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-   -   Bulb keel VS Bulb Keel Hanged With 2 Foil (http://www.cruisersforum.com/forums/f47/bulb-keel-vs-bulb-keel-hanged-with-2-foil-161000.html)

stefano_ita 11-02-2016 07:58

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 :biggrin:
- 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

http://www.mondialbroker.com/Immagin...401-10_400.jpg

stefano_ita 11-02-2016 08:31

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.

UNCIVILIZED 11-02-2016 09:09

Re: Bulb keel VS Bulb Keel Hanged With 2 Foil
 
Quote:

Originally Posted by stefano_ita (Post 2042726)
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 :biggrin:
- 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

http://www.mondialbroker.com/Immagin...401-10_400.jpg

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

stefano_ita 11-02-2016 09:23

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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