A 7 metre variant of SibLim

  • 30 Dec 2018 04:48
    Reply # 6976596 on 6975958
    David Tyler wrote:

    OK, winter holiday time is nearly over for me, and I should be getting back to thinking about fairing and adding the hull panels. Yet here, I think we're getting into an area where there are multiple choice questions to be answered: puzzle joints or scarphs or butt joints? Hull panels applied all in one piece or in single sheets of ply or something in between? Framing on the bulkheads or stitch fillet and tape?  Chine logs and sheer clamps or stitch fillet and tape? I don't think I should be making those choices for a prospective builder, as much will depend on tooling and machinery available, skill levels and manpower available, space available, time available and other such factors. I think that I've demonstrated by getting this far with the model that everything in the internal structure fits; and that the method of building "inside out and upside down" and doing the bulk of the accommodation first, from precut components, is going to speed up the build and improve accuracy and rigidity. From that practical viewpoint I needn't take the model any further. If I do, it will be for my own amusement only, not because it's necessary to facilitate building a full size boat.

    As an exercise in thought, I would be inclined to assemble the hull panels ahead of time and apply in one piece as giving the fairest curves unless I was doing two layers in which case the second layer could act as a butt block and I would install piece at a time. Having read the G bros. epoxy guide, I might be inclined to use puzzle joints (also looking at the RM Yachts build) because they should be quicker and I suspect no weaker than scarphs (I think butt blocks may be weaker because the join is only one ply of the plywood) Most of the final strength will come from the epoxy impregnated glass tape on either side anyway.

    I feel that chine logs and sheer clamps should be thickened epoxy with tape covering both sides.  Anything else would go anti-purpose to the method of build followed so far as hand shaping would become needed. I think the chine joint should have the plywood panels leave a gap between each other of 6mm or so (I was about to say 1/4 inch :) ). This way the hull panels should just get their shape from the bulkheads and turn out fair without having to fit the edges. I would start with the bottom as you have done, but I am not sure if putting the sheer on next or the bilge would be better. In either case the last one would have to be filleted from inside... perhaps if the bulkheads had tabs that fit through the panels so wedges could hold them on that would help. My other thought is that if the sheer panels are added first, it may be possible to do the turn over before adding the bilge panels. I am probably over thinking all this.

    Finally, I agree that you have proven this method of assembly. While speed of build may not matter to some people, others may have partners where speed from first cut to water will mean the difference between still having that partner or not. I do not think this is "cheating". The same materials are used either way (plywood and epoxy) with similar amounts being used in either case.

    Having built 1.5 boats in similar manner (the dingy has a hull but needs finishing), I think I am more likely to start with a cheap/free glass hull and change it. I am realizing that the main thing I don't like about most glass boats is the cockpit. After seeing what Roger Taylor has done with his MingMings, I think I could just fix that... Yes, more work that I think even still ;)

    If I do build another boat, this is the method I would use. No fine finishes for me, paint covers lap joints no problem

  • 29 Dec 2018 09:27
    Reply # 6975958 on 6010674

    All good points, Gary. High rotary moment of inertia is important, and that is a function of having a deep keel and high mast, which give good windward ability in good conditions - but then when the wind versus current chips are down, in a Strine southerly buster, or the Gulf Stream, or the Agulhas Current, or something more local like the Pentland Firth, Corryvreckan or Portland Bill on a bad day, then I'd rather be in a heavy shoal draught boat with a low, stout mast. Always compromises to be made, but I think we've got things about right for your usage.

    Currently, the draught is 0.58m at 2 tonnes displacement, with the 125mm deep steel plate ballast. I think I'd add another 0.1m for oceanic use, but not more, to avoid the "tripping up" scenario; and if it makes things more difficult for most of the cruising you want the boat to be able to do, trailing the boat to faraway places or poking into the upper reaches of eg Port Stephens or Port Hacking, then I'd stick with 0.58m. I think that's a wholesome compromise.

    OK, winter holiday time is nearly over for me, and I should be getting back to thinking about fairing and adding the hull panels. Yet here, I think we're getting into an area where there are multiple choice questions to be answered: puzzle joints or scarphs or butt joints? Hull panels applied all in one piece or in single sheets of ply or something in between? Framing on the bulkheads or stitch fillet and tape?  Chine logs and sheer clamps or stitch fillet and tape? I don't think I should be making those choices for a prospective builder, as much will depend on tooling and machinery available, skill levels and manpower available, space available, time available and other such factors. I think that I've demonstrated by getting this far with the model that everything in the internal structure fits; and that the method of building "inside out and upside down" and doing the bulk of the accommodation first, from precut components, is going to speed up the build and improve accuracy and rigidity. From that practical viewpoint I needn't take the model any further. If I do, it will be for my own amusement only, not because it's necessary to facilitate building a full size boat.

  • 29 Dec 2018 01:36
    Reply # 6975718 on 6974050

    Oooohh, let's hope that the poor fellow has good resistance against getting seasick!...

    Arne

    This caused me to look up going over Niagara falls in a barrel (not sure why) but I reckon that is even crazier. Best not to google that.

    On a slightly more serious note the question of stability has been raised, as well as the question of scaling. 

    Firstly I should say that the 7 metre version is not just a scaled down version. Look closely and you will see the proportions are different. The two metrics were that beam was restricted to 2.5 metres (for Oz-trine non-escorted towing or shipping) and that gave rise to a length which would just fit into the garage I am about to build. 

    As for stability or rather sea keeping ability its all about compromises but anyone who thinks that the bureaucrats will keep you safe with something like the EU RCD better think again. Statistically the edges of bodies of water are where most vessels are lost and hence an ability to continue to plug to windward in horrid conditions is probably the most important aspect of safety at sea. An ability to run off safely if sufficient sea room exists probably comes second.

    Both requirements are amply met IMO by raisable leeboards which when raised give the ability get pushed along by the breaking head of the wave ...... however if you get a "washing machine" type sea state such as around Australia's east coast sea mounts where a 4-5 knot south going current hits a northward going East Coast low then a vessel this small (and some much larger) is highly likely to be rolled.

    In that case while self righting is highly desirable - and I believe this vessel will self right fairly easily - you will be not be a whole lot better off than the French man in the barrel if at least a large part of the rig does not survive. In the first instance a rig is necessary to remove oneself from the ocean but primarily its the rig that provides a much higher roll moment of inertia. Sans rig in a nasty sea would not be pleasant at all and that motion is sufficient to make people want to abandon an otherwise sound vessel.

    Generally a smaller rig has a much better chance of surviving. I think its a scale thing. But in any event keeping some rig is something you really need to plan for probably just as much as self righting.

    Now I do recall a comment from David saying that he might add a bit more deadwood beneath the keel were it he but I don't recall how much he suggested (if indeed he listed any dimension any at all) - clearly you would not want to add too much or one might trip over it, and Selden's online righting moment calculator ( http://www.seldenmast.com/en/services/calculators/rm_calculator.html ) indicates that deepening the keel a small amount does not make a huge amount of difference. Its this piece of the puzzle I am pondering at the moment.

  • 28 Dec 2018 22:47
    Reply # 6975615 on 6974050
    Anonymous wrote:
    Anonymous wrote:
    David T wrote:
    What was I saying about going to sea in a barrel??

    Oooohh, let's hope that the poor fellow has good resistance against getting seasick!...

    Arne

    " ... a mattress with straps to keep him from being tossed about by rough seas."

    It seems like he will be on board mostly for added ballast?

  • 28 Dec 2018 22:07
    Reply # 6975543 on 6010674

    The nutters get nuttier.  Excuse my lack of imagination, but why on earth would anyone want to do such a thing??!!  Especially when building a 7 metre variant of SibLimwould probably been quicker and cheaper!!  And a lot more comfortable.

  • 27 Dec 2018 23:15
    Reply # 6974050 on 6973990
    Anonymous member (Administrator)
    Anonymous wrote:
    David T wrote:
    What was I saying about going to sea in a barrel??

    Oooohh, let's hope that the poor fellow has good resistance against getting seasick!...

    Arne

  • 27 Dec 2018 22:07
    Reply # 6973990 on 6962762
    David T wrote:
    David W wrote:

    David,

    with the buoyant and relatively high topsides and the well rounded deck I would imagine that neither Annie's version or the 7 meter version of SibLim will be inverted stable. Is this correct? If so then I would not question their ability to undertake moderate ocean crossings.

    David.


    I can't give a numerical answer to this, only to say that we're about as good as we can get with the deck shape, and the range of inverted stability should be small, and much less than boats with a wide, flat deck. Yes, the closer one can approach to "a barrel with a keel on it", the better for extreme sailing, but in reality, we have to be able to walk along the deck. Moderation in all things.
    What was I saying about going to sea in a barrel??
  • 20 Dec 2018 18:38
    Reply # 6967916 on 6967254
    Anonymous wrote:

    I've used 20˚ from the horizontal on the side decks, the same as on the foredeck of Tystie, and I've observed that some people are nervous and uncomfortable with that much, if they're not roofers or hillwalkers/mountaineers. It's not sensible to put anything down without securing it, and loading stores and water containers onto the deck from a pontoon or dinghy is difficult. I proposed to Annie that the area outboard of the bilgeboard cases should be horizontal, for those reasons. She didn't go for it, but I'm going to do it on my model so that Gary can see it and make his own decision.

    As you say, David, flat deck area is handy to have, and in the case mentioned, there were flat deck areas forward and aft of this(arched) deck part, and admittedly, this did require a a hop or two getting over. But did provide a stiff structure with low mass and CG, to make a contribution to the stability system.

    The hull had  low/no deadrise and not much depth of keel, along with external ballast of only about 15%.


    Last modified: 20 Dec 2018 18:54 | Anonymous member
  • 20 Dec 2018 09:32
    Reply # 6967254 on 6010674

    I've used 20˚ from the horizontal on the side decks, the same as on the foredeck of Tystie, and I've observed that some people are nervous and uncomfortable with that much, if they're not roofers or hillwalkers/mountaineers. It's not sensible to put anything down without securing it, and loading stores and water containers onto the deck from a pontoon or dinghy is difficult. I proposed to Annie that the area outboard of the bilgeboard cases should be horizontal, for those reasons. She didn't go for it, but I'm going to do it on my model so that Gary can see it and make his own decision.

    Last modified: 20 Dec 2018 13:30 | Anonymous member
  • 19 Dec 2018 20:56
    Reply # 6966830 on 6010674

    More deck camber than I see in this design, is ergonomically manageable, and combined wwith the buoyancy of the mast and battens, should provide enough floatation to aid righting, from a bit more than 90deg....depends, of course, on ballast and CG at the other, or bottom side of the hull.

    Coniderably less than 40% has worked for me. 

    Last modified: 19 Dec 2018 20:59 | Anonymous member
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