Robert wrote: > In Derek Hutchinson's "complete book of Sea Kayaking", > he describes different hull designs, and their characteristics. > On the subject of hard chined hull shapes, the description has > me a bit puzzled, because, it is contrary to what a lot of > kayak owners say about their hard chined kayaks. > (From Derek Hutchinsons book "Complete book of Sea Kayaking") > "The V-shaped or chine hull, the traditional shape of many > Eskimo kayaks, gives directional stability as long as the amount > of rocker is not excessive. However, with such a hull it is > unfortunately almost impossible to maintain fast forward speeds > because as the speed increases the boat tends to plane on the > flat chines. The kayak thus retards itself on its own bow wave". First I strongly recommend you tear out Derek's pages on boat design. He has packed an enormous amount of inaccurate nformation in a few pages. Next you need a useful definition of planing. Most naval architects use Savitsky's definition who defines it as the point at which wave making resistance begins to diminish with increasing speed. I am not sure if this definition is published some where. Mr Savitsky gave it to me at a meeting of the Society of Naval Architects and Marine Engineers. Why do planing boats achieve such high speeds? They displace so little water and generally have lower wetted surface. Do not confuse a change in trim as planing. Planing is a function of the vertical rise of the center of gravity not a change in trim. Many people think they are planing or approaching planing when the trim changes. Sorry, just ain't so. As the dynamic forces increase the CG rises vertically out of the water. This requires a lot of horsepower since initially the boat sinks due to the increase in kinetic energy across the bottom. Experts argue over the onset of semi-planing. Purists say it begins once the CG rises above the static position. This definition has the virtue of precision but probably overly restricts the advertising department's freedom. Do not confuse surfing with planing. A surfing boat makes use of the power in the wave to reach higher speeds. A planing boat can plane without help from waves. This may seem like a trivial matter but it is important to people who design planing boats. Do not make the mistake of thinking that the boat climbs the bow wave or is obstructed by the bow wave. The boat makes the bow wave and the bow wave's size indicates the energy expended in its formation. Thus a large bow wave indicates large resistance but it does not cause more resistance. This problem with cause and effect often gets confusing. Despite many claims no one has ever proven that they can plane a boat using paddles. The test is simple. Set up a surveyor's level on shore aimed at a vertical measuring tape affixed over the CG. Now observe the change in the CG's vertical position at top speed. If the boat has risen you can then calculate the displacement and, from that, calculate the resistance using KAPER (or other formulas) to see if the wave making resistance has diminished. This is not precise but if you detect a significant change you can then use more sophisticated methods to confirm it (I.E. tank testing using pulsed power). In my research I could find very little difference between round bilged and chined boats so long as all other parameters were kept equal (hard to do). Most hard chined boats suffer form higher wetted surface and lower prismatic coefficients (less efficiency at higher speeds) which result from the restrictions imposed by the hard chine configuration. I did discover that chine immersion in the fore body caused an increase in resistance so it seems to pay to keep chines nicely sloped and high forward. People should not draw too many conclusions from limited tests of a type unless you know that the boats have significant similarities in form coefficients. For displacement hulls naval architects generally prefer round bilge boats for lowest resistance and will not use chines unless they have a specific reason for doing so like ease of constitution, spray shedding, etc. etc. Michael mentioned Marchaj's comments on sailing canoes. Unlike kayaks they are true planing hulls and canoes only by virtue of having a more or less rounded stern. They use chines aft to promote planing. *************************************************************************** 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/ ***************************************************************************
At 10:05 AM 1/16/02 -0500, John Winters wrote: >Robert wrote: > > >First I strongly recommend you tear out Derek's pages on boat design. He has >packed an enormous amount of inaccurate nformation in a few pages. I realize that it's fashionable to bash Derek, but for someone that wrote a lot of "inaccurate" information about boat design, he sure has designed a lot of really nice kayaks. I know that a lot of people find Derek strongly opinionated but in the class that I took with him, and the time my wife and I spend with him just chatting I found him to be quite charming. Speaking of Derek's boat designs, has anyone paddled the new Current Designs Andromeda or the poly version, Sirocco? *************************************************************************** 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/ ***************************************************************************
At 10:49 AM -0500 1/16/02, John Fereira wrote: >At 10:05 AM 1/16/02 -0500, John Winters wrote: >>Robert wrote: >> >> >>First I strongly recommend you tear out Derek's pages on boat design. He has >>packed an enormous amount of inaccurate nformation in a few pages. > >I realize that it's fashionable to bash Derek, but for someone that >wrote a lot of "inaccurate" information about boat design, he sure >has designed a lot of really nice kayaks. I know that a lot of >people find Derek strongly opinionated but in the class that I took >with him, and the time my wife and I spend with him just chatting I >found him to be quite charming. An inaccurate understanding of what is really going on does not preclude you from doing the right thing anyway. Sometimes even incorrect models will yield correct answers. Doctors in London thought the bad air caused colera so they sent their patients to the country where they happened to drink clean water and didn't get sick. Wrong reason, right result. I have used the "climbing over the wave" explanation for "hull speed". It is not accurate, just brief and easy. A boat can't really climb over something it is creating itself, yet as a quick and really dirty way of making other people understand what is going on it works fairly well. If you can do something that makes the wave smaller "(easier to climb over") the boat will go faster. The fact that this is because the boat is wasting less energy making waves and the waves are just a by-product of an inefficiency in the design doesn't really matter. The cause and effect get mixed up and for any real understanding it is a poor model. But if you don't want to go into a lot of detail and and get bogged down in the physics of how water moves and wave creation, "climbing over the wave" works. It is possible Derek understands better than he lets on and he is just being lazy and doesn't want to write a in-depth text on hydrodynamics. -- Nick Schade Guillemot Kayaks 824 Thompson St Glastonbury, CT 06033 (860) 659-8847 *************************************************************************** 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/ ***************************************************************************
At 10:05 AM -0500 1/16/02, John Winters wrote: > >Do not confuse surfing with planing. A surfing boat makes use of the power >in the wave to reach higher speeds. A planing boat can plane without help >from waves. This may seem like a trivial matter but it is important to >people who design planing boats. But a surfing kayak is often planing, in that it's CG has been lifted and the drag is reduced. The fact that it is only capable of doing it by getting a power boost from an outside source doesn't change the way the boat moves through/over the water. Maybe not all kayaks actually start planing in surf, but those that do so more readily are faster and generally considered "better" surfing boats. This ability can be designed into the kayak. "Surfing" and "planing" are not be the same thing, but they can happen at the same time. -- Nick Schade Guillemot Kayaks 824 Thompson St Glastonbury, CT 06033 (860) 659-8847 *************************************************************************** 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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