Motion Comfort Ratio for Power Vessels

Discussion in 'Stability' started by makobuilders, Sep 5, 2015.

  1. makobuilders
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    makobuilders Member

    How applicable is Brewer's MCR for power boats, as an indication of ease of motion? My trawler is computing at over 100 points (with a roll period of 4.6 seconds), which seems ridiculously high compared to most of the heavy sailboats which seem to be in the 40-50 range.
     
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Ted Brewer, I think, intended MCR to be applied to sailing yachts, but technically, the factors that go into the MCR equation are indifferent to the type of boat: displacement, length on the waterline, length overall, and beam. So the equation works no matter what type of boat you are analyzing. The subjectivity comes in the divisions between "Greater," "Average," or "Lesser" comfort--these are definitely defined for sailing yachts, in my opinion. Certainly, the MCR could be applied to motoryachts and trawlers which necessarily will show higher on the scale of MCR because they generally have less displacement and are usually wider than sailing yachts, and they don't have ballast keels and masts. So the standard definitions probably don't apply, and your definition of what are acceptable ranges for life aboard a trawler have to change.

    For example, you may rate an MCR of 100+ on your boat, but do you find it comfortable in a seaway, or not? Certainly, it is true that the accelerations of motion on a motoryacht or trawler are going to be higher (= higher MCR) than for a sailing yacht,. So in absolute terms, the MCR reveals the differences between power and sail boats. I do not know of any database of collected MCRs for motoryachts and trawlers. You'll have to figure that out on your own based on comparisons of various types of boats and any experience you may have on their behavior. Ted Brewer, I think, probably had a pretty good idea of behavior of motion on sailing yachts designs since that is what he specialized in. But with respect to motoryachts and trawlers, he probably left that up to others, such as yourself, so define.

    I wrote a fairly complete discussion of MCR in my booklet "The Design Ratios" which was first published on this forum about 5 years ago. The discussion of MCR is in Chapter 8 starting on page 28. I upload a copy of it for you here.

    I hope that helps.

    Eric
     

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  3. Rurudyne
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    Rurudyne Senior Member

    Btw, Thank you again for that essay....
     
  4. makobuilders
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    makobuilders Member

    Yes, thank you for your essay. I notice that when reviewing the various displacement speed predicting formulas that I cannot find any correlation to the charts developed by Robert Beebe.

    On my previous boat I found Beebe's charts to be relatively accurate. However, iIn this case, Gerr's formulas are yielding an answer for me of V/L = 1.18 yet Beebe's computes to 1.28. A huge difference.
     
  5. Eric Sponberg
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    Eric Sponberg Senior Member

    I don't hold a lot of high regard for Bob Beebe's numerical work. For example, his A/B ratio is nonsense, and for speed-power prediction, he does not define his Factors F1 and F2. Although if I took the time to analyze them fully, I could probably figure them out, but he should have been more forthcoming with their definitions and meanings. To do so was not worthwhile in my write-up of The Design Ratios.

    This is not to say that Bob Beebe's work is worthless, it's not. His book is full of practical advice about voyaging powerboats.

    A difference in speed-length ratio of 1.18 vs. 1.28 is not really that great--I would not call it a "huge difference"--in the overall schemes of naval architecture, considering that there are many hull factors that influence speed-length ratio.

    Eric
     

  6. makobuilders
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    makobuilders Member

    Good point. Beebe's formulas were probably developed around his hull form - a long, lean "sailboat" type hull. Perhaps that's why is was accurate for my last boat of L/B of 4.1.

    My current design however is a beamy trawler of L/B 3.0 which obviously has a different powering profile.

    Thanks for the input.
     
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