Cash prize of 250 GBP - Dinghy Design Competition

  • 03 Jun 2021 13:57
    Reply # 10586816 on 10211344

    I think you're talking yourself into the job of judging, Graeme! The only way to do that job properly is to build and test them all in the water, even if at model size as you're doing.

  • 01 Jun 2021 13:28
    Reply # 10579847 on 10211344

    I started making some more models of the various dinghies

    Its rather fun, they are all quite different, its quite a learning exercise.

    Youyou is dead simple, in the same category as KISS.

    Oyster (the inverted V bottom) with its high topsides might be the most able of them all - I have no idea how it will perform, but it is easier to build than it looks.  There is no rocker in the centreline - it gets its rocker from its chines.

    Halibut looks simple but not quite so simple to build (at least, in small size).


    I am starting to like this one best, for a number of reasons, including the construction method which is self-jigging - its a first for me - I found it very easy to build (but difficult to do with thickened epoxy. I can see now why the intended method was conventional stringers). Building this way, you get a very robust dinghy - with built-in buoyancy tanks as a bonus. I discovered that the side planks are simple rectangles (you trim the ends later. The bow transom is a square.



    Its proving to be a nice little indoors job.

    There's a couple more I want to try, and then maybe try to find a way to test them for towing, stability etc in a rough sort of way.


  • 25 May 2021 03:40
    Reply # 10547522 on 10211344

    Yes David W I think you are right, and its one more of the features that I am learning to appreciate in this design. The bow is really a continuation of the bottom, sweeping up, no doubt contributing to those powerful fore quarters, and maybe deceiving the water that it is not a transom at all.

    At first the whole thing looked ugly to me, but its appeal growing. Also at first I did not think the hull looked very junk-like (still don't) - but realised later that many of the junks recorded by Worcester do have this square bow transom, raked forward.

    Last modified: 25 May 2021 03:45 | Anonymous member
  • 25 May 2021 03:17
    Reply # 10547417 on 10211344

    Graeme, one thing that you have not considered is the angle of the bow transoms. On my Webb 8 design the bow transom is at and angle of 45%. I found that this lifted the bow when rowing and did not butt into the chop as a vertical transom does. David Tyler's Siblim T has virtually the same angle to the bow transom and this will make her much easier to row into a chop and will tend to lift the bow over waves, rather than butt into them as a vertical transom does. It will also make her easier to sail at an angle of heel as the bow will become a V shape and go through the water much easier than a vertical transom. I think this is one of the reasons that my Webb 8 sails so well. I always sailed her bow down to ensure that the stern transom was not immersed. When others sailed her they did not do this and performance suffered accordingly.

    Last modified: 25 May 2021 03:20 | Anonymous member
  • 24 May 2021 16:06
    Reply # 10544668 on 10211344

    Thanks David.

    I would be reasonably confident about planking the bow. When I built Havoc I did two skins, diagonal planking – but when it came to the bow, some of the diagonal planks had to be tapered to follow the cone surface around – and in that region, the second skin had to go on, almost on the same angle – the curve was too great to bend around it. In that part of the bow it had to be single diagonal (approximately radial) but doubled up, if you get what I mean - not terribly satisfactory.

    On the scow I am currently building I wanted to use plywood, but there was no way it would go round that curve – and I wasn’t confident I could do it in two skins and not get voids. In the end I did it in one skin, full thickness (15mm) by making saw cuts in the outside surface of the ply, approximately along the radial lines of that cone sector. It turned out to be simple and easy. The saw cuts opened up a little. I filled them with thickened epoxy and the result looked good (looked like diagonal planking) – I was sorry to cover it with sheathing.


    On the inside the curved surface is smooth and continuous, which is what I wanted. I fitted extra framing to it afterwards, although that part of the hull is very strong.

    No mould is necessary, the only difficult part is lofting the panel, which I did not have the drafting ability to do. I made a model, and made the panel with cardboard to begin with, trimming with scissors until it fitted. Then, on the real thing, I fitted a 5mm panel made from cheap plywood, which I then used as a pattern for the 15mm ply. It all proved in the end to be very simple to do, and very strong, I think.

    On a dinghy, I would do the bow part the same way (radial saw cuts). with 5mm ply, and leave it frameless. 

    I am referring here to just that part of the bow which fans out from the fore foot, to reach that upswept fore part of the chine. The rest of the hull is either flat, or simple roll and easy to do with plywood.

    The bottom and bow, well curved and "triangulated", would need no framing for a dinghy, I think. However, the sides would be flat and flimsy and I think your advice regarding stringer is a good idea, and would be simple. As for the gunnel - it also might call for some stiffening - even a narrow side deck perhaps?

     I take your point regarding beam. On that drawing I actually copied the beam (and the run) of Sibling Tender, and maybe it could even be reduced a little.

    Arne - thanks too.

    I like the pram bow for a small dinghy, better than the "pointy" - and I think I agree with you, except that the NZScow bow doesn't really fit into either category. The first working scows in New Zealand had transom bows, but with the short chop which is so common on our NE Coast, the model very quickly evolved into that bluff, semi-pointed bow which seems to be a compromise between sharp and square. I just thought it might be fun to try it on a dinghy - it does seem to me like a good compromise, retaining a lot of power and buoyancy at the bow, yet maybe handling a chop just a little better than a pram. It still tramples over the waves rather than cutting through them - but maybe does it with a little less fuss. I don't know, I just thought it might be worth to try but I am not interested in something as extreme as a Bolger Brick.

    And thanks for suggesting to David about the measuring stick. I don't have good software here and spent a couple of hours this afternoon trial-and-error trying to scale people's drawings up or down so they come out on an A4 sheet as 1:10 scale. If they all had measuring sticks it would be a sight easier! If I make any more models, I want to make them 1:5 and use that super thin plywood I have a stack of - so it still means a trip to the print shop to get everything doubled and onto A3 then paste them onto the plywood. (1:10 cardboard is no good for my clumsy fingers). I made the KISS model that way - but did not need to do any lofting for that, and none of the others will be as easy to assemble as KISS. I wish I had time, it would be rather fun to make more of these models.

    PS I can't imagine how to strike diagonals onto a NZScow hull - anyway, I suspect this type of hull follows its own set of rules - I do know that the type can sail quite well, similar to any of the old workboat types.

    Edit - a diagonal on the bow section would be interesting. The waterlines are surprisingly hollow. I don't know how to do a diagonal.

    Last modified: 26 May 2021 02:29 | Anonymous member
  • 24 May 2021 15:11
    Reply # 10544493 on 10544380
    Anonymous member (Administrator)
    David wrote:

    Graeme,


    PS to make a model of the SibLing tender, I'd scale up the three drawings of the plywood layouts, and take them to a copyshop to be printed on card - equivalent to getting the plywood CNC cut.


    David,

    If a model of an 8-footer is made at 1:10 scale (like when I made the 112% KISS model), the bits can easily be printed onto A4 pages. I did that with KISS and then glued the cut-out bits to an old calendar, before cutting them out. Much quicker than going to a copy shop.

    Could I suggest that you add a one-metre stick to all your drawings? That would make it easier to ensure that the printed out bits are to the same scale.

    Arne


  • 24 May 2021 14:57
    Reply # 10544458 on 10211344
    Anonymous member (Administrator)

    Graeme.
    I am not convinced that dinghies will row better if fitted with pointy bows. The prams can be made with lower entry angles at the bow (at the wl.) than many pointy bow skiffs and dinghies. In Norway the majority of small traditional everyman’s boats were the double-ended 14-17’ færings. In some areas, mostly sheltered, people preferred prams. Later test-rowing of the prams (Holmsbu type) against færings have shown the prams to be the fastest. The pointy bows are better in a head sea, in my view. I once had a 17.5’ Nordlandsfæring, with vertical stems, very fine entry and generally slim lines. It was a marvellous rower against a nasty chop.

    Yacht-designers often operate with diagonals when drawing their yachts. These stand at closer to right angle to the hull sides. The designers aim for the straightest possible diagonals. A round-bottom pram would have the straightest diagonals of all.

    Sooo, entry angles are important here. I just found that angle to be 18° on ‘my Boys’. This surely is wide enough, but remember that we are talking about load-carrying 8-footers here.
    The entry angles of the topsides are identical to that of the bottom, so this discourage waterflow to be forced to cross the chines and cause turbulence. It will take some rowing to see if this is an important factor or not.

    Arne


    Last modified: 24 May 2021 15:12 | Anonymous member (Administrator)
  • 24 May 2021 14:37
    Reply # 10544380 on 10211344

    Graeme,

    The NZ scow is a workboat - and therefore well suited to being a workhorse tender. The slab sides would be good for coming alongside for loading/unloading. Query: how would you plank the bow? Double diagonal 2 x 3mm perhaps? Or commercially available bendy ply? I wouldn't use more than 5mm ply for the rest, but I'd put stringers on edge halfway down the topsides, which would come in handy to lodge the thwarts on. Will it sail? Certainly, there's nothing wrong with sailing scows. With the very high stability and load capacity that it has, I'll put in a plea to not make it too beamy. I'm going to stick to my guns and declare that 3ft 6in beam with 7ft oars is spot-on for a working tender

    PS to make a model of the SibLing tender, I'd scale up the three drawings of the plywood layouts, and take them to a copyshop to be printed on card - equivalent to getting the plywood CNC cut.

    Last modified: 24 May 2021 14:47 | Anonymous member
  • 24 May 2021 13:50
    Reply # 10544271 on 10211344

    Arne – sorry, I thought it would have just been a click of the button. Still, when you get time, it would be good to see Medium Boy worked out fully – with the Halibut rig of course!

    For resin thickener, I have used the fine wood flour salvaged from the collector bag of a belt sander – but only with epoxy, I’ve never used polyester. (Now I want follow Slieve’s experience and try polyester. I’m not epoxified yet, but I dislike the stuff and its expensive).

    David – yes, I couldn’t put my finger on it, but Sibling Tender almost looks “cods head mackerel tail” (whereas KISS is quite the opposite, actually more junk-like). Now I am slowly starting to understand that design. I spent some time this evening thinking about it in terms of making a model – clearly there would be more work in it,

    Now I would like some more advice – and blunt criticism. I looked at a video this evening of a Bolger Brick (8’x4’) sailing under a flat junk rig.

    https://www.youtube.com/watch?v=zyUg5sOj58o

    You want to skip the first 25% of it. Its tootling along, demonstrating reefing etc. It seemed to sail moderately well, which kind of validates what I have tended to believe, that at 8’ just about any shape will do.

    This has got me thinking again about the NZScow – not as an entry, its not suitable - but simply as an experiment, or at least a topic for discussion here. I am sure it would sail and row at least as well (if not better) than a Bolger Brick.

    For a tiny vessel, I think it has a better bow than either a stem dinghy or a pram dinghy.

    Its initial stability would be very high. As for the rest of it, I really don’t know.

    The Bolger Brick tells me that it ought to heel a little and sail on its chine as well as any other 3-plank. As for initial stability, and load carrying capacity, it ought to be up there with the best. Lofting and construction can be very simple (I know that much already). Its flat and flabby sections mean it might need to be built a little heavier than a conventional dinghy.

    I’ve looked at the run and the sheer line of Sibling Tender, and its proportions – and had another go at trying to cobble together a NZscow dinghy. I’m not looking for anything other than bunt criticism – do you see any particular virtues or vices?


    Last modified: 24 May 2021 14:07 | Anonymous member
  • 24 May 2021 11:20
    Reply # 10543904 on 10211344

    When I last used stitched-and-glue with polyester resin I just used the resin 'as mixed' and it did the job. The inside didn't look all that pretty, but it stuck and held well. That of course was some years back.

    Since then I've thickened resins with a variety of fillers, and even used fine sawdust which worked but was a bit coarse. I've heard of people using wheat flour, as used in baking. and had some success. It's worth having some filler if the ply edge joins are not a tight fit, but many books show big fillets which add weight. I tended to brush pure resin and then a very small fillet of peanut butter consistency filler pressed in with an old knife before the tape. 

    There ae many books on the subject, but they tend to use expensive fillers.

    Cheers, Slieve.

       " ...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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