On May 13th and 14th a team from Antelope Point Marina and Beach Bags engineering performed wind load testing on a large vessel on Lake Powell. Weather forecast for these days called for constant winds of 15 to 20 mph with gusts up to 40 mph. Our test vehicle was Gettaway 3… a 75 foot long, 22 foot wide, and 29 foot tall Adonia. Gettaway 3 is primarily a solid structure on all stories with little sun screen. An accurate measurement of the Port side of the vessel resulted in 1685 square feet of wind loading from the water line to the top, front to back.
The team anchored Gettaway 3 such that the port side of the vessel would be directly perpendicular to the forecast wind. The nose was placed on a sand beach with about 10 percent slope. The boat was anchored with two lines on both Port and Starboard sides and two lines on each side of the bow directly onto the beach. The team installed two load cells on the rear Port side anchor lines as well as wind speed and direction instrumentation. All this data was gathered at 200 samples/ sec and recorded for detailed analysis.
During the test period we recorded constant wind speeds at 90 degrees to Port of 20 mph to 25 mph. Multiple wind gust events of 35 mph occurred with the strongest gust measuring 40 mph.
During the test period the team experimented with various rope conditions. The initial test set up was with all anchor lines pre tensioned to the optimum levels of 400 to 500 pounds / anchor line. Additional tests were conducted with slack lines to understand the line loads when gusts hit the boat and the lines were slack. As anticipated, there was significant movement of the boat, as much as 10 to 15 feet side to side during these slack line gust tests.
The maximum loads recorded during the two day test was 1079 and 1044 pounds for both rear anchor lines, for a combined total of 2123 pounds. With the wind hitting the boat at a 90 degree angle, the bow anchors were doing nothing but keeping the bow of the boat on the beach. The force that was held by the bow of the boat while on the beach is difficult to calculate but during analysis of all data, engineering estimated that the nose of the boat was taking 15% of the of the total wind load.