The effects of a wavy keel

[Leo Lazauskas]

Kim Klaka, in "On the Performance of a Wavy Keel", concludes that "...waviness of more than 1 mm makes a significant difference in performance" on a sailing yacht of 10m length.

See the interesting (albeit rough-and-ready approach) to the problem on page 45 of the May 2010 issue of Australian Naval Architect at:
http://www.rina.org.uk/australian_naval_architect.html

Does anyone know of any other theoretical or experimental work on this topic?
Any experiences you'd like to share?

Leo.

[Ad Hoc]

It is not a very scientific conclusion, since based upon a sample size of one! No references to what and how measurements were taken before and after….so more anecdotal than anything else. Despite it being “interesting”..

But as he noted historically often points the way.

“Full scale tank tests of an International 10 Sq. meter Class canoe” by T.Tanner in RINA Trans 1961.

He varied centreboard shape and profile. The biggest conclusion was the effect on the ratio of Lift/(speed)^2 and Drag/(speed)^2 was yaw.

Yaw had a considerable effect on the results to those without.

Thus how did Kim measure the yaw and its effects?

[daiquiri]

It is an interesting, but unfortunately too short report. However, it teases us to search for more info on the subject, and as such is more than welcome.

On the other side, thanks to Leo's link I've discovered another very interesting fact - See the Letter to Editor at page 4, by mr. Martin Grimm. The concept of Lateral Force Estimator and of it's relation to passenger comfort is probably well-known to NA's, but not so much to YD's - because it's effects are mostly felt on the big-size vessels. I was one of ignorants up untill now.

So thank you Leo for the unexpected development in my ethernal research for knowledge, I have something to read about in the next days.

Cheers!

[Gary Baigent]

When Cote d'Or was in Auckland during a Whitbread stopover, skippered by Eric Tabarly with a crew of hotshot stars in the making as crew: Halvard Mabire, Roland Jourdane, Michel Desjoyeaux, Francois Carpente and numbers of others; when the big red maxi was on the hard, sharp eyed kiwis immediately saw there was a twist in the keel (I think it was Halvard's brother who made it) but the French guys just shrugged it off with a laugh - apparently they, and Cote d'Or's instruments could not ascertain any difference whether on port or starboard, upwind or down. Hmmmm!

[Leo Lazauskas]

Originally Posted by daiquiri

It is an interesting, but unfortunately too short report. However, it teases us to search for more info on the subject, and as such is more than welcome.

Yes, it is a real tease! As Ad Hoc noted, we have a sketch of a sample of one at the moment.

I thought some of the aerodynamicists that lurk here might also have something to add. There would be a similar loss of lift on wings and tails of planes.

Leo.

[Leo Lazauskas]

Originally Posted by Ad Hoc

It is not a very scientific conclusion, since based upon a sample size of one! No references to what and how measurements were taken before and after….so more anecdotal than anything else. Despite it being “interesting”..

But as he noted historically often points the way.

“Full scale tank tests of an International 10 Sq. meter Class canoe” by T.Tanner in RINA Trans 1961.

He varied centreboard shape and profile. The biggest conclusion was the effect on the ratio of Lift/(speed)^2 and Drag/(speed)^2 was yaw.

Yaw had a considerable effect on the results to those without.

Thus how did Kim measure the yaw and its effects?

Sorry, but I have no idea how anything was measured. I started the thread hoping it might provide some further interesting anecdotes if nothing else.

Of course, there is also the waviness in the keel planform to consider. I suspect that a straight-edged planform could be made very accurately, but curved dorsal fin-like keels might be a bit tougher to get right, especially at the tip of the keel. That, of course, is where all the interesting stuff happens.

All the best,
Leo.

[Ad Hoc]

Originally Posted by Leo Lazauskas

Sorry, but I have no idea how anything was measured.

Im not suggesting you do. It was just an observation.

However your other comment is more relevant and ‘juicy’. What roughness and straightness does a shark or dolphin have on its dorsal fin? Nature generally shows us the way before anyone can work out why!

[Leo Lazauskas]

Originally Posted by Ad Hoc

Im not suggesting you do. It was just an observation.

However your other comment is more relevant and ‘juicy’. What roughness and straightness does a shark or dolphin have on its dorsal fin? Nature generally shows us the way before anyone can work out why!

Yes, but animals tend to have non-rigid fins and they have clever feedback systems.
If only there were more days in an hour!

Leo.

[daiquiri]

Originally Posted by Leo Lazauskas

I thought some of the aerodynamicists that lurk here might also have something to add. There would be a similar loss of lift on wings and tails of planes.

Dimensional tollerances of single aircraft mechanical parts are of the order of +/-0.01" or +/- 0.25 mm. For a wing of, say, 2 meters chord and 0.25 meters thickness it gives an approximate "waviness" equal to 0.1% of the thickness or 0.013% of the chord length. When you stack up the errors along very long structures, such as a wing or a fuselage, you can arrive to +/- 10 mm error in total length, but for a thickness the error is very small - of the order of 0.25-0.50 mm.

0.35% of the chord (mentioned in the Australian Naval Architect article) would give +/-7 mm of thickness error for a 2 m chord, which is unacceptable by aeronautical standards. Guess thats why there is not much data available from that source.

Cheers!

[Leo Lazauskas]

Originally Posted by daiquiri

Dimensional tollerances of single aircraft mechanical parts are of the order of +/-0.01" or +/- 0.25 mm. For a wing of, say, 2 meters chord and 0.25 meters thickness it gives an approximate "waviness" equal to 0.1% of the thickness or 0.013% of the chord length. When you stack up the errors along very long structures, such as a wing or a fuselage, you can arrive to +/- 10 mm error in total length, but for a thickness the error is very small - of the order of 0.25-0.50 mm.

0.35% of the chord (mentioned in the Australian Naval Architect article) would give +/-7 mm of thickness error for a 2 m chord, which is unacceptable by aeronautical standards. Guess thats why there is not much data available from that source.

Cheers!

Thanks, Slavi. Those tolerances are good numbers to keep in the back pocket.

Leo.

[sorenfdk]

IIRC, C.J. Obara and C.P. van Dam wrote about wavyness in "Keel Design for Low Viscous Drag". Tha paper was presented at a Chesapeake Symposium and can be found on the SNAME Small Craft Papers CD.

[DMacPherson]

I'm not sure how you conclude it is a data set of one. While Kim's personal test was singular, this follows the empirical data presented in the references he noted - Ward, Hoerner (probably presenting Ward), and Powell. This article prompted our own interest in checking if waviness was significant on propeller performance. With the advent of pretty reasonable propeller blade scanning devices, we can begin to quantify this. I'll keep you updated on progress.

Don

[baeckmo]

Originally Posted by DMacPherson

This article prompted our own interest in checking if waviness was significant on propeller performance. With the advent of pretty reasonable propeller blade scanning devices, we can begin to quantify this. I'll keep you updated on progress. Don

Appreciate, and looking forward to that!

[Leo Lazauskas]

Originally Posted by DMacPherson

This article prompted our own interest in checking if waviness was significant on propeller performance. With the advent of pretty reasonable propeller blade scanning devices, we can begin to quantify this. I'll keep you updated on progress.

Don

I'm glad Kim's work inspired you to look at other lifting surfaces.

I'm not quite as practical, so I'll just see how slight "wobbles" in planform shape affects the lift of a thin wing with circular planform (because there is an exact solution available). I just need to work out a systematic way of introducing wobbles.

All the best,
Leo.

[DMacPherson]

My biggest question is with the geometric definition of "waviness". What wave length and amplitude are we talking about? There are a number of papers about propeller CAM tooling marks and performance, but is this "waviness" or something else? When does "waviness" become roughness?

Don