Slipstream

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

According to Merriam-Webster, slipstream is defined as “a stream of fluid (as air or water) driven aft by a propeller.”  We often times use the term “prop wash” to describe the phenomenon , however, slipstream is the more correct term.  Slipstream is easy to see.  Simply shift the transmission into forward, and watch the water behind the boat.  The current travelling backward from the boat is slipstream, and is one of the most valuable forces at your disposal while motoring.  Control 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.

To understand slipstream, lets start with a very basic discussion of the propeller.  A propeller is generally defined by four criteria; number and shape of the blades, diameter of the circle the propeller turns within, pitch, and direction of rotation. A 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.  Basically, a propeller is a screw (which is actually another name for the propeller).  Pitch is an indication of how far the prop would move forward if there was no slipping thru the water.  In this case, 10 inches of forward movement for each revolution of the prop.  Unfortunately, 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.  When the prop begins to turn on a boat that is stopped, there is a great deal of slippage, with a lot of slipstream resulting.  As the boat begins to move thru the water slipstream decreases as the prop slips less and less.  Introducing a rudder into the slipstream allows directional control.  As forward speed increases, slipstream decreases, and the current from the forward movement replaces the slipstream as the controling force.

The proper amount of throttle to get a boat moving at a speed where control is possible seems to involve more art than science.  A prop turning too fast too soon slips more than it grabs.  Too slow and the boat doesn’t want to overcome inertia and begin to move, much less obtain a speed sufficient to have control.  Knowing 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.

Which I think is enough of the theory, now we need some practical tips.

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.  Throttle control is of paramount importance while in reverse.  Too little throttle and the boat will not want to overcome its resting inertia and will be at the mercy of wind and prop walk.  Too much throttle is wasted due to the propeller slipping through the water.

Second … slipstream while in forward provides a great deal of control, reducing the amount of throttle needed to obtain rudder control.

Third (and I feel the most important of the three) … once sufficient rudder control has been achieved, you MUST reduce the throttle to a point where control is maintained.  Continued use of the throttle after that point results in excess speed, and in close quarters, loss of control!  Also, while in reverse, more throttle results in more prop walk.  Reduce the throttle and you reduce prop walk.  Shift into neutral and eliminate it alltogether.

In closing, here are a trio of cliche’s to think about:

  • “Slow is pro”
  • “Too slow don’t go”
  • “Don’t go any faster than you are willing to hit something and don’t hit anything any harder than you are willing to sign a check for”
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4 Responses to Slipstream

  1. Gene Stangel says:

    Hi Don, another very useful article! As you know I back my boat, Exhale, a 38′ Catalina into my slip. I have observed that in turning the boat in the marina to transition from forward to reverse I need to allow several boat lengths (4-5) to make the transition. I point about 10-15 degrees to the right to account for prop walk during the transition (depending on the wind). And it takes initially about twice the throttle in reverse to start backing up. Once moving, I can throttle back to about 1.5 the throttle I would normally use for going forward thru a fairway.
    Thanks for another thoughtful article.
    Gene

  2. Robin Berrin says:

    Thanks Don. I’m always learning from you. Robin

  3. JD says:

    Excellent information! Although I feel as though this article was written with me in mind….

    Another example of sailing a boat being more than a great day on the bay-always new frontiers of learning.

  4. Chris Owczarek says:

    Great article Don!

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