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      A Few Simple Rules   05/22/2017

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Sea Scouter

Keel fairing and trailing edge

19 posts in this topic

What is the best profile for the trailing edge of a keel? Small radius ~ 1/8"? Knife edge (< 1/64" radius)? Double bevel? Subject boat is a Beneteau First 345. Would like some opinions... would LOVE some data!

 

Just as pretty,

 

just not as well sailed.

post-1-1073965142.jpg

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I've always been told a squared off trailing edge is the best way to achieve a cleaner flow seperation from the keel without creating as many turbies and therefore drag.

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Squared off edge up to 1/8th inch is fastest - no radius or bevel.

 

Sorry but a Beneteau is not my idea of pretty.

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Easy - give Steve Thompson a call - he shares his ideas & toys without demanding a bloody huge cheque to talk to anyone.....

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I've built a few foils, and a few boats. I did a lot of research first, and all the info sez a squared off trailing edge.

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On our boat the keel would start humming at 12 knots and and cease at 16, we put a 30 degree bevel on the trailing edge and now it starts to hum a little at about 15 and stops at 16.

 

The big difference is that the boat gets to 15 a lot sooner and breaks through the sound barrier a lot quicker.

 

The top speed may not have increased but the acceleration certainly has. It is even noticeable in upwind conditions, the boat just seems to accelerate just a little bit better.

 

We plan to remove the hum altogether by increasing the bevel to 45 next time the boats out of the water.

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On our boat the keel would start humming at 12 knots and and cease at 16, we put a 30 degree bevel on the trailing edge and now it starts to hum a little at about 15 and stops at 16.

 

The big difference is that the boat gets to 15 a lot sooner and breaks through the sound barrier a lot quicker.

 

The top speed may not have increased but the acceleration certainly has. It is even noticeable in upwind conditions, the boat just seems to accelerate just a little bit better.

 

We plan to remove the hum altogether by increasing the bevel to 45 next time the boats out of the water.

can i get a huge bag of whatever cnc is smoking?

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Keel Hum

 

There is some other stuff around on the web about it also, ther is an article some were about the musician (non sailor) who first explained it and why it happens.

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The folks at Waterline left the edge on mine pretty small and squared off. Again in that 1/8" range.

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Am currently doing my 325 keel, any days above freezing, grinding cast iron, I don't think there enough discs in creation to get to the 1/4 trailing edge, but am trying, I got the universal templates from computer keels,1/8 is a little thin isn't it? I thought as long as the corners are sharp and square the thickness isn't a big deal.

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Hmmm..... the question still remains.... what is the fastest shape without a hum, the square edge or the beveled one???

A friend of mine owns a 40-foot Open with had a hum from 12 knots and up, when refinishing the trailing edge with a glued fine sailbatten/square edge and fairing it, the hum completely disappeared !!!

This week looking closely at a Pogo (minitransat) keel, I saw that the trailing edge is beveled!!

Probably an easier way to finish a keel coming out from the yard!!

And X-yachts have nice keelshapes with fine squared trailing edge!!

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Am currently doing my 325 keel, any days above freezing, grinding cast iron, I don't think there enough discs in creation to get to the 1/4 trailing edge, but am trying, I got the universal templates from computer keels,1/8 is a little thin isn't it? I thought as long as the corners are sharp and square the thickness isn't a big deal.

well.......... thickness is everthing, with a 6mm edge it will hum!!! so lengthen your keel with a glued sailbatten about 20 to 40mm width, (to match your trailing edge ) and 2 mm thick...( using epoxy) fair it and the result is guaranteed!!!! You will thank me after your first seasonsailing!!!!!!

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We prepped lots of keels when I ran Newport Marine and a square 1/8" trailing edge was always the recommendations we got from the designers. Today I make injection molded surfboard fins. And, we have found that if the trailing edge of the fins exceeds .060" or has any hollows it will hum. We always square the trailing edges.

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Interesting to note, the first season I had with the boat there was a distinct hum/vibration when going over about eight knots or so. This was when the rtrailing edge was huge.

 

Last summer after the bottom job and slimmed down ends, it appears to have gone away. I never heard it anyway, it's tough to say it's gone for good.

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OK... Thanks NZ Moose for the Thompson recommendation. Also to Culigula for the keel links. Best web reading is at http://www.tboat.com/sport-boat.html. Steve Thompson seems to understand a thing or two. I am going with the squared off trailing edge, less than 0.080" in width (which is close to a knife edge in epoxy). If this will make up for stupid tactical errors, failure of the skipper to successfully communicate, inability to judge wind patterns, tacking on lifts (upwind), jibing on headers (downwind), and a few other shortcomings then we have a winner.

 

For Grimreaper, it might be a lot easier to add a little rather than take off a lot. The batten trick seems like the hot idea, but I added with some high density fill and used creased mylar to form the edge.

 

Thanks to all who replied...

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On our boat the keel would start humming at 12 knots and and cease at 16, we put a 30 degree bevel on the trailing edge and now it starts to hum a little at about 15 and stops at 16.

 

The big difference is that the boat gets to 15 a lot sooner and breaks through the sound barrier a lot quicker.

 

The top speed may not have increased but the acceleration certainly has. It is even noticeable in upwind conditions, the boat just seems to accelerate just a little bit better.

 

We plan to remove the hum altogether by increasing the bevel to 45 next time the boats out of the water.

can i get a huge bag of whatever cnc is smoking?

 

Didn't we see that yacht? Wasn't it called "Hum Job?" LOL :blink:

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