Thought Experiments:
- a thought experiment involves thinking about a hypothetical situation which has a tangential relationship to iceboating
- by thinking about the situation you gain valuable insight into a technical matter
- for example: we are trying to analyze why sharp longer runners have better lateral resistance than sharp shorter runners
- constants for both long and short runners
- hard black ice at 30 deg F
- 90 degree face angle
- runners are stoned to very smooth straight edge with a 600 grit
- 500 pounds of weight on special sled (runners plus sled)
- two runners on the sled mounted in friction less pillow blocks
- runners are separated by 20 inches on the sled
- a wizard is pulling the slid forward at 50 MPH
- think about Runner A:
- traits
- runner that is 40" long
- very little crown
- 90 deg facet
- stoned to a very sharp tip
- smooth hard black ice with temp of 30 deg F
- weight on the runners = 550 pounds
- the runner digs a groove into the hard black ice
- the depth of this groove is a function of the runner length and the hardness of the ice
- friction force of runner acting in opposite direction to boat travel is about (.01 * weight) on the runners, or 5.5 pounds
- the .01 is called coef. friction of steel on ice
- friction force at right angles to the runners is about (.75 * weight), or about 412 pounds of lateral resistance
- this force is much higher than forward resistance because the tip of the runner is tearing up the ice along the whole runner edge as it is pulled sideways
- in this tearing up process the tip is slicing into the bonds of the ice crystals and separating the chemical bonds
- most of the tearing up process is going on at the extreme tip of the runner
- once a little bit of ice is torn up at the extreme tip it is easily pushed up the face of the groove in the ice
- the 412 pounds of side bite amounts to 10.3 pounds per inch of runner length (i.e. 412/40 = 10.3)
- now think about Runner B:
- traits
- everything is the same as Runner A:, except the length is only 20" long
- the coef. of lateral resistance is going to be similar to the longer runner
- the runner tip is digging in twice as deep because there is twice as much weight per inch on the runner tip
- each inch of the shorter runner must dig up twice as much ice as each inch of the longer runner
- if we assume the side bite of the shorter runner is 412 pounds then the side bite per inch of runner length = 412/20 = 20.6 pounds
- as it gouges out a portion of the ice the whole ice structure higher up is weakened
