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I want to attach a mast and sail to a dinghy. The mast length would be 10.5 feet tall to accommodate a 20-22 sq ft sail, but for transportation purpose would need to be collapsible into at least two pieces.

I have to decide between a tapered mast or one single diameter. If tapered, I would insert 0.75" OD aluminum into the 0.875 which will have a 0.875 - 2 x(0.058 wall) = 0.759 ID (maybe too tight of a fit and difficult to (dis)assemble?

Using 6061 aluminum, I have a few options:

  1. a two-piece tapered mast with 0.875 OD and 0.75 OD tubes (wall=0.058" for both)
  2. a two-piece, non-tapered mast 0.875 OD, 0.058 wall using a 0.75 OD ferrule/connector tube that I would secure on one side with pop rivet.
  3. a 3 piece, mast using the same method as above
  4. or 4 sections?

Which mast construction would offer the best strength without causing a permanent bend? How long do the overlaps need to be when using connector tube? Would 0.058 wall thickness be strong enough?

***NOTE: I tried to calculate wind pressure on the sail using F= A x P x Cd and P=0.00256 x v^2 F (in lbs), A (sail area in sq/ft), Cd (Drag coefficient: I used 0.8 and 1.2)

F=22 x 0.36864 x 0.8 = 6.5 lbs (for 12 mph wind) F=22 x 0.576 x 1.2 = 15.2 lbs (for 15 mph wind) These numbers seemed low, but then again, the sail is small. enter image description here

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  • $\begingroup$ A 10 foot long .875" thin-wall aluminum tube doesn't pass the sense test for me. I feel like leaning against it would bend it over. I agree with @MeEngineerTrustMe as to how to approach the solution but I think you need a substantially larger mast than what you have chosen. $\endgroup$ – Tiger Guy Aug 3 '20 at 15:35
  • $\begingroup$ You need to add " T 6 " condition to your 6061 aluminum ( age hardened). $\endgroup$ – blacksmith37 Aug 3 '20 at 16:49
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The force on your mast comes from:

  1. Lift force on the sails - the sails are a kind of aerofoil
  2. Drag force on the sails - which you mentioned above
  3. Inertial forces - if you imagine the boat rocking in calm conditions, you still get loads on the mast

Probably the easiest way to estimate the forces on the sails would be to find out what the maximum speed of your dinghy is under extreme winds, and then calculate the drag force on the dinghy. If we assume the accelerating forces from the sails are balanced out by the drag forces, then you've got your forces on the sails. It is an approximation; the boat's structure also catches the wind. So apply a safety factor of say 2.

The failure mode of the two-part mast you described above, which I would be most concerned about, would be buckling under bending of a tube (Google for pictures). Basically all the bending resistance of your mast comes from the smaller tube, with most concentrated at the hinge point. Some friction between the tubes (depending on the fit) will transfer shear to the smaller tube and reduce the radial forces on the inner tube.

If the smaller tube buckles, it will pop itself out of the larger tube.

I presume the mast you have is sized to the dinghy, in which case if we assume further that whoever did the sums knew what they were doing (these days this is a bit of an assumption!) then all you need to do is make the join stronger than the rest of the mast.

How I would join it would be via Option 2, with a solid inner aluminum rod, 5" long with a collar to prevent it falling into the mast. If possible, have an outer sleeve with an 0.75" ID + tolerance, and comparable wall thickness. Secure everything axially with shear pins.

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  • $\begingroup$ You can't estimate the forces from the maximum speed of the dinghy, because the relationship is extremely nonlinear. In simple terms, the speed is limited by the fact that the boat can not go faster except by climbing up its own bow wave, and then operating as a hydrofoil instead of a conventional boat hull. Of course some large racing yachts can sail as hydrofoils, but not a dinghy with a 10-foot mast! $\endgroup$ – alephzero Aug 3 '20 at 11:31
  • $\begingroup$ Hence the fudge factor of 2. How would you suggest one estimate the forces on the mast then? Or just go with the plan to make the reinforcing joint stronger than the rest of the mast, and skip the calculations? $\endgroup$ – MeEngineerTrustMe Aug 3 '20 at 11:56
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Thank you for all the input given so far.

"Some friction between the tubes (depending on the fit) will transfer shear to the smaller tube and reduce the radial forces on the inner tube. If the smaller tube buckles, it will pop itself out of the larger tube."

  1. Could this be mitigated by choosing a larger wall thickness of the thinner tube? (I will probably not go with option 1)

  2. My second thought was to use carbon fiber tube, but I have no personal experience working with carbon. I see it offered in 1, 1.5 and 2 mm thickness, at 1 meter lengths.

Learning a bit about sail shape, one way to aid in creation of aerofoil ( besides curve cutting the sail) is by mast bend. My sail, however, is so small that perhaps I can opt to have a single diameter mast rather than tapered. I don't expect to sail in 20 knot winds and (probably boom-less) sailing would just be an added fun utility.

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  • $\begingroup$ Best non-permanent way to transfer shear forces is with a bolt or pin through the tubes. I am not a fan of glass or carbon fiber composites, even though the former is common for hulls nowadays. Their damage tolerance is low. If they sustain an impact, you might not see any damage on visual inspection but they might sustain BVID (barely visible impact damage) where strengths can be reduced by up to 70%. If your mast breaks, on a yacht it is possible to improvise a sail mast from something...on a dinghy your options are a bit more limited. $\endgroup$ – MeEngineerTrustMe Aug 3 '20 at 17:30

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