Proper rudder shape please

[BrianR]

Is the trailing edge of my Melonseed Skiff rudder supposed to be tapered? There's no suggestion either way in the plans. Thanks.

[Bruce Hooke]

It's not that big a deal if you are not building a racing boat, but a taper is a nice touch. A feather edge would be ideal from a hydrodynamic perspective but since a feather edge is fragile and hard to maintain it seems that the best thing to do is to square off the back edge at a reasonable width. A squared off edge appears to cause less vibration than a rounded over edge...

[Bob Smalser]

I'd shape the blade into a reasonable airfoil and use 5oz glass tape and epoxy on the edges for durability.

http://pic3.picturetrail.com/VOL12/1104763/6490387/85769044.jpg

This q-sawn solid, drifted mahogany rudder ranges from 5/8" to 1/4" with 1/2 to 1/4 on the wetted surface. One layer of 4oz glass and epoxy....followed by heavier edge tape and epoxy....followed by a second layer of 4oz glass and epoxy....followed by 6 coats of spar.

I'ts exactly as John Gardner drew it except every surface is 1/8th thicker. Gardner called for stronger Dynel fabric reinforcement....and I can't get Dynel and varnish together satisfactorily.

http://www.woodenboat-ubb.co m/cgi-bin/UBB/ultimatebb.cgi?ubb=get_topic&f=1&t=011020&p= (http://www.woodenboat-ubb.com/cgi-bin/UBB/ultimatebb.cgi?ubb=get_topic&f=1&t=011020&p=)

[ 04-12-2005, 05:04 PM: Message edited by: Bob Smalser ]

[John E Hardiman]

Originally posted by Bruce Hooke:
A feather edge would be ideal from a hydrodynamic perspective but since a feather edge is fragile and hard to maintain it seems that the best thing to do is to square off the back edge at a reasonable width. .Actually a feather edge is the WORST thing for a rudder because it cause the rudder to stall earlier and is harder for the flow to reattach properly. YES! to a squared off tapered trailing edge. Thickness at the square off should be ~20-25% of the maximum thickness. Taper to the square off should be about 1 in 5-7.

Or so I think

[paladin]

well now John....how fast izz this thing gonna go and would a GAW-1 or GAW-2 foil be better...do we need leading edge slots or flaps...trailing edge flaps and trim tab or both......?

[John E Hardiman]

Originally posted by paladin:
well now John....how fast...Yeah, yeah, yeah. tongue.gif

The whole idea about a rudder is to provide the most turning moment with the least drag and work.

Leaving the rudder unfaired causes boundary layer growth and vorteices at the TE which reduct the rudders effiency at sailing speeds. Likewise, if the rudder is too thin at the TE, the vorticies are not stabilized and again the rudder efficency is reduced. What I recommend is the minimum amout of work to maximize the moment and minimize the drag.

Actual optimum shape would a hyperbolic shape optimized for boundary layer growth at sailing speed with a trailing edge radius selected based upon instantious flow velocities at seperation to minimize strouhal effects. :rolleyes:

[George Roberts]

As I recall the Strouhal number is L/tV. Keep it low.

[paladin]

maybe it needs wing fences for vortex control.....a negative 7 degrees off the bottom would reduce tip drag....and the possible use of a splate...

[N. Scheuer]

The rudder on my lapstrake cedar Melon Seed has a taper along the aft edge, but not tapered to a point, somewhat blunt.

With all the transom rake these boats have, I don't think tip fences are a good idea as the fence would incline rather severely when turned one way or the other.

Moby Nick

[paladin]

Nick...I was only joking......

[Lion]

John

Does the squared off trailing edge requirement extend to foil shaped c/boards as well?

Lion

[Bruce Hooke]

John,

Thank you for correcting me. I knew there were good reason not to go with a feather edge but I had forgotten about the stalling problem.

Lion,

With the caveat that my memory for this stuff is clearly not what it could be :D , I am fairly certain that the same rule applies to centerboards. I don't think stalling would be as much of an issue with a centerboard but vibration could still be a problem...

- Bruce

[John E Hardiman]

Bruce is right, for a CB or low angle of attack foil, the end should still be squared off, but can be thinner (say 5-7% of thickness) than a rudder. And it is almost a given that you can never totally get rid of the "humming" so pick a pitch that you can live with... ;) (note, a small diameter wire bedded in the finish about 1/4-1/2 inch ahead of the TE will do wonderful things to end the humming)