possible to decrease rolling motion?

Discussion in 'Stability' started by kapnD, Sep 25, 2017.

  1. kapnD
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    kapnD Senior Member

    IMG_0215.JPG IMG_1501.JPG IMG_1502.JPG
    My new to me boat is an ex navy utility boat, she's 50 feet, looks like an older lobster boat design with a 2/3 keel and rounded chine, so she has a pretty good amount of roll in quartering seas.
    I have added a house, taking care to keep it light, and would like to add a small flying bridge on top of that, but am concerned that it may contribute to more roll.
    The engine and 200 gallons of fuel are located curiously high, at floor level, and there is at least 18" under the motor, so my question is, if I were to move the motor back and down (shortening the driveshaft) and the tanks down and center (where the motor is now, and lower), would this help with the rolling, and offset addition of the upper station?
    It baffles me that it was built like that, unless the navy had some silly spec requirement that would keep the motor running with the boat full of water.
    I don't think fore/aft trim will be a problem, I've been experimenting with drums of water, and a ton at the transom only give up a couple of inches waterline.
    Any input welcome, this is my pet project, and I am determined to make an awesome boat out of it!
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Is it the period or amplitude of the roll you dislike ? Dropping everything lower ( sounds like a big job) will make it roll on a snappier shorter cycle, and less inclination, presumably.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    kapnD

    There are several issues here.
    Lowering the KG will help, always does. What this does, is to increase the GM, the metacentric height. However, the period of roll for a vessel is simply proportional to the 1/(GM) to the power 1/2.
    So to this into simple numbers for you..if the GM is now say 0.25m, the period of roll would be in the order of 2 seconds. If the KG is lowered so the GM increases to say 0.5m, the period of roll woudl be 1.4 seconds.

    Lowering the KG, increasing the GM, makes your boat more stable, but the motions will be more snappy. As it is said to become "stiffer".

    So, your original question relates to the stability in terms of will she roll over and capsize, and will the rolling make me sea sick.

    Without knowing any details for your boat, lowering the KG is always good, makes it safe, since a larger GM means you have more ability to right the vessel, than if the GM is low, with a high KG. But if you then add a flybridge, if the amount of weight you're adding is roughly the same or less than the movement of the weights you noted, then it may well be equalled out. In other words, you may notice no major difference. But, given the amount of relative movement available, and the fact that fuel is a consumable, ie it is burnt off and thus losing weight low down, I strongly suspect you may not like the difference once the fuel tank is near empty. It could be a whole lot worse!

    You can counter this to some extent by adding bilge keels.

    So, in short....moving the engine and fuel lower down helps...but it may not be the total panacea you are after, if your fly bridge is high up and heavy..
     
  4. kapnD
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    kapnD Senior Member

    Mr Efficiency, Ad Hoc,
    Thanks for the speedy replies!
    The roll is noticeable, but not quick, or snappy as on the similarly dimensioned planing hulls I have previously had.
    I did not intend to imply that the roll seemed dangerous, just a somewhat uncomfortable to some passengers.
    This hull fits somewhere in that nebulous land of semi somethingorother, not slow, not fast, but very efficient.
    I had no idea that lowering the weight in the hull would make the roll snappier, but, thanks to your explanation, it does make sense.
    I think the keel may be weighted already, possibly influencing what I perceive as enhanced roll recovery or snap.
    Moving the motor and tanks around is not that big a job, as this boat is a mostly empty shell, it's original form being very utilitarian. The navy, in fact, officially calls it a "utility boat".
    I'm just trying to personalize it to my needs and comfort.
    I have looked at similar boats that have been fitted with bilge keels, but nobody seems to have any solid "before and after " data to show how much it helped, if at all.
    I think that extending the keel back a few feet might prove to be a more "streamlined" means of roll reduction, and a big gain in prop protection.
    I have included a photo to show the current keel configuration.
     

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  5. TANSL
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    TANSL Senior Member

    Balance keels are a very effective system to increase the balance period. They increase the work required to tilt the boat because of the mass of water associated with the hull, which increases, and bbecause part of the energy of the movement is used to create eddies around the keels.
    Extending the keel back a few feet will have similar effect to bilge keels.
    Try to lengthen the keel, which is simpler, and if it is not enough, try to lower some weights.
    Have a look on this thread : How to decrease the inclining oscillation and increase the stability on the boat https://www.boatdesign.net/threads/how-to-decrease-the-inclining-oscillation-and-increase-the-stability-on-the-boat.58026/page-7#post-813798
    PM @Nickname123, he has practical experience in this matter.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    It doesn't appear to be a very heavy boat, looking at the pics, and the waterline mark. I don't follow why the engine and fuel tank would be elevated.
     
  7. kapnD
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    kapnD Senior Member

    Yes, the boat is light, possibly lighter than the designer intended.
    I have not been able to get any technical design information on the vessel, other than the rating plate, which simply says " 20,000 lbs dry weight, max 150 persons".
    The cabin I built, full fuel/oil, beer, ice, crew, fishing gear etc. adds up to about the weight I removed from the boat in cutting down the cockpit sheer and built in seating to make room in the cockpit for fishing gear and fish holds.
    The line of the bottom paint in the stern is about 6" above actual waterline at rest, that was a seat of the pants guesstimate. The transom is barely submerged, and as mentioned before, gains buoyancy rapidly as it is loaded.
    Tansl,
    I agree with your comment on extending the keel, seems that would be much more satisfactory than bilge keels. I don't understand why it was built that way (short), other than the propeller definitely runs in clean water.
    The thread you referenced is not a vey good comparison to the situation I have, but was an education for me in number crunching! Thank you.
     
  8. TANSL
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    TANSL Senior Member

    @kapnD, the thread itself does not say much. The real fact is that the OP, after all this discussion, placed bilge keels on his boat and the result was satisfactory. But it must be said that the stability of the boat continued to be very poor.
     
  9. kapnD
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    kapnD Senior Member

    Yes, the OP failed to perform his due diligence and paid to be screwed by his engineer.
    This world is overflowing with unscrupulous nincompoops!

    Despite all the quibbling and banter on this site, there is much good information available, and I thank the posters for sharing freely on a public forum.
     
  10. PAR
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    PAR Yacht Designer/Builder

    The proposed flybridge will likely slow down your roll quite a bit, much like a heavier mast does on a sailboat. The roll period will increase and the motion slows. I don't think lengthening the skeg will do much, though it should help to a degree, as would runners or bilge keels, both of which will also add some drag. If it is lengthened, taper it down as much as practical to get reasonable flow to the prop. On this aspect, I'd consider runners, first, rather than lengthening the skeg. These can be run nearly perpendicular to the hull shell, making them quite effective for their area.

    I agree with John, in that lowering the tank(s) and engine will help, though it's possible the motion may be quicker.
     
  11. Blueknarr
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    Blueknarr Senior Member

    The engine and 200 gallons of fuel are located curiously high, at floor level, and there is at least 18" under the motor,
    ...

    It baffles me that it was built like that, unless the navy had some silly spec requirement that would keep the motor running with the boat full of water.

    I was once partners on a former minesweeper. Its engines were quite elevated as well. Presumably, to avoid detonating magnetically triggered mines.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Needless to say, the perception that a boat is rolling "excessively", does have a significant subjective element. Therefore, for more clarity, a smartphone "app" is needed that can gauge the degree of inclination, and the time period. Some nerd will do it, if they haven't already.
     
  13. PAR
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    PAR Yacht Designer/Builder

    Roll timing can be done at the dock and I think you're correct, with there being an app available for it.
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    It is often very hard to second guess why such things are on boats, especially old ones. Until one understands that every boat is designed for a specific task or duty. Understand THAT and it'll help. Comparing two boats of similar length size side by side, is often useless in terms of understanding too. Since it is like comparing a Ford Mondeo with a Ferrari 458, simply because they are the same length width and have 4 wheels. Yet the reality is totally different SOR, or objectives.

    However, having said that, adding length in a keelson:
    i) adds drag
    ii) adds weight
    iii) makes the boat TOO directionally stable - ie hard to steer

    these are just the main ones.

    It is a nice looking boat you have, treat her well and modify it professionally, not bodge it, and she'll give you many years of good service.
     

  15. kapnD
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    kapnD Senior Member

    PAR,
    Thanks, I was at first inclined to address the roll issue before adding an upper station, but may switch priorities back to the flying bridge in hopes that it will act as a slowdown mechanism. Grill cooler combo.jpg

    Ad Hoc,
    When moving ahead, she's pretty nimble, seems the shortened keel moves the pivot point forward, but astern movement suffers severely from prop walk.
    Backing into my slip is dicey, and more than once, I've had to do the dreaded "circle of shame", even though I am no stranger to single engine boat handling.
    Thanks for the kind words, she is indeed a keeper!
    Here's a picture of one of the high end amenities I hope to have aboard soon. The coals are courtesy of the nice Oak seat, you might call it "pre-seasoned" Oak.
     
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