Anonymous wrote:

The civil aeronautics manual 19 (CAM 18), if my memory serves me correctly lists a 15:1 splice as being equivalent to an unspliced piece of wood in terms of strength.  This of course assumes a good glue joint.  In other words you can make a wing spar from a piece of virgin aircraft grade spruce, or a piece spliced with a 15:1 taper, and they will be equivalent.

Interesting, I had never read that much on building aircraft. My information on scarf lengths are all from boat building references. In particular "The Gougeon Brothers on Boat Construction":

"In many boatbuilding situations,
we use a ratio of 8-to-1 to determine the size of the
bevel, so that a 1" (25mm) thick board will have a
bevel 8" (200mm) long. When high-density, high-
strength wood is used in a critical area, a mast for
example, 12-to-1 proportions may be required. For
extra strength and safety, we increasingly recommend
12-to-1 scarfs for lumber."

However, to be clear, the mast is intended to be aluminum. I want it light as it can be without the cost of C.F. Wood makes a wonderful mast but not so much where it is moved and handled a lot. It is perhaps a mistake to bring splicing into the conversation at all but I felt it shows a similarity as to the overlap when joining things together. Mainly that 10% bury is well in line with other joining practices, though perhaps if anything a little less.

      I do have the gut feeling that where a tabernacle is involved, giving the mast itself bury into the hull is probably not worth the trouble.

I would prefer not to. My only reason for even thinking about it is clearance while on a trailer. I would prefer either a permanently mounted mast or a tabernacle where the mast is all above the deck. However I have to work with what realities are there.

  I'd build the strength into the tabernacle.   The thing that bothers me about bury in the hull itself in this case is that you are providing a place for water to enter or puddle, and that is never a good thing.  Eliminate that well, and you eliminate a host of problems IMHO.    Do you make a scupper type drain over the side or drain into the bilge?   I'd be curious

I would send and water over the side as the bottom would be above the water line and the bilge is quite shallow, this being a boat designed for trailing. (actually it is compromise boat with a capital C... and so probably more suitable for a junk rig than ever)

Thanks to all for the great responses. Really, it looks like I need to get the boat on the trailer I will be using first and see where my maximum height above deck will be in that case. I need it to be less than 14 feet from the ground legally.. though lower would be better. The keel is only 22 inches below the hull but the cabin is a full 6 feet high. I expect to keep the keel as close to the axles as possible so about 16 inches off the ground.

Anonymous wrote:

Let me dare to show my ignorance here again ;-)    There is always talk about bury... the holy grail of free standing masts.  You must have a bury of x% of mast height.   looking at things honestly, the actual mast doesn't know how much bury there is.... the loading (on the mast) at the partner is the same weather there is 5' of bury of 5" of bury.  The boat structure however is a different matter.  Roughly speaking, the distance from the vertical center of effort to the partner divided by the amount of bury, approximates the multiplier of the load on the partner and step versus the load at the vertical CofE.  While that is not completely accurate, it is sufficiently accurate to approximate the  structural loads for most purposes.   In a tabernacle, it is the bury of the tabernacle, and the strength of the tabernacle itself that matters, not how deeply the mast is buried into the boat except in so far as the mast itself contributes to the strength of the tabernacle.   

     At least that's how I see it........... Thus if the mast itself terminates at deck level, or extends down into the boat, is of little consequence, so long as the tabernacle itself distributes the loads to the frames of the boat adequately.   I would say build the strength needed into the tabernacle, and design things such that the mast need not "drop in" or lift up........ that at least is my view........ for what it's worth.

**** This does not take into account the flex of a  mast between the partner and step that is present in a "normal" free standing installation. This ability to flex through the length of the bury between a keel step and a deck partner looks to me as though it adds significant resilience.

The tabernacle bury will be from cabin top to keel in any case. I think you are probably right about the over all bury being the important thing... but because the mast is so important to the well being of the boat and because I do not intend to provide enough tankage to get from "anywhere" to port. I want to have an over strong mast.

From a materials point of view, scarf joints of any kind in the books I have read are 12 to 1. That is the length of the scarf is 12 times the thickness of the material. So for a solid wood mast of 4 inches diameter the bury would be 4 feet... longer than the 10% bury recomended. However, the same 4 inch mast made of aluminum tube of 1/8 thickness might have only an inch and a half bury or join to the tabernacle. That may be ok for a backing part for a weld but in this case the mast is only loosely joined to the tabernacle and relies on the leverage arm of a long mating surface to keep the mast from distorting. I am only looking at a 30 foot mast and 3 foot of join does not seem overly much. I do think in this case the diameter of the mast should be used for overlap/bury calculation or stick with the tried and true 10% of length. Both seem to be similar.

The one thing I would like to do differently to the tabernacles I have seen, is to make the hinge point and mast heel the same shape as the mast even if the overall tabernacle shape is square. I feel that this will support to the mast for almost it's whole diameter rather than just point contact.

I guess the other possibility is a hinged mast with a moving "tabernacle" or sleeve that has 10% "bury" or overlap on each side of the hinge. This could be slid down below the cabin roof line with no extra cabin space taken.

Let me dare to show my ignorance here again ;-)    There is always talk about bury... the holy grail of free standing masts.  You must have a bury of x% of mast height.   looking at things honestly, the actual mast doesn't know how much bury there is.... the loading (on the mast) at the partner is the same weather there is 5' of bury of 5" of bury.  The boat structure however is a different matter.  Roughly speaking, the distance from the vertical center of effort to the partner divided by the amount of bury, approximates the multiplier of the load on the partner and step versus the load at the vertical CofE.  While that is not completely accurate, it is sufficiently accurate to approximate the  structural loads for most purposes.   In a tabernacle, it is the bury of the tabernacle, and the strength of the tabernacle itself that matters, not how deeply the mast is buried into the boat except in so far as the mast itself contributes to the strength of the tabernacle.   

     At least that's how I see it........... Thus if the mast itself terminates at deck level, or extends down into the boat, is of little consequence, so long as the tabernacle itself distributes the loads to the frames of the boat adequately.   I would say build the strength needed into the tabernacle, and design things such that the mast need not "drop in" or lift up........ that at least is my view........ for what it's worth.

**** This does not take into account the flex of a  mast between the partner and step that is present in a "normal" free standing installation. This ability to flex through the length of the bury between a keel step and a deck partner looks to me as though it adds significant resilience.   These comments are based on years of experience with structures other than boats, or junk rigs......but I don't see that the basics are different at all.  A post or pole .cast in solid concrete will be stiffer than one supported at the surface and solidly tamped at the base, but will fail sooner as it cannot flex.  The deeper it is buried, the less the probability that it will move........ simple leverage.

      If the "functional tabernacle" (the part that contains the foot of the mast) is entirely above deck, everything becomes simpler.  Like an iceberg, that is merely the tip.

                                          H.W.

The engineering that’s going into Befur is impressive I must say, she’s going to be an interesting and lovely boat.

I’ve raised a solid wooden mast on a gaff rigged boat (slightly longer than Befur’s) on a number of occasions just using the gaff (reversed and placed forward facing on the mast) instead of a bipod - and a 2-1 purchase on the foreguy running back to a small electric deck capstan. There was no tabernacle as such, only a hinge at the heel. I was afraid of it toppling sideways, but after a couple of goes I found that there was no need for lateral support. It was quick and easy, although I suppose the bipod is a safer way if there are other boats or other people around.

For those who do not have welding equipment, here is a metal tabernacle made from aluminium sheet, just folded. I have been advised the thickness needs to be about equal to that of the aluminium mast. I’ve had a couple of these folded up – very simple and quite economical, I thought the idea worked quite well.

If your mast is positioned in front of a deck house, then the forward-opening tabernacle is probably the best. If the mast position is aft of a deck house or some other structure, then the aft opening tabernacle makes sense. If your mast position is on top of a deck house (as I suspect yours might be) or a flush deck, then you have to make a choice and maybe the internal accommodation vis-a-vis slot-trunk or other geometry will make it obvious which is best for you.