tag:blogger.com,1999:blog-349800934689829416.post6438534982248514354..comments2020-09-10T20:49:57.161-07:00Comments on jwboatdesigns: Progress, life gets in the way a bit.John Welsfordhttp://www.blogger.com/profile/07058513465858775154noreply@blogger.comBlogger6125tag:blogger.com,1999:blog-349800934689829416.post-35773142622691727922014-04-27T06:56:21.338-07:002014-04-27T06:56:21.338-07:00After thinking abut this, I remembered that the ma...After thinking abut this, I remembered that the maximum volume and minimum surface area is a sphere... So, of course, rounded hulls will always produce less surface area for a given displacemnt.<br />I think I'll extend this spreadsheet to included 'round bottom' hulls, adding it to the "V" bottom hulls currently in the spreadsheet.Tom Clarkehttps://www.blogger.com/profile/14491526364202641521noreply@blogger.comtag:blogger.com,1999:blog-349800934689829416.post-30459234930489181032014-04-26T06:57:09.188-07:002014-04-26T06:57:09.188-07:00That's interesting that you say use a 'rou...That's interesting that you say use a 'rounded' hull to minimize wetted area. I was curious about displacement vs. wetted area as a function of deadrise... created an Excel spreadsheet and found, for a constant displacement, the least wetted area was a 45 degree deadrise for a "V" bottom hull... Here are a couple of examples... all with a constant displacement...<br />DEGREE DEADRISE PERCENT INCREASE IN SURFACE AREA<br /> 85 58%<br /> 75 29%<br /> 65 12%<br /> 55 3% <br /> 45 0%<br /> 35 3%<br /> 25 13%<br /> 15 29%<br /> 5 58% <br />John, if you want the spreadsheet, send me your email address to <br />tlclarke at optonline dot net Tom Clarkehttps://www.blogger.com/profile/14491526364202641521noreply@blogger.comtag:blogger.com,1999:blog-349800934689829416.post-62503726276517562632014-04-23T17:39:42.684-07:002014-04-23T17:39:42.684-07:00Tom, this is a long and involved subject, and gets...Tom, this is a long and involved subject, and gets really involved when the more esoteric elements of hull design are considered.<br />But do remember that SEI is a sail and oar boat, so there are a whole set of conflicting requirements to be worked out in the shape, the result will inevitably be quite different to a pure rowing boat.<br />I'd suggest for your rowing boat, that you draw your shape and do the numbers, being only 12 ft long you'll row her close to hull speed so her prismatic c/f will ideally be higher than say a 16 ft waterline rowing boat.<br />Try for 0.54 for starters and see what that looks like.<br />Keep the shape fairly rounded which will minimise wetted area.<br />I cant really think of one book that will give you all of the information that you'll need, I have an extensive library including access to several professional societies literature and each book or publication seems to have one or two interesting points to make.<br />But do read everything that you can, the more of those "interesting points" that you can gather the better.<br /><br />JJohn Welsfordhttps://www.blogger.com/profile/07058513465858775154noreply@blogger.comtag:blogger.com,1999:blog-349800934689829416.post-40214556706059072022014-04-17T06:38:04.599-07:002014-04-17T06:38:04.599-07:00Thanks, John...I'm thinking of a pure rowing b...Thanks, John...I'm thinking of a pure rowing boat of 12 feet... either double ended (DE) or square stern (SS)... The DE would have constant deadrise stem to stern of let's say 20 degrees... The SS would have 20 degree deadrise forward trailing out to let's say 5 degrees at the transom.<br />Given the above, the SS would have more surface area friction (all other factors being equal).<br />If I understand you correctly, the stern wave generated by the SS will be further aft than that generated by the DE... this will result in a higher "hull speed".<br />But the difference can't be very much... Let's assume the distance between the bow wave and stern wave on the SS is 14 feet (Hull speed = 14^-2 (3.74) X 1.3 = 4.86) and for the DE 12 feet (Hull speed = 12^-2 (3.46) X 1.3 = 4.5). So the difference in "hull speed" is .36 knots based on my asumptions above. <br />And, more importantly, I'm not going to be rowing at "hull speed" for very long. So at cruising speed, let's say 3 knots, the "hull speed" is not a factor in this scenario. But the skin friction IS a factor.<br />Do I have this right?Tom Clarkehttps://www.blogger.com/profile/14491526364202641521noreply@blogger.comtag:blogger.com,1999:blog-349800934689829416.post-9040230242040229592014-04-16T13:54:29.448-07:002014-04-16T13:54:29.448-07:00There is a point where wave making becomes more a ...There is a point where wave making becomes more a restriction than surface area friction, and the square stern boats will make their stern wave further from the bow wave, the distance between the two being what sets "hull speed".<br />Double ends work ok on heavier boats, or on rowing boats, and to some extent surfboats, but if you want a really fast sailing boat check out the Sydney Harbour 18 ft skiffs shape.<br />In general the wider stern contributes a lot of stability which enables the boat to carry more sail, plus manages the stern wave form better.<br />Its a long and complex subject though, and if you want to pursue it there is a book called "High Speed Sailing" buy Frank Bethwaite that would help explain a lot of it.<br /><br />JJohn Welsfordhttps://www.blogger.com/profile/07058513465858775154noreply@blogger.comtag:blogger.com,1999:blog-349800934689829416.post-32154458081289176362014-04-16T05:49:15.104-07:002014-04-16T05:49:15.104-07:00John, could you comment on this from your post abo...John, could you comment on this from your post above: <br />"...double ended means that it will be slower than boats with near parallel sections from the widest beam aft..."<br />Is this also true for a pure rowing boat? Or is it true only for sail boats? <br />I'd also like to understand the underlying rationale for why double enders are slower vs parallel sections running aft. I would think they would be faster because of the reduced surface friction.<br /><br />TomTom Clarkehttps://www.blogger.com/profile/14491526364202641521noreply@blogger.com