Deep keel vs centerboard

RECESS · Dec 12, 2010

Two main reasons. If you are trailering or if you live in a place with a lot of shallow cruising water like Florida. I could not imagine that there is any performance gains over a full keeled boat when sailing a centerboard boat. Perhaps running dead down wind, but I doubt that makes up for the differences in other places.

I draft 2 1/2 feet with the CB up and 6 feet with it down on the O'Day. With it up I can get the bow of the boat almost to the shore on a lake. Do not think I would want to do that often when tides are involved.

I think your biggest determining factor is going to be the conditions of the place you sail.

Roger Long · Dec 12, 2010

One reason you don't see many keel centerboard boats any more is the surprising results with vessels like my Endeavour 32 which were built in both keel centerboard versions and with just 6" or so added to the keel. It turned out that the keel versions were faster to windward.

It is counter intuitive because you look at that high aspect foil reaching far down into the water and it seems like it would make a huge difference. The problem is that neither the keel nor the centerboard are large enough to provide the necessary side force on their own and each has a different optimum angle of attack (provided by the leeway). The result is neither foil providing optimum lift. The leeway that all boats must have in order to sail to windward also means that the long centerboard slot is experiencing cross flow and produces considerable resistance.

A few inches of draft is a pretty small price to pay for eliminating the knocking, maintenance, and cruise ending failure modes that accompany a centerboard. Better windward performance is a bonus as is significant construction cost savings.

A true centerboarder, in which the board is large enough to do all the work and there is no, or minimal, keel to share the side forces, is a different proposition.

Ed Schenck · Dec 12, 2010

Very well stated Roger. That is good information for anyone looking at racing or cruising a larger boat. As you indicated, doesn't apply to dinghys. Never considered the turbulence from that large slot opening.

Alan · Dec 12, 2010

Just to clarify, there are 3 different keel types being mentioned here, keel centerboard, fin or deep keel and full keel. The full keel has pretty much gone the way of the square rigger. A long (almost the length of the boat) keel with moderate draft. Its advantage is very straight tracking. Its con is the huge increase in wetted surface area, inability to point, and increased weight.
The fin keel moves the CG much lower for increased righting moment, high lift to draft ratio for increased pointing and lower leeway. Its obvious down side is in shallow water.

As for pointing and leeway make no mistake. After I changed keels from shoal to fin on my boat the increased pointing and loss of leeway was dramatic. Also the lower wetted surface area gave the boat a significant speed gain and the lower CG made the boat stiffer in a breeze. No, I can't beach it, that's pretty much the domain of beach cats. I'm relegated to sitting at anchor and using the dink if I need to make shore.

RichH · Dec 12, 2010

DC Daniel said:
Deep keel vs centerboard, given same boat make\model.
Which is faster\better in light air? IE: cb easier to heel to get initial flow? Has less drag with board up?
Is stability difference that great? cb easier to capsize?
Is any sailing to windward performance difference significantly noticeable or just on paper?
Why would a person go with a cb option, other than the foot or so less of grounding risk?
Anyone who has sailed a centerboard, ballasted boat (I never have), any additional comments.
Thank you

Is stability difference that great? *** slightly less in a CB if 'form stabiliity' is lacking. (modern large beam boats depend more on 'form stability' than on keel weight.

cb easier to capsize? yes due to the higher 'center of mass' of the boat (for comparing same boat)

The generalized FOIL shape of a deep fin keel is better due to the hydrodynamics/aerodynamics: Less drag'; and if properly designed, the foil can provide LIFT (to windward) - yes indeed, a properly designed keel shape will provide LIFT to windward; thus better enabling 'pointing ability' as such a keel 'flying' to windward. The shape, length to 'depth', thickness, etc. all combine to affect the 'efficiency' of the foil. Modern keels mostly follow the shape as defined by NACA (predecessor of NASA) of the "NACA 63A010" for sailing (or flying)... there are other NACA shapes that have less drag, more lift, etc. but the '010' shape is the overall best shape .... for the conditions in which such a keel-foil shape that most sailboats are operating. The 63A010 shape is the accepted 'standard' shape for the most efficient keel for a boat, all other shapes (for keels on boats) are less efficient overall. do websearch: "NACA + 63A010"

Cautionary note: forget all the USA high school 'stuff' about air or water streams traveling faster over one side of a foil/wind/keel causing 'lift' -- as that 'bunk' concept has been wrong since the time of the Wright Brothers. Just totally ignore this widespread erroneous concept when analyzing foils

Centerboards (heavy weight nor not) are generally flat or 'flat-ish' plates (and usualy not even remotely close to the "010" shape profile, ...... PLUS, the 'trunk' of the centerboard, not being 'faired' to the hull creates significant turbulence hence drag. Further if the centerboard trunk is allowed to come in contact with the atmosphere / air will allow the centerboard to 'suck air down' into the trunk and will result in cavitation of the foil (added to the turbulence of the 'trunk opening').

For full or cutaway full keels the length of the keel to come close to the width dimensions of the 010 would necessitate an extremely wide keel ... that this 'profile shape' about 3 ft. wide in a 010 full keel on a ~40 ft. boat would produce immense ''profile drag", add to that the immense amount of 'skin friction' due to the large LENGTH shape .... and the optimzation quickly falls off. Secondly the long 'bottom edge' creates a lot of 'tip vortice' (small 'tornados') as the high pressure side 'short circuits' across the edge to the low pressure side.

The best general shape for a sail boat operating between 0 to about 15 kts. is the NACA 63A010 ALL other shapes are less efficient.

Summary:
• deep keel comes closest to the 63A010 shape
• centerboards --- generally too flat, and have high turbulence due the shape of the 'throat' of the centerboard trunk .... plus they can easily 'cavitate' if air can get 'sucked into the trunk'
• Full keels .... wrong aspect ratio (depth to length), too flat, too narrow of width re: "010 shape, subject to substantial 'tip vortices' and 'shunting of high' to low pressure across the 'bottom'. However can better optimize at speeds MUCH MUCH MUCH greater than is possible due to 'hull speed' - a functional impossibility ---- not optimized for the normal operational speeds of sailboats.
• FWIW --- the shape of the leading edge of the keel/foil/wing is the most important.

"Anyone who has sailed a centerboard, ballasted boat (I never have), any additional comments". If you mean a CB (dagger-board, actually) with a 'weight bulb' at the bottom which is 'cranked up/down' - generally the foil section above the bulb is too narrow to generate optimum lift and generally the trunks are open to the atmosphere & can suck air at times, and thus are less efficient than a boat with out an 'air communicating' trunk, etc - ie.: fixed keels.

The best general purpose keel shape is the NACA 63A010, and the deeper (Length—Depth aspect ratio) the better the efficiency. Nature has already profoundly designed/evolved these shapes ..... appendages of whales, fins on sharks, wings of an abatross or other 'soaring' birds, etc. You dont find many full keeled or centerboard birds/fish, etc. in nature do you?

Ross · Dec 12, 2010

Sparkman and Stevens designed Finisterre is a center board boat and won several races to Bermuda.
It is my opinion that generalizing keels is as useful as the debate over front wheel drive versus rear wheel drive. There are simply too many variables to allow a broad statement that one is always better than the other.

Prime Time · Dec 12, 2010

Ross said it all here

I agree with Ross, It depends on a lot of variables.
For the Bristol 40, the CB version started as the keel (internal lead) version with 5.5 draft and the fiberglass was cut and the keel was then converted to a centerboard type.
The PHRF ratings are very similar:
162 deep keel
168 centerboard

My Bristol 43.3 draws 4.5 with the board up but 10.5 with the board down. I point very well. Not like a J 109, but we do ok.

John Nantz · Dec 12, 2010

Centerboard on a Daysailer

First off, RichH, that was an impressive write-up! A lot of information there.

Second, the next thing I wanted to say was about my experience with a centerboard on our Falcon 16 daysailer. The centerboard is a swing-up type that goes into a centerboard trunk and leaves the bottom of the boat flat.

In a good breeze when one is sailing off the wind or downwind and swings the centerboard all the way up, you can feel the boat almost rocket ahead, like turning on the afterburner.

Depending on the angle of sail, say beam reach, it works well to run with the centerboard a tad over about 45 degrees, further off the wind one can swing it up progressively higher until it is all the way up.

dkamens2 · Dec 13, 2010

Definitely Keel or Modified Keel....unless you are aground.

mauruuru · Dec 13, 2010

Shoal Draft is a bonus

I have owned three shoal draft boats. The first a Catalina 22 swing keel and the second an O'day 30 swing keel. My current boat-a Hake 32RK has a vertically lifting keel. Draft goes from 20" to about 7'. Unlike swing keels, center of effort never changes regardless of keel depth. The boat can go to the beach--and we have gone over the bar and up on the beach at Sandy Hook just like the power boats.

The vertically retracting keel is the way to go for any shoal water sailors out there who want to maintain max stability in thin water and still be able to gunkhole. And it is trailerable for those who like to travel to new overland destinations!

Blaise · Dec 13, 2010

Vey nice Rich. When you have an airfoil in air, you can create lift because air is compressible. Aiefoil shapes in keels and rudders are all about maintaining attached flow. Once you lose that you cavitate and cause drag. One thing discovered years ago was that fatter is faster. A fatter airfoil shape is much better at maintaining laminar flow that a flat plate. You would think that a flat plate 1/2" thick would have less drag that a three inch thick airfoil but that is not the case. That is why so many one designs have maximum thickness measurments for rudders and centboards but no minimums. All your rudder does is deflect the flow of water. As long as you do that without cavitating you are fast. If you stall the rudder, (too great fo angle of attack) you cavitate and loose a huge amount of effectivness. If you stall a wing, you loose lift (cavitate in the air)you fall out of the sky, which any pilot can tell you. This is extremely simplified but hopefully is helpfull.

timxgo · Dec 13, 2010

Good question

I have been sailing an Irwin 28 keel centerboard version since1984. The lifting mechanism is a constant source of problems. The original board even disappeared once between haulouts. I had to fabricate a replacement. It has never worked well enought to be able to document even anecdotally the differences between up and down. As most of my sailing is in the notoriously thin water of Galveston Bay, I can say that the 3' board up draft is most welcome. Needless to say, working to windward is tiresome in a breeze, but oh well, I'm out on the water.

capta · Dec 13, 2010

I've a Pearson 530 w/ a weighted centerboard & I love the centerboard.
I do not race so pointing is not a factor, but comfort definitely is.
At anchor I can increase my draft by 4+ feet and sit very comfortably while most boats around me roll their guts out.
On the passage from Ct. to the VI, I experimented w/ the board a lot & found, again, w/ the board down comfort was increased immensely, but speed was reduced slightly (a trade I'll happily make).
Of course, cruising in the tropics can be a real pain w/ a draft over 9 feet (I've done the Bahamas w/ 9.5' & ate a lot of Rolaids), so once again the board is an advantage.
It can be a bit noisy, but I've finally found the right position for anchoring, & you've got to remember to bring it up when necessary, but I'll never have another cruising boat w/o a board.
A centerboard requires a lot of experimenting as even a foot or two can change the sailing characteristics of the vessel, considerably. Remember, it's not an all or nothing proposition, there is infinite adjustment available.

RichH · Dec 13, 2010

Blaise makes some very valid points .... attached flow.

I have a sportboat with flat plate 'centerboards' and flat plate rudders ... A big ILYA Scow with 2 bilge-boards and twin rudders. The bilge-boards arent parallel to the centerlilne but angled to improve windward performance by many degrees; only one bilge-board is 'down' at a time. This boat is capable of 20++kts and I have occasionally pulled water skiers with it.
Once the flow attachment on a board begins to fail .... the boat will begin to loudly and audibly 'groan' or hummmmmm and will begin to noticeably 'vibrate'. Once this (separation) begins the speed, and helm control, will begin to noticeably fall off .... until I correct the helm pressure and re-establish the attachment. These bilgeboards are in trunks and do sometimes loudly 'suck air' ... and that can also be felt in the helm due to cavitation and its accompanying loss of control; luckily the boat is sooooo fast that the cavitation is usually 'behind' the foil. More modern versions of this boat now have replaced the flat-plate rudders with more true 'foil shapes' to alleviate this important problem.

I have raced 'short keeled' boats (H27, etc.) which when they lose their flow attachment can actually totally lose their 'bite' and seemingly 'violently broach' when beating to weather if really pressed 'hard' !!!!!!!!

So, once a keel (or rudder) begins to generate considerable 'lift' ... and the NACA 63A010 shape is one of the better all-around one ... if you begin to lose the flow attachment, things can quickly become unstable & 'hairy'.

Interestingly also I find, that most of these foils will seldom generate much 'lift' .... if the boat has any significant WEATHER HELM which apparently keeps the foils/keel at a 'too high angle of attack'. It seems, for on almost all boats, that if the rudder has to be held at more than ~4°+ degrees to keep the boat going straight ... the keels (and rudders) simply wont 'fly' very well. .... the answer to that issue begins with close to perfect sail set/shape/trim: SAIL TRIM forum with Don Guillette --http://forums.sbo.sailboatowners.com/forumdisplay.php?f=17
;-)

James DAddio · Dec 13, 2010

Wow I learned a lot from these masters of sailing..Sailing a 28 O’day for about 7 years with a drop keel have always guessed at the advantages with it down. This design drops out like a jackknife design so there must be air going up there, which I haven’t really, noticed the difference.. 3.3 to 6.6 but the drop keel is very thin.. We however have had difficulty during weather helm of more than 4 degrees steering in strong wind and felt we were loosing speed with this acqward compensation. You can’t beat the access in shallow area for mooring we sail the Noank-Martha’s Vineyard area large current and swells on the way always feel comfortable with that drop keel but failed to recognize the factual differences these were great bits of information thank you Jim

geneWj · Dec 13, 2010

Well with a PhD in Fluid dynamics from Cal tech and the tank testing of Intrepid twice and all the Mull 6 meters under my belt. I take a different view!
My dear friend Morgan Embroden designed a 32' boat called Teacher's Pet III. He wanted a shoal keel with the ability of a dagger board. draft was a very important consideration. In between Intrepid tests we put his design on the force balance after about 500 plus hours wer developed a keel which after it reached a certain speed developed a lifting moment. It was different than any other keel on the market at that time.
The design was stolen by french designer and incorporated in his class of boats called the Trismas class. There were over 255 of them buit in fiberglass and aluminum.
The 32' boat was the very first small boat allowed in the Transpac and for the first 1800 miles she was boat for boat #!. The U.S Coast Guard clocked her at 16 knots going down Moloki Channel.I have posted pictures of her keel on the Trailer sailer board on many occasion.I will now post a picture of her Again on the trailer sailers board.
respectfully geneWj

hfngotsail · Dec 13, 2010

Being that I sail exclusively in fresh water, and on Lake Chargoggaggoggmanchauggagoggchaubunagungamaugg, where depth can go from 35 feet to 2 feet in a matter of seconds, a swing keel (CB) is essential. I sail an Oday Mariner with CB, at Webster Sailing Association, keep it on a trailer, and once again, a CB is essential for launching and retrieving. My experiences with high performance Catamarans (NACRA) has always been that the high aspect ratio dagger boards out performed most other types, especially the HC 18 with the pie shaped CB's, but the NACRA dagger boards don't swing up if they encounter the bottom of the lake, but rather create a costly repair. It all depends on where you are going to spend your days sailing.

hfngotsail · Dec 13, 2010

Sorry, one last concern. In the high performance NACRA designs, the trailing edges of the rudders, and the daggerboards had to be filed to a nice sharp edge to avoid cavitation. My 5.2 could easily reach 22 knots, but when she was new, and the trailing edges were rounded (factory stock) it would cavitate to the point of actually feel like somebody threw out an anchor and was dragging it on the bottom, excessive vibration, and stall. I seriously think that a sailboat that can only make 6 knots would experience very little cavitation regardless what the trailing edge shape is.

Roger Long · Dec 13, 2010

Blaise said:
If you stall a wing, you loose lift (cavitate in the air)you fall out of the sky, which any pilot can tell you.

Funny, I used to fly and stalled many times without falling out of the sky. I never even lost lift. Instead, the airfoil would lose it's stability in the airstream and suddenly seek a new angle of attack where lift and weight were equal. This would involve a brief and sudden pitching down of the nose after which I would ease the yoke and push the throttle in to recover.

Sometimes, just for fun, I wouldn't recover but would keep pulling back on the yoke and put the plane into a mushing descent where gentle pushes on the rudder pedals would rock the wings back and forth by speeding up and slowing down the airflow over their tips. The wings were fully stalled yet developing exactly the same amount of lift as in normal, level flight because I had reached a steady decent angle and speed. You only want to do this in a well behave plane like a Cessna 172 that is well rigged unless you are proficient at spin recovery.

My vertical speed in the 172 was actually less than the speed at which the much touted Cirrus descends under its emergency parachute. However, I still had a residual forward speed of about 40 knots and didn't have the shock absorbing seats so going all the way to the ground this way would hurt.

Stalls usually kill because one wing stalls before the other and the plane loses lift because the airflow starts being mostly sideways over the wings as the nose drops and the aircraft goes inverted.