Twin keel research

[RHKo]

Take a V-hulled, single chine sailboat with twin keels. Inside, the area between the keels is sloped and narrow, reducing its usefulness. Now, what if, just for the length of the keels, the hull shape between them were flat athwartships but with the same rocker longitudinally as the standard design? The usefulness of the interior would be enhanced by allowing a lower and wider cabin sole with more headroom and less awkward stowage. The hull would have less wetted surface and slightly more volume. The forward and aft ends of this section would have to be faired into the rest of the hull, making the keels a little longer at each end.
Would there be any predictable effect on the hydrodynamics or performance or handling of the vessel?

[Brent Swain]

There would be a slight presure bubble at the root of the inside of the keels , but I can't see what you propose making a huge difference as long as the ends are faired in well.
The problem with wetted surface on twin keels was because they made each keel the same size as they would make a single keel , ignoring the advantage that ,with the lee keel upright when heeled, it was 100% more effective in lateral resistance. Twin keels angled out at 25 degrees need only half the area of a single keel , giving them roughly the same total area, and far less wetted surface that a full length keel.
Twin keels , by making cleaning easier, are far more likely to spend more of their sailing time with a clean bottom.
Brent

[Boston]

hmmmm
I noticed that the anti roll fins were mistaken for bilge keels
gotta admit they do look awfully similar
whats the subtle difference kids
looked like the roll fins were at about a 45 deg angle and the bilge keels at about a suggested 27 deg angle
roll fins looked narrower in section
and placed farther forward than a bilge keel

also tests mentioned showed that at anchor roll was actually worse with bilge keels yet anti roll fins are sooooooo similar

any thoughts
B

[Brent Swain]

Just after launching my current twin keeler,I motored her up Georgia Strait from Nanaimo to Cortes Island. Off Comox, the wind increased to SE 25 knots. I was motoring with no ballast, a bare shell. I put her beam on to a 3 ft chop and she barely rolled at all. I crossed the north end of Georgia Strait in a 25 knot beam wind , with no mast or rig , with amazingly little roll. My single keeler would have been rolling her guts out. She is far more comfortable in an anchorage and rolls far less than single keelers, in any swell.
Why? The keel on a boat tends to be the centre around which she rolls. With two centres opposing each other to be the one she rolls around , rolling is greatly reduced.
Brent

[MikeJohns]

Originally Posted by Boston

.............
also tests mentioned showed that at anchor roll was actually worse with bilge keels yet anti roll fins are sooooooo similar

any thoughts
B

There are lots of tests on anti-roll keels and they make a huge difference to the vessels roll characteristics.

Tests on twin keel sailboats are few and far between. See my post #84 if this is the test you are referring to I wouldn't take it very seriously, it wasn't so much research as an attempt to use a hydro package as a rough prediction tool.

He simply correlated a fluid dynamics roll model for a single keel between a full sized vessel and a computer model (with significant error). He then proceeded to use this model to suggest a predicted roll motion for one configuration of twin keels on the same hull for a limited wave spectra.

This approach is an interesting illustration of hydrodynamic prediction but not a useful study of twin keels and roll behavior.

Even if the computer model correlated accurately with a full size twin keel vessel ( not done ) consider the variables that should be run through to reach such a conclusion........what if the keels were angled differently, had a different aspect ratio, were positioned differently, had different end shapes.....

There was sufficient error for the conclusion to be wrong for the singular design predicted, and he never closed the loop with a valid verification of the computer model.

[Boston]

Brent
Pictures and details of your keel configuration would be a perfect match to your experience with it

thanks
B

also the original question still remains
both bilge keels and anti roll fins are easily mistaken for one another
yet one is reported in some cases to have little effect on roll dampening and the other reported consistently to have a positive effect on roll dampening
one is said to modify wave formation favorably and yet little is ever mentioned about the effects on wave formation that anti roll fins might have
is that because of its angle to the vertical axis or is it a function of aspect ratio, whats the defining characteristic that gives one roll properties and not the other

[MikeJohns]

Originally Posted by Boston

..............yet one is reported in some cases to have little effect on roll dampening and the other reported consistently to have a positive effect on roll dampening ..........

Boston
I'm always interested in studies and reports of all aspects of twin keels.
But for roll characteristics there are no decent studies that I know of, and who's reports ?

There's very little in the research field of any use WRT rolling of twin keels because there are so many variables in their design.

Anecdote I've heard suggests most twin keel owners find a reduced roll at anchor and underway.

Look at the picture in post #83, I'd expect this to be an effective roll damper which shares some of the characteristic of the more effective anti-roll fin designs where there is less entraining of the water and more 'mixing'.

[Boston]

some of the stabilizing fins seem to be powered and some not
is that accurate
so among the fixed fins what is the defining characteristic separating the fin from the bilge keel

also if there is a lift effect to the bilge keel why would a submerged chine not accomplish the same lifting effect
and be at least somewhat effective against roll

[Boston]

I found a fantastic article and thought you guys might be interested
seems to quantify bilge keels and there effect on roll period pretty precisely

http://encyclopedia.jrank.org/RHY_RON/ROLLING_OF.html

[Tad]

Originally Posted by Boston

fantastic article

From Encyclopedia Britannica of 1911???

There's been considerable research since then...check with SNAME for a list of publications. Find a copy of Kenneth C. Barnaby's Basic Naval Architecture.

I have three papers published by SNAME
Roll Damping on Two New England Trawlers, Goudey & Venugopal, 1989
Selection and Evaluation of Ship Roll Stabilization Systems, Sellars & Martin, 1992
Roll Stabilization for Small Fishing Vessels Using Paravanes and Anti-Roll Tanks, Bass, 1998

[Boston]

do you have the key to the calculations
its not in the article and the formulas are confusing with out them

I think I can calculate the roll of that old elco if I had the key
then I could math out the effects of twin keels on the hull

B

[Brent Swain]

The angle to vertical has no bearing , nor does aspect ratio, as it is the simple fact that having two keels eliminates the possibility of either becoming the sole centre of the roll, period.
A friend crossed the Atlantic in a British twin keeler. While sailing downwind in the Carribean, alongside a sister ship wiith a single keel, the single keeler was rolling twice as far as the twin keeler. This is self evident in rolly anchorages, with twin keelers anchored near single keelers
Brent

[brian eiland]

Going thru some material I had stored on my computer I ran into this little 48 footer I was seriously thinking of buying a few years back. She's quite a nice looking vessel.

[Brent Swain]

Looks good . Cant see any obvious problem with her, as long as support inside is adequate.
Brent

[lewisboats]

Take a V-hulled, single chine sailboat with twin keels. Inside, the area between the keels is sloped and narrow, reducing its usefulness. Now, what if, just for the length of the keels, the hull shape between them were flat athwartships but with the same rocker longitudinally as the standard design? The usefulness of the interior would be enhanced by allowing a lower and wider cabin sole with more headroom and less awkward stowage. The hull would have less wetted surface and slightly more volume. The forward and aft ends of this section would have to be faired into the rest of the hull, making the keels a little longer at each end.
Would there be any predictable effect on the hydrodynamics or performance or handling of the vessel?

The way I am reading this (correct me if I am wrong) it that you want to eliminate the V section between the keel blades and create a flat spot in there. I also suspect that you are suggesting that the bottom of the keels is the bottom of the flat between them creating a box between the keels? If this is so...then you are creating a massive box keel with a tremendous amount of volume. You would need to bolt massive amounts of lead to it to get it to settle on its lines. It would be a pig to sail with all that weight and you would have created a very movement un-friendly shape. Great for sitting at a mooring or dock but crappy for sailing.