John Winters refers to a flatness ratio as the displacement (as a volume)/(.01*Length)^3 This is closely related to the concept of displacement length ratio differing only by a constant multiplier. Does anyone know if this is just an invention of John's or is the fatness ratio actually an accepted nautical measurement term. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Robert Livingston <bearboat2_at_comcast.net> asked: >>>>>John Winters refers to a flatness ratio as the displacement (as a volume)/(.01*Length)^3 This is closely related to the concept of displacement length ratio differing only by a constant multiplier. Does anyone know if this is just an invention of John's or is the fatness ratio actually an accepted nautical measurement term.<<<<< I think the constant multiplier is .1 (rather than the .01 you used) which after being cubed in the divisor becomes a multiplier of 1000. Therefore the "Fatness ratio" could be defined as 1000 times the "Volumetric Coefficient" (which is a non-dimensional version of the displacement-length ratio). I think John probably made up the term "Fatness Ratio". Multiplying the "Volumetric Coefficient" by 1000 makes it a number without several zeros after a decimal point (which makes it easier to think about). The Volumetric Coefficient is a very important ratio in regards the drag performance of a hull. It is one of the most major ones we take into account in determining the drag calculations for Sea Kayaker. If you take the total displacement (the weight of the boat and its contents) and convert it into volume by dividing that weight by the weight of the volume of sea water in a cubic unit (that you are measuring length in) and then divide that by the waterline length cubed you get the Volumetric Coefficient. (It will be the same what ever units you choose to measure in.) For instance, using feet as the unit we know that a cubic foot of sea water weighs 64 pounds so by dividing the total weight of the boat in pounds (its displacement) by 64 we get its volume in cubic feet. Dividing that volume by the waterline length in feet to the 3rd power (making it into a big cube of the same units) we get the volumetric coefficient (which is the ratio between the volumes of those respective cubes). Multiplied by 1000 and kayaks will likely fall into a range of .9 to 3.0 with a few extreme exceptions. Most fall between 1.0 and 2.0. Of the first 82 kayaks measured by Sea Kayaker magazine (with 250 pounds of paddler and gear weight added) the average "Fatness ratio" was 1.42, the median was 1.32, and the highest was 4.27 (Dawn Trakker--small rec. kayak) and the lowest was .77 (Futura II--surfski). Matt Broze www.marinerkayaks.com *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
> Therefore the > "Fatness ratio" could be defined as 1000 times the "Volumetric Coefficient" > (which is a non-dimensional version of the displacement-length ratio). I am surprised how little I can find on the Web about Volumetric Coefficient relevant to boats. >If you take the total displacement (the weight of the boat and its contents) >and convert it into volume by dividing that weight by the weight of the >volume of sea water in a cubic unit (that you are measuring length in) and >then divide that by the waterline length cubed you get the Volumetric >Coefficient. Is this part of the formal definition that SALT water density be used. And for that matter is there an official density of SALT WATER that everyone uses? (Baltic Sea a lot less salty than other places for example) Does Volumetric Coefficient have an OFFICIAL definition? >I did not invent it. The Society of Naval Architects and Marine Enginers >proposed a standardized presentation format for model resistance in >"Explanatory Notes for Resistance and Propulsion Data" which includes the >fatness ratio (Volume displacement/(0.10L)^3 . They also include the >displacement/length ratio (They call it the fineness ratio - go figure) for >North Americans more familiar with the dimensional term. >Because I live in Canada where we use metric units but design for US >builders etc. I prefer the non-dimensional terms. Thanks John. I certainly share your preference for non-dimensional terms. But on the other hand I prefer fine to fat :) Now is this fatness ratio -- is it linked to SALT water density *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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