CB 10 degrees-Keel 20 degrees

RichH · Mar 27, 2012

A centerboard is a flat plate shape .... which generates maximum lift at only one precise angle of attack.

A keel generally is a parabolic cross sectional shape which was 'borrowed' from the NACA (predecessor of NASA) aircraft wing shapes. The typical keel shape from the NACA compendium is a similar to the NACA 63A010-xxx shape - a wing of an 'all-purpose' airplane and when used as a keel has a broad range of angles of attack that are efficient for generating lift.

http://www.hanleyinnovations.com/keelanalysis2.pdf

Here's what the 63A010 looks like: http://www.worldofkrauss.com/foils/1737

Bill Roosa · Mar 27, 2012

Also, keel boats right themselves better than CBs. So if you don't want to end up in the water you have to be more prudent in a CB craft.

RichH · Mar 27, 2012

The CB / keel amount of heel is only a 'general guideline' for a 'general' shaped boats.

Many loooooong or 'stretched' waterline and hard chined boats, both CB and keel boats, sail better when heeled 'way over' ... the 'side' of the boat digs-in and increases the surface resistance while simultaneously much of the windward side of the boat comes out of the water for less skin friction/drag. .... all depends on hull shape.

If your boat is 'raced' on a widescale basis, each 'class' will usually have recommendations of how much heel such a boat should be sailed ... if you're looking for 'optimum'.
Many such 'classes', especially 'open developmental' classes, have done hydrodynamic 'tank testing' to find the 'sweet spot' of where speed vs. heel angle is 'best'.

RobertLangdirect · Mar 27, 2012

RichH said:
A . . . keel generally is a parabolic cross sectional shape which ... has a broad range of angles of attack that are efficient for generating lift . . .

Rich;

It isn't that I don't believe that a foil causes lift, but I don't see how a symetric shaped foil does -- produce lift. Did we not study the Wright Flyer and how the wing is longer on top than on the bottom and how air flowing over the top has to travel faster than air over the bottom, Thus low pressure on top producing lift?

I think, my boat, at rest, if hit by a 10 MPH puff, will heel over 10 degrees. And my boat, under sail and at speed, when hit by a 10 MPH puff, will heel over at 10 degrees. The heeling come from the sail -- an asymetrical foil.

Or, is there some other aspect of geometry that comes into play? When heeled over, does the aspect of the fold present itself in the plane of movement as an asymetrical wing and actually provide lift?

RichH · Mar 27, 2012

RobertLangdirect said:
Rich;

It isn't that I don't believe that a foil causes lift, but I don't see how a symetric shaped foil does -- produce lift. Did we not study the Wright Flyer and how the wing is longer on top than on the bottom and how air flowing over the top has to travel faster than air over the bottom, Thus low pressure on top producing lift?

The Wright Flyer's wing was like a sail having virtually NO thickness and the 'top' is/was just as long as the 'bottom' is .... as a sail is. The concepts of 'lift from a foil' that are taught in US public Highschools, etc. that explain that 'the air moves faster on the top side' is entirely WRONG ... and as was originally discovered by the Wright Brothers to be wrong.

The flow or air across a foil/wing is a vectorial CIRCULATING sum of flow and the lift is caused solely by that vectorial circulating flow field because the air/fluid has VISCOSITY.
In all practicality, the flow of air, etc. across the 'low pressure' (lee) side has MORE flow, not 'faster' flow.

Air with unconstrained / unducted flow at atmospheric pressure is essentially non-compressible with changing pressure; so if incompressible it doesnt 'speed-up' relative to the wing due to 'shape' and pressure differences! If the air 'sped up' it would be compressible at atmospheric pressure in 'open flow' ... and thats not the case.

Yup, generations of american public high school 'science' students, and a lot of pilots, etc., have been WRONGLY taught about 'lift' ... the 'curriculum' that includes this is and has been WRONG for almost 100 years.

Origins of LIFT: http://www.arvelgentry.com/techs/origins_of_lift.pdf Although written for the 'layman' this is a 'hard' technical read for those without math / fluid dynamics backgrounds. You really have to have a 2 or 3 dimensional geometric perspective when reading this explanation.

Rob2435 · Mar 27, 2012

Funny, many people get on my boat (Beneteau 352, winged keel) and want to bury the rail because they think the boat sails faster, it doesn't. The drag caused by the rudder to hold the boat on course will almost certainly cancel any thing gained on the 'waterline'. I've heard that Beneteaus are best sailed between 8 to 12 degrees of heel.

Of course a racing boat is a totally different hull design.

RichH · Mar 28, 2012

robfmcc said:
Funny, many people get on my boat (Beneteau 352, winged keel) and want to bury the rail because they think the boat sails faster, it doesn't. The drag caused by the rudder to hold the boat on course will almost certainly cancel any thing gained on the 'waterline'. I've heard that Beneteaus are best sailed between 8 to 12 degrees of heel.

Of course a racing boat is a totally different hull design.

I used to crew on a 'hot' Bene First 351 WK. Our best heel to windward was at about 15-20+° ... rudder drag is not a problem and weather helm is easily corrected by increased halyard / cunningham tension to bring the position of max., draft well forward to attain a 'light' rudder pressure / helm balance, the second from the top batten aft section was approx. aligned to be parallel with the boat's centerline .... like you do for 'most' boats.

RobertLangdirect · Apr 11, 2012

RichH said:
The Wright Flyer's wing was like a sail having virtually NO thickness and the 'top' is/was just as long as the 'bottom' is .... as a sail is. The concepts of 'lift from a foil' that are taught in US public Highschools, etc. that explain that 'the air moves faster on the top side' is entirely WRONG ... and as was originally discovered by the Wright Brothers to be wrong.

The flow or air across a foil/wing is a vectorial CIRCULATING sum of flow and the lift is caused solely by that vectorial circulating flow field because the air/fluid has VISCOSITY.
In all practicality, the flow of air, etc. across the 'low pressure' (lee) side has MORE flow, not 'faster' flow.

Air with unconstrained / unducted flow at atmospheric pressure is essentially non-compressible with changing pressure; so if incompressible it doesnt 'speed-up' relative to the wing due to 'shape' and pressure differences! If the air 'sped up' it would be compressible at atmospheric pressure in 'open flow' ... and thats not the case.

Yup, generations of american public high school 'science' students, and a lot of pilots, etc., have been WRONGLY taught about 'lift' ... the 'curriculum' that includes this is and has been WRONG for almost 100 years.

Origins of LIFT: http://www.arvelgentry.com/techs/origins_of_lift.pdf Although written for the 'layman' this is a 'hard' technical read for those without math / fluid dynamics backgrounds. You really have to have a 2 or 3 dimensional geometric perspective when reading this explanation.

Rich: "Faster was a very poor choice of words for me to use. I see what you say abut "flow." Regarding the Wright Flyer, I ask not to debate the shape of the wing because in words, I do not think I can understand what you may explain to me. Rather, I want to talk foils shaped keels.

Doing a drawing, I can see the flow. I see the way it creates lift when pointed anyway other than directly into the current -- er . . . flow.

Now the 64 dollar question. With flow over the keel producing lift to starboard and the resulting air flow, apparent wind on the sail to starboard, can the sum of the vectors, in any practical sense, move a boat forward? That is, on an otherwise zero wond day, can a skillfull skipper use current to produce apparent wind, the tack back and forth across a flowing channel, to actually move up against the current?

Thanks!

RichH · Apr 11, 2012

RobertLangdirect said:
Now the 64 dollar question. With flow over the keel producing lift to starboard and the resulting air flow, apparent wind on the sail to starboard, can the sum of the vectors, in any practical sense, move a boat forward? That is, on an otherwise zero wond day, can a skillfull skipper use current to produce apparent wind, the tack back and forth across a flowing channel, to actually move up against the current?

Thanks!

With zero wind you simply wont have the developed force from the sails to move the boat and hull drag in the oncoming current will probably overcome any 'forward' motion against the current.
Is it possible to sail solely on the current? I suppose its theoretically possible and it would be similar to the 'boat' that sailed FASTER downwind than the speed of the wind using a propeller as its means of propulsion ... but that was a 'mathematical trick' .... the 'pumpkin seed' effect where TWO opposing forces are forced together in a trigonometric 'opposition' that mathematically tends to produces almost 'infinite' resultant force (dividing by the SINE of the intercepting exceedingly small angles). Squeezing the slippery (almost frictionless) pumpkin seed with opposing fingers ... to 'rocket' the seed outward. Boats arent so 'frictionless', .... unless youre illegally dumping 'strange polymers/surfactants' plus are blowing 'air bubbles' into the water at the hull to overcome the hull drag.

Todd Smith · Apr 12, 2012

RichH said:
With zero wind you simply wont have the developed force from the sails to move the boat and hull drag in the oncoming current will probably overcome any 'forward' motion against the current.
Is it possible to sail solely on the current? I suppose its theoretically possible and it would be similar to the 'boat' that sailed FASTER downwind than the speed of the wind using a propeller as its means of propulsion ... but that was a 'mathematical trick' .... the 'pumpkin seed' effect where TWO opposing forces are forced together in a trigonometric 'opposition' that mathematically tends to produces almost 'infinite' resultant force (dividing by the SINE of the intercepting exceedingly small angles). Squeezing the slippery (almost frictionless) pumpkin seed with opposing fingers ... to 'rocket' the seed outward. Boats arent so 'frictionless', .... unless youre illegally dumping 'strange polymers/surfactants' plus are blowing 'air bubbles' into the water at the hull to overcome the hull drag.

I wouldn't call it a "mathematical trick". It is an actually "wind machine" called the blackbird.

http://en.wikipedia.org/wiki/Blackbird_(land_yacht)
http://www.fasterthanthewind.org/

A boat could do it, iceboats do it all the time, but as Rich says, the drag of a hull is probably too high to allow it. Maybe a foiler could do it.

By the way, there is absolutely no difference between
A) still water and a 5 knot wind
B) 5 knots of current and a still wind ( calm relative to shore )

Dan Johnson · Apr 12, 2012

Eh, what about weight...

I have a keel boat and raced on a centerboard boat. Big difference in righting moment between the two due to weight of the keel. With either, 20* of heel seems to produce more sideways movement than 10 to 12* provides.

RichH, bringing a Gemini cat into WCM tomorrow; off to North East on Sat.

Crazy Dave Condon · Apr 13, 2012

Quoddy;

I have more experience than most on the Hunter 260 and will speak in layman's terms.

12 to 14 degree heel maximum as 10-12 degree is the best. Remember this is a water ballast and cennter of effort is higher than a keel boat. Thus the old addage flatter-faster. The trick is in sail control and make sure there is hardly any pressure on the rudder.

Never believe anyone who says you can empty the water ballast in order to go faster. You will definitely go over, harder to control and you will not effectively compete others in racing.

The centerboard is basically a finn that keeps you from tracking sideways. Sometimes if you want to go from point a to point b, you can for better lack of terms "crab the boat just like an airplane.

I was very much involved with this boat. Check archives.

crazy dave condon

RobertLangdirect · Apr 13, 2012

Todd Smith said:
. . .By the way, there is absolutely no difference between
A) still water and a 5 knot wind
B) 5 knots of current and a still wind ( calm relative to shore )

Todd -- Hmmm. I will need to think that through but for the moment and for the sake of what I am trying to get my head around, I will accept that as factual and say thanks.

THANKS because, most obvious to me, I can sail my boat from any Point A to any Point B, regardless of where the wind is from. I just have to tack a bit, at the worst.

So, what you are really saying is the 5 knots of current will create apparent wind and I should, just as I would with the raw wind, sail from A to B. Accept with the current dead on the nose and in that case, I simply go backwards.

THANKS! That really gives me something to think about.

Robert

RichH · Apr 13, 2012

Dan Johnson said:
RichH, bringing a Gemini cat into WCM tomorrow; off to North East on Sat.

Dan - just remember that the reason that cats dont heel over is ... they are so ugly that the earth is actually rejecting them. :-O

RobertLangdirect · Apr 13, 2012

Use a 3x5 card and a cup of coffee

Dan Johnson said:
I have a keel boat and raced on a centerboard boat. Big difference in righting moment between the two due to weight of the keel. With either, 20* of heel seems to produce more sideways movement than 10 to 12* provides.

RichH, bringing a Gemini cat into WCM tomorrow; off to North East on Sat.

If you use a 3x5 card to make a make a foil, or even better, a tapered foild such as the fin keel on the Pearson 31, and dip it into coffee. Now, from the center, you can measure the resulting line. The port and starboard will be equal.

Next, change the angle of attack. The port or starboard will be greater and, if we had flow over the foil, the inequity will produce lift to the greater measured side.

Now, redip the foil but his time, do it as if the boat is heeled over. The new line will make the larger measured side even larger (by 6 percent in my experiment). The greater draft should produce more power (just as greater draft does on a sail).

I expect that as one lifts the keel, this greater draft approaches it maximum, but at the same time, the resulting movement lifts the rudder up and the boat rounds up, so we try not to go there.

So, in very light wind, we are taught to put weight on the leeward side. Ask why, and most sail instructors say it is to better shape the sail. But now, seeing the reshaping of the foild in the coffee cup, I think it has everything to do with increasing the lifting force of the keel.

Dan Johnson · Apr 15, 2012

Rich...

Point taken, but with a 14 foot beam and 34 feet of hulls, it was a very stable platform for the two days of motoring up the Bay. Hot scrambled eggs and sausage sitting in the cockpit under way in the cabin lee made up for the difference in esthetics. Besides, it's not my boat!