Subj : Re: Skeg Design To : rec.sport.rowing From : James HS Date : Tue Sep 14 2021 01:49 am On Friday, 10 September 2021 at 13:01:41 UTC+1, carl wrote: > 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: ca...@carldouglasrowing.com Tel: +44(0)1932-570946 Fax: -563682 > URLs: carldouglasrowing.com & now on Facebook @ CarlDouglasRacingShells It is not all user error :) (but quite a bit is) At low tide, the inshore zone becomes quite narrow, and those using the (correct) part of the fairway are quite close - so I do have a tendency to occupy a safe zone close'ish' to the bank. Often what looks like a proper depth of water has a shock (Granite sets that hold the wall up have crumbled and migrated into the inshore zone) so you can have blade depth on both blades one minute, then your riverside blade will hit a rock (some 3-4 metres from the shoreline) and then your shoreside blade (in what looks to be the proper depth), and then your fin Normally I know where they are, but occasionally when I am concentrating on technique (yeah, fairly rare) I forget. Go further from shore - no chance, the buoys which define the inshore zone are there to remind you, and navigation of others is often more perilous than losing a fin. I once lost a fin on an outcrop (this one I should have known better) and did my 12K outing (boy does it point out any power imbalance) - returned to the club, wandered to the now dry outcrop, retrieved my fin and replaced it! Will replace the silicone :) James --- SBBSecho 3.06-Win32 * Origin: SportNet Gateway Site (24:150/2) .