Van de Stadt 36 Seal - JR conversion

<< First  < Prev   1   2   Next >  Last >> 
  • 19 Nov 2020 19:23
    Reply # 9376044 on 9338306
    Anonymous member (Administrator)


    now I had a look at that calculation tool, made by Oscar Fröberg.

    When choosing mast material as Norwegian Spruce, its density is given as 0.43.
    Then I look at your .xls file where you say the density of spruce is 0.63 (630 kg/m3). That makes a whole lot of difference:
    If you multiply your found weight of 298kg with 0.43/0.63, the mast will come out with

    Wm = 298kg x 0.43 : 0.63 = 203kg.

    The rule of fancy algorithms is: “Put garbage in and you will get garbage out”.

    In my world we reckon the density of dry spruce to be around 0.52, give or take  2%.
    If the other algorithms are ok and the density is 0.52, then the weight should come out at

    Wm = 298 x 0.52 : 0.63 =246kg.

    That would only add about 2.5% to the boat’s displacement, so should be good.


    PS: I also note that the Sigma of spruce is set to 72MPa. In my calculations I have used the much more modest 45MPa. Maybe I am too pessimistic...

  • 19 Nov 2020 17:23
    Reply # 9375812 on 9374993
    Anonymous wrote:

    according to the junk rig calculation tool of oscar froberg a steel mast for 16300kpm could be a 300/6mm tube – a bit heavy…


    Nice tool!

    With this tool i would estimate the weight of spruce mast roughly to 200 kg. 

    I wonder?? I have tried to estimate it and I am always around 300 kg. Attached.

    1 file
  • 19 Nov 2020 11:30
    Reply # 9374993 on 9338306

    according to the junk rig calculation tool of oscar froberg a steel mast for 16300kpm could be a 300/6mm tube – a bit heavy…


    Last modified: 19 Nov 2020 11:33 | Anonymous member
  • 19 Nov 2020 09:36
    Reply # 9374844 on 9338306
    Anonymous member (Administrator)

    Hans-Erik, that is the hard part.

    To find the scantlings of a mast, one can approach it from two angles.

    Either do as in Chapter 6b of TCPJR and start with guessing on the boat’s ultimate righting moment:
    With a displacement of 9250kg and with 3300kg ballast, I would guess the righting arm to be 1/5 of the beam of vessel (3.54m).

    The ultimate righting moment, should then be
    Mr = 9250kg x 3.54m x 0.20 = 6549kpm.

    If we choose the mast strength to be 2.5 times Mr, the mast must take 16373kpm.Then one can produce a mast  in any material with this strength in mind.

    This method doesn’t count with sail area or LAP, but as long as the LAP is not extreme, I think we get away with it. A very tall mast will suffer quite some additional loads due to pitching, but this factor is hard to quantify. That's why we have safety factors...

    The other approach to find a wooden mast, is to go to Chapter 6 and use my modified Hasler-McLeod method. This would result in a hollow spruce mast with 34cm diameter (check it, yourself). Since Finland is full of spruce trees, that could be an affordable alternative.

    On such a big steel (?) vessel, a steel tube mast could be an alternative, but that is outside my area of experience. I hope Paul Thompson could help us there.


  • 19 Nov 2020 05:12
    Reply # 9374619 on 9338306

    Greetings Arne,

    In your various suggestions for a large sail sloop rig for Falkor you state “The biggest challenge is to build a tall and strong enough mast at a moderate weight and cost.”

    What would be the specs for a suitable mast in, for example, your most recent iteration of a 65.6 square metre sail?

  • 18 Nov 2020 15:30
    Reply # 9372907 on 9338306
    Anonymous member (Administrator)

    Good, Eero.
    Then I guess I would suggest one of the two last rigs I have presented.
    The light winds in the Bay of Finland during summer, speaks for the last one; the one with AR= 2.40 and SA= 65.6sqm.
    However, we don’t get younger as the years go by, and the shorter 13.4m mast is cheaper and easier to deal with than the one at 14.9m...

    As for performance; even the smallest JR will outperform the masthead Bermuda rig on the downwind leg. Close-hauled, I guess I would put my money on the BR, in light winds.

    The only ‘hard’ job when sailing with a JR, is hoisting the sail. On your boat, that can easily be overcome with an electric capstan. Some of these are meant for hauling up the anchor, and are not terribly expensive. Hoisting the sail from bottom to top will drain a 12V battery with less than an Ampère-hour.

    Anyway, rigging the boat with two sticks will cost a lot more  -  I would say, forget that.

    When organising the sheet, it is important to give them a good layout to make them easy to haul at by hand and via manual winches.


    PS: I have sent the drawing in QCAD (.DXF format) to David, to let him suggest a rig. If others will have a go, and can handle that format, just let me know.

    PPS: In case the mast position I have suggested results in awkward access to the fore-peak, the mast may well be moved 10-15cm forward, and it can even be offset 10-15cm to one side, without suffering any handling problems.

    Last modified: 18 Nov 2020 23:21 | Anonymous member (Administrator)
  • 18 Nov 2020 14:36
    Reply # 9372814 on 9338306

    Arne, David, Graeme,

    Thanks for your comments and input. Very much appreciated!

    Arne, in the scale (0-100) I would say I am 45% sailor. Pure cruising. Focus is on safe, comfortable, and enjoyable sailing. The crew is 60 year-old couple, whose idea is to leave the regular work within say 2-3 years and the boat will be our liveaboard for maybe 6 months per year. Nowadays we sail the ‘normal’ summer holiday sailing i.e. daily trips along coasts of Gulf of Finland and Northern Baltic Sea. But once we get rid of the tight schedules, the trips will get longer but leisured. We can keep the boat abroad and no need to dock the boat for every winter. Perhaps not crossing oceans but exploring canals, rivers etc might be nice. For that purpose, a shorter mast(s) would be nice as they could be carried on deck if the overhang is not too much. For that reason, we have also considered if we should switch the boat to a smaller one with a smaller draught. But on the other hand, this is a nice and cozy liveaboard boat giving very safe and comfort ride at open sea.

    The present sail area is main 25.7, furling genoa 37.3 making 63m2 in total (hopefully I managed to measure them correctly). We have a few more sails, but in practice we keep those home. I would guess that a JR of the same size would be much more efficient downwind, as without a spinnaker/gennaker we loose a lot of effective area of the genoa. The present balance is quite good I would say based on the rudder angle. Difficult to say because the hydraulic steering gives almost none sense of the pressure. I am used to tiller, which I keep as my favorite way for steering.



  • 18 Nov 2020 11:42
    Reply # 9372402 on 9338306

    Good, Arne. I like the reasoning, and I like the end result.

    Except for the missing forward rake ...

  • 18 Nov 2020 11:00
    Reply # 9372358 on 9338306
    Anonymous member (Administrator)

    OK, I put the blame on the Corona half-shutdown and the never-ending rain here. Now I have sketched up yet another JR for that 11.0m/9.25 ton Seal.

    David Tyler’s notes below made me sober up a bit. I can see his point that handling a very big, single sail can be too hard as the size of the boat grows. Problem is that we people stay at the same size no matter if we are sailing a nutshell or a ship. This time I have kept the chord of the sail moderate (5.71m), and made the sail taller by simply adding a panel to the last sail I showed to you. This way the sail area goes up from 57.4 to 65.6sqm and the SA/displacement  now reaches 15.1. The AR goes up from 2.15 to 2.40.The mast position is un-changed.

    What I like with this is that we end up with a taller sail than the original, but still with a shorter mast. In addition, there is plenty of space for the (double, upper-lower) sheets. My hunch is that the boat will be at least as fast as with the original rig on all courses, and thanks to the moderately short chord of the sail, the downwind steering will be easy.

    The biggest challenge is to build a tall and strong enough mast at a moderate weight and cost.


    Last modified: 18 Nov 2020 12:51 | Anonymous member (Administrator)
  • 17 Nov 2020 21:04
    Reply # 9371005 on 9338306
    Anonymous member (Administrator)

    I realise that I jumped into drawing mode too early. Before presenting any sketches, I should have asked a few questions (I see now that you have hinted about it in your profile):

    • ·         Where on a scale from 0-100 do you regard yourself to be (0 = motorboatman, 100 = die-hard pure sailor)? How much do you focus on sailing itself?
    • ·         What waters will be your preferred to sail in for the next five years?
    • ·         How many people will you be on board, mostly?
    • ·         How big SA do you actually make use of today, with the original rig?
    • ·         How is the helm balance with the original rig?

    I will not argue for or against any rig or sail area until I learn more about your needs.

    However, just for fun, I have now added a ‘minimum rig’ of 57sqm (same mast position and CE). This would result in a SA/Disp.=13.2. See below.

    As for rudder incorporated in the CLR or not, I generally avoid that, in particular with sloop rigs. I try to end up with about neutral helm when close-hauled in light winds, and keep the rudder ‘as a spare’ for controlling the boat on a beam-to-broad reach in rising winds.


    Last modified: 17 Nov 2020 21:13 | Anonymous member (Administrator)
<< 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