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Old 11-02-2016, 08:58   #1
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Bulb keel VS Bulb Keel Hanged With 2 Foil

Hi all

I was wondering wich are the difference of between the "ordinary" thin keel and this one what is on the same boat ( a 42 ft) weight .

As far as i know:
PROS
-Shallow draft (30 inch less or half meter)

CONS
-I GUESS, less bolt that keep the keel in place, so more stressed area
-Heavier (3.100 kg vs 2.700kg, 800lbs about of difference )on a boat total of light displacement: 9.000 kg (18.000 lbs)
-less sailing ability (especially at 90* wind angle), less heading capcity on reaching

I don't know region
- less drag due less wetted surface?
- more drag due the vortex from the first "fin keel" that goes on to the second?
-more lift as flatter bulb...so more resistence again..
-in case of unwanted grounding maybe the flatter bulb help ? (shaft drive transmission)

Any idea or advice?
Thanks
Stefano

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Old 11-02-2016, 09:31   #2
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Re: Bulb keel VS Bulb Keel Hanged With 2 Foil

P.S.

-capsizing/rightening moment/positive stability angle: according to me the shallow draft is better because the water can pass through the hole, without act on the longer keel as a " lever" , making the boat roll more, so i guess it could be better with it.
It's not a fact of the assolute number for angle of vanishing stability is a fact of how easy or hard is to reach it.
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Old 11-02-2016, 10:09   #3
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Re: Bulb keel VS Bulb Keel Hanged With 2 Foil

Quote:
Originally Posted by stefano_ita View Post
Hi all

I was wondering wich are the difference of between the "ordinary" thin keel and this one what is on the same boat ( a 42 ft) weight .

As far as i know:
PROS
-Shallow draft (30 inch less or half meter)
The benefits to such a "keel" would be;
- getting a lot of weight down low, with a shoal draft
- a shoal draft

CONS
-I GUESS, less bolt that keep the keel in place, so more stressed area
Stress on keel bolts is a function of the lever arm/righting moment of the keel, vs. the number of, & offset distance between the keel's bolts.
Any level of stresses created by a keel, can be minimized by the bolt pattern (transverse spacing distance) & the number (& size) of the bolts. That, & by designing very strong keel floors into the boat.

-Heavier (3.100 kg vs 2.700kg, 800lbs about of difference )on a boat total of light displacement: 9.000 kg (18.000 lbs)
Either way, those numbers yield a Very light keel for a boat of the stated displacement. Unless you have a Very deep draft.

For example, a Class 40 racer has a very light weight keel bulb, but also, a 10' draft.
And, the bulb in said attached picture is much more than 1.5t of lead (or most any type of ballast).

-less sailing ability (especially at 90* wind angle), less heading capcity on reaching
Yes, such a keel will definitely produce Much less lift than a conventional fin.

I don't know region
- less drag due less wetted surface?
From the image, it likely has more wetted surface than a fin does. If you want minimum wetted surface, go with a small foil section, & a big bulb on it's end.

- more drag due the vortex from the first "fin keel" that goes on to the second?
The drag produced by such a keel would far exceed anything I've seen to date.

-more lift as flatter bulb...so more resistence again..
The purpose of a bulb is primarily to get lead down low. And pretty much the only bulb keels which produce more lift than a standard fin keel, are those with carefully shaped, "Beavertail" bulbs. Such as this one Magic Keel

-in case of unwanted grounding maybe the flatter bulb help ? (shaft drive transmission)
With this shape, when you go aground, the keel will act much akin to a wing keel. Which are known for as acting very much like anchors. Meaning that they make it more difficult to get unstuck.

Any idea or advice?
Thanks
Stefano

This keel looks like a (poor) attempt to add more weight down low to a Warwick Collins Tandem Keel. As popularized by author Annie Hill, in her book Voyaging on A Small Income.
In her book, she & her husband built a shallow draft, junk rigged, dory design. Drawn by the Benfford Design Group. And fitted a Warwick Collins type keel to it.

On the Tandem Keel as I've seen them, there are 2 vertical foils, with a gap in between them. And a flat'ish bottom plate which connects the two together.
Tandem Keel...? - Boat Design Forums
http://www.yachtsnet.co.uk/archives/...t7147-keel.jpg
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Old 11-02-2016, 10:23   #4
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Re: Bulb keel VS Bulb Keel Hanged With 2 Foil

Perfect thanks, i didn't know the name in english

Getting lot of information from your link and the link's inside

I'll update the question

Does anyone has a boat with it or ever sailed one?
How it was?
Thanks
Stef
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