Hull Is Slower Without The Keel

[SuperPiper]

I sail a micro-cruiser. It is a Sandpiper 565, 18'6" LOA with a 15'0" LWL. This little boat has 300 lbs of lead ballast in a keel that raises vertically into the cabin table. When completely retracted, the bottom of the keel is flush with the bottom of the hull.

Recently while sailing straight downwind wing-on-wing, the keel was cranked up to reduce wetted surface and to reduce drag. To our amazement, the boat speed appeared to decrease! So the experiment was repeated a 2nd time and the result was inconclusive. But, the speed did not increase as expected.

The experiment was later repeated by a different boat using its outboard motor. Again, there was no detectable change in speed.

The boat speed was being measured using GPS with resolution to 0.1 knot. The sailing experiment was conducted at about 2.8 - 3.1 knots. The motoring experiment was conducted at 4.7 - 4.8 knots. Hull speed is estimated at 5.3 - 5.7 knots.

Is there an explanation why withdrawing the keel should not increase the boat speed? Especially while travelling below hull speed? Does the keel somehow change the boat's wave-making ability to give the illusion of a longer boat? Does the appendage sticking out the bottom of the hull somehow mimic the winglet on a bulb keel? Does it provide less drag despite its increased wetted and frontal areas?

[alan white]

More likely the keel in the up position is creating a very disturbing flow pattern.
I don't know if this could be the problem, but you might experiment with taping over the slot and using that basis for keel-up speed tests.
it's doubtful the raising of the keel has lowered the boat more than a tiny bit, so that can't be measurable.
Nor could pitch characteristics be responsible if you also checked with an outboard. It seems it wouldn't even require measuring at all, that you could and should feel the increase in speed with board up.


Alan

[alan white]

If you take a drinking glass and submerge it upside down (making sure it fills to the same height as the surrounding water), and move it laterally across the water, and then do the same with the glass right side up, there is an appreciable difference, in that the upside down glass has far more resistence to sideways travel. I would wager there's another dynamic that would come up if an open tube were used and no air cushion were present.
Is the only vent a very tight opening for the cable to pass through?

[Paul Scott]

super, what DOES the bottom look like with the keel all the way up? I like the tape try. Either that or you may have found an arguement for leeboards.

Paul

[alan white]

Boy, after doing all that work to make the keel...
A lifting ballast is really nice. Leeboards are okay, but you will find, once you reduce wetted surface AND whatever else is slowing you, that you can have your cake and eat it too.
My only problem with centerboards or lifting ballast is the loss of space in a small boat. I have a very unusual 15 foot daysailer myself, with full shallow (22" draft) keel, lead down low.
I could do better to windward, however (though it's not bad). I may rig leeboards as a consequence, because I love the space in the cockpit, much like an H-12 1/2.

[Paul Scott]

Alan, if I remember right, aren't nearly all of the Sea Pearls now leeboarded (hope that's a word)? I've heard that owners were actually converting their centerboard Sea Pearls to the leeboard configuration, to the point that almost none of the centerboard(ed) models are around. I wonder how the two configurations compare going to windward? Someone's GOT to know.

Paul

[alan white]

Here's a link with a guy who had a sea pearl with centerboard, and later one with leeboards.

http://www.stevemorrill.com/sp.html

I've never owned a leeboard boat. To answer your question, though, I believe the current sea pearls are leeboarded. In grounding on hard beaches, they must be handier, as well as offering more cockpit room and fewer chances for leaks.
However, the article aforementioned mentions the hassle of changing boards--- I can agree on that score. I change every boat I own, it seems, to self-tending jib for that very reason--- especially in narrow waterways, tacking should be as simple as putting the rudder over as far as I'm concerned.
Note I have neither c'board or leebds, but full shallow keel. I love the simplicity, as said, and I can launch and retrieve alone.