Has anyone taken the brakes off a C16?

[Citroen/Dave]

Capt. Nemo,

My very first thought when I saw the underpowered C16 was to go to a gaft main sail.  More drive at a lower center of effort seemed very appealing.  Then I started to read about the weather helm tendency.

After I install a new rudder, I may look for a used gaft sail to play with.  I know that a gaft will move aft the center of effort and make a weather helm more likely. But perhaps that helm response will fall in the range of tolerable with the addition of a proper foil rudder.

I wonder if anyone has tried a gaft main on a C16 with a foil rudder? I know that all the subsequent renditions of the C16 try to move the center of sail effort forward, but like Bob I am always thinking about the next improvement in boat speed.  I am always in a race with, me.

Skip 1930,

I think we are on the same page.

Kickingbug1,

The centerboard goes through the bottom of the stub keel in the pictures that I have seen. I bet it would be improved, also, by making a hydraulically smooth keel bottom.

Citroen has been marketing a turbo charged three cylinder diesel that get 63-79 miles per gallon. I would like to try one of those anchors . . .  Citroen typically wins the world championship of rally racing each year.  Real world racing, not that oval stuff.

Thanks all for your thoughts and experiences.

Dave

[Citroen/Dave]

Skip,

The Hotckiss was a remarkable car for its time.  It a shame the did not progress forward through time.

Dave

[skip1930]

" a remarkable car for its time " Well jesting aside, yes it was remarkable. Remarkable that it killed no one that I knew of.

skip.

[Citroen/Dave]

I just saw a program about submarines.  You should take a look at the latest Virginia class subs, in particular the conning tower with periscopes withdrawn.  The sub's sail looks very much like the C 16's stub keel upside down.  Please note the rounding of the top of the sail for an example of a proven rounding of the sail (or rounding of the C 16's stub keel).  The sub's designers would not allow turbulence to give away the silent operation . . .   As expected, there is only a slight addition in height, probably a little less than half of the cross section width of the conning tower.

Dave

[skip1930]

Saw the same program, good point.
Read the book " Blind man's Bluff' ? Good book.

skip.

[Pacman]

The sub's sail looks very much like the C 16's stub keel upside down.  Please note the rounding of the top of the sail for an example of a proven rounding of the sail (or rounding of the C 16's stub keel). 

How fast were  the Virginia class subs designed to travel underwater?

My reason for asking is that optimum foil shape depends on design speed.

I suspect that the submarines were designed to travel quite a bit faster underwater than my Com Pac 16 was designed to sail.

[Tim Gardner]

The specs say the Virginia class subs were designed for 25+ knots.  Word has it they can travel in excess of hull speed with the propulsor drive system.

Note: 26knts is fast! one 24hr day = 624 nautical miles

tg

[skip1930]

Nukes underwater top speed is classified. I heard, but cannot swear to it, that one of these vessels was timed from a nautical mile out to under the keel of a aircraft Carrier in 60 seconds. That's splitting some atoms indeed. I wonder if it's true?

Steam...or rather the coefficient of expansion of saturated steam is tremendous. Steam locomotives were said to out pull diesel/electric locomotives routinely.

skip.

[Pacman]

That is what I was thinking. 

If we were to design a foil shape for the bottom of our keels the curve would be quite different than the curve in the much faster submarine design.

However, I do like the concept.

[dwkfym]

Dave,
I don't have the actual engineering experience as you do and I am completely new to sailboats, but I did work on race cars in college as the president of a formula race team (international race car engineering (and racing!) competition).  I did a bit of studying in aerodynamics, particularly on race cars (and definitely not on sailboats).

Maybe I am lacking in understanding of what you were saying in the OP, but are you talking about vortex drag?  My understanding is that vortex drag is only appreciable in lift generating surfaces with an angle of attack, or of course at supersonic speeds or other conditions where drag is increased by many factors.  Now from what little I understand about keels, yes it does see some sort of angle of attack, probably all the time due to lateral drift, but it is really not much.  A cleverly designed horizontal winglet arrangement may work, kind of like wing keels although from what I understand, wing keels try to achieve a different goal.  

However, I do understand that a properly trimmed rudder will work with the hull and keel to work as one large airfoil.  I would be curious on how reducing vortices generated by the keel would affect lift generated by the keel/hull/rudder working in conjunction.

Of course with minimal effect, but one thing I did learn in racing cars was every little bit counts and it all adds up.

One thing I do remember is that water flow around most solid objects is almost never laminar, relative to how laminar it can be with air.  Except on some palegic fish.  I don't know how that would play into reducing vortex drag on a keel or its effect in overall drag.  From what I understand, fluid dynamics (which I know almost nothing about) is a completely different game in water and boat design is more of an art than say, race car design.  Which is more of an art than aircraft design.. (predictability of medium being the spectrum here)

I hope I didn't misunderstand your whole post.

[skip1930]

Very interesting discussion about drag, vortex, air and water...when I worked at Palmer Johnson Yachts I glanced at an article in Yachting Magizine dealing with the impracticality of using a variable speed transmission [ like in cars/trucks ] in place of our normal Twin Disc transmissions that hook-up between the V-16 diesels and the fixed props. For many reasons the hp is not available when transmitted to the props through  multi-speed transmissions. Water and it's viscosity totally make this not possible. For what it's worth we did manage to take a 150 foot~130 ton P.J. Yacht with three V-16 [ 4000 kW each ] and break the 50 knot barrier but all with constant speed set-ups and 64,000 gallons of diesel oil to fill her up. The purchaser wanted to go from Florance, Italy to Marcellis, France in a day. And to take his house along too.

As any Air Force guy will tell you, resistance through air squares as speed doubles...I wonder if this applies in water? I read in Skunk Works that the SR-71 Blackbird spy plane, when at speed of mach 3+ needs the air entering the jet engines to be slowed down in able to 'breath', so these cones were used to close off the air intake, thus slowing the air down. Otherwise the air entering is stacked up to a consistency of molasses.

skip.

[dwkfym]

As any Air Force guy will tell you, resistance through air squares as speed doubles...I wonder if this applies in water?

Yes for water as well.  I forget the formula but it goes something like coeff. drag x cross section x fluid density x velocity^2 for all fluids.   Not just for drag, but I think for a lot of forces generated by fluid motion is done with v^2.

I think for low speeds drag formula becomes different, but I don't know anything about it and where sailboat speeds will fall.

[dwkfym]

Just wanted to update this thread a bit after doing some thinking.  Rounding off the trailing edge of the keel will probably increase the vortices generated by the keel when it is moving at a slight angle of attack, therefore, increasing drag.  When sails are under power, boat will almost always be moving at a small AoA.  Under motor power, the trailing corner will not have much negative effect on drag because the cross section of the keel is small and effectively becomes a 2D flow environment.  This is from what little i know about fluids, if someone knows more please chime in.