Surface Interval wrote:
Oldav8tor wrote:
I suspected it was something like that. In prop airplanes you have to add rudder correction to offset turning forces generated by the propeller but once the correction is in, the plane stabilizes. I couldn't find any happy position of the stern drive where it wouldn't wander.
Thanks for your input!
Tim
As one old airplane guy to another, the big difference here is speed. The boat is stable at cruising speed, just like the airplane. At no-wake speeds the boat is much more affected by the water flowing around the hull and other factors. Just like a baseball curves or a knuckle-ball moves unpredictably, the water flowing past the hull can have an unexpected effect. For example, to stop a yaw to the left, the wheel is turned to the right. The slight course correction yaws the boat to the right. Remember that it takes more of a steering input to stop a yaw and bring the boat back on course than to maintain the course. This course correction eventually will be too much. Just like air flowing over an airplane wing at different angles of attack or an aircraft fuselage in a slip, the water flowing by one side of the boat may behave differently than on the other side. This constantly changing difference in water flowing past the hull causes the boat to wander. The boat's thrust at the stern which means that you are always pushing the boat. If/when the bow strays off-course the stern has to get around behind the boat to push it back on course. This can cause the boat to move somewhat sideways until it re-establishes a new course. At this point you realize you have too much steering input and start to turn back the other way. Any wind, water current, wave, etc. can push the bow off course and require another steering correction. Unlike an airplane at cruise, the boat is not "dynamically stable" at idle. It's somewhat like trying to push a wheel barrow or back a trailer. There are many "hydro-dynamic" forces at work on a boat at any speed. Like with an airplane, slower speeds usually take more control input. Sorry for the long-winded explanation. Hope this helps. Gary
This is as good of an explanation/theory as I have heard. But I take exception to a few parts:
1. Overcorrection: while it is easy to overcorrect for the wander, it does not explain the wander. You can leave the steering wheel in one position, and have the bow wander left to right to left to right to etc. The amount of turn stays about the same each time.
2. Some of the other differences in "water flowing by one side of the boat may behave differently than the other side" make sense, but these differences would be the same with an outboard engine as well as an I/O. However, outboards do not seem to have this wander, and I/O's do.
So here is one difference I can think of: Think of what happens when water passing below the hull reaches the transom. It is suddenly free to "pop back up" to surface level right at the transom. This rush of water UP the transom would also be angled towards the centerline of the boat (perpendicular to the V-Hull bottom). Water at the centerline of the transom would compete for position up the center. With an outboard, this Upward rush of water would be unimpeded. With an I/O, it would hit the transom bracket on the way up. Some of the water would be forced to port of the outdrive and some would be forced to starboard. My theory is that this upward flow of water up the transom may shift back and forth to the current side of least resistance. There is also the propeller interaction to make it more complicated, with the right side rotating down and the left side rotating up.
...just rambling on instead of getting my work done...
Ray (another old airplane guy)
_________________
"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings
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