A 7 metre variant of SibLim

  • 31 Dec 2018 13:35
    Reply # 6977744 on 6977460
    Jeremy wrote:

    Seven metres length and 2.5M beam is more than my garage space will allow, but I have just heard that David has a 6M design as well ...

    Well, that might be overstating it a bit. In response to a query about a size to fit within a 20ft shipping container, I had a quick look to see that it was feasible. I think it is, with  sizes of 5.7m LOA, 2.2m beam, 2.2m maximum depth. But that's as far as I've taken it, and would need a serious declaration of intent to build, and then a design brief (as with Annie and Gary) to take it any further.
  • 31 Dec 2018 04:21
    Reply # 6977460 on 6975958
    Anonymous wrote:

    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.


    Going back a few years when I had about 20 sheets of 9mm plywood and a 6x3m garage building space limit, the plan to build a small junk rigged boat took hold. So this 7M design is interesting and  characteristics along with dimensions mentioned above are relevant to design ideas still bouncing around in my head.

    Seven metres length and 2.5M beam is more than my garage space will allow, but I have just heard that David has a 6M design as well, so a few thoughts  related to the above quote might not go amiss.

    Having a short 'stout' mast is good for surviving a rolling (IMO) if shrouds are included, along with stepping firmly between key and partners. But then w/ward performance and handling ease is compromised. So a  very strong mast (especially in the lower half) is the suggested option.

    Usually(in this day and age) a carbon composite item is touted as the answer, but a hybrid is  most likely the more affordable option.

    Swapping an alloy flagpole for a grown spar is something I do not regret having done, which was a move that included shrouds set up lightly (rather than hard and taught).Since that time(opting for a grown spar), my composites experience has been considerably augmented, so a taller mast having a variably thick wall reinforcement strikes me as the way to go.


    Last modified: 31 Dec 2018 04:40 | Anonymous member
  • 30 Dec 2018 22:33
    Reply # 6977201 on 6976845
    David wrote:
    Len wrote: 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.

    Fair curves are still possible with the panels added in pieces. But thin plywood doesn't lay in a fair curve over frames alone. There's nothing that identifies an amateur-built boat more than wobbly chines, and although it's easy enough to adjust them in a dinghy before filleting, it's harder in this size of boat.



    I'm sorry, I have been reading BBBB too much :) I think G Buelhler would probably not go less than 1/2inch for a dingy and a 7 metre boat he would use over 25mm (over an inch) thickness. So I was thinking two layers of 9 to 11 mm ply. I agree 9mm is about as thin as one can get and get fair curves. For this kind of build avoiding double layering is the right way to go. (and considering the abuse our almost 20year old hull with 9mm unsupported ply has gone through 9mm is probably fine)
  • 30 Dec 2018 20:21
    Reply # 6977088 on 6010674

    Another way to do the chines is to put a light stringer on edge about 50mm either side of the actual chine position. After the ply panels either side of the chine have been fitted, the chine edge is rounded over and several layers of fiberglass tape are applied along the exterior length of the chine.  After the hull is turned upright the valley between the two chine stringers receives a heavy layup of fiberglass. Thus the actual chine corner itself is mostly fiberglass. This is a way to create a chine without needing to use a heavy timber chine stringer. The frames or bulkheads need to be no more than about a meter apart in order to maintain a fair curve for the chine.

  • 30 Dec 2018 15:07
    Reply # 6976845 on 6976596
    Len wrote: 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.

    Fair curves are still possible with the panels added in pieces. But thin plywood doesn't lay in a fair curve over frames alone. There's nothing that identifies an amateur-built boat more than wobbly chines, and although it's easy enough to adjust them in a dinghy before filleting, it's harder in this size of boat.

    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. 

    Using them in practice might indicate to you that though they are quicker than scarphs because you don't have to cut them, you lose that in extra cleaning up time. Both scarphs and puzzle joints need great care to clamp them flat with a stiff strongback. I would use butt joints for adding the hull panels one sheet at a time.

    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 :) ).

    I did the rough fairing of the chine logs on Annie's Fánshì (as we must now learn to call her) using an electric plane. It took quite a while, and I made buckets and buckets of shavings. On the model, there's less physical work, but it's harder to work accurately. I'd like to get away from that, but retain the fairness of having solid timber there. So I went back to the Gougeon Bros and looked at their stringer-on-frame method. They put 1 1/2in x 3/4in stringers on edge, to get the maximum stiffness, and therefore fairness, using the least material. I think we can do that here:


    putting fillets on either side to make up for the loss of the bonding area of a conventional chine log. There is only minimal checking for fairness to do, not major fairing in.

    This way the hull panels should just get their shape from the bulkheads and turn out fair without having to fit the edges.

    Not the case. See above.

    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.

    It has to be topsides first, bilges second. I strongly suspect that the bilge panels are going to be difficult to do in one piece, because small inaccuracies in the setup are going to be magnified in the bow area due to the twist. Also, I have to work around the bilge board cases. I may end up getting more plywood and fairing the bottom and topsides flush with the bulkheads, so that the bilge panel sheets can be put on one sheet at a time, cut roughly, a little oversize, glued on and then trimmed. I think this would be safer. Butt joints would be easiest and also fairest.

    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.

    I think you are. Conjecture only takes you so far, getting hands-on takes you further - that's why I'm making this model.


  • 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.

       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

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