Keel Problems

[DJPierrot]

I am doing an experiment in which I will be investigating different keels with the same hull. The problem is that I need to find a constant element on the keel, and a variable one.
I was wondering in the first place if any people reading this message would have any idea about the subject, any knowledge on it or experience.
I am especially worried about re-defining a research question (To make a long story short: I started researching a minimal TDF hull, then chose one element of the hull that would vary: the keel. But the keel's problem isn't about least TDF but about making the boat go straight and about preventing it to tip over.) Therefore, I need to re-define what exactly I am looking for, and by varying which parameters am I going to investigate this?

Any idea or advice?

Thank you in advance

Pierre Badin

PS:
My experiment would be designed this way:
The boat is placed in a tank full of water, and attached to a string. At the other side of the tank, a pulley, on which there is the string, and the string is attached to a certain mass. (basically, I'm re-editing the smart pulley experiment) So by releasing the mass, the boat would have a constant force acting on it making it move on. Light gates would give me the acceleration of the hull and the keel.
I forgot to mention that the hull, tank, volume of water, and pulley would be constant, and that the mass and keel (width, length, shape or any other parameter chosen to be the variable) would be the controled variables.

[Karsten]

Hmmm.
Sounds like no keel would generate the lowest drag since I don't think you can put a side load (lift) on your keel in the experiment. Most of the drag will be surface friction and if there is no surface there is no friction.
If you want a keel at a fixed size and optimise it for the no lift condition you basically have to optimise the airfoil profile. Other people have done that for you already. Problem in your experiment will be that the Reynolds number is very small (small keel, low speed) and therefore the results are useless for a real boat.
If you want lift on your keel and minimum drag that has also been researched. With some math you can proove that an elliptical side view with a maximum aspect ratio will create the lowest drag.
If you also want some ballast as low as possible on your keel and you have to make it work in reality you are restricted by the depth of the common harbour, the strength and stiffness of the keel fin and you need a boulb at the bottom. You first have to work out how deep you can make your keel. Then you work out how much surface area you need to generate enough lift. This basically gives you the chord length of the profile. Then you work out what your lift coefficient and your Reynolds Number is and get a profile catalogue and look for a profile that has the lowest drag at the given lift coefficient and Reynolds Number. To finish your keel you produce the boulb by rotating your profile around the chord and pop it under the fin. The result should be pretty much the optimum for the given conditions (lift coefficient) and therfore for a single wind speed, heading and boat speed. Other conditions will result in a bigger or smaller keel area or different profile. You will probably find that the optimum lift coefficient for a given profile is pretty high. Problem in reality is that your keel will stall whan the boat accelerates.
Do you just want a low drag keel for your experiment or something that is usefull in reality?