Rotating mast with tabernacle

[Howard]

For what it's worth, did a little mockup for a sprit boom and mast...testing for rotation. The sprit boom is a surprisingly strong lever. As I thought, the mast is the fulcrum and the force acts on the end of the sprit boom, through the snotter to generate a lot of rotational torque......you engineers probably call that a turning "moment". As long as the end of the sprit boom is built to take it and the attachment point on the mast for the snotter is strong, it should hold up.

The other thing I realized from my mockup is that the forward compression strain of the sprit boom on the snotter has to be offset by something anchored to the mast, and that something is the sail itself. That may not be as great as I thought, but it has to be there, else the snotter just spiral wraps around the mast. Might be a good idea to sew some type of webbing to the sail, so when the whole thing is in tension to the point the sail is stretched out flat, it won't go further, stretching the fabric of the sail out of shape.

On the tabernacled/rotating mast idea, I've been wondering about a simple collar at the top of the tabernacle.  Mast slides through this collar......I think Jeff did something similar on his Core Sound 20's....then drops into a round mast step on deck. If the "bury" of the main is not sufficient to take the strain, drop it all the way to the bottom of the anchor well. Water intrusion won't matter, as it drains out the bottom of the anchor well. The mizzen has plenty of bury and drains into the cockpit. The collar these masts slide through would pivot on an off center bolt, (aft edge on the main) at the top of the tabernacle. If you feel it's necessary, when in the "up" position, run another bolt through the front half of the pivot sleeve. The sleeve could be a pretty beefy block locked into place a the top of the tabernacle with that second bolt through the sides of the forward edge of the tabernacle.  If stepped on deck, the main would only have to slide a few inches....the amount of drop into the mast step. If stepped inside the anchor well, a few extra feet. Put a sliding, locking collar underneath the pivoting sleeve so it can't jump out. There are plenty of low friction bearing surfaces that can be used on the mast step and sliding collar.

Seems to me that to get the range of rotation you want from this, all sail controls except the sheet are ended on the mast itself and rotate with the mast.

BTW, rotation also complicates any wiring for the mast. Lights, etc. Masthead tri-colors are out. 

[Garry]

Interesting ideas Howard. I see some good points and have some questions but I will take a little time to respond. I may create another sketch, that seems to help me focus and work out some of the details in my mind.

[Garry]

Here are some comments Howard and I have shared offline (reproduced with howard's permission). The quotes are Howard's:

What got me going on that rotating mast thing was recollection of Graham's incident on the EC challenge when they broke a sprit. I'd like to see that eliminated as a possibility.

I would like to know more about how that happened. Anyone?

New idea. Rather than rotating the mast, if the snotter attachment point is the issue, how about a bail or loop for the mast end snotter block to rotate on? Rather heavy (maybe 1/2 inch stock), with bolt eyes on the ends. Would loop around the front of the mast and bolt to the mast along each side of the sail track.

That would let the snotter block rotate with the sprit boom and snotter nearly 270 to 300 degrees....more than enough to prevent any strain, even if the sails were let out forward. That way the snotter line could be run to the base of the mast and back to the cockpit. Sail track would be fixed and sail would have to wrap around the mast, unless you skipped the track and laced it on. If laced on, skip the bail and use a lashing for the snotter block. A lashing could be held up with a simple dead eye.

This could probably work but there are several other advantages to rotating masts (see list above quoted from Eric Sponberg) that make me think it might be worthwhile.

BTW, my idea for the mast is 100% birdsmouth wood. Solid 5' or 6' insert at the bottom for strength and solid attachment point for hardware, and solid plug at the top. Maybe....and I stress maybe......carbon fiber on the outside. Seems like I can recall a thread on the Woodenboat forum where carbon fiber over wood masts were discussed. None other than Tom Lathrop panned the idea of carbon fiber on wooden masts. There were chronic failures as I recall. Was surprised the issue didn't come up on the "Skimmer". Intended as a short term racing rocket, so maybe it didn't matter as was justified to save weight. I'd want to hash that out.

As I understand it, the critical thing for these freestanding masts is that they must flex a bit at the top to flatten the sails in gusts. The flex has to be dialed in just right for them to work their best. Whatever it takes to get that right is what I'd do.

I would need to know exactly what was discussed before basing any opinion on the concerns you remember about the WoodenBoat forum. I'll have to take some time to do some searching on that forum. I'll report back any good stuff I find.

I was impressed with the design of Skimmer's masts. Graham showed me a cut off section and it made sense to me about combining the strengths of wood and carbon fiber in an optimum way. You are right about getting the design right and I feel confident in Graham's ability to do that. We'll be talking about that as we get into the rig design.

[Tom Lathrop]

BTW, my idea for the mast is 100% birdsmouth wood. Solid 5' or 6' insert at the bottom for strength and solid attachment point for hardware, and solid plug at the top. Maybe....and I stress maybe......carbon fiber on the outside. Seems like I can recall a thread on the Woodenboat forum where carbon fiber over wood masts were discussed. None other than Tom Lathrop panned the idea of carbon fiber on wooden masts. There were chronic failures as I recall. Was surprised the issue didn't come up on the "Skimmer". Intended as a short term racing rocket, so maybe it didn't matter as was justified to save weight. I'd want to hash that out.

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Howard,  I did have reservations about just laying some carbon fiber unidirectional tows on a wood mast or other spar.  This is quite different from what we did on Skimmer where the carbon was a woven sleeve with the fibers at a great angle for the mast axis.  this primarily added hoop strength and did not stiffen the mast an appreciable degree.

To see the problem, remember that the tensile strength of carbon is much greater than wood fiber and its elongation to stress is far less than wood.  If the carbon is on the outside of a bending spar, it will take nearly all of the initial elongation stress of tension with the wood taking hardly any.  For this reason such a layup will require that the carbon be strong enough in tension to take all the initial load without failure.  Just adding a little bit of carbon will not do that.  It is my untested opinion that failure of too little carbon on a wood mast could actually cause failure of the otherwise strong enough mast.  This would happen because rupture of the carbon  would cause a shock load at a single point and could break the spar.  this would depend on how much carbon and how much load but it makes sense to me.

[Howard]

Tom:

In the design you are talking about, is the mast simply a mandrel for the carbon fiber sock? The mast just being something to give the carbon fiber some shape and perhaps carry the compression load of the mast (weight of the sail hanging from the halyard). The next thing to ponder is how much weight savings is there? Worth the substantial expense of the carbon fiber?

BTW, in many respects, this is just silly academics. The only way this is going to get resolved is commission Graham to get busy with the design and see what falls out. Fun to kick around in the dead of winter though.

Howard

[Tom Lathrop]

Tom:

In the design you are talking about, is the mast simply a mandrel for the carbon fiber sock? The mast just being something to give the carbon fiber some shape and perhaps carry the compression load of the mast (weight of the sail hanging from the halyard). The next thing to ponder is how much weight savings is there? Worth the substantial expense of the carbon fiber?

BTW, in many respects, this is just silly academics. The only way this is going to get resolved is commission Graham to get busy with the design and see what falls out. Fun to kick around in the dead of winter though.

Howard

Howard

There is compression in any bending mast way beyond the weight of anything on it.  When any beam bends there is tension on the convex side and compression on the concave side.  Failure can occur on either side.  Carbon is great in tension and not so good in compression.

The carbon sock was put on the birds mouth because Graham had it on hand.  If you have to buy it for that particular use, it's not worth the money.  Fiberglass will do as well in this use and for much less cash.  The wood in the birds mouth upper spar on Skimmer takes about all of both compression and tension loads.  The sock is for hoop strength.  It helps prevent point stress loading by spreading it to the adjacent members.

Commercial carbon spars are built in the same way.  That is, they have unidirectional fibers on the longitudinal axis inside (like the wood inside the birds mouth spar) and diagonal fibers outside to hold the thing together.  Just looking at a windsurfer spar doesn't show this since all you can see is the outside layers.

[Garry]

BTW, in many respects, this is just silly academics. The only way this is going to get resolved is commission Graham to get busy with the design and see what falls out. Fun to kick around in the dead of winter though.

Howard

Both Graham and I have let other priorities take precendence for some time since we started on this design. However, recently we have started up again and we should be addressing the rig in time. Right now I am working on the spreadsheet for weights and moment arms to use in the final design. I will keep you posted. I am very interested in getting the design finalized and perhaps start construction this spring. Graham says there is another person who is eager to build one also, so you should be seeing things develop rapidly. I'm excited!

[Garry]

Graham and I discussed this recently and settled on the non-rotating design. One of the biggest advantages to me for rotating masts would be the potential to reef or drop sails on any heading by letting the sheets slack. Graham explained that even with a fixed mast, the absence of shrouds and spreaders would mean the only resistance to dropping sails while headed downwind comes from the track and the sail against the mast. He felt strongly that for these size sails, this would be readily done.

Now that I think about it, I realize he is correct. On my sloop, it was necessary to head into the wind, sometimes a tricky proposition after getting caught on a run with too much sail. I needed to head upwind to keep the jib on deck, and to keep the main off the spreaders and shrouds. With freestanding masts, the standing rigging is gone. And with wishbone booms, the loose sail will be contained in the lazyjacks under the boom even if its swung out over the water.

Sounds good to me!