Design and Use of Drogues

  • 14 Mar 2018 19:15
    Reply # 5978881 on 833331

    Thanks for the video link, Darren. It illustrates very clearly how vital it is to get any kind of drogue to sink well below the surface. A drogue that's in the frothy, tumbling top of a wave is no use at all. I see that when they used a length of chain, it looked like about 5 metres of 8mm or 10mm, which would weigh about 7 - 10kg. That isn't enough. The JSD website has an illustration showing a 30lb weight well below the surface, but that's only a hand-drawn picture. To do any good, I'd want to see a weight equal to the weight of the boat's main anchor - about 20kg in the case of that test boat, at a guess. In fact, a scoop, plough or claw anchor would have something of a paravane effect, drawing the drogue deeper, as well as stabilising it against yaw and rotation, so this ought to be the necessary weight, IMHO.

    Not enough attention was paid to launching the drogues, with the result that some of them got tangled and didn't deploy properly.

    And another thing that's graphically illustrated is just what a PITA it is to recover a JSD.

    Last modified: 14 Mar 2018 19:25 | Anonymous member
  • 14 Mar 2018 16:59
    Reply # 5978576 on 833331

    I've watched this thread without participating as I've never looked a full-blown storm in the face from the deck of a sailboat.  However, as an aquatic ecologist I have towed a fair number of things behind boats.  Cones (and some other shapes, especially spheres) can behave erratically when towed at speed.  I suspect the small size and large number of cones in the JSD allows them to offset one and others behaviour.  My limited experience makes me think that the larger cones seem to behave worse than smaller ones.  So, although the JSD cones seem smaller than necessary, I wouldn't want to go with too small a number of larger cones.  Making the cones out of a mesh might improve their behaviour without a large decrease in drag.  Although this video is a bit biased, and doesn't include regular cones, it does show some of the behaviours you want to try and avoid as you experiment.

  • 14 Mar 2018 16:49
    Reply # 5978571 on 833331
    Anonymous member (Administrator)

    After having sewed up four six-segment parachute drogues, ranging from 0.125 to 1.00sqm, I am inclined to think that these are more practical in use than a serial drogue. Losing a parachute is not such a big deal with a few in spare. I imagine that an anchor chum tied to the middle of the rope, would ensure good immersion as well as some shock damping.

    Today I just finished the biggest one. My plan for it is not storm survival offshore (no way!), but either to give it to a friend who has a motorboat. He could need the drogue to give him some precious time and easier movement while sorting out the clogged diesel filter before hitting granite... For myself, I could use it for deep-water cod fishing without drifting away (our fjords are often too deep for anchoring).

    I am looking forward to testing the drogues (plus ball fenders), and will let you know the results.

    Arne


    Last modified: 11 Jun 2018 08:41 | Anonymous member (Administrator)
  • 14 Mar 2018 12:50
    Reply # 5978034 on 5977962
    Graham Cox wrote:
    David, how many of Jimmy Green's 30cm drogues do you think would be needed in series to have the same effect as a JSD for a 6 tonne boat?
    Without any hard science or experience to go by, I'd have to compare frontal areas. For this size of boat, the JSD recommendation is for 107 cones of 5in diameter, and I calculate the total frontal area to be 13554 sq cm, or 2100 sq in. (the Para sea anchor size would be 9ft diameter, which is about twice that area). That would be the equivalent of 18 x 30cm cones, or preferably (since smaller cones take just as much labour to make as larger cones, since I get bored when I have make more than a dozen of anything, and since the main aim is to spread a number of cones out along a long line) 11 x 38cm cones on a 55 metre line, with a 25 metre leader.
    Last modified: 14 Mar 2018 12:54 | Anonymous member
  • 14 Mar 2018 12:03
    Reply # 5977962 on 5967060
    David Tyler wrote:
    David Tyler wrote:
     Mr Jordan was asked if it might not be just as good, and easier, to make a smaller number of cones, to a larger size so as to give the same effective area, and his reply was along the lines of: "if it ain't broke, don't fix it". I would be inclined towards using (say) ten big cones, more stoutly constructed, and spread out along the line so that the finished length of the drogue was the same as for a hundred cones, thus putting all the cones in different parts of the waves, and again, averaging out the pull exerted, moment by moment, on the boat. .
    I've just received an email from Jimmy Green Marine, advising me of their new website. I browsed it a little, and came across the interesting fact that they make the heavily-constructed PVC drogues that have been around for a very long time, in sizes from 30cm to 76cm mouth diameter, and include this in the product information:
     Smaller sized drogues can be trailed one behind the other on the same warp by making them off at different points along the rode using the stainless ring.

    David, how many of Jimmy Green's 30cm drogues do you think would be needed in series to have the same effect as a JSD for a 6 tonne boat?
  • 12 Mar 2018 12:59
    Reply # 5967060 on 5885768
    David Tyler wrote:
     Mr Jordan was asked if it might not be just as good, and easier, to make a smaller number of cones, to a larger size so as to give the same effective area, and his reply was along the lines of: "if it ain't broke, don't fix it". I would be inclined towards using (say) ten big cones, more stoutly constructed, and spread out along the line so that the finished length of the drogue was the same as for a hundred cones, thus putting all the cones in different parts of the waves, and again, averaging out the pull exerted, moment by moment, on the boat. .
    I've just received an email from Jimmy Green Marine, advising me of their new website. I browsed it a little, and came across the interesting fact that they make the heavily-constructed PVC drogues that have been around for a very long time, in sizes from 30cm to 76cm mouth diameter, and include this in the product information:
     Smaller sized drogues can be trailed one behind the other on the same warp by making them off at different points along the rode using the stainless ring.
  • 11 Mar 2018 05:33
    Reply # 5957651 on 5933012
    Jim Creighton wrote:
    Could you expand on this a bit for us? How was the boat steered in heavy wind? By hand mostly or self steering?
    With the exception of one occasion, where we had a something break on the self-steering gear (I can't remember what it was) Badger always ran before a gale steered by the windvane - unless, of course, we were close to shore.  Sometimes we would let the lowered mainsail out, sometimes it was sheeted in, depending on the conditions.  It was quite wonderful to stand in the pramhood and watch the waves thunder down towards her and then see how she would rise up their faces with only perhaps a bit of foam from the crest spilling over the stern, while the self-steering gear kept her on course.
  • 09 Mar 2018 15:24
    Reply # 5933012 on 5887770
    Annie Hill wrote:
        We sailed 110,000 miles in Badger and never dragged anything behind us and we didn't stick strictly to the Trade Wind route.

    Could you expand on this a bit for us? How was the boat steered in heavy wind? By hand mostly or self steering?
  • 04 Mar 2018 16:03
    Reply # 5888318 on 833331
    hi arne

    Arne Kverneland wrote:

    my experience is that the wind strength appears to be over-reported by boats around me…

    …I think the reason is that they have their wind sensor up in the mast top, some 12-15m up.

    the position of the sensor may be one reason. but i had the same experience (sailing in more moderate winds than other people in the same region) even when my own wind sensor was something around 18 meter above sea level.

    i think, some sailors tell you about the highest reading in a gust – but that's not what produces the sea state…

    ueli

  • 04 Mar 2018 12:19
    Reply # 5888115 on 5888112
    Arne Kverneland wrote:

    Annie,

    my experience is that the wind strength appears to be over-reported by boats around me. When I have sailed in something I estimate as a stiff breeze, other have told me that they recorded 30-35kts winds (F7-F8). I think the reason is that they have their wind sensor up in the mast top, some 12-15m up. I wonder what height is the standard height used by the met office? On the airports I worked on, the wind sensors seemed to sit on a standard mast, some 6-8m above the ground. I used to maintain them.


    10 metres is the meteorologist's standard height, so a wind instrument at the masthead will be giving a similar readout to what the forecast is saying. This will always be much more than we can feel on our faces, or on a hand-held instrument, at deck level.

    https://www.metoffice.gov.uk/guide/
    weather/observations-guide/
    how-we-measure-wind

    In Aero-hydrodynamics of Sailing, fig 3.11, C A Marchaj gives a graph showing that, with a reference of 100% wind speed at 100ft above sea level, the wind speed at 33ft (10 m) is 80% and at deck level is about 54%. This must vary with actual wind speed and turbulence of the air flow, but does at least indicate that at deck level, we feel about 70%, or less, of that which a masthead instrument indicates. So, 35kts (F8) at mast head would be felt as 24kts (F6) at deck level.

    Last modified: 04 Mar 2018 12:58 | Anonymous member
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