Monday, August 8, 2011

External Chine Logs

An anonymous contributor has posted a comment on one of my earlier posts about flat bottomed boats. The comment was made in relation to my assertion that Phil Bolger had a theory that external chine logs may reduce resistance. The contributor has pointed out with a quote Phil made in his essay on the Light Dory Type V that he was not claiming a reduction in resistance: -

Ross, Bolger had a theory of water flow and how that applied to sharpies. With respect, what he wrote about exterior chine logs of relevance here was: "The external chine log... adds a minute amount to the stability, which certainly needs anything it can get (Light Dory Type V); I don't think it increases the resistance but I can't prove it yet." He worried that it may increase resistance, not reduce it.

Well, I have another quote from Phil Bolger on the subject, and I think I have more available but I can't remember where to find them right now. Anyway, here is my reply to the person making the comment: -

Yes, and he also wrote, regarding Poohsticks, on page eight of "Small Boats",

"This was the first design I made using the outside chine log, which I've since adopted quite often in constant-deadrise boats: it saves some labor in fitting the log, and gives better bearing and fastening for the size of the log. It also gains a bit of stability, and I have an idea, just conjecture, that it reduces eddying along under the chine by carrying the side flow aft......"

I don't have the qualifications in hydrodynamics to give an educated guess one way or the other, but this I do believe: -
Jim Michalak's Mayfly 14 design, showing off her external chine logs
  • External chine logs are much easier to fit, for a whole range of reasons I'm not going to write about here;
  • External chine logs do protect the vulnerable topside planking at the chine;
  • External chine logs provide a very useful hand grip when the boat is capsized, and a good toe-hold when climbing aboard;
  • I am still in two minds about drag reduction, but I do know that a hard chine with an internal chine log can pull a long and powerful underwater eddy (I've experienced it), and I see no reason why Phil Bolger's statement above should not be worth thinking about;
  • On a sailing boat, the increased lateral resistance from external chine logs may well give back more than is lost to resistance - if there is any loss in the first place.
If anybody has some worthwhile information on this subject, I'd be keen to learn more.

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