Monday, June 8, 2015

A 37' LNVT's Roll Period: Theory vs Reality

Dave Gerr in The Nature of Boats, discusses a heuristic that states a boat's most comfortable roll period, i.e. the time it takes to roll from one side to the other and back again, can be determined by dividing the maximum waterline beam (in feet) by the constant 3.28.   A boat that rolls faster than the ideal is called "stiff". A slower rolling boat is called "tender".   For a 37' LNVT the ideal roll period is 12' / 3.28 = 3.7 seconds.

When tested, Brave Duck 37VT67 and Victorious 37VT26 both had a roll period of 2.5 seconds.   Using the definitions above, the tugs are a little stiff.

The roll period can be extended by several means.  One method is to add weight high on the tug.  As an experiment, Brave Duck put about 500 lbs on the dinghy deck and found this extended the roll period to 2.8 seconds.  Another method is to add bilge keels.   Knock Off 37VT66's bilge keels add 14 sq-ft of roll resistance.   As can be seen in this video (https://www.youtube.com/watch?v=xFlk8Qst-aI) her roll period is 3.0 seconds.


Next to be tested is the effect a riding sail on an LNVT's roll period.  


References:
1.  Gerr, Dave, The Nature of Boats, September 1995
http://www.amazon.com/The-Nature-Boats-Esoterica-Nautically/dp/007024233X
2.  Using the Rolling-Period Test to Estimate Stability in Small Fishing Vessels
http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/5573/SR%20no.%20779_ocr.pdf?sequence=1 


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5 comments:

  1. LNVT Owners AssociationJune 10, 2015 at 8:27 AM

    From Sally W: We did the experiment and found the roll period to be 2.58. You would have howled at our method. Allan resistant because it wasn’t his idea - ha ha. I, on the other hand, always interested in learning more about these tug creatures.

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  2. LNVT Owners AssociationJune 13, 2015 at 9:13 AM

    Roll tested Victory 37VT02 this morning. She's 3 seconds. Why longer than the 2.5 seconds of the rest of the fleet? Bob said there's a bunch of pig iron outboard and in the engine room (close to where Jim Backus called for it to be). Perhaps Tommy put it in the called for location and then discontinued the practice. I'll get a better look at the ballast today. Standby....

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  3. LNVT Owners AssociationJune 13, 2015 at 1:57 PM

    The ballast in Victory 37VT02's engine room is made up of metal punchings--like from a punching press. Outboard from each side of the engine, under the battery storage shelves, in the compartment closest to the hull, is where the punchings are found. The areas are literally overflowing with them. Given the weight of steel (490 lb/ft3), there could easily be 500lb of punchings on each side of the hull. And, that would slow Victory's roll rate.

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  4. Bicki and Dave HowellAugust 9, 2015 at 9:50 AM

    After talking with the owners of hulls #2, 3 and 4, and Tommy Chen, I believe all the tugs were built with the ballast in the keel.

    ReplyDelete
  5. Bicki and Dave HowellAugust 9, 2015 at 9:51 AM

    This comment has been removed by the author.

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