Slide rule for computing motor boat performance

Disclosed herein is a slide rule device including a base member and a slide member which is slidably carried by the base member. Each member has a pair of logarithmic scales which are located in spaced relation on the front faces thereof and are calibrated to respectively indicate boat gross weight, engine horsepower, boat speed and a boat performance factor which is a function of the boat gross weight, engine horsepower and boat speed and is indicative of boat performance. These scales are arranged in interrelationship on the members so that the performance factor for a boat having a known gross weight, propelled by an outboard motor or stern drive having a known horsepower, and having a known speed can be easily computed by simply moving the slide member relative to the base member to align the appropriate values of the gross weight and engine horsepower scales. Once the performance factor for a boat has been computed or estimated, the predicted speed of the boat, when there is a change in gross weight and/or engine horsepower, can be computed by aligning the appropriate values on the gross weight and engine horsepower scales.