Has Weston Farmer's theory of "comfort" been confirmed?

Discussion in 'Boat Design' started by John Riddle, Dec 9, 2010.

  1. John Riddle
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    John Riddle Junior Member

    With regard to what he referred to as a "medium-sized" hull's motion at sea, and its effect on the comfort of those aboard, he offered the theory that displacement should equal 64 lbs. per square foot of waterplane - too much lighter results in a "corky" motion and too much heavier can be too sluggishly out of sync with wave patterns. He presented that as a theory based on practical experience but without extensive data to back it up.

    Has that principle been proven to be a reliable indicator of a comfortable motion? Is it a factor that has a significant influence in creating a design, either as a target weight or as a measure of compromise when other factors (like speed) are the priority?

    I'm asking because I have been involved with a client who has a 11.4' x 36.5' (waterplane), round-bilged, planing lobster boat (>25 kts), loaded at just 55 lbs that rolls too quickly in a beam sea. I recently came across Weston Farmer's theory and wonder how much of a consideration it would be in designing a new, improved lobster boat of the same size that needs to have an easier motion.

    Any thoughts?
  2. CatBuilder

    CatBuilder Previous Member

    John, just a thought... since you are talking about a lobster boat, they are designed to be at their best with a heavy load of traps or a catch aboard. They will be built to have the proper roll period in a loaded condition for safety and comfort.

    I think the motion with no load is given less consideration, since it's a work boat and to keep it safe, it should be designed to be at its best when loaded. Lobster guys are kind of tough, usually. They can take a fast roll motion unloaded, but spend very little time in the boat that way. It's usually loaded down completely with traps or a catch, therefore, increasing its overall mass and slowing its roll period.
  3. mark775

    mark775 Guest

    Wow - Cat is spot on.
  4. PAR
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    PAR Yacht Designer/Builder

    None of these "comfort" formulas really work. It would be nice it it did, but they don't, at least not on all hull forms and configurations. Ted Brewer's motion comfort ratio (Disp / (2/3*((7/10 * LWL)+(1/3 *LOA))*Beam4/3 )) is another and fairly well accepted as a way to compare similar yachts. I'd do an roll and incline test on the boat and see how she shook out, before making any rash decisions about ballast or something.
  5. wet feet
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    wet feet Senior Member

    The second post is absolutely correct,a working lobster boat will be designed to work efficiently as a lobster boat.My recollection of the Weston Farmer article is that he was describing the level of comfort of a displacement Elco cruiser (a 34?) and this would be a very different boat to a planing lobster boat,with different priorities for the designer to consider.
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    The 64 lbs per sq ft looks pretty meaningless to me without reference to hull shape and size, height of COG, whether at rest or underway......
  7. Ad Hoc
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    Ad Hoc Naval Architect


    Not to mention...what sea state; what kind of spectrum? One of the key issues is not even mentioned, wave period....so without this, can't gleen much from very very rough rules of thumb. Farr too wolly to be used as a "measure"

  8. Alfonso
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    Alfonso Junior Member

    Right! Trying o get something with such rough aproximations without knowing a sea state, i.e. wave spectrum is just time consuming and You will probably get nothing. But You can try to find some articles with regression coef. for seakeeping of fishing vessels and try to use them....

    You determine wave spectrum for sailing area, then RAO-s using regression....
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