"Easy" mast stepping - alternatives for a tabernacle?

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  • 30 May 2018 11:30
    Reply # 6272847 on 4611808

    Thanks David. I am afraid I did not use the triangular blocks of wood, as I thought the purpose was purely to save volume/cost of material and I did not realise there was another reason, to put a limit to thickness. Some parts of my casting are thicker than 1/2" so perhaps this was my mistake - though I saw no evidence of increased temperature, none at all.

    Interesting. The swelling seems to have deflated now and although some distortion remains, I can force the casting back into the cavity, so all is not lost and I think the casting will still do the job satisfactorily. 

    I can not think of any further explanation. There may have been some other error on my part of which I was not aware. I am tempted to buy some more, and see if I can do a better job next time, and learn something, but as you say, it is expensive stuff.

    It has been a worthwhile job, if a little unsatisfying not to know exactly what should have been done better. Thank you for your advice and for taking an interest, I do appreciate that.

  • 30 May 2018 10:56
    Reply # 6272829 on 4611808

    When I've poured a large moulding, I've done in several stages, for two reasons:

    1. The quicker setting polyurethanes particularly, get hot as they set, and this causes distortion. The instructions specify a maximum thickness of ½". Did you use a triangular block of wood to reduce the thickness and volume, as discussed?

    2. This stuff costs money, and I didn't want to mix too much and waste some. It's easier to mix and pour  small cupfuls, as soon as the previous mix is set but not fully cured.

  • 27 May 2018 13:23
    Reply # 6266440 on 4611808

    As an adjunct to my 15th May posting I have to report a further failure of my urethane casting. On 15th May I reported that the results were good except for the last of the four castings in which I ran out of material and used dregs from the containers, probably no longer the correct mixture and possibly not stirred enough as I was panicking a bit at that stage. Unsurprisingly that casting did not cure properly and parts of it remain sticky and soft.

    Now I have found a problem with the second of the four castings: over a period of a few days a kind of gaseous reaction must have taken place as a large blister is appearing, which has also had the effect of distorting the casting out of shape. See photographs here.

    Now, I am sure the proportions were correct, as I was not even measuring at this stage but mixing complete 2-part packs. And I am also pretty sure I did stir it well. The usage time is given as about 4 minutes for this product, and I am pretty sure I was manually mixing quite thoroughly for about two minutes before pouring. The first casting I did, which remains deliberately bonded into the tabernacle, seems to be good. The double photograph shown is of the second casting I did. Now the only difference in procedure between the two pours, is that the first one was done in one shot, what was left over was poured into the next mould but was not enough to fill it. So I went away and mixed up the next batch and after a few minutes competed the pour. I noticed that the previous pour was still liquid or soft at that stage, but in any case I had been told that it would be OK to pour in two stages. I am not sure now if that was good advice, because that is the only difference in procedure that I can see between a good casting and one which some days later is showing a defect.

    In this case I am not too worried as I only need two good castings – and even that one which has now gone a bit out of shape I can probably release that gas and bend it back into shape – these blocks are going back into the cavity they came from, locked in and bolted up and they will still do an OK job. Also I am still in a position where I can cast another one if necessary. I still think the method is good where a little flexibility is useful, eg packing at the top of a tabernacle. And if there is any fault it is mine.

    However I do want to let David know, and to add this to the bank of information we have. My personal conclusion is I will only use this stuff where there is good reason to prefer it over the familiar old epoxy – be very, very careful about proportions and thorough mixing – and unless a better explanation comes forward, try to calculate the volume required correctly and do the pour in one shot.


    Last modified: 27 May 2018 23:10 | Anonymous member
  • 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 | Anonymous member
  • 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. 


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