Slip is the percentage (-ve) of the distance theoretical that the propeller should advance through the water in one revolution that the PITCH dictates versus the ACTUAL distance the prop advances through the water in one revolution. ie If the propeller has a designed pitch of 10inches it should advance 10 inces per revolution, If it actually only advances 9 inches it would have a Slip factor of 10 percent.(Pretty good!!)<br />OOps see this was posted to Dhadley. Hope I not jumping in too quick.<br />Sorry
No, I dont mind, but how do you figure it, Ive seen Dhadley figure it out with information given on some posts. There must be a formula having something to do with speed RPM and pitch.
Try actual speed ( GPS or radar gun ) minus calculated speed based on prop RPM/pitch. Result is SLIP( MPH ) amount. A little math to figure out slip percentage. Or use those figures in one of the prop calculators.<br /><br />
Whan I was deep sea slip was something that was worked out on a daily basis by the Chief Engineer and this was long before GPS systems were available or even that accurate. It was calculated by taking the total Revs done over 24 hours multiplied by the pitch to give the "Engine distance" done in 24 hours Minus the actual distance run Noon to Noon expressed as a percentage of the Engine distance. The "Slip" figure was thus produced and this gave a trend when taken over a period of time as to the Hull condition amongst other things. Obviously if the ship happened to be in a favourable current the slip figure would be small (sometimes even +ve) as the current would be adding to the ground speed / distance. Conversely if the current was adverse and the hull was becoming fouled then the slip figure would be high. Discounting the current out of the equation it would give an indication that the hull was due a scrub or we had inadvertently forgot to stow the anchor!!<br />I do not believe you can ever say that for any given boat/ propeller/ hull application that the slip factor is a fixed amount as too many factors come into account. A general average would give an indication of what you could expect for a particular application but it would not be set in stone. I would suggest that the faster the prop spins the higher may be the loss of efficiency ie higher slip factor.<br />Going to lie down now... Exhausted!!
