Can a sharpie-inspired trawler's lively movement be tamed?

Discussion in 'Hydrodynamics and Aerodynamics' started by Kit_L, Dec 25, 2019.

  1. Kit_L
    Joined: Nov 2016
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    Location: Greenwell Point, Australia

    Kit_L Junior Member

    Seabiscuit (40', 13' beam, 4' draft, 15 tons) is 'tender' or 'lively' in confused seas. The flat aft sections are trying to conform to the changing surfaces, and the net effect is very rapid back-and-forth rolling on her longitudinal axis.

    On a trip back from Brisbane to Greenwell Point, about 35nm offshore (to catch the East Australian Current) we had a small Easterly swell (~1 metre); and a 1–1.5 m Southerly swell. The movement was so fast that crossing the cabin sole was difficult, at times. Rough is a good description.

    The big question is this: can this kind of hull be effectively stabilised by paravanes? Or is the most cost effective solution to get another boat?

    If this question is posted in the wrong sub-forum, can a moderator move it, please?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    So what is it, a converted trawler ? A fast rolling motion suggests a low centre of gravity, which would be the case if all the ironmongery of trawling was removed. If this was your first experience with the boat, it might be you just struck a sea state that just happened to amplify the motion. But if it regularly behaves like that, it would seem the best angle might be bilge keels to dampen it.
     
  3. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    If your partner doesn't like the motion of the boat, bilge keels are not going to change it enough to enjoy it.
    I'm not sure it's the most "cost effective" but it may be the best value for your money to get another boat.
     
    Last edited: Dec 25, 2019
  4. bajansailor
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    bajansailor Marine Surveyor

    Could you perhaps post a photo or two of your vessel please Kit?
    If you want to keep the boat, and you are not so keen on paravanes, would it be feasible to perhaps add a small steadying sail rig?
     
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  5. fallguy
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    fallguy Senior Member

    Decisions cannot be made unless the value of the boat is understood first. If the value of the boat is $250,000, then an expense of $40,000 is only 15% of the cost and you could even add a gyro. The same is true of the paravanes. You really need to start off with more details to make an informed decision. If the boat is under a value of $100k, the gyro becomes too much a portion of cost to justify, etc.

    Anyone can give you ideas, but without an understanding of the real economics, it makes it more difficult. I am not your advisor, just lurking.
     
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  6. Kit_L
    Joined: Nov 2016
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    Location: Greenwell Point, Australia

    Kit_L Junior Member

    Details of weight and length, etc., in first post.

    Designed as a Bass Strait cray boat, the original builder extended the wheelhouse, put a Seawasp generator where the fish well otherwise would have been, and two 600 litre fuel tanks in the aft section. So, apart from the Perkins 6.3544, in the engine compartment ahead of the fuel tanks, no real weight amidships.

    Thanks everyone for your comments; great to wake up to! And Merry Christmas to you all, too (we get Xmas before you).

    I think fallguy's comments are spot on: we have about $100,000 AUD in Sb now, and the cost of a gyro big enough to calm her down would be 40% of the boat.

    There is zero rigging for a stay sail (and everything I have read suggests that these doe not work that well) and no one in our local area can fit bilge keels (and their efficiency in this hull shape is not certain, either).

    I am not worried about the rapid movement; it only happens when you have two swells, but my partner is. My concern re. fitting paravanes (which is possible) is that Sb's movements are rapid—and this would require some serious engineering for strength. My understanding is that paravanes work better on slower roll periods, like many of the round-bilge fishing boats we have around here, not these flat Sharpie-type hulls.

    Selling her and finding another boat might be the least expensive solution, I am thinking.
     

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  7. Kit_L
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    Kit_L Junior Member

    And Mr Efficiency's comments I think are spot on, too: if Sb had a heavy mast, or rigging above decks, then the motion should be slowed. I have heard of boats having had paravane gear installed and the motion is calmed to some extent just by extending the poles, without deploying the fish. Still thinking on this.
     
  8. fallguy
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    fallguy Senior Member

    The picture of her stern and high headroom cabin tells the entire story to me. I think the trouble you might encounter is this boat is always going to be that way and you won't want to run the vanes in shallower water and in shallower water, she'd likely do worse. You can certainly deploy fish, but it really seems like it should not be needed in one meter seas and I have a feeling you'll find them a bigger hassle than changing boats. I am not wise enough to speak to the issue, but another option would be lead placement onto the hull edges. Perhaps I will trip someone into the discussion. I have no idea how she runs against her designed waterline now, but you might have already been overbuilt and I would avoid drowning her further if she is already under.
     
  9. Kit_L
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    Kit_L Junior Member

    It was the approx. 90° intersection of a 1m and a 1.5m swell; as they interacted, much higher, but transient, waves were encountered, and this is where it moves the fastest. In a single swell with a modest chop on top, she's very comfortable.

    My partner and I are not averse to changing boats, and the big picture is it's all a valuable learning experience.

    Re. lead placement: are you talking about fixing lead bars (say) up under the deck where it meets the hull, to have mass as far away from the roll centre as possible?
     
  10. bajansailor
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    bajansailor Marine Surveyor

    If you do decide to keep this boat - you note that "There is zero rigging for a stay sail (and everything I have read suggests that these doe not work that well)"
    But would it be feasible to retrofit a mast with some rigging to create a fairly basic motor sailer rig?
    I think you would be surprised by how effective it would be at reducing the rolling on all headings apart from dead upwind or downwind.
    This Fales 38 has a relatively large rig - I don't think you would need anything as substantial as this, as you would then need to have additional ballast on the keel to compensate.
    For Sale: 1976 Fales Navigator 38 - Trawler Forum http://www.trawlerforum.com/forums/s69/1976-fales-navigator-38-a-7931.html

    So perhaps something a bit smaller, more like a Finnsailer 35 rig?
    SailboatData.com - FINNSAILER 35 Sailboat https://sailboatdata.com/sailboat/finnsailer-35
     
  11. fallguy
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    fallguy Senior Member

    I believe I have read where lead is placed near the waterlines athwartships, but I have zero knowledge on the subject. I believe many trawlers have used lead to dampen roll. It is very counterintuitive for me, so I am fairly useless, but it has to do with energy. The boat requires energy to roll and the lead, I believe requires more energy and there isn't sufficient energy, but if the boat is constantly getting such energy, then lead might only help at anchor. I simply do not know enough about the approach. There might also be something happening with centers of mass changing using lead that affects centers, well beyond my paygrade. Just enough to mention to prime the pump here.

    Bilge keels are another low cost solution. I am a member of a facebook forum where this gets a lot of discussion is all. Bilge keels might actually be something worth at least asking about on this forum and it is even possible to get something like them drawn up. Let's say you have 30k invested into the boat that you don't feel is recoverable or would be lost investment at resale. If bilge keels could be done for say $3-5k, then you might be in business. Again, I just don't know if these help enough underway, but once we get away from the holiday, perhaps someone else might discuss it more.
    Bilge keel - Wikipedia https://en.wikipedia.org/wiki/Bilge_keel
     
  12. gonzo
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    gonzo Senior Member

    A steadying sail would work, without having to drag anything in the water or increase draft.
     
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  13. Kit_L
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    Kit_L Junior Member

    We feel we have spent enough on her (sound familiar?), and we are getting her ready for sale presently. The utility of the single main cabin, and the very cramped engine room, and her lively movements all point to a new vessel. Thanks to everyone who contributed, and best wishes for the New Year.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Better make it a bigger boat, I would say, a displacement motor boat of 40 feet is naturally going to be subject to "lively" movements, on the open sea.
     

  15. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell "Whatever..."

    You're welcome.
    Good luck with the new vessel.
     
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