"Easy" mast stepping - alternatives for a tabernacle?

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  • 17 May 2018 10:53
    Reply # 6238904 on 6238619
    Deleted user
    David Tyler wrote:

    Yes. In a lifetime of designing and making things, I've machined most of the common engineering materials, including the plastics. Your Delrin bar is extruded, not cast. 

    These bushings I am referring to aren't, you can see this clearly. I'll try post a photo if it matters.  Simple small batch injection moulding perhaps? I don't know, the top surface is shiny, irregular and feathered at the edges, like it was cast in a simple open cup.

    --

    For the record, re the collar, what I was thinking of was how Dremel or die grinders work (see below).

    It strikes me, although most folks just enjoy messing about with their boats, what is missing to really popularise the genre, is an easily adaptable 'off-the-shelf' kit, eg as we've seen with alternative mast sources, and ALC (lighting columns) pulling out.

    OK, fine for few, they can downsize their properties or spend their inheritances on bespoke stuff but for the rest of we mere normal human beings, that's beyond our means.

    It seems what folks are doing is the prototype development work a commercial entity would have to spend $100,000s if not millions on. But there's no sort of "bolt-on" kit, like the kit car industry used to offer to make it easier for handy amateurs. No Folkjunks. 

    In theory, a 'Folkjunk' should be a cheaper, easier gateway to sailing for, if not 'the masses', at least more people to sail, especially if it 'environmentally' re-cycles cheaply available old boats.

    It strikes me that 'someone' or someones should be concentrating on this idea, ie fairly standardised kits that can be applied to a wide range of boats, eg 22', 25', 30' or, like the kit car industry that took widely popular (and cheap) VW or Ford engines and suspension, chose specific suitable seaworthy boats from amongst the most popular marques, eg Hurley 22, Centaur, Vega, Folkboat derivatives. 

    Then it would be possible to get batches of CNC items made up cheaply to make it happen.

    Amongst those "someones" is, of course, here - the NPO or NGO of the Junkworld.

    Do you archive 'Open Source' plans for boats that have had conversions resolved and made, or are "we" still in the R&D phase? 

    (If so, I can't find where yet).

    There is the 'custom car' world, then there is/was the 'kit car' world and it threw the door wide open for many to learn skills, have fun, and end up with working projects.

    For example, I know Sunbird did a load of conversions, yes? What happened to their intellectual property after Robin died?

    What's going to happen to everything in Arne's head once he dies? 

    I wonder if there might even be some kind of EU funding to start such a project?


    Last modified: 19 May 2018 06:56 | Anonymous member (Administrator)
  • 17 May 2018 08:13
    Reply # 6238619 on 6235103
    Anonymous wrote:

    I have "personally" used cast delrin bushings, e.g. on motor vehicle suspension, also machined it for other purposes. Indeed, I have a set about 3' away from me as I type and some raw bar in the next room. Have you?

    Yes. In a lifetime of designing and making things, I've machined most of the common engineering materials, including the plastics. Your Delrin bar is extruded, not cast. 

    The reason I mentioned castable is that I understood the original poster wanted a square block rather than a cylinder (or rod) as it is usually supplied. I don't know of anyone supplying larger square blocks.

    I buy my engineering plastics from https://www.directplastics.co.uk , and they list several materials in 50mm thickness of sheet. I would choose https://www.directplastics.co.uk/pvc-sheet for this application, for a suitable balance of material properties and economy. Still, the price is high, at £72.85 + carriage + VAT for a square block 250mm x 250mm x 50mm, and a big bandsaw is going to be needed to cut the hole for the mast. All in all, casting a 2 part polyurethane material is the better way for those without access to a well-equipped workshop, if a plastic is preferred; but a stack of plywood squares, epoxied together, is also a workable solution.


  • 16 May 2018 13:24
    Reply # 6235103 on 4611808
    Deleted user

    Did I say or infer home casting?

    I have "personally" used cast delrin bushings, e.g. on motor vehicle suspension, also machined it for other purposes. Indeed, I have a set about 3' away from me as I type and some raw bar in the next room. Have you?

    The reason I mentioned castable is that I understood the original poster wanted a square block rather than a cylinder (or rod) as it is usually supplied. I don't know of anyone supplying larger square blocks.

    Do I get to respond to the insults, or do I just have to keep accepting them?

    Last modified: 16 May 2018 13:30 | Deleted user
  • 16 May 2018 08:50
    Reply # 6227437 on 6192258
    Graeme Kenyon wrote:

    Pouring urethane:  some feedback for David

    Thanks, Graeme. There's nothing like getting "hands-on" with a material, to find out the advantages and limitations for oneself.
  • 16 May 2018 08:46
    Reply # 6227305 on 6194151
    Anonymous wrote:

    Have not had time to fully read and consider your post but, just as a headsup, look up acetal  or POM (Polyoxymethylene). Generic terms for Delrin like products (it's the Dupont trademarked version).

    There are slightly cheaper and castable options, such as Acetal Copolymer (also here as an example - although that's in a 1 ton batch). I have not grasped what you are yet but I actually don't think you need the higher properties of Delrin, although it could be used elsewhere, eg rudder bushings, replacing corroding metal gears, in windvanes etc.

    However, you can buy rods of it up to 200mm diameter.

    As aways with such materials, find out who is using them locally and scout around for offcuts and half-open tins. 

    But, what do I know ... I ain't even got a name.

    If only "None Given" would speak of what he knows from personal experience, and not continually demonstrate his dangerous lack of grasp of engineering basics. I'm beginning to think that his real name is Manuel: "I know nothing; I am from Barthelona".

    Acetal is emphatically not castable. Do not try this at home. It is highly viscous in molten form, and will not pour. It is processed by injection moulding, under closely controlled conditions of temperature. If it is overheated, it decomposes and releases gaseous formaldehyde, which is toxic and extremely unpleasant. Back at the start of my professional design career, I made the mistake of specifying Delrin for a small component to be injection moulded by a rather unsophisticated machine without such temperature control. The entire workforce downed tools and walked out, until the material was changed to something easier to process, that did not release noxious gases. 

  • 15 May 2018 21:49
    Reply # 6209705 on 6192258
    Wow! What a write-up Graeme.  Nothing like reading about what someone's done, instead of what someone speculates could possibly work if only they knew what they were talking about.

    David Tyler is also your 'Go-to' man about solid plastics, having used them in his self-steering gears, rudders and leeboards in the past.  In your neck of the woods, of course, there are a number of industrial plastics specialists.  I used to deal with one in Nelson, but they don't appear to be there any more: I tried to find the name.  But they were very helpful.

    BTW, your comment:"Failure to do so does not just result in delayed curing and possibly slightly lower quality (as would happen with epoxy)" is not quite correct as applies to epoxy.  Incorrect mixing ratios and insufficient stirring means that epoxy will not cure to full strength and may well be considerably weaker.  I have had it happen twice on this project: in the first place because I hadn't correctly appreciated the ratios a set of pumps mixed at and the second because one of the pumps had a blockage that I only noticed when I came to mix up another batch.  The first one I remedied (I can hardly say corrected) with mechanical measures, the second I took apart and glued again.

    Being a female of the species, I have a tendency not only to read the instructions, but also the spec sheets.  Always worth it with expensive, two-part products!

    It's great to hear that you are making good progress, again. I'm looking forward to seeing your boat come sailing up the river in the not-too-distant future!!

    Last modified: 15 May 2018 21:51 | Annie
  • 15 May 2018 21:32
    Reply # 6209246 on 6194151
    Anonymous wrote:

    But, what do I know ... I ain't even got a name.

    Just so.  But you could do something about both, you know. 


    If you don't want to fully participate in an Association, I suggest that you should not join in the first place.
  • 15 May 2018 12:41
    Reply # 6194151 on 4611808
    Deleted user

    Have not had time to fully read and consider your post but, just as a headsup, look up acetal  or POM (Polyoxymethylene). Generic terms for Delrin like products (it's the Dupont trademarked version).

    There are slightly cheaper and castable options, such as Acetal Copolymer (also here as an example - although that's in a 1 ton batch). I have not grasped what you are yet but I actually don't think you need the higher properties of Delrin, although it could be used elsewhere, eg rudder bushings, replacing corroding metal gears, in windvanes etc.

    However, you can buy rods of it up to 200mm diameter.

    As aways with such materials, find out who is using them locally and scout around for offcuts and half-open tins. 

    But, what do I know ... I ain't even got a name.

  • 15 May 2018 11:26
    Reply # 6192258 on 4611808

    Pouring urethane:  some feedback for David

    Following David’s useful suggestions I decided to fit the mast pole to the upper tabernacle  by using castable urethane to pour a clamp block in two halves. It is expensive stuff to use in any volume, and it might have been more economical to make the blocks from a combination of wood and epoxy, while still using the mast and tabernacle (in horizontal position) as a cavity mould. However, urethane is new to me: its flexibility appealed, as did the opportunity to learn something by trying a new material. I decided not to mess around trying to reduce the amount of resin needed by embedding triangular wooden blocks. The over-all result was  a (qualified) success. See pictures here: Two half blocks cast; and  The two blocks clamping the mast

    I will give a run down on what I found, as it may be helpful if some else decides to do the same at some time in the future.

    I made plywood dams and divided up the space between the mast and tabernacle in the manner suggested by David. But I was unable to fit them nicely, watertight and coved with modelling clay – because with the mast lying in place I could not get my fingers down into the space to do that. Instead I put plumber's silicone rubber around all the edges, stuck the dams in place and settled the mast in position, hoping the silicone rubber would set, hold everything in position and contain the urethane when I poured it. It was a mistake. It worked all right, but the silicone rubber stuck so well to the aluminium surfaces that it was a very tedious job to clean up afterwards, and also made the finished urethane casting a little rough around some of the edges. After the dams were set in place I used a rather expensive spray can product to coat the inside of all the casting surfaces with a release agent – and this worked extremely well, the castings later came away beautifully, by comparison with the silicone rubber which had stuck firmly to the dry aluminium as described above.

    (As an aside, David pointed out the value of having the tabernacle sides flaring slightly to allow the castings to pop out. As it happens, due to slight inaccuracy, the sides of my tabernacle do flare out, by about 1mm. But it would not matter in this case. Due to the shape of the castings and the flexibility of the rubber, they can be deformed and easily removed from the mould. I suspect even negative flaring would not have caused a problem - in this case – though a solid block might not pop out so easily.)

    I made the two half-blocks successfully (see photographs above). They are 150mm in length, and used about two litres of urethane to make the two halves. I actually also cast two spare blocks at the same time, but miscalculated and ran out of urethane before completion. I quickly mixed up the dregs from the containers it came in (without measuring) and threw that in to help - the result was some soft spots where the material did not cure properly. The lesson to learn from this is: this stuff needs to be measured accurately and stirred properly. Failure to do so does not just result in delayed curing and possibly slightly lower quality (as would happen with epoxy) – it simply does not cure at all in patches, and that simply means failure.

    A word on the material used: I decided Spartite was too expensive, and PCM790 was not available locally, so I used what I assume to be a similar product called Simpact 85A (A “Smooth-on” Product.)

    Its was claimed to be shore-hardness 85A, and very low viscosity for easy pouring. In fact the viscosity was quite resinous and it flowed quite slowly. But it did fill all the spaces and left no voids.

    PCM90 is very slightly harder (90 on the shore A scale) – but neither is as hard as Spartite, which I found out later. Spartite is rated shore hardness 60 D – but this is a different scale. A chart showing how these two hardness scales relate can be seen here. All three products are close in hardness, somewhere between a rubber shoe heel and a supermarket cart wheel - and harder than car tyre rubber. But Spartite (60D) is the hardest and Simpact 85A is the softest. I think all three products will do the same job where they are in a confined space and subject only to compression forces. My feeling is they will all work out about the same for a mast collar arrangement. In hindsight I would have preferred something harder for a tabernacle– something like the V-blocks and rollers in boat trailers would be better, I think. Still, the mast will fit snugly when the tabernacle is bolted up tight, and there will be no shocks or rattling at that end of the tabernacle At the heel I want something hard and tough, and I am going to cast a solid block around the mast, made from wood and epoxy. (Interestingly, the reverse of what Pol is doing.)

    I need the mast heel to be tough and hard, as I have decided now to follow another of David’s suggestions and face the tabernacle backwards, the mast pivoting at the deck (with no hinge.) More on that later.

    Delrin. As a further afterthought, the material referred to by our anonymous friend (in one of his posts) called Delrin sounds to me to have the most perfect properties for both the top and bottom of the tabernacle. But I don’t think it is available in kitset castable form, and machining a solid block (even if I had the machinery and the skill) would, I think result in far too expensive a pile of shavings.

    I would use urethane again, where shock absorbing, tight, accurate fit and slight flexibility are required. In most cases though, I would stick to the tried and trusted epoxy/wood/glass combination.

    Thanks again for David's help and suggestions.

    Last modified: 15 May 2018 11:42 | Anonymous member
  • 14 Mar 2018 23:51
    Reply # 5979319 on 4611808

    The triangular section wooden bocks.

    David wrote:

    The problem with the wooden blocks would be to hold them in the correct position, but if you can solve that, I don't see any reason not to use them. 

    How about just stick them to the mast with Simpsons?

    Purely for volume displacement, slightly shorter than the surrounding urethane casting so they would be entirely immersed.

    And would that add anything to the adhesion to the mast, of the finished casting?


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