Transom Edge

[lohwaikin]

Guys,

How sharp do you finish your transom edge adjacent to hull bottom?
I read somewhere that it's gotta be as sharp as possible for speed (or hull drag) advantage.
My concern is the inevitable knocks this edge will sustain in the course of usage and exposing the plywood end-grains to water.
I intend to chamfer this edge to one ply thickness (1/4") then build it back with epoxy, followed by sanding it off to a 1/4" radius.
Some radius also makes the glass cloth sheathing easier.

2013-02-17-2.JPG

Your thoughts or experience please.

Cheers,

[Chad]

My sequence:
Radius edge about 3/8"
Glass bottom, wrapping a little onto transom
Build up silica filler to sharp edge
Glass transom (glass now somewhat supports the corner)
Sand and fair, leaving about a 1/32+" radius

[ryderp]

I think that the theory is that fluids like to adhere to surfaces (the Coanda effect), and that a radius on the trailing edge will draw water up resulting in a less clean separation from the back of the transom. However I used a 1/4" radius also for the reasons you mentioned, and I think that we're talking about a trivial amount of water, and who knows, maybe the rounder edge is faster when the boat is not going fast enough for the water to separate cleanly.

I think that 1/4" is the smallest radius that I could reasonably bend the glass around. You could always create a sharp edge using some fairing compound, but I didn't bother to do that.

Phil

[Tim Ford]

I did the Chad method, I guess, or something close to it. Glass the seam inside and out and then built up the edge with fairing and then 2-part epoxy paint. It's pretty sharp edge, guessing in the 1/32nd range. Will probably get busted up quickly though and look ugly after a month

transonBottomEdge.jpg

[Kevin]

A rounded edge will pull water up around it creating turbulence which creates drag. Anywhere you have water leaving the hull you want the sharpest edge you are comfortable with. In my experience some part of the hull bottom to transom edge is below the water line at almost all times. I rounded, glassed and then faired back to a sharp edge.
Cheers, Kevin.

[ryderp]

Sorry to be disagreeable, but fluid dynamics are not so simply described. Dimples are put on golfballs and vortex generators on wings specifically to create turbulence allowing the void behind the ball or wing to be more easily filled, so it is wrong to say that turbulence creates drag (it depends on the degree of turbulence). I strongly doubt that there is any information to verify that sharp trailing edges are better than slightly rounded or if they are, how much better. What is helpful to a boat on a plane may not be so beneficial for one that is trundling along at a couple of knots in light wind.

Boat design is all one big compromise. In many areas of construction, I went with aesthetics and ease of use over absolute speed. I would also guess that the effect on boat speed of all of these slight differences, is well within that caused by the skill of the skipper.

Phil

[Chad]

Yeah, "turbulence" is probably not quite the right concept. The coanda, or adhesion idea seems to be what drives the need for sharp corners, where release is beneficial. If your transom is immersed, and there is some amount of water being sucked upward around that corner, then by the simple concept of opposing forces the transom is being pulled downward by the amount the water is being pulled upward. Therefore, the hull is plowing a deeper furrow through the water.

Similarly, at the chines if water bends around the corner and travels up the hull side, there is both an effective buoyancy loss as well as the parasitic (wetted surface) drag of the attached water. This applies in full planing mode, but probably not at all at displacement speeds.

These ideas were well tested by the Bethwaites as they developed their 18's and the 49er, using full sized examples for towing experiments.

(My estimation is that the sharp chine hurts upwind since we don't plane, so I radiused them as much as possible. Like LeMans, things often go better around circuits if you optimize the slower part of the course, rather than shoot for higher top speed. I imagine an i550 spends twice as long going uphill as it does down...)

.....fluid dynamics are not so simply described. Dimples are put on golfballs.....

I reckon a golfball's dimple scaled to an i550 hull ought to be about the footprint of a 4" grinder disc. I'll loan you mine (grinder, not hull!!) if you want to give it a try?

[Tim Ford]

That settles it, I'm fairing again and wet-sanding 600 to a radius of 1/256ths inch (approx 0.1 mm or 100 microns). I hope one day to race my sharp-edged tranny against some you old boy's rounded ones. That'll put an end to this-here debate (wink wink nudge nudge)

If I lose, I promise to shave my face with the edge of my boat.

[Kevin]

If dimples worked best, then all the ac72's would be dimpled, or have dimpled foils since they are all foiling now.
If rounded trailing edges worked best, all the ac72's would have them too.
That being said, I don't plan to change my trailing edge nor do I plan to dimple my bottom to prove/disprove any theories mentioned herein. Not because I'm convinced that my way is the best, but because I'm lazy. I'd rather go sailing and my punch list is already too long to make that happen.

[ryderp]

Not to speak for Chad, but I believe that he was kidding about loaning me he grinder.

[Kevin]

Oh, I think he is more than willing to lend you that grinder. He might even bring a generator so you can use it at the launch for any last minute "adjustment" to the dimple pattern.

Cheers, Kevin.

[ryderp]

It doesn't seem like the AC72's care too much about their transom edges:

http://www.youtube.com/watch?v=Y6dnOlE9sjk&feature=player_embedded

Pretty awesome video.

[Tim Ford]

wonder if my boat will foil.....

[Kevin]

Glad to see oracle looking so good with the modified and fixed boat. TNZ looked really good early like they hit the mark pretty well on the foiling out of the gate. That steady smooth ride is what it's going to take to win. And man what a race they will have at 40 knots. man o man.

[TexLex]

From what I read, for what its worth, the edge will be who can tack while foiling. ETNZ looks so balanced and smooth and I read the have been tacking while foiling. I hope Oracle comes out of the shed with a boat 2 that can make the difference.