Help with Olympic project

I have been struggling with a project and am over my head. In doing m research I came across this site, my good fortune, My son has qualified in the Finn for Rio (Caleb Paine USA6). We are working to optimize the boat. There is a bailer design developed by the Canadian team to replace the Elvstrom bailers in the boat that are foil shaped and are reported to provide drainage as well as lift and lower drag when deployed. The design would have a hollow foil shaped section with drain holes and would be pulled up inside the boat when not in use (bottom edge flush with the bottom of the boat), and then would be pushed down under the boat hull when deployed. I am attempting to develop the same advantage for our team. I am clearly not an engineer.

My first goal has been to calculate the drag developed by the current bailer and to use that as a top drag limit for the new design. The deployed Elvstrom has a cross sectional area of 36mm by 41 mm for an area of .001476 sq. m. My research lead me to use a C(drag) of .191 (from an equation .01112(.5 x wedge angle) + .162. This calculation led to a drag force of .615 N. Is this close to right?

The next attempt I made was to figure out a section to use and how deep it could be without exceeding the drag of the Elvstrom. I have used NACA 00XX sections (0010 and 0012 foils) on Sabot leeboards and rudders, my servo blade for my self steering etc. My reading indicated it was stable over a range of angles of attack that will occur working a Finn upwind through the waves. In order to get enough drainage through its center I was planning to go to a 0018 section or wider as necessary. Haven't got that far yet. I was assuming 4 knt. velocity and a 5 degree angle of attack.

Here is where I really ran into trouble. I have had a really difficult time finding C(drag) for the NACA sections. Not that I could not find any, but I found many different ones. Using a calculator on line I figured the Reynolds number of 252,162, and that translated into a C(drag) of .191 (http://library.propdesigner.co.uk/html/naca_0012_charateristics.html) which tallied with the NASA Turbulence Modeling Resources graph.

When I work this through I get a possible foil depth of the bailer that seems way to long to be real (5333 mm). The foil has a possible length of 55 mm and at 12% chord the width would be 6.6 mm. Do these numbers sound right? Where did I zig when I should have zagged?

My kid, and his support group, has always had to do more with less, yet he has persevered and succeeded. That is why he is stuck with me trying to do what an engineer should be doing. But ignorance has not stopped us yet!

The big questions are: will the project in fact produce improved performance, can we get large enough flow out of the boat to drain it, can we generate enough lift to make it worth while (the ability to pinch off someone to weather at a start gives you big tactical options), how do we design the drainage holes (slot at the bottom of the foil to avoid compromising the foil above or holes all along the trailing edge or ???).

One of his supporters has a CnC machine to make the units, our thoughts were that Delrin or Teflon would work well.

Please contribute any and all ideas/suggestions/corrections to the above. It really is all about winning a medal (gold is good) in Rio. He is doing his part working hard. I hope we can help him by giving him the fastest Finn in existence. We need to develop this in the next two months.

Thank you for your time and interest.

 

You maybe trying to reinvent the wheel. Have you see the Supersuck bailer, developed by a friend of mine in the 1970's - I used to sell them

http://www.sea-sure.co.uk/configproduct/externally-mounted-supersuck-bailer

Richard Woods

 

Thank you for your response, I had looked at it and it is quite similar to the Elvstrom. The hope is develop one that provides lift as well as drainage.

 

I would be greatly surprised if an in hull bailer could develop measureable lift that did not create drag that negated any lift potential.

As anl Olympic candidate you and you son will know more about competitive sailing than I do. However, being either clever enough or lucky enough to be on the favored side of a microburst is going to create more windward advantage than any mechanical device that I can imagine.

 

Do you have a sketch or an existing design that you are working off of?
Not quite sure what the configuration is.

 

I think by lift he might be refering to lift to windward as opposed to vertical lift. Very interesting idea for the Finn Class, that can an efficient shaped rudder but a very primitive flat plat delta centerboard with a half round leading edge. The half round becomes an ellipse because of the angle of the leading edge, so it is not quite as bad as it sounds, but I know it could make a real difference if you took a file and made sure it was kept well rounded and not too blunt. A little extra lift by the bailers can reduce induced drag from the centerboard. I am not sure at what point the automatic bailers would be considered leeboards as opposed to bailers.

I remember way back in the 1980s when my brother was campaigning in the Finn class there was talk about how the bailers effected the centerboard and rudder, and which of the four supersuck bailers might be best used when going to windward. There were two on each side, and one on each side was located very close to the centerboard. Probably the same arrangement is used today, but with CFD analysis you could make improvements in boat design and design.

p.s. Good luck Caleb, but Go Team Canada ;-)

 

Thank you all for your input. I got the number clarification I needed from a parent I used to know from junior sailing days who works for Boeing. There seems to be little gain with respect to lift (less than .3N for a 55m deep bailer) but there does seem to be worthwhile gains in the reduction of drag. The wedge of the Elvstrom generates about 4.55N of drag, while a foil of the same frontal section drops that down to .29N. Saves about a pound of force, which in the light stuff makes a difference. At that level, any small advantage is important.

The bailer/centerboard interaction is also interesting and worth exploring for the next quad, but frankly beyond my current expertise or ability to pursue. Time is short and we need to act to get this done.

FYI Jamie Kennedy, One of Caleb's closest friends and best training partner was Greg Douglas. They trained together for a few years and Caleb's success owes much to Greg. I know he would have really loved to train through this quad all the way to Rio with Greg. Sadly Greg retired from Finn sailing when the funding got scarce. So, yea, go Canada all the way to the silver medal!

 

They are a tough breed.

 

I strongly doubt that something the size of a bailer can produce lift AND reduce drag. There is a drag penalty for producing lift, and even in ideal circumstances this drag is inversely proportional to the square of the span of the lifting surface. The width (span) of a bailer is small, so the drag due to lift will be huge in comparison. The drag due to lift is also inversely proportional to the square of the speed, and at the speed the Finn sails, the drag due to lift is also quite high.

If you were looking to design a rule dodge, by installing a hydrofoil whose span was comparable to the beam of the boat and calling it a bailer, then you might have a chance. But it doesn't sound like that is the direction you're going.

Probably the best thing you can do is to work on a bailer that can be effective while minimizing the drag. Perhaps a "floating" bailer that would automatically open and close, depending on the hydrostatic pressure of the water inside the hull, and not be open any more than was necessary to drain the water. The Laser had a bailer that was just a molded recess in the hull and was not particularly effective. But perhaps a Laser-like recess with a flap on its lip could be more effective while not extending down as far as an Elvstrom bailer.

You might also experiment with the aspect ratio of the bailer. Which has less drag for the same exhaust area, a wide shallow bailer or a narrow deep bailer? Does it help to make the bailer longer or shorter in the longitudinal direction?

At the same time, Paul Elvstrom was a pretty savy dinghy sailor, so maybe he knew a thing or two when he originally developed his bailer.

 

I am pretty sure they are talking about improving lift to windward, not vertical lift. The Finn's windward performance is very sensitive to changes around the centerboard, which is rather primitive and class rules restricted. Two of the Four automatic bailers are traditionally located in the center of the boat very close to the trailing edge of the centerboard.

It's an interesting problem and folks have wondered for years whether to have the windward or leeward one open when needed when sailing upwind, but this is the first I have heard of someone redesigning the bailers to effect the Finn's performance.

 

Very interesting little project, Solo Sailer.
It is indeed possible to use a foil-shaped bailer for more efficient bailing, even at low speeds. After reading your description of the Canadian system, I have realized that there are actually several possible bailer layouts which could achieve the goal of faster bailing with less drag. However, you should forget the idea of creating any sensible vertical lift as a by-product. The bailer is too small and the boat speed, especially upwind is too low for that. You should IMO concentrate your efforts on minimizing drag and maximizing the water flow through the bailer. That's where you will IMO get the biggest performance gain.

 

The working part of the bailer needs to project below the hull boundary layer to be exposed to faster flow / lower pressure. I think that is why the Elvstrom type bailer has the flap displaced vertically downwards with a solid barrier above to "block" the b.l. pressure from the duct. The production bailers are one size fits all, so the height of the barrier possibly could be reduced to match the b.l. thickness for the Finn - easy to test by lowering it incrementally.

Separately from the bailer drag, what happens to the momentum of the cockpit water in the boat?
When water lands on the boat it is accelerated to boatspeed, clearly giving rise to a drag force. Then to "suck" the water out the reverse occurs - cockpit water slowed from boatspeed to zero. This must be caused by a pressure, which intuitively acts as a thrust, but I don't really understand.

 

Consider the bailer as a short column of water open both at the top and the bottom, the small rectangle in this picture:


On the upper surface the total pressure is:

p_up = p_atm + rho g H
​

At the bottom surface (submerged to a depth Z relative to the local free surface along the hull) the pressure is given by:

p_down = p_atm + rho g Z + Dp_bailer
​

where Dp_bailer is the low pressure created by the bailer flap due to the water flow below the hull (hence, it has a negative value). For the sake of clarity, I am not considering the pressure losses in the bailer duct.

The overall pressure differential (positive when giving upward-directed thrust) which acts on the column of water is the difference between these two pressures:

Dp_tot = p_down - p_up = rho g (Z - H) + Dp_bailer
​

If it is positive, the water will want to flow upwards into the cockpit. If it is negative, it will flow down and outside. The higger the absolute value of Dp_tot, the higher the flow through the bailer duct (and hence, the quicker the cockpit bailing).
So in order for a bailer to work, the following has to be true:

Dp_tot < 0,
​

or

Dp_bailer < - rho g (Z - H)
​

In other words, the negative gauge pressure created by the bailer has to overcome the term rho g (Z - H).

This equation tells you that you have two options for increasing the outflow:
1) help lift the boat out of water (decrease the term Z-H)
2) decrease the low pressure created by the bailer (the term Dp_flap).

And of course, there is the third option, not shown by this equation but obvious:
3) decrese pressure losses in the bailer duct.

The trick consists in finding the bailer configuration such that the term Dp_tot becomes as negative as possible, while maintaining the overall drag as low as possible.

That's why the idea which the Canadian team has come up with is really excellent.

 

It's an interesting puzzle. I think we can say that the work needed to create the void in the freestream to receive the bilge water is minimized when the bilge water dump velocity matches the freestream velocity. And we can design the acceleration duct so that the work needed to accelerate the bilge water is recovered as thrust. This requires a bit of thickness to pull off, either hull thickness or bailer draft will work. If we consider pressure drag to be a measure of the work done building the void, then we want the bailer shape with the greatest ratio of pressure drag to friction drag which also produces a bilge dump velocity equal to the free stream. If the bilge dump velocity doesn't match the freestream, then the pressure drag won't all be constructive work creating a hole for the bilge water.

 

I thought a NACA duct would be the lowest possible drag if sized appropriately.