{"id":1567,"date":"2017-08-21T15:03:06","date_gmt":"2017-08-21T22:03:06","guid":{"rendered":"http:\/\/tradewindssailing.com\/wordpress\/?p=1567"},"modified":"2017-08-21T15:03:06","modified_gmt":"2017-08-21T22:03:06","slug":"slipstream","status":"publish","type":"post","link":"https:\/\/www.tradewindssailing.com\/wordpress\/general\/slipstream\/","title":{"rendered":"Slipstream"},"content":{"rendered":"

A better title for this Skippers Tip might be Slipstream, Prop Walk, and Rudder Control<\/strong>, however that seemed a bit long and tedious, so lets just call it Slipstream<\/strong>.<\/p>\n

According to Merriam-Webster, slipstream is defined as “a stream of fluid (as air or water) driven aft by a propeller.” \u00a0We often times use the term “prop wash” to describe the phenomenon\u00a0, however, slipstream is the more correct term. \u00a0Slipstream is easy to see. \u00a0Simply shift the transmission into forward, and watch the water behind the boat. \u00a0The current travelling backward from the boat is slipstream, and is one of the\u00a0most\u00a0valuable forces at your disposal while motoring. \u00a0Control of a sailboat is obtained by either getting current flowing across the rudder due to movement thru the water, or by slipstream created by the propeller forcing a stream of water aft over the rudder.<\/p>\n

To understand slipstream, lets start with a very basic discussion of the propeller. \u00a0A propeller is generally defined by four\u00a0criteria; number and shape of the blades, diameter of the circle the propeller turns within, pitch, and direction of rotation.\u00a0A notation stamped into the propeller such as 12 X 10 RH indicates a clockwise (or right hand) turning, 12 inch diameter prop with a 10 inch pitch. \u00a0Basically, a propeller is a screw (which is actually another name for the propeller). \u00a0Pitch is an indication of how far the prop would move forward if there was no slipping thru the water. \u00a0In this case, 10 inches of forward movement for each revolution of the prop. \u00a0Unfortunately, water is soft and slippery, so as the prop turns it does not make it 10 inches and any excess water it grabs is forced backwards away from the prop resulting in slipstream. \u00a0When the prop begins to turn on a boat that is stopped, there is a great deal of slippage, with a lot of slipstream resulting. \u00a0As the boat begins to move thru the water slipstream decreases as the prop slips less and less. \u00a0Introducing a rudder into the slipstream allows directional control. \u00a0As forward speed increases, slipstream decreases, and the current from the forward movement replaces the slipstream as the controling force.<\/p>\n

The proper amount of throttle to get a boat moving at a speed where control is possible seems to involve more art than science. \u00a0A prop turning too fast too soon slips more than it grabs. \u00a0Too slow and the boat doesn’t want to overcome inertia and begin to move, much less obtain a speed sufficient to have control. \u00a0Knowing how much throttle is needed to overcome a boat’s inertia, and once moving, how much is required to maintain it can only be learned through practice.<\/p>\n

Which I think is enough of the theory, now we need some practical tips.<\/p>\n

First … slipstream exists while in reverse, however, there is no way to translate the slipstream into boat control because there is no way to introduce a rudder into the flow. Therefore, in order to control the boat while in reverse, sufficient speed is needed to promote rudder control. \u00a0Throttle control is of paramount importance while in reverse. \u00a0Too little throttle and the boat will not want to overcome its resting inertia and will be at the mercy of wind and prop walk. \u00a0Too much throttle is wasted due to the propeller slipping through the water.<\/p>\n

Second … slipstream while in forward provides a great deal of control, reducing the amount of throttle needed to obtain rudder control.<\/p>\n

Third (and I feel the most important of the three) … once sufficient rudder control has been achieved, you MUST reduce the throttle<\/strong> to a point where control is maintained. \u00a0Continued use of the throttle after that point results in excess speed, and in close quarters, loss of control! \u00a0Also, while in reverse, more throttle results in more prop walk. \u00a0Reduce the throttle and you reduce prop walk. \u00a0Shift into neutral and eliminate it alltogether.<\/p>\n

In closing, here are a trio of cliche’s to think about:<\/p>\n