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Old 30-03-2016, 13:12   #1
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Unusual Keel design

My Bavaria 37 has this unusual double fin with bulb keel design. I came across quite a good discussion about it on a different forum once but for the life of me I cannot find it now.

It is not a Bilge keel on the sides of the hull like the discusson earlier this month. The keels are inline directly under the hull and the bulb links across between them. I remember the name of the design starts with a W.

Picture here https://flic.kr/p/DJEuqu.

Any information would be appreciated.


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Old 30-03-2016, 14:53   #2
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Re: Unusual Keel design

mausgras,
I suggest you try this: https://www.google.com/search?as_q=%...pe=&as_rights=

Alain
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Old 30-03-2016, 16:33   #3
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Re: Unusual Keel design

Looks like it will the kelp off...


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Old 30-03-2016, 16:48   #4
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Re: Unusual Keel design

Quote:
Originally Posted by Hydra View Post
mausgras,
I suggest you try this: https://www.google.com/search?as_q=%...pe=&as_rights=

Alain
Thanks for that .. perfect.
I tried googling, dual keel, twin keel, double keel and kept getting links to barge keels. Doh!!!!!!

Tandem keel ... got it.

Ian

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Old 30-03-2016, 16:50   #5
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Re: Unusual Keel design

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Originally Posted by Marpessa View Post
Looks like it will the kelp off...


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No problem with kelp in my waters but still get lots a barnacles


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Old 30-03-2016, 17:21   #6
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Re: Unusual Keel design

That is a well known and studied type of keel, a tandem keel. Bavaria used it for some time and stopped some years ago I suppose because it is more expensive than a normal keel. It gives a better performance on shallow draft keels with less wet surface for the same job.

Not very usual due to cost on mass production boats. Etap used it as well as some Harmony.

There are some nice custom boats using them. Yours seems a nicely designed one. Here have a look at some others:








Even a racing one used on the America's cup:
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Old 31-03-2016, 01:18   #7
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Re: Unusual Keel design

Can anyone explain the advantage of this design over a normal strut and bulb type fin?

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Old 31-03-2016, 01:37   #8
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Re: Unusual Keel design

Here's my guesses...

Less resistance to lateral forces. Like when sliding sideways surfing down a wave front.

Less wetted surface.

Less weight, same strength since the remaining sections compared to conventional are the areas where forces are concentrated, in the fore and aft vertical plane.

Less torsional loading at the hull interface, especially with a high aspect fin and long bulb.

Purely WAGs, since today is the first time to see one.
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Old 31-03-2016, 04:43   #9
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Re: Unusual Keel design

I thought it might also distribute the weight more evenly along the hull so there is less heaving (?) which will somehow impact on performance. But that is just a laymans guess.

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Old 31-03-2016, 04:49   #10
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Re: Unusual Keel design

This type of keel has not any advantage regarding a deep torpedo keel and it only makes sense and offers advantages if a swallow keel is needed. in what regards that the two best alternatives regarding fixed keels are the tandem keel and the twin keel.

One of the contemporary NA that hade made more research about this type of keel is Martin Defline, a French very creative and innovative NA. He is not a fundamentalist and use all types of modern keels in his designs, including tandem keels and when I say all types that include some that most never heard about:


This beauty (a 43ft voyage boat) has a tandem keel.






He had experienced extensively with a First 345 (swallow draft) one that he had modified with a tandem keel and only more 10cms of draft. The improvement in stability and performance was very substantial, including a better pointing ability. The increase in stability allowed 3m2 more on the sail area.

Here you have more comparative and detailed information about that:
quille tandem first 345- defline yacht architecture

Basically this type of keel versus a torpedo keel with the same shallow draft provides more lift with less drag. Structurally it can also contribute to distribute the efforts of the keel by a bigger part of the hull, allowing for a lighter and more efficient keel support structure.
quille tandem - defline yacht architecture
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Old 31-03-2016, 06:00   #11
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Re: Unusual Keel design

I seem to recall that tandem keels, along with other things like winged keels, came out of that very innovative era of the early 80's America's Cup boats. When more rigorous scientific testing was performed, the benefits were found to be either non-existent or so minor as to be not worth the extra cost of putting them on production boats.
Similarly, would there be any point in a swinging keel on a production cruiser? Or a 'moustache' hydrofoil? I certainly wouldn't want the extra complication.


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Old 31-03-2016, 16:28   #12
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Re: Unusual Keel design

I believe that the original/first version of this keel type, was the Warwick Collins Tandem Keel. And in it's original form, the lower, horizontal piece, is pretty much a flat plate (or wing section).

There's some history behind it, due to the noteriaty of the boat "Badger", in Annie Hill's book Voyaging On a Small Income, wherein, they fitted one to their 34' Benford designed, Dory.

Evidently, when the original was being designed, a Lot of testing went into the optimal sizing of, & spacing of, the two foils. And those who fit them, tend to report much better handling of their vessels.
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Old 31-03-2016, 17:26   #13
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Re: Unusual Keel design

I can understand the possible structural advantages, but as yet, no one has addressed the question of why there would be a better lift/drag ratio with this configuration. I'm still wondering! The one with the saildrive between the two foils is really odd to my eye... but it would give good prop protection!

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Old 31-03-2016, 17:34   #14
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Re: Unusual Keel design

Quote:
Originally Posted by UNCIVILIZED View Post
I believe that the original/first version of this keel type, was the Warwick Collins Tandem Keel. And in it's original form, the lower, horizontal piece, is pretty much a flat plate (or wing section).

There's some history behind it, due to the noteriaty of the boat "Badger", in Annie Hill's book Voyaging On a Small Income, wherein, they fitted one to their 34' Benford designed, Dory.

Evidently, when the original was being designed, a Lot of testing went into the optimal sizing of, & spacing of, the two foils. And those who fit them, tend to report much better handling of their vessels.
Tandem keels come from far and are linked to the America's cup. The first experiments happened on the late 80's....from the XIX century and it seems Herreshoff himself toyed with them when he designed the first true modern keel boat.

Some few years later some more experiences were made with a dinghy with surprisingly good results and Captain Marony in 1989 designed an incredible advanced 13 meter boat, with chines and a true tandem keel. However the design was so unusual that it frightened everybody and it was never built.

The critics said about the boat "We do not mean to commit ourselves by offering any opinion as to her merits, but we would most decidedly hesitate to make a voyage in her from the Solent to the Clyde."

A pity because it was a beautiful design with the only "defect" to be too advanced for its time:

You can read more about it on the blog of François Chevalier:
Chevalier Taglang: AMERICA'S CUP - HISTORY OF YACHT DESIGNS - TWIN-KEEL - YACHT DESIGN INNOVATION - FIN-KEEL - BALLAST
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Old 31-03-2016, 17:37   #15
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Re: Unusual Keel design

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I can understand the possible structural advantages, but as yet, no one has addressed the question of why there would be a better lift/drag ratio with this configuration. I'm still wondering! The one with the saildrive between the two foils is really odd to my eye... but it would give good prop protection!

Jim
Not to mention the weight of the engine almost over the boat CG contributing for a better balance

Regarding more lift if you have foil with a suspended torpedo it would be dificult, and not very good structurally, to make really a long torpedo suspended only by a foil with a small width. That problem does not arise if the torpedo (with a flat bottom) is suspended by two narrow foils well apart.

That will allow a longer torpedo that will provide more lift without significantly more drag. You can understand that looking to the keel of the America's cup NZ boat.

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