatever it is. But tomorrow, under the influence of coffee and a nights rest I shall redo my ciphering and see if the figures agree :-)