ST300 Thru-hull Shorty#153. The S300/ST300 speed sensors are our lowest cost and shortest thru-hull speed sensors. As part of our Shorty#153 Series, they are designed for boats with low headroom. The plastic P371 low profile outer housing option is nearly flush and minimizes drag with only 5 mm (3/16 ) extending outside of the hull. The plastic P398 countersunk outer housing option mounts flush, eliminating turbulence for more accurate speed readings..