Stability of power dory

Discussion in 'Boat Design' started by davflaws, Feb 13, 2006.

  1. safewalrus
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    safewalrus Ancient Marriner

    Which goes to prove what PAR and myself where saying earlier, we all presumed that this was an old style 'Banks' dory - these don't look bug*** all like them, so we needs to find out what the original on this post was/is? Before we can pass judgement, and yes for a time I was a sguilty as the next man (they all got flat bottoms instead of keels so I guess they kinda fill the definition?!
     
  2. bhnautika
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    bhnautika Senior Member

    Safewalrus not all beaches have pebbles. What is wrong with bags of wet sand flattened out on the sole? It is safer than water ballast (if its not baffled correctly) you can keep it in place quite easily and in small volumes.
     
  3. PAR
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    PAR Yacht Designer/Builder

    I pretty familiar with Jay's cruising dories and they are typical of the modified forms seen today, having wider bottoms and other (ballasted fin keels, etc.) things done to them. If I remember correctly, he did the first of them while literally sitting at the desk behind "Billy" Atkins. The original poster, Davflaws, hasn't been back in a week, I hope he hasn't lost interest.
     
  4. davflaws
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    davflaws Junior Member

    Nope - I'm checking the thread most days. I have no way of posting pictures at present and other committments got in the way of more sea trials with ballast as suggested, so I haven't felt as if I had much to contribute. I have one query about the suggestions though. Why put ballast in the mid line rather than out on the chines? I haven't done any calculations, but it seems intuitively that locating ballast as low and far from the midline as possible gives the best righting moment.
    Since I haven't any pictures yet, I suppose I can describe the boat and that will give you an idea. On one level, she is a bit of a dogs breakfast, but all the various modifications have had some rationality underpimming them, even if the result has not always been too flash.
    She started off in 1985 as an 18ft Pelin sailing dory, with about 4'' rocker in the otherwise flat bottom, a maximum chine beam of 42" and chine beam of 34" at the strongly raked transom. from chine to gunwale she was 22" midships, and the sheer was quite pronounced so she had more freeboard forward and aft. There was lotsa flare in the topsides, with maximum gunwale beam of 50" .
    She changed hands and the new owner wanted a power boat, so he blocked off the centreboard casing, installed a heavier transom with less rake, and added another strake to give 4" more freeboard. With 25hp she shifted lotsa water trying to climb her own bow wave, but eventually made 15kts and pounded in head seas. So the then owner put a birdsmouth in the bottom forward (removing the forward part of the rocker in the process) and "V"ed that part of the bottom. She then planed reasonably well - albiet "head up".
    The then owner's wife wanted shelter and storage so a "barrel" cabin was added (a pretty nice job structurally, but not to my aesthetic taste), balanced with a "powder horn" addition to the stem (sickmaking!), and some cockpit lockers.
    The cockpit lockers were installed with zinc plated screws, and by the time we bought her for the price of the trailer last year, there were a few soft spots in the ply of sides and bottom aft and the transom. In hindsight, a gallon of diesel and a match would have been best, but there's no fool like an old fool, and I thought we could make her a capable wee boat and we got stuck in with a grinder, and by the time we were finished there was nothing left aft of the cabin bulkhead. So the rocker came out of the bottom aft so she runs pretty nearly flat from the birdsmouth "V" to the transom. Maximum chine beam stayed at 42", but we ran the new chines out to a widened transom at 38", and since the gunwale beam stayed the same, the flare in the topsides was reduced aft. The "barrel" cabin was altered forward, the "powderhorn stem" was sawed off, a sole was added with buoyancy underneath, and new cockpit lockers were constructed with more buoyancy, less storage and stainless fastenings.
    I hoped to get an easily driven, seakindly hull capable of carrying four adults out at 10kt to catch Groper. The performance under power was fine, the stability was a worry.
    I can live with her tenderness, and when she rolled onto her beam ends during the sea trial I was both pleased and relieved that she didn't turn turtle - I wouldn't bet on the 14ft tinny I had previously coming back from that angle, but I won't feel comfortable taking her offshore until I am satisfied that
    A. She won't swamp in a hurry
    B. She won't turn turtle if she does
    C. She is bailable in the event that we take a greenie.
    That's quite a rave. A picture might indeed be better than a thousand words, but since I can't post one yet my dimensions will have to do.
     
  5. safewalrus
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    safewalrus Ancient Marriner

    Davflaws

    By your discription I'm not sure if your a fool or a brave man with a sense of humour, from your comments in the thread above I'd be inclined to actually state the later rather than the former (which I know makes a change for me, but I actually admire youe sense of humour - your comment on the gallon of diesel may be the right thing to have done at a guess but **** we gotta have a crack at insanity sometimes! :D

    Further the comments on 'beach' above, you could use sand if thats what you've got, the idea is to get a cheap environmental firendly ballast for your initial assessment! The main problem is that sand tends to fairly fine, gets in everywhere (thus is a bugger to get out, especially of mechanical things) so needs to be stowed in tight bags (plastic tends to be good but the usual plastic bag is very thin and a little bit of wet sand will bust the bag, so this bag needs to be in a stronger bag - more expense, with a plastic bag don't forget chafe as well!) so stones tends to be better but depends on your definition of 'stones' if you see what I mean! The other good thing with beach as opposed to gravel is the fact that beach stones tend to be smooth and round (chafe) whereas gravel tens to be angular - nasty sharp points!:cool:
     
  6. davflaws
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    davflaws Junior Member

    Stability test

    Hi Safewalrus
    I don't think I 'm a fool in relation to drowning. I hope to die in my bed (or perhaps someone elses) as a very old man. I ham getting to the age when I am sometimes forced to acknowledge I am not as physically capable as I used to be and foolishly imagine I still am.
    A wee while ago I posted about my dory. After much messing about inside my head and since I had lotsa lead slugs I decided on about fifty kilos of lead in her bottom hard out against the chines and then some static stabilty tests while safely on a mooring. I took the motor off and replaced it with a very big hunk of lead lashed to the transom to keep the weight distribution roughly the same.
    Yesterday was vaguely unpleasant, the sort of day when you go to sea to work or because you haven't been fishing for three months, rather than because its a nice day. We launched without trouble, and stuck the dory on the mooring. Marcus (boatbuilder) did the deed while I watched from the dinghy, ready to render assistance. We had her washboards in so the cabin was closed off.
    Big rocking and Marcus standing on the gunwale eventually put water over the coamings and heroic efforts after that gradually got her to about 110-120 degrees and then she slowly turned turtle and floated upside down. Marcus waited a minute. Then he grabbed the keel, stood on the gunwale belting, and she turned right side up again, a damned sight easier than she turned turtle. So Marcus climbed aboard and bailed her (perhaps 100 lit).
    I felt reasonably OK. The tinny I have been using for the last ten years will turn turtle a lot easier than that, and won't bail if it swamps.
    Then he did it again. After unsuccessful attempts to roll her with his weight right forward and then right aft he went back amidships. Eventually she rolled upside down again. This time we left her to settle a bit. Bad idea, but I'm glad we did it. She took a lot more water into the cabin, since washboards are not designed to keep water out while submerged. She floated a wee bit lower upside down. Now she wouldnt roll back. Very stable upside down. ****! Marcus couldn't right her and even when I jumped out of the dinghy and swam over to help it was still a real mission for the two of us to right her. When we eventually did, she was bailable - for a younger, (or at very least) lighter, fitter man than me, cos I couldn't get back into her from the water without rolling her again (since now she had not much stability at all). Marcus bailed her.About 300 litres this time.
    The additional ballast certainly helped her initial stability, but the fact that she turned turtle in extremis really worries me. Unless I can do something to make it harder to roll her right over, she is going to be a strictly sheltered water boat.
    Cheers (damply)
    DEF
     
  7. SamSam
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    SamSam Senior Member

    You tried it with the weight on the chines and didn't like it, now try it the other way. My intuition would be to put it all in the middle. Putting half on each chine would seem to sort of cancel itself out and just make the boat heavier and sluggish. Sam
     
  8. davflaws
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    davflaws Junior Member

    Thanks Samsam, there isn't actually anywhere to put ballast in the midline. The bottom is flat and there is a cockpit sole over it forming a false bottom and providing sealed in buoyancy. I shared your discomfort about weights in the chine cancelling each other out until I drew some diagrams and made some rough calculations. They don't cancel out. The placement of ballast that gives the best righting moment is at the chine (unless you have external ballast on a fin or proper keel).
    Cheers
    DEF
     
  9. davflaws
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    davflaws Junior Member

    Hi Guys
    Many adventures over the past eighteen months, so I thought I'd post and update anyone who is interested on progress.

    First considered some big trim tabs to provide dynamic stability under way. This never got off the drawing board, since we decided to bite the bullet and make a structural alteration. First we did a 'quick and dirty' trial by extending the chine from the powith sponsons made from packing grade polystyrene foam, building adhesive, and duct tape. Worked well. So set up for a major alteration.

    First sanded back to the original glass on topsides and chine edges of the bottom form max beam aft. Then extended the bottom with 9mm ply past the existing chines from nothing at the point of max beam to 4" per side at the transom, making the new chine beam 46" at the transom. Epoxy/glass tape joint.

    Then laid the 2" thick planks of High Density polystyrene foam on edge like a stack of cards or biscuits on top of the extended bottom, offering them, marking them, fairing them and epoxying them to the topsides and each other as the stack was built, then fairing the stack (which was taken away to nothing at the gunwale beltings). All quicker and easier said than done.

    The finish could have been painful, but the boatbuilder showed me something new. Troweled on a stiff bog from epoxy and microballons, faired off while wet and then laid 6oz glass and wet out with resin in the normal way.
    Vinylester bog and longboard, then primer, undercoat, paint.

    Then a further innovation (boatbuilder again). Rubbing strakes along the new wide chines, from nothing at max to add a further 2" at the transom. Cut from 2"x3/4" tanalised pine and stuck on with a "goop that is normally used for sealing and sticking aluminium windows. Spray rails from 3/4"x3/4" gooped on 12" above the chines from the stem to 24" aft of max beam.

    Sea trials in the harbour were impressive. 15hp engine gave 14.6kts (by GPS) with one crew and ususal gear, down to 9.8kts with three crew. Tenderness all gone, and a stable and comfortable fishing platform. The modifications have pushed the deadweight of the hull up to the point where she doesnt feel nice with 4 adults aboard. Not a matter of HP, more a matter of fundamental stability, in that she wallows and the righting moment is so small when she is heavily loaded that she feels ponderous. So I only take two when I go offshore.

    So far she has punched her way to windward in a 20kt breeze with the resulting sea on top of a 1.5m swell pretty comfortably and dry when trimmed by the lee and the stern and throttled back to 10-11kt. Rather like going to windward in a little keeler, but much faster. She goes fine in the tide races that are pretty much par for the course at the mouth of the harbour, but I have to throttle back to 8kt when the wind is against the tide.

    I'll try to attach some photos.

    DEF
     

    Attached Files:

  10. safewalrus
    Joined: Feb 2005
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    safewalrus Ancient Marriner

    Looks like you've rebuilt the whole boat - your bungalow is now a house kinda thing! Whilst many will comment to me the important thing is the fact that you've taken your time, experimented and am now reasonably happy with your creation! whilst not brilliant (in shiny glossy hi tech boatbuilders speak) she works! Stay with it! well done for the trials - just one thing, so now your with the chop and change sort of thing, but please do know when to stop!
     
  11. davflaws
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    davflaws Junior Member

    Thanks for your comments Safewalrus. There will be no more changes to the Dory! I'll live with what I've got, and keep reminding myself that my boat for adventuring a wee way off shore is sorted. Now I am trying to sort the hp requirements for a 3.4m polyethylene dinghy to launch off the beach for easy evening trips in sheltered water over a 2-3nm radius to catch a fish after work. (separate thread)
     
  12. SamSam
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    SamSam Senior Member

    Thanks for the update, it's good to see solutions to problems. Now that you've experienced the before and after, do you have ideas on what the next would be as far as to what you originally wanted?
     
  13. Loveofsea
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    Loveofsea New Member

    I designed and built a 19ft modified dory and i routienly take it a 100nm offshore. There are things you can do to a dory to make them work exceptionally well in rough seas. I have logged over 90,000nm open sea miles in two modified dorys since '84 and i have pretty much refined the hull configuration as far as practical for my needs. I do underwater photography as a hobby and rarely take a trip less than 75nm from port. i always spend at least 2 nights per trip so getting home on that third morning can be somewhat of a guessing game, weather wise. I needed a boat that could get me home regardlless of the weather, that's why i chose the flattbottom dory style hull with a tiller engine.

    The skiff was featured in a web article here:

    http://www.oceanskiffjournal.com/Su...02Issue01/General/GoodSkiff/boatprofile1.aspx

    One of the biggest mistakes people make with a flatbottom is to draw the bow to a 'V'. A flatbottom requires exagerated transom lift and if you have a 'v' in the bow, it causes bow-steering. A flat entry is the only way to maintain stability in seas. Weight distribution is another important factor. For that flat entry to work, i hold all of my fuel (54gals) in the fwd 1/3 of the hull. Flatbottoms are a great hull configuration for running the seas, but you have to do it right.

    Out of time for the moment...
     
  14. ATCSchaefdog
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    ATCSchaefdog Junior Member

    Dory

    You have a really tough problem here. I just got back from a trip with a 16' Dory. I found her to be great....when loaded. I went into VERY heavy weather with a fully loaded boat and she was a rock. I know you have found this answer often times thus far...sorry.:(

    They also used a dory type boat on the rivers of maine for log drives because they were so good loaded.

    Goodluck.
     

  15. davflaws
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    davflaws Junior Member

    I won't go any further with the dory I've got. It does everything I set out to do (10-15nm offshore and confident of being able to come home if it turns to custard) except that I can only take two rather than the four I wanted to accommodate. I should have done the "gallon of diesel and a match" referrred to earlier in the thread when I found the rot originally. If I were to do that project again I would have taken the chine beam out to its present 46" in the original rebuild, and kept the construction a great deal lighter.

    The dory project was a marital compromise from the beginning. I set out to build a John Welsford Clarence River Dory before my wife vetoed the idea, and I still have the plans. Since my wife sailed here from LA and is basically a sailboat girl, I suspect that we will eventually wind up with a motorsailer, but at present she is not up for that committment in $s and time, and meantime I wanted (and now have) a craft that I can go offshore and fish and adventure in at very low cost (15hp fuel economy). The Welsford Dory would have been cheaper in $s and time, and would do the job better - but 20/20 hindsight is a wonderful thing.
     
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