The "Sib-Lim" Challenge

  • 02 Feb 2015 01:58
    Reply # 3215480 on 3144241

    D: Here’s a sketch for a companionway arrangement that I’m hoping might suit. A flat slide (cyan) goes into a garage in the usual way on modern boats. There’s either a sliding captive washboard like mine, or a hinged washboard (red). The cutout in the deck is a little more than half a circle 600mm diameter, and above it on the garage is an upstand onto which a semicircular, but non-rotating pramhood can be mounted. The top of the garage can be the location of your solar panel.

    A: I'm afraid not.  Why?  Well probably because I'm infatuated with my existing arrangement.  Why can't I have a fully-rotating pram hood, please.  I could very much do with one now, while I'm writing.  It's a hot, sticky, humid day and if I had a pramhood facing into the wind it would be a lot more pleasant.  I really don't like sliding hatches.  You lean against them - and they slide.  You go over a big wash and they slide the other way and trap your finger.  Or, more often than not, they don't damn well slide at all and you have to push and shove.  And with driving rain from astern (and don't forget I spend a lot of time in tidal anchorages), the water starts to percolate along the runner and drip.  So can we try and avoid one, please?  I'd rather have a lifting hatch if I have to have one at all.  Your beautiful files don't have a scale (and I daren't use dividers on my computer screen, anyway!), so I'm not sure how high the back of the cabin is.  That will decide whether or not I need a hatch.

    Talking of which, I suppose it would make it all a lot more difficult to rake the cabin back like Fantail's?

    D: And for ballast, you could buy a ton of these 20kg ballast bars and bond them below the cabin sole, all same like Tystie:

    Or maybe price up lead to go under the boat herself as ballast and grounding shoe combined?

    I've also been having thoughts about the davits: bringing a sort of lute stern up to the top of them and across as a platform on which to sheet.  I think it would look very Chinese and would be a great way of getting the sheets out of the cockpit.  I had to abandon my idea for a hoop on Fantail, because anyone over 5ft 6ins would probably knock themselves out on it.

    D: The slide into a garage is the best way of closing off a rotating pramhood, IMHO. You weren’t thinking of having a loose disc to dog down over the hole, were you?

    A: In truth, I hadn't got to that extent of detail.  More likely I would have done something like the bubble on Badger, except with with a piece of ply: ie a lid that would fit over and be held down (possibly) with over-centre catches. 

    D: Some time ago, I invented a companionway with a fully rotating pramhood, with a slide for closure, and yet with a washboard so that you didn’t have to climb right up through it to exit. I fear, though, that it would be difficult to make, and I didn’t want to foist it on you.

    A: If I have to have a hatch, I can't really see anything wrong with a lifting one.  The major reason that most people don't use them, I think, is that they are difficult to use when you just want a quick glance out.  But the pramhood obviates that problem.

    D:I don’t secure my washboard at part-closed, though I could, with a bolt each side. I use my pramhood, with its slide, for partial ventilation as required.

    A: I like to be able to have the lower part of the hatch up when it's rough and there's water about.  It makes it a lot less likely for any to get below and I can still peer through and look at the chart.  Would it be possible to make your washboard arrangement out of acrylic?  I would find it a bit gloomy otherwise, especially in winter.

    D: I’d thought about a lute stern, as I’d seen one on a genuine sampan, but I’ll have to ask for an extra 8in of LOA to add it. I was just going to put on a stern deck 8in wide, to mount the vane and sheet blocks on. Either way, a pair of wooden davits could be fastened on top.

    A: I think I would waive the extra 8 in quite happily.  By LOA, I was using it as one would of a gaffer, ie length on deck, as they say these days.  I like the look of the lute stern you've drawn, but would happily see it even more so. And davits would be great.  While we're on the subject, and having studied the development of the topsides, it occurs to me that there would be sufficient spare plywood to be able to have 'wings' at the bow, like a real junk.  It would be practical, too, preventing the bucket from falling off.

    Talking of the developments: what about turning the plywood through 90° at that area where it's not quite wide enough.  It wouldn't matter would it?

  • 02 Feb 2015 00:42
    Reply # 3215465 on 3144241
    David and I have been batting ideas backwards and forwards.  I have been frantically busy since Christmas - it's hard work, living a life of leisure, so I've had a bit of catching up to do: 

    D: I think you’d better remain flexible over the ballast, for the time being. Yes, you could get a foundry to cast it in iron, or even lead, if you make a wooden pattern ( a plywood box made to the right shape, with shrinkage allowance added). Yes, you could have a box welded up to the right shape (⅛” copper or bronze sheet would be brilliant, if it were affordable) and melt lead into it, a bit at a time - lead is 1.5 times as heavy as iron, so that could be 100m thick. But the ballast is an add-on, so the question of inside or outside needn’t be agonised over at this stage. However, I’d like to have 10 - 20% inside, to correct the longitudinal trim after launching.

    D: My idea with the outboard is that it would be mounted on a mini bulkhead about 500mm forward of the transom, and would pivot up out of the water in the normal way, but between twin rudders, set far apart enough not to foul the prop. There would be just a small U-shaped slot in the bottom of the hull, just big enough to let the motor leg drop into place, with the cavitation plate just below the hull. The motor head would remain forward of the transom, and could be covered, or partially boxed in, and out of the elements, as you say. The prop would protrude a little aft of the transom, but not as much as a normal transom-mounted motor. What size had you in mind? 6HP to 9.9HP, I guess. I had the Volvo Penta 7.5HP saildrive with the Honda engine, in my Sadler 25, and that was enough.

    A: Outboard motor arrangement  Now that I understand it, I like your idea of the outboard and prefer it to Arne's suggestion: the further into the boat it is, the less likely it is to cavitate.  I was thinking of a 6hp four-stroke.  From all accounts that would be substantially more powerful than the 7.5 hp sail drive that you had on the Sadler.  It would be completely impossible for me to lift anything larger (I doubt I could lift this too readily, which is why it has to be semi-permanent).  Yamaha seem to have a good reputation.  You can get a second-hand one 'never used in saltwater' from a reputable dealer for just over $900.  A brand new 6hp four-stroke seems to go for about $2,500, which is a gulp until you think of the price of a diesel engine.

    D: Motor - I know nothing about outboards, and their brands. At random, I looked at Suzuki, and saw that the 9.9 4 stroke high thrust had all the bells and whistles one could desire: 4 blade 10” prop, 12A alternator, electric start, power tilt even. Probably have to sell your soul to the devil to afford it, though. Some of the problems with inboard diesels are soluble: PSS drip free shaft seal (I fitted an early one of that type to Ivory Gull and i think it’s still OK), keel cooling, using  your grounding plate: two 4ft x 1ft  x 1/2in steel plates, spaced apart a little. The diesel engine itself’s OK, it’s putting salt water through a strainer, impeller, heat exchanger and wet exhaust that’s not OK. Fifty million workboats and canal boats can’t be wrong. But financially, the outboard wins.

    A: Certainly whatever outboard I would go for, I would choose the high thrust type.  I think 6 should do it and don't want the complications of alternator, electric start or power tilt.  I couldn't include KISS in the name, but maybe she should be called Sib-Lim-Kiss!  Yes, the problems with inboard diesels are soluble.  But I've seen some of those drip-free shaft seals fail: a terrifying thought.  Yes, keel cooling, but it's a lot of extra work.  Yes, 50 million work boats and canal boats can't be wrong.  On the other hand, price apart, there is a huge extra layer of complexity with an inboard engine: gear box to be bought and matched, control cables, tank and extra lines, electrics, to say nothing of the space that it all requires.  Plus, of course at least one extra hole in the hull.  And all for what?  Something that I use about one trip in six.  No, it makes neither common nor financial sense to me.

    D: You could have twin rudders, each with their trim tab. There can be a crossbar linking the tillers of both rudders and trim tabs. I’m keeping the transom vertical partly to make things easy in that department, with no funny angles in the linkages. I imagine a vane turret like the one you have, mounted on top of the transom with dyneema cords down and across to the tab tiller crossbar. Easy-peasy. Easier than my servo linkages.

    A: Twin rudders I am growing into the idea of the twin rudders and ditto trim tabs.  I can see these working very well on a stiff little boat like you have designed.  And redundancy, too!  Seriously though, I have of course, had experience of twin rudders linked by a cross bar, in Stormalong.  They work well, but it's not so easy to get the feel of it as with a tiller, on which, essentially, one just rests two or three fingers and merely suggests what it should do.  I would always be a bit more cloddish on the helm, but you can't win 'em all.

    D: I had in mind to put the crossbar close to the rudders, so that the cockpit is not taken up by it. Then you just have the two tillers to grab hold of (one at a time!) or stand between. If you want to lift the tillers to clear the cockpit for entertaining, the bar has to be aft of the hinge anyway.

    A: we need to think a little more about the tiller/crossbar arrangement.  (A detail, I know, but this is part of the fun.)  I steer from the windward side of the boat.  I don't see how I could do that holding one of the tillers.  I dare say that I'm not looking at it properly, but you know me and visualising in three dimensions :-(

    D: Think of Brent Swain and his origami steel boats much bigger than Sib-Lim. Not a problem, to bend three sheets of 12mm ply that have been scarfed end to end, and there has turned out to be very little twist and hollow in the bow area. The bottom panel, though, would need two layers of 6mm around the stem, running up further to form the bow transom.

    A: Stitch and glue  Right, of course, you scarf the panels together first.  That makes so much sense.  I don't see the bow panels adding too much grief and I'm sure if I shop around, I can find 5-ply 6mm.  The building method would be very familiar: Badger and China Moon.  24mm for the bottom is bullet-proof - Badger was designed for less - and we may, yet, have my big steel shoe on it. 

    D: Here’s the development of the panels that Freeship spits out. You get all the offsets, to draw them out. You can make these up, flat on the floor, and then drop them on top of the bulkhead and longitudinals assembly. The topsides come out of a 4ft sheet nicely, but the middle panels will need a bit adding on, to make up the width. After assembling all five panels, I’d add an extra bottom panel, to make up the thickness to 24mm, then fair it in.

    A: Gosh! It's a terribly clever program, that Freeship.  They will be big pieces of wood to handle, but I should be able to rope in the occasional helper :-)

    D: Yes, you’ll have to get a team of coolies to lift the hull panels into place, but that should all go fairly quickly. You may need some number eight wire to stitch them! I like Dave Zeiger’s 3/32” copper sheath for the bottom panel, so you wouldn’t necessarily have any steel outside. One steel plate doesn’t add much ballast, and it would take an awful lot of steel plates to make a tonne; it doesn’t seem reasonable to go that way. However, no need to deal with that now. 

    Last modified: 02 Feb 2015 00:45 | Anonymous member
  • 01 Feb 2015 18:43
    Reply # 3215285 on 3211935
    Arne Kverneland wrote:

    Talking about which  -  I surely hope I will never be bitten by the boatbuilding bug. My only ambition is to build ... maybe, at most, a 5-6m pram-shaped sailing dinghy.



    That got me thinking, and I scaled down the 3D model to 2/3 of full size. I ended up with a micro-cruiser or dayboat LOA 5.43m, DWL 4.55m, beam 1.97m, draught 0.33m, displacement 886kg. Could be water-ballasted?


  • 29 Jan 2015 20:34
    Reply # 3213271 on 3212933
    Mark Thomasson wrote:

    Looking good,  should be quite a 'slippery' hull.  One question:

    As there is quite an angle to the first panels, will she be a bit tender, then harden up when heeled.  Will she roll when running?

    The waterline beam is actually quite large, so she should have plenty of initial stiffness, so as to be comfortable at anchor, but when the chine immerses, she will gain a lot more stiffness to carry sail. This triangular midship section is actually the best for sea-kindliness. When David Thomas was drawing Tystie's hull I gave him a sketch of a triangular midship section, but he initially drew a midship section closer to a semicircle. I looked at it in horror, and said "But David, she'll roll like a pig!" He muttered about the triangular midship section having a greater wetted surface, but complied with my wishes. Both rectangular and triangular sections have good roll damping, but triangular is better in a seaway. This design draws strongly on my experience with Tystie, whose sea-kind behaviour I like very much.
  • 29 Jan 2015 20:19
    Reply # 3213259 on 3212895
    Rudolf van der Brug wrote:

    well done David!

    This is a good looking boat and it seems up to the job

    I do have a question about the daggerboards. In the drawings it seems they aren't in line with the waterflow around the hull, which is very important for bilge keels/boards. 

    Also i wonder if the rudders wouldn't be more effective if they would be angled outwards. That make them more complicated to build of course, but they would give more stability when drying out and when sailing with the hull at an angle one rudder would be straight down into the water as opposed to two at an angle with one at the surface ventilating.


    Good questions, Rudolf.

    Freeship permits me to put flowlines around the underwater part of the hull, and I've added an image to my photo album. The manual says that they are not as accurate as full-on Computation fluid dynamics, but still useful for visualising the flow. The flow near the bilgeboards is along a line which is still diverging from the centreline a little, and it may be advisable to toe them in a little. Tystie's boards are toed-in a degree or two. However, the situation will be different when heeled, and then there is no reason to suppose that the waterflow will be anything other than in line with the leeside board.

    On a wheel-steered boat, it is of course standard practice to cant twin rudders. But in this case, it is a major requirement to keep things simple. Twin tillers that rise and fall would be awkward, and both the crossbars, for the tillers and the trimtab tillers, would need universal joints, rather than simple fork/eye/pin joints. Also, a major requirement is to be able to careen for hull cleaning, and this would become impossible. 

    Last modified: 29 Jan 2015 22:04 | Anonymous member
  • 29 Jan 2015 13:16
    Reply # 3212933 on 3144241

    Looking good,  should be quite a 'slippery' hull.  One question:

    As there is quite an angle to the first panels, will she be a bit tender, then harden up when healed.  Will she roll when running?

  • 29 Jan 2015 11:47
    Reply # 3212895 on 3144241

    well done David!

    This is a good looking boat and it seems up to the job

    I do have a question about the daggerboards. In the drawings it seems they aren't 

    in line with the waterflow around the hull, which is very important for bilge keels/boards. 

    Also i wonder if the rudders wouln't be more effective if they would be angled outwards.

    That make them more complicated to build of course, but they would give more 

    stability when drying out and when sailing with the hull at an angle one rudder would

    be straight down into the water as opposed to two at an angle with one at the surface ventilating.



  • 28 Jan 2015 16:59
    Reply # 3212210 on 3144241

    Thanks, Arne, you just saved me a job by laying out very accurately my reasons for choosing this hull form and rudder/motor configuration.

    Freeship is indicating that the hull, deck and cockpit weight (bottom panel 2 x 12mm ply, sloping panels 12mm or even 2 x 9mm ply, topsides 12mm ply, deck 6mm ply/foam/6mm ply sandwich) should be around half a tonne. Add around a quarter of a tonne for the fixed internal structure, and 1.2 tonnes of ballast (40% of displacement)(1 tonne of lead ingots and 200kg of lead shot, all set in resin, would be my choice for internal ballast). That leaves a tonne for the rig, engine, fittings and equipment, Annie and all her possessions.

    Yes the motor could go directly onto the transom, but the controls would be further from the reach of the skipper, and I wanted to put a strong partial bulkhead at the forward end of the skegs anyway.

  • 28 Jan 2015 10:26
    Reply # 3211935 on 3144241
    Anonymous member (Administrator)

    David, I surely like much of your Sib-Lim design.

    Most important, the main section with the second chine above the waterline. This will give a lot of useful hull stability without too wide beam at the waterline, at rest (good in light winds). It also lets one fit the internal ballast (or outside ballast plate) quite deep. I bet the righting arm at 90° heal will be long. This shape also lets one have a low cabin sole to give a decent headroom without the need for a towering superstructure.

    Another thing is your combination of twin rudders and a centrally fitted outboard engine. This solves a number of problems in one go:

    • Much cheaper than fitting a diesel inboard
    • modern outboard engines are very reliable and have quite efficient propellers
    • no propeller drag with the swung up engine.
    • the centrally fitted engine ensures better grip on either tack. Not needed often, but it is still good to have.
    • added harbour manoeuvrability if the engine can be swung a bit to the sides (20° is enough)
    • easier to sort out a fouled propeller
    • the engine slot gives a huge self-draining “pipe” compared to the usual 1 – 2” pipes
    • better rudder authority with shallow rudders (I recommend adding endplates)

    I guess I would have moved the engine a bit aft to let the transom itself be the o.b. bracket. This way one could get away with just fitting a hole in the transom to let one swing up (and in) the engine. This would simplify the building  -  an important factor with my (lack of) skills at carpentry.

    Talking about which  -  I surely hope I will never be bitten by the boatbuilding bug. My only ambition is to build a plywood tender (the cardboard model is lying around here), a kayak, and maybe, at most, a 5-6m pram-shaped sailing dinghy.

    Good luck with the design work!



  • 27 Jan 2015 19:52
    Reply # 3211511 on 3144241

    I've added a sketch for the companionway to my Sib-Lim photo album. Annie may not like it, because contrary to her spec., it includes a sliding hatch, but I believe it may offer a way to incorporate a Hasler rotating pramhood into a more conventional companionway, with easier entry and exit than the pramhood used on Jester, and so more suitable for a boat that is more of a home than an ocean voyager.

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