Moving the rig aft, and supersizing the rudder.

  • 13 Mar 2019 21:14
    Reply # 7217737 on 7216702
    Anonymous wrote:

    David,
    Phil Bolger actually had the 11.5’ dinghy, “Dart Dinghy” built with an aft-set rig, a huge rudder and a small cb. near the bow. It worked, but he regretted he didn’t make the cb. bigger.

    Apart from that boat, most of his rudders are fixed low-aspect-ratio jobs with a big endplate on them. His first endplate was fitted to a traditional American catboat with the usual barn-door rudder. The added endplate improved the handling of the cat hugely, so from then on Bolger used these on many of his designs.

    My motive for thinking along these lines are different on big and small boats.

    On a 2-ton keelboat like my present IF, Ingeborg I see no need for any changes. The lateral resistance is very good on her, and the weight of the mast through the foredeck hardly does any harm, at only 2.5% of the weight of the boat. With the bow water tank out of use, the boat trims perfectly on her waterline, and the mast’s short length probably adds less to the inertia than the 1.2m longer Bermuda mast. My motive here is rather to give a designer or rig converter more options on where to put the mast, with respect to interior.

    On dinghies I feel that the cb. trunk is a real space thief. In addition, the JR sheets sweeping over the helmsman makes the tiny cockpit very busy and crowded. By shifting the cb. forward and the mast and rig aft, one will gain free room at both ends, so that even a 10-footer can be sailed by 2-3 people. My 18’ Oslo Dinghy, Broremann was really a one-man boat because of that cb. case (but then is was designed for racing with two agile youngsters on board).

    Sooo, no cutting in wood, here for a while. I am more tempted to find a decent 15-16’ færing (double-ended row-boat) and see if that can be fitted with a big rudder and a low-AR JR with the mast through the middle thwart. A long boomkin would ensure perfect sheeting. That rudder would count for much of the lateral resistance, and thus keep the shallow-draft boat from sailing crab-wise.

    Arne


    Here's a vote for this rudder and balance..... it worked for me on a bigger Junk and I am still going down that track, although it has got to be amphidromic (shunting capable) on my current craft.

    Suggestion is to experiment along the lines of traditional Junks - which have used ropes under the hull to hold the rudder up against the transom (such as on the Junk Keying).

    In theory a foil shaped plank could be  held against a saddle on the  very bottom centre of the transom:using a U bolt would be  a strong and easy to fit experimental method, which could be improved upon once the geometry has been sorted.

    This is the lower pintle/gudgeon point of the rudder(its pivotal fulcrum) when the holding line is hauled home, and when let go, it allows the rudder to kick up.

    Bracing the rudder at the upper pintle/gudgeon point, is a notch and another string/line collar to capture the rudder stock.

    Raising this upper point on a strong sheet horse or gallows structure, keeps things in the traditional Junk style and practice.

    Also, I would suggest using an extruded composites tube or alloy bar bound to the leading edge of the plank foil, in place of the stock.

    Once sorted for size and geometry, the foil may be built from composite skinned foam, with a carbon fibre bar stock bonded on.

    An area forward of the pivotal axis will balance the blade to ease helm/tiller pressure, and raking of the stock, along with the sweep of the blade/foil, will provide additional control over the flow and dynamics.

    Achieving a broad range of adjustment, depends on how and where the upper end of the 

    stock is captured...... here a Junk style raised and extended poop deck helps, which also gets the sheeting point moved in the right direction.

    Reading the above, a I have just now done(a few days after posting it) the description certainly needs a drawing to convey what I am getting at, but facilities to post such a drawing are not possible right now, so I have only the option of more words added in an attempt at clarification - Having made and test sailed an approx 1:20 scale model of a double chine sheet stock hull with Junk rig and large/deep rudder and a daggerboard, there is some material evidence for a very simple lash-up rudder as mentioned above.

    This model was made in Portugal, after I had lost Jung Jung, and was intended for a future boat.

    Jung Jung had sailed with a big drop rudder as the primary lateral area board, augmented by fairly large forefoot as additional lateral plane up front, which worked fine for directional stability, but was certainly a hindrance when sailing through the wind when tacking.

    Bolger's designs had been something of an inspiration in using a  lateral plane area up forward along with a rudder, then there was also the Hobie beach cat, that relied much on their deep rudders.... besides the historic Junks.

    My model had a shaped piece wood tied on with string, and worked just fine.Then as well, it did have a daggerboard just forward of the mainmast and did have a smaller, forward raked foremost, allowing a headsail to be carried.

    The daggerboard adds considerably to the lateral area for leeway resistance, and  then is helpful in pivoting the hull when coming about.



    Last modified: 16 Mar 2019 19:33 | Anonymous member
  • 13 Mar 2019 13:15
    Reply # 7216702 on 7214895
    Anonymous member (Administrator)

    David,
    Phil Bolger actually had the 11.5’ dinghy, “Dart Dinghy” built with an aft-set rig, a huge rudder and a small cb. near the bow. It worked, but he regretted he didn’t make the cb. bigger.

    Apart from that boat, most of his rudders are fixed low-aspect-ratio jobs with a big endplate on them. His first endplate was fitted to a traditional American catboat with the usual barn-door rudder. The added endplate improved the handling of the cat hugely, so from then on Bolger used these on many of his designs.

    My motive for thinking along these lines are different on big and small boats.

    On a 2-ton keelboat like my present IF, Ingeborg I see no need for any changes. The lateral resistance is very good on her, and the weight of the mast through the foredeck hardly does any harm, at only 2.5% of the weight of the boat. With the bow water tank out of use, the boat trims perfectly on her waterline, and the mast’s short length probably adds less to the inertia than the 1.2m longer Bermuda mast. My motive here is rather to give a designer or rig converter more options on where to put the mast, with respect to interior.

    On dinghies I feel that the cb. trunk is a real space thief. In addition, the JR sheets sweeping over the helmsman makes the tiny cockpit very busy and crowded. By shifting the cb. forward and the mast and rig aft, one will gain free room at both ends, so that even a 10-footer can be sailed by 2-3 people. My 18’ Oslo Dinghy, Broremann was really a one-man boat because of that cb. trunk (but then is was designed for racing with two agile youngsters on board).

    Sooo, no cutting in wood, here for a while. I am more tempted to find a decent 15-16’ færing (double-ended row-boat) and see if that can be fitted with a big rudder and a low-AR JR with the mast through the middle thwart. A long boomkin would ensure perfect sheeting. That rudder would count for much of the lateral resistance, and thus keep the shallow-draft boat from sailing crab-wise.

    Arne


    Last modified: 13 Mar 2019 21:48 | Anonymous member (Administrator)
  • 13 Mar 2019 10:04
    Reply # 7216475 on 7214895

    Here's a boat you probably already know about, which has mast (and sail area) placed well aft, and has a large rudder which evidently contributes to lateral resistance.


    (In this picture the large rudder is folded up into the vertical position.)

    Its the Paradox  design by Matt Layden. It seems likely that the rudder provides a fair amount of the lateral resistance, as this thing does not have a keel or centreboard, and is flat-bottomed - but evidently goes to windward on chine runners (external chine log extended to become tiny winglets).

    Prior to designing Paradox Matt also built a small boat with large rudder aft and centreboard placed well forward. I found a picture of it at http://www.microcruising.com/lc1.htm                                                             Little Cruiser is junk rigged.

    Loved by her owners Dave and Mindy Buldoc who have done extensive coastal cruises, she seems to be proof that the concept of small centreboard placed in the bow, and large rudder aft, is a viable one.

    Last modified: 13 Mar 2019 11:47 | Anonymous member
  • 13 Mar 2019 09:26
    Reply # 7216419 on 7214895

    It is a truth universally acknowledged that getting the weight of the mast back to the middle of the waterline makes for better performance and comfort in larger seagoing boats, and that should be true for dinghies, too. The ballast of a larger boat shouldn't be too far from central, either, but an unballasted centreboard could be further forward. I can see how redistribution of areas between centreboard and rudder might be a good thing. At the same time, I can't help thinking that if there was something to be gained, Phil Bolger would have designed it already.

    Sooo, this is something that just might be worth some experimentation on a dinghy, initially. When do you start cutting timber, Arne?

  • 12 Mar 2019 23:28
    Reply # 7215804 on 7214895
    Anonymous member (Administrator)

    Graeme,

    I may be known for my rule of thumbs, but not this time. I am very low on the learning curve here. I can think of three or four ways of getting more rudder power.

    ·         The La Chica rudder. This rudder was moved aft and away from the long keel, so the water flow around it was probably faster than before, when sitting right behind a long keel. Its good foil shape and the use of endplates helped too. Moreover, Paul avoided high tiller forces by giving the rudder 20% balance. The rudder was supported in the lower end, so was strong and safe.

    ·         My big-rudder design (not built yet  -  at least not by me) on a swing-up ‘false transom’. I guess that could be built as strong and safe as you can dream up, but it requires a transom stern of some width.

    ·         The improved dinghy rudder, which someone pointed at here, recently. Maybe not for crossing oceans, but it should be possible to make them sturdier than traditional swing-up rudders.

    ·         Twin rudders, of course. They both saves draft, and lets one have an outboard engine on the cl.

    Until more experience is gained, I guess I would feel quite safe if the combined CLR of the cb. and rudder is well aft (15-20% of wl.?) of the sail’s CE. The fine thing is that one is free to build a bigger or smaller rudder if it is called for, unlike a cb, which sits in a slot.

    One must of course give a big rudder enough balance to keep the tiller forces down, and then one must have a handy tiller lock to prevent the boat from rounding up if one leaves the tiller for a (split-)second.

    Arne


    Last modified: 14 Mar 2019 13:47 | Anonymous member (Administrator)
  • 12 Mar 2019 22:09
    Reply # 7215671 on 7214895

    Arne this is very interesting. Improved handling from a larger rudder makes sense. The idea of a larger rudder to enable shifting the mast position is an interesting thought. I presume you have a rule of thumb for adjusting the so-called centre of lateral resistance when designing for the supersize rudder?

    Two more questions:

    1. For the purpose of improved lateral resistance, would you consider twin rudders to be the equivalent of a supersize rudder?

    2. Nowadays it is quite easy to make a big, strong rudder, which will kick up, if needed.

    Are you referring here to dinghy rudders, or something suitable for, say, a 4 tonne boat? I have seen a drawing of your (rather clever) fixed rudder on a swing-up transom. Do you have any other ideas for a "big strong rudder, which will kick up if needed"?



    Last modified: 12 Mar 2019 22:10 | Anonymous member
  • 12 Mar 2019 15:17
    Message # 7214895
    Anonymous member (Administrator)

    Moving the rig aft, and supersizing the rudder.

    It is a well-known fact that the Chinese boatmen used the rudder of their craft both for steering and as a second centre-board. That is why their rudders are so much bigger than western rudders. Frankly, I think it is a pity that we haven’t learnt to appreciate and use this principle in our own boats.

    When reading through the last JRA-Magazine (#79), filled with dinghies and small tenders with JRs, it inspired me to sketch up a three-metre dinghy with a slightly aft-set rig and with a  big rudder to compensate. By introducing a retractable boomkin, one kills to flies in one smack:

    ·         One gets all the space one needs for good sheeting angles. No problem with any D-min.

    ·         One gets the sheet out of the little boat, and thus makes it feel much less crowded.

    On my sketch, I have also moved the cb. further forward, and this could be enough to get proper sitting-down room for two people. This cb.-rudder combination is not radical at all: It has been used on zillions of craft in the East. Nowadays it is quite easy to make a big, strong rudder, which will kick up, if needed.

    Since the Buccaneer 24 trimaran has been discussed lately, I gave this much the same treatment, shifting the whole rig 63cm aft. This moves the mast away from the bow, and also moves the sheet out of the cockpit. The area of the modified rudder is 0.36sqm, versus 0.17 for the original.

    The Split JR and Aero JR surely are alternatives, in this case. I just wanted to see how the Johanna-style rig would look like in this role.

    I have to remind you that Paul J Thompson reported that he could control his (then) schooner, La Chica with only her mainsail set, after he made a proper rudder for her. 

    Cheers,
    Arne


       " ...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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