SibLim 4 metre dinghy

<< First  < Prev   1   2   Next >  Last >> 
  • 13 Jul 2021 11:54
    Reply # 10748078 on 10741742
    Anonymous member (Administrator)

    Slieve writes “…it is not the wind strength that causes the problems in a dinghy. It is the gusts that catch a dinghy out…." This is particularly true in sheltered waters, where the dinghy is in the shelter of hills and valleys. I would not expect to be sailing a 4m dinghy in really strong wind conditions, and would not expect to be far from shelter at any time, if it cuts up rough.

    I still think it is good to be able to reduce sail in a hurry, though,  if one gets caught out in a sudden weather change, especially as this allows one to be a little generous with sail area for the fine, light weather days. That being said, I do not see the need for multiple reefing options as most junk rigs provide. One or two reefing positions at the most ought to be enough for a cruising dinghy, oughtn't it?

    This ties in with the sheeting issue which, again, Slieve has sensibly raised. It is possible to speculate that the Halibut rig as David has drawn it, with its low balance, only three lower panels, and relatively simple sheeting arrangement, ought to make it possible to ease sheets freely and quickly. I am not so sure about the 33% balance,  5-panel SJR though. The sheeting forces will be pretty light…

    For a couple of reasons, the above sheeting issue being one of them, I think I am going to divide the lower part of my SJR into no more than three – and possibly just two - lower panels, hoping that, among other things, there will be a simplification in the running rigging and less friction in the mainsheet.

    (The other reason is: I don’t want to be the first to try jibs with an aspect ratio of less than 1, but that’s another discussion.)

    If it still turns out that these little junk sails won’t “let fly” in a hurry, there is another solution, isn’t there? I seem to recall reading in one of the magazines of a dinghy (I can’t remember which it was, now) which had been given a sort of “two tier” sheeting system. As I recall, and as I think I understood it, the conventional junk sheeting system came back to a jamb-cleat-block, whereby small adjustments to the sheeting could be made. But the jamb-cleat-block was not given a fixed mounting – it was attached to a second-tier sheet – probably 1-1 and just a single block – which the helmsman kept in hand and could release in a hurry if necessary. Would that do it?


    Last modified: 13 Jul 2021 12:02 | Anonymous member (Administrator)
  • 13 Jul 2021 11:44
    Reply # 10748072 on 10741742

    It's interesting to put three possible rigs side by side:


    1 file
  • 13 Jul 2021 11:37
    Reply # 10748068 on 10741742

    David W,

    Bearing in mind that this boat is to be used in a shallow estuary where quick reactions with the sheeting will be needed if I touch bottom, I don't think two rigs will work for me. The smaller the sail, the harder I find it to sew up. I'd rather just make one rig of each type at 5.9 sqm. Since the river channels are narrow, I have to be able to skim over the sandbanks, so the board needs to be broad and shallow. I can't immediately visualise a push-up dagger board in that kind of shape, can you?

  • 13 Jul 2021 11:17
    Reply # 10748017 on 10741742

    David,

    may I suggest that you look at possibly using a two masted rig on your dinghy, similar to what I used on my Webb 14. I can see several advantages.

    The rig would be lower so easier to go under the bridges.

    Being lower the rig has less heeling moment and is less demanding to sail.

    The two sails make it possible to balance the boat and sail for extended periods without touching the helm (as I often did with the prototype Webb 14).

    You could use different sails on each mast and so have a direct comparison of say a Split Junk rig and a Halibut rig. With sails of about 45 sq ft each, or use the same shape for both if preferred, say something like the four panel sail on my dinghy designs entered in the dinghy design competition.

    Everything would stow easily within the boat and would be easy for you to rig as all of the parts are smaller and lighter than for a single sail.

    Regarding the centerboard, have you thought of a forward sloped dagger board like the one on my competition dinghy designs. This is simple to construct, easy to work from the center of the dinghy and automatically retracts if you touch bottom. It also helps to push the boat to windward.

    Anyway just some food for thought.

    David.

  • 13 Jul 2021 09:39
    Reply # 10747870 on 10741742

    Thanks for your remarks, Slieve. I too raced dinghies in my youth: Enterprise (I still have a couple of pewter tankard trophies from 1962!), OK dinghy, Fireflies at college. Then I taught children in Enterprises (with the smaller rig) and Wayfarers. On our days off, if it was windy, we instructors would scorch around the lake single handed in the Wayfarers, so I know something about keeping a dinghy upright. Then after I married, we raced a National 12 at Hamble River SC. At age 25, I was comfortable with leaping around a racing dinghy to keep it on its feet; at 75, I'm not. Something nearer to a ballasted dayboat is more my style for sailing these days, but anyway, the SibLim 4m is designed just as much a rowing skiff for exercise and an outboard skiff for family picnics upriver, as a sailing boat.. 

    You might remember my Oughtred dinghy Dorothy, 11ft 6in, that was successfully rigged with several junk rigs. I could handle breezy days in the Solent with no trouble. Last year, I took her out with a western balanced lug on a windy day, with a reef in, and found it unmanageable. I quit and rowed home, before I went for an unintentional swim. JR is much, much easier to handle by a geriatric sailor than a western balanced lug.

    You mention the need for a free-running sheet. That's true. As you are well aware, this is what's always worried me about using a SJR on a small, lightly ballasted boat that can quite easily be blown down in a gust, if the sheet has some friction in it and the sail is so near the point of balance that there is little or no tension in the sheet. So, I've drawn a SJR with 33% balance, with the feeling that this might be too much; but equally, I'm well aware that I need to prove and demonstrate the point, even if it means going for a swim.

    On the other hand, I would expect the extended Halibut rig to have a lot of sheet tension, so if I'm alert, I should be able to let it run easily in a gust. But on a run, the Halibut rig will twist a lot, I suspect (indeed, its camber would come partly from permitting it to twist), so getting into a death roll and capsizing to windward is a strong possibility. 

    These two rigs are at opposite ends of the JR spectrum. I don't know at this stage whether I'd build either, or both, but the boat should be designed such that I could. I think that if I simply wanted the easiest, safest possible rig for this boat, and didn't want to experiment and compare some different rigs, I'd just make a Weaverbird mk 1 kind of rig, with a 45˚ yard, balance somewhere in the middle of the possible range and two fewer panels.


    1 file
  • 13 Jul 2021 09:26
    Reply # 10747865 on 10741742
    Anonymous member (Administrator)

    To avoid clogging this topic, which was meant for the SibLim dinghy, I suggest we jump to this more "general dinghy topic":


    Arne

  • 12 Jul 2021 23:21
    Reply # 10747073 on 10741742

    Hi Guys,

    This looks like an interesting thread and it will be interesting to see how things develop. I'm not trying to rock the boat (sorry) but I would like to make a couple of points purely because the letters SJR have appeared.

    Let me explain where I am coming from, but first the stats. on some dinghies I will refer to.

    Boat LOA m LOA Feet Beam m Beam Feet SailArea sq.m Sail Area sq.ft Weight Lbs Length/ Beam









    Wayfarer 4.827 15.84 1.855 6'1” 11.61 125 373 2.60
    Wanderer 4.3 14'0” 1.8 5'11” 10.7 115 291 2.39
    Enterprise 4.04 13'3” 1.6 5'3” 10.7 115
    2.53
    SibLim 4m 4 13'1” 1.22 4'0” 5.9 63.5
    3.28
    Golden bay 3.9 12'9” 1.3 4'4” 6.9 74.3
    3.00
    Mirror Dinghy 3.3 10'10” 1.39 4'7” 6.5 70 100 2.37
    Bumble Bee
    8'5”
    4'

    ~ 60 2.10
    Optimist 2.36 7'9” 1.12 3'8” 2.26 35
    2.11

    The Wayfarer is a much respected cruising dinghy, and the Wanderer was designed as a lighter version for the same job. The Mirror Dinghy is another popular boat that is sometimes used for dinghy cruising, and all three get mentions by the Dinghy Cruising Association. All three of these have good static stability. The Enterprise is a design I raced for a number of years and will mention below. The Optimist is the baby of the list, but the rig is of interest. The Jack Holt Bumble Bee will be mentioned later.

    Looking back over a life time of falling into and out of dinghies I cannot remember ever taking a reef in one. Taking the Enterprise which is a competitive one design class but not overly high performance boat, but it is remarkably similar to the two boats above it but just a little less stable. We raced it is conditions from, 'Are we moving?' to 'Are we totally mad?' and all with the full standard main/ jib rig.

    In a keel boat in the extreme conditions we would reef, but in a crew ballasted boat it is the movable ballast that keeps the boat upright, and I mean upright. As the wind gets stronger it is more important to keep the hull directly below the mast or a capsize will be staring you in the face. When the wind is too strong for the crew weight to keep it upright then it is up to the helmsman to play the sheet and spill wind and continue to drive the boat. Dead runs are not on so tacking down wind is the order of the day. The windy gybe is fine, as long as it is not attempted exactly at the gybe mark, but by taking a sweeping turn it can be done with completely slack sheet when the boat is at maximum speed in a lull rather than when the boat is at lower speed from a lull and the wind has gusted up again, and the pressure on the rig is much higher.

    I know this is teaching granny about eggs, but it is not the wind strength that causes the problems in a dinghy. It is the gusts that catch a dinghy out. If it is a strong wind day then a cruising dinghy can set out with a reduced rig if desired, but the gusts have to be handled by shifting the crew weight and easing the sheet quickly. It is unlikely that reefs would be adjusted frequently in a dinghy.

    The one boat in the list above that could reduce sail easily would probably be the Optimist, where the sprit could be dropped to let the top half of the sail collapse quickly. A two line system could be rigged to release the peak and pull it down towards the tack.

    What I'm suggesting here is that it would seem unwise to sail a small crew ballasted boat in windy conditions with a main sheet that would not let fly immediately, and I feel a junk sheeting system might be too slow in a small dinghy.

    I fully recognise the interest in trying a small junk rig, but before I bought Poppy I had already started to install the free standing mast into a Mirror dinghy when I realised this, and that there would be far too much sewing required for a 70 sq.ft rig. The Mirror is a very stable design, but if capsized it can be exhausting to get up and climb back on board (single handed).

    I included the Jack Holt Bumble Bee above because of its rig. You could describe it as a one panel SJR. The jib is simply barrel cut with no broadseam built into the corners, but either way there seemed to be little drive from the jib when I saw one sailing. I reckon a simple balanced lug would be more practical, with a simple main sheet. It will be interesting to see what Arne comes up with if he plays with a cambered panel and varies the balance.

    I've listed the length/beam ratio above simply because this ratio tends to reduce as LOA decreases. Simply scaling a design down tends to result is a narrower boat than desired. With only human ballast and quick sheeting to keep a dinghy upright then extra bean just might make it easier for a light non-energetic crew to keep the boat on its feet. The Mirror Dinghy is a remarkably practical dinghy, and at 100 lb could be loaded on the roofrack of a Renault 5 by one person (when I was younger). With a LOA/Beam of 2.37 I found it a remarkable boat, and I wish I had one now.

    Please note that I'm only offering these as points to ponder.

    Cheers, Slieve.


    Last modified: 13 Jul 2021 07:13 | Anonymous member
  • 11 Jul 2021 09:10
    Reply # 10743564 on 10741742

    Rig:

    Maybe I can stretch a point and make the mast 4.5 metres. Most of the sailing would be below the bridges anyway. That makes it possible to add a panel to the SJR, reducing the batten length to 2000mm to keep the area at 5.9 sqm. Since the aim would be for me to learn something by making and using a SJR of my own, it had better not be a sub-standard version of it. 


    Centreboard:

    Seeking for inspiration, I got out my copy of Little Ships and Shoal Waters, by Maurice Griffiths, a gentleman who knew more than a little about them. He, in turn, borrowed heavily from the Eastern USA types such as the Chesapeake skipjack. I think this type of centreboard will do nicely. There's no point in ignoring the accumulated wisdom of a great many shoal water sailors, and trying to reinvent the wheel. Or centreboard. Being brutally honest with myself, I won't be going dinghy camping in this boat, so some protrusion above the cockpit sole won't be a problem, but a rectangular version of the centreboard could be completely below the sole. 

    Rudders:

    If there are runners on the bottom strake, to protect it and provide somewhere to put a centreboard bolt, the hull fixed draught is 200mm at 208kg displacement. It would be nice to think that twin rudders with end plates would be enough, at this depth, and obviate the need for a hinged rudder blade (or two).

    2 files
    Last modified: 12 Jul 2021 07:53 | Anonymous member
  • 11 Jul 2021 04:41
    Reply # 10743380 on 10741742
    Anonymous member (Administrator)

    This is going to be very interesting. I love the SibLim hull shape (especially with its potential for usefully carrying just a little bit of ballast).  Also I am currently still mulling over a sail plan for my similar length (but very different hull shape) 3-plank Golden Bay.

    I was doing the same, thinking about the same two rigs and making possible provision for both.

    The "Serendipity rig" is, of course, just a scaled down dead copy of the Mkll Amiina rig, designed by Slieve. Here it is, with one reef in, just for comparison. With one reef in the sail still has a reasonably moderate aspect ratio, and Serendipity goes well in this mode.



    Its a pity about the bridge, and the need to have such a short mast. Its brought the proportions down to about that of Amiina ll with two reefs in. Serendipity doesn't seem to sail so well with two reefs, but this could be for a number of reasons. Another thing, Serendipity's sail just won't set well unless the mast height is up a bit higher than shown here, but again, I am not altogether sure why and it too might be for a number of reasons.

    All of this is making me consider, for my Golden Bay, a slightly smaller 1-reef Amiina sail, set in the forward position, and some sort of mizzen sail to balance things up. I don't know what to do, because I don't really want to be fooling around with two masts. Although, Marcus reckons a mizzen sail earns its keep - and as a bonus, makes it easier to sling a tent between the two masts.

    Alternatively (or better still, - as well as -) I too am tempted to give David's extension of Arne's Halibut rig a try. It might be more comfortable with the shorter mast - and at this small scale might be more sensible than a split rig. Then it will be back at the bottom of the learning curve all over again - though at the rate I work, I might yet have David to lead the way!

    I suffer from what James G calls "analysis paralysis", and admire David's way of rationalising through the various constraints, explaining his reasons and making decisions seem logical. Looking forward to following this.

    The evolution of a 4m SibLim design will be a highlight for the coming year


    PS Here is the stage Little Dipper is currently at.


    Last modified: 11 Jul 2021 07:32 | Anonymous member (Administrator)
  • 10 Jul 2021 09:28
    Message # 10741742

    Messing about on the river

    On Wednesday and yesterday, I took my kayak up the two major rivers that feed the Ravenglass estuary, the Irt and the Esk (and discovered how weak my paddling muscles have got - but that's another story), to think about my requirements, should I build a SibLim 4 metres.

    I'm thinking of a beam of 1,22 metres/4ft, to make it convenient to row and slippery if using an electric outboard. This is the same as for Iain Oughtred's whitehall-type Acorn skiffs, so like those, sailing with a 6 sqm area low AR rig should be OK. 

    An air draught of 4 metres will fit comfortably under the railway viaducts and the road bridge on the Esk, at a tide height of 6 metres, the minimum sensible height of tide to be afloat and sailing ( we get 6 metres at neaps and 8 metres at springs, commonly). So since a 4 metre mast is convenient as a cover ridge pole on a 4 metre dinghy, that decides that.

    I've sketched the two extremes of possible sail plans, as I see them, to decide on the extremes of the possible mast positions. They are a SJR of the Serendipity type, with a yard angle of 30˚ and a fanned sail of the extended Halibut type, with a yard angle of 75˚. A slot in the foredeck should make it possible to arrange partners in any position between these two extremes.



    At 6 metres rise of tide, a draught of 300mm will skim over most of the sandbanks without touching, and the hull draught is 186mm at 208 kg displacement. So the question of what kind of lateral resistance surface I should add is exercising my mind. A fixed keel of 300mm draught might be an embarrassment when coming back down river on a falling tide, if I touch the uptide side of a sandbank. A lifting board of some kind would be safer. So, a broad shallow dagger board with an end plate? a centreboard of the sector of a circle type?

    I find that I can nest all the major components onto 4 sheets of 5 or 6mm, 1 sheet of 9mm and 1 sheet of 3mm plywood, if the sheet length is 2500mm as it is for Robbins Elite Okoume. 


    Last modified: 12 Jul 2021 10:26 | Anonymous member
<< First  < Prev   1   2   Next >  Last >> 
       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

                                                              Site contents © the Junk Rig Association and/or individual authors

Powered by Wild Apricot Membership Software