Airmar has a new style speed sensor that doesnt use a paddle wheel, check it out CS4500 uses an ultrasonic speed sensor, data is collected 5700 times per second for tracking rapid changes in vessel speed <a href="http://www.airmar.com/Product_Catalog/marine_catalog/thru_hull/thcat_index.html" target="_blank">http://www.airmar.com/Product_Catalog/marine_catalog/thru_hull/thcat_index.html</a> <BR>download catalog then go to page 9