by Bob Dill - US 3904 - February 1993
All sail boats have performance characteristics associated with their design, sailing conditions and the amount of wind. Like most of us I have been curious about how fast we are truly going on the ice. Last fall I bought a radar gun. It can accurately measure the speed of a DN about 500 feet away. Wind speed was measured with a Davis Turbometer. The following are some measurements we have made so far this year, mostly from the Gold Cup.
Making measurements of wind speed or boat speed is easy. What is more difficult is measuring the wind the boat experienced to attain a given boat speed and sailing angle. For the purposes of this discussion I estimated the wind speed the boat saw based on the range of winds I measured on my anemometer over an extended period of time.
At the low end of the wind range the boats will sail in winds as light as about 2-1/2 MPH. The down wind boat speed is about 10 MPH and the up wind boat speed is about 12 MPH. Peak running speed is about 14 MPH so when boat speeds get slower than 10 MPH the races degenerate into running races. Races sailed in the North Americans on Wednesday in 2.5 to 3.6 MPH wind had peak boat speeds of 15 MPH and relatively little running. Races sailed on Monday in slightly less wind had too much running.
By 5 MPH wind the peak down wind boat speeds are in the low 20's. By 15 MPH wind the down wind boat speeds are in the mid to high 40's. The peak speed we saw at the Worlds was 56 MPH in wind that was 15 to 18 with gusts into the low '20s. Peak up wind boat speeds in this wind were in the low 30's Even in 25 to 30+ winds a few weeks ago Peter Hill and Bob Schumacher were sailing in the high '50s on a deep down wind angle.
On a broad reach with the sail sheeted as much as the boat would handle Bob got up to 68 MPH. A limiting factor is that the balance of the helm is very dependent on how much of the sail is luffing. This resulted in the need for rapid and radical corrections with the tiller. When Bob let out the sheet at the end of his run the runner lifted completely and he dragged the stern of boat for 150 feet or so. The drag of the sail exceeded the weight on the front runner.
We measured a couple of high wind indicators that will be handy when you do not have an anemometer in your hand. At about 22 MPH the sails on a parked boat will start to flog (modern racing sails). At about 32 MPH the boat will start to hop backwards on a rear mounted break.
Suggestions for racing winds on good ice: Wind that dips below 2.5 will result in excessive running. 3 MPH should eliminate running by skilled sailors. Of course, on sticky/snowy ice higher minimum wind speeds are appropriate. On the high wind end, 25 MPH in regular gusts seems like a reasonable top speed for racing. The Europeans set 12 meters per second (26.8 MPH) as their upper limit.
In terms of performance on a race course, it looks like the best boat speed to wind speed performance is at low wind speeds. A 4 or 5 to l ratio is possible at 3 to 5 MPH wind. At 15 MPH wind the ratio is probably closer to 3 or 3.5 to 1. By 30 MPH wind the ratio is only about 2:1.
I am also trying to get some data on other types of iceboats. If you would like to help with this project please get in touch with me.