Subj : Re: Skeg Design To : rec.sport.rowing From : carl Date : Fri Sep 10 2021 01:01 pm On 10/09/2021 07:48, James HS wrote: > On Thursday, 9 September 2021 at 23:34:07 UTC+1, carl wrote: >> On 09/09/2021 03:39, Glenn Engel wrote: >>> How about the design of a skeg? >>> >>> - Matt C mentioned there is no difference between a 1.5mm and sharp trailing edge. What about the leading edge? >>> - Should the back edge be angled back or perpendicular to the boat? >>> - Is there an ideal angle on the leading edge? >>> - What about height? How is the appropriate height determined? >>> - Does thickness matter? >>> - Are there standard shapes documented somewhere? >>> >> Interesting discussion! May I first respond to James' comments on the >> fin in his boat? We bond them in with polyurethane mastic, James, never >> silicone. And we rely on ligaments of PU embedded in keyhole slots in >> the fin root to provide excellent retention but with loss of fin rather >> than damage to boat if you indulge in heavy-duty dry-land sculling. >> NB Nothing bonds well to the hard-anodised finish on our fins, so those >> PU rubber ligaments are vital to fin retention. Silicone rubber is soft >> & doesn't have what it takes to resist the normal knocks, let alone the >> PTRC hard groundings at low tide ;) >> >> Now to Glenn's comments: >> The leading edge should be sloped backwards sufficiently to minimise the >> risk of the fin retaining grass, weeds and man-made detritus. Also to >> enable it to hop over floating debris with minimal damage to anything. >> You're welcome to look at fins in our web shop to what we find works well. >> >> If the leading edge is sloped, then it makes sense to slope the trailing >> edge as you want to preserve a reasonable amount of area out near the >> tip, which is doing a lot of the work out there in the undisturbed main >> flow. It is the right length from leading to trailing edge (the minor >> chord) that matters for fin performance (all other things being equal), >> as the area thus provided has to resist side loads (i.e. generate lift) >> until the sculler wants to make a significant change of direction >> (whereupon it should stall). The slope of those edges is based on other >> considerations, as indicated. >> >> Fin depth (major chord, as with a wing) & area are largely down to >> experiment & experience. >> >> Thickness does matter as a fin that's too thin is easily damaged, >> impossible to fully straighten & is too thin at its leading edge for >> good performance (see below). >> >> Thickness matters for other reasons. A plate fin must generate enough >> lift when at a small angle to the flow to control the position of the >> stern & the boat's direction under normal the off-axis forces, but be >> capable of stalling if the sculler wants to change direction. And the >> flow must reattach to pull the boat straight once the course correction >> is done. A ~2mm aluminium plate works well & resists damage. The >> leading edge should be radiused, never sharp - & not just to protect >> fish & swimmers. A radiused leading edge better accommodates slightly >> off-axis flows without premature stalling & minimises drag. But the >> trailing edge should be moderately sharp, to prevent generation of >> trailing vortices which generate a buzz & may diminish fin efficiency. >> >> There are no standard shapes or locations for fins. You could say it's >> an under-researched topic, but I might disagree ;) >> >> Cheers - >> Carl >> >> -- >> Carl Douglas Racing Shells - >> Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories >> Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK >> Find: tinyurl.com/2tqujf >> Email: ca...@carldouglasrowing.com Tel: +44(0)1932-570946 Fax: -563682 >> URLs: carldouglasrowing.com & now on Facebook @ CarlDouglasRacingShells >> >> --- >> This email has been checked for viruses by AVG. >> https://www.avg.com > > Thanks Carl for the info on the mastic - perhaps that is why they come out so easily now as I have been using silicone - will seek out something more durable :) (oh, and anticipate beeches better!) > So the first loss took one heck of a bang while every subsequent (how many!!?) was much too easy? You will have a problem, however, unless you fully eliminate all traces of silicone rubber as it's an excellent release agent for anything else. Cheers - Carl -- Carl Douglas Racing Shells - Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories Write: Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK Find: tinyurl.com/2tqujf Email: carl@carldouglasrowing.com Tel: +44(0)1932-570946 Fax: -563682 URLs: carldouglasrowing.com & now on Facebook @ CarlDouglasRacingShells --- SBBSecho 3.06-Win32 * Origin: SportNet Gateway Site (24:150/2) .