mast tabernacle for minimal cabin space use.

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  • 23 Jan 2020 22:22
    Reply # 8663539 on 8662055
    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)


  • 23 Jan 2020 19:31
    Reply # 8662055 on 8616630

    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.  The glue used, and the gluing process are critical however.    In flying, your life depends on the wing spar, so aviation standards are an excellent  set of standards to adopt.  The EAA offers a superb book on aircraft building with wood, that I would recommend for anybody building a wooden mast.  It discusses many issues the general public are not aware of.    Of great importance is a failure mode caused by invisible faults in lumber resulting from wind stresses that was first discovered by ladder manufacturers, who also used aircraft grade spruce, and how to find such faults and prevent them.   My personal favorite wood glue is an epoxy called  T88, made by System 3.  It is an aviation grade wood glue, it is reliable, and very tolerant of less than perfect joints, unlike some of the older glues.   I'm sure West System Epoxy glues are equivalent, and in fact I use a West System epoxy called GFlex for numerous projects as well.  It is one of my favorite epoxies because it does give and flex rather than being totally rigid, so it works well with other materials rather than shearing away in an environment where there is give in the part.   On a wood mast this could be valuable....... I actually just replenished my supply of this a couple of days ago from Amazon.  I consider ordinary wood glues like Elmers, as well as urea formaldehyde glues and resourcinol obsolete......... but that's just me.  The Urea formaldehyde glues like Weldwood and Aerolite are downright dangerous, as the deteriorate progressively at elevated temps...... it's epoxy ONLY for me these days.

          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'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 about what Pete did on Oryx, as what photos I've seem of Oryx seem to suggest that the tabernacle has a well....... it is a boat I'd love to have a chance to "crawl over and around in" examining every detail, but doubt I'll ever have that opportunity.


                                                                     H.W.

  • 22 Jan 2020 17:19
    Reply # 8652204 on 8651282

    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.


  • 22 Jan 2020 16:32
    Reply # 8651840 on 8616630

    Howard, I can't argue with your reasoning at all. 

    I guess (and it really is a guess) the argument for "more bury" is to some extent a desire to reduce the loading at the base of the mast (where it is clamped to the tabernacle - as that's going to be many times (5-ish) the load at the centre of effort (assuming CE is 50% of the distance between mast pivot point and the top of the mast).

    The thing I struggled with more was trying to compute the maximum mast loading - I finally I realised the mast isn't buried in concrete, so all you need to be able to do is capsize (or god forbid) pitchpole the boat!!! ... and there's another set of imponderables. Either way I have put my faith in PJR and advice from other members... 

    Last modified: 22 Jan 2020 16:32 | Anonymous member
  • 22 Jan 2020 15:22
    Reply # 8651282 on 8616630

    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.

       

    Last modified: 22 Jan 2020 15:24 | Anonymous member
  • 21 Jan 2020 15:28
    Reply # 8641219 on 8641168
    Anonymous wrote:

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

    Thanks for that Graeme, the engineering I can manage - I think it's the sailing that's going to be a challenge here!

    We are hoping to keep her on Windermere this year - a bit calmer than Ullswater (where she rode out a blow with gusts over 100mph last year on her swinging mooring).

    We're mulling over the need to add another 500kg of ballast - probably just see how tender  she feels with the rig on....

  • 21 Jan 2020 15:22
    Reply # 8641168 on 8616630

    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.


    Last modified: 21 Jan 2020 15:29 | Anonymous member
  • 21 Jan 2020 13:57
    Reply # 8640525 on 8616630

    As it happens, we hoisted the mast on Befur today (first time with pictures) The tabernacle is a welded steel arrangement with the pivot above the cabin, a 3-sided steel box in the cabin, on a pole set into the hog. It has box section steel braces to the deck beam at the aft end of the foredeck, and on both sides to the deck clamp.

    This leaves the two v-berths clear, and seems pretty strong...

    Here is our blog post on the rig to hoist the mast.


    And here is a much earlier two on the tabernacle construction - No 1, No 2.

    ...and one with pictures showing it in place - note the cabin is not really attached to the mast support structure, it was build around it.

    We used a running foreguy and bipod as recommended by PJR and this worked very well.

  • 19 Jan 2020 19:11
    Reply # 8623074 on 8616630

    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. 


    Last modified: 19 Jan 2020 19:20 | Anonymous member
  • 19 Jan 2020 11:02
    Reply # 8618753 on 8616630

    This type of mast step is similar to the way a pole vaulter plants the pole, and something similar is seen on the fishing cobles of north-east England, so it will work to get the mast up. I've done it on a 24ft Yorkshire Coble. The heel of the mast has to be controlled, but this can be solved, perhaps by something as easy as having someone sit on it. I'd make the after face of the tabernacle at an angle but straight, not curved, so that the mast heel drops into place as soon as possible. As Graeme correctly identifies, there's a bit of an issue around getting it down, with the curved face. The safe solution has to be an A frame or single pole as a derrick to lift the mast vertically - in which case, the curved after face of the tabernacle seems to be redundant, it seems that a simple parallel sided hole would do as well. Whereas if the after face is angled but straight, the mast pivots around the heel somewhat, without having to be lifted until it's at an angle of ~30˚.

    For the SibLim 7m design, I drew a tabernacle that only extends a little way above the deck, with the hinge point at the top of that and an open forward face so that the heel of the mast can swing forwards and upwards over the foredeck. Is that a viable alternative?

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