Very good explanation
You and grayyak “get it”, and I’m glad I didn’t come out of this feeling all alone in the world. Both of you have done a nice job trying to explain this concept, and you exercised more patience than I did. Like that physics professor told me in college, this is a concept that tends to escape “misdirected practical observation”, and people misinterpret what they see mostly because they don’t realize what it is that they are missing. I love the challenge of explaining scientific principles and stuff like that (all of my friends in graduate school urged me to be a teacher), but trying to do explain this with nothing more that the written word (along with my self-imposed limit on post length) made me give up in this case. Just wanted to say, that to someone who “gets it”, your explanations are very good. Thanks for trying.
put the sand bags on wheels
Put the sandbags on wheels on the walk instead.
then they’d resemble giant water
Then they’d just resemble giant water molecules responding to shallow water waves (oscillating to and fro since water molecules in a deep water wave oscillate in a circle).
you have a great way
of NOT answering my simple questions.
Don't get me wrong, I fully understand the concept of a hull's interaction on a moving body of water and it's balance that is achieved, You a guideguy have pretty much run it into the dirt. but if I "Find a wide river on a calm day. Paddle straight across. Note whether or not there's more turbulence on the downstream side of the hull. (there won't be) Angle upstream a bit."
And what if I build up some speed at this new ferry angle then stop paddling. where will my boat go? Now deploy the skeg and do the same friggin' course. Now where does my boat go? same place? I don't think so.
Please, just answer this question.
steve
I was done, and after this I really am.
The answer to the question (which you directed at scombrid, not me, and *he* already provided the answer) is "yes", your boat does go the same place. Skeg up or skeg down, your boat goes straight through the water with no side slip *relative to the water*. The skeg WILL help you maintain your heading as you slide down the face of a large wave or get blown by the wind, but it does not have the effect on your boat's "aim point" when in current that you say it does. For your explanation to be true, an outside force would need to be acting on the boat, trying to keep it in place relative landbound objects and therefore resisting the current. If that were true, you could ferry across any stream, no matter how fast the current, using your boat the same way a sailboat uses its sail in a crosswind. But it doesn't work that way: If the current is faster than you can paddle, you can't ferry straight across the stream at a right angle. Or think of this example: You are drifting with the current, with no paddle-induced velocity relative to the surrounding water. If your claims were true, you could deploy your skeg or dip your paddle in the water like a rudder and your boat would spin so that the skeg or paddle-end of the boat would aim downstream. But that won't happen. The boat will just continue to turn to face random directions as the result of minor turbulence that is always present. Whether you understand these examples or not (and I'm pretty sure you do), if you don't see how they are related to my point (and that of grayyak and scombrid), it means you're missing the point, and from what I've seen, there's nothing I can do to remedy that with the communication medium at hand.
The only thing I can think of that might be making you think you've experienced a difference in ferrying with and without a skeg, is how the boat feels when you *change* the ferry angle. In that case, the boat will definitely "feel" different, but just during the time you change the angle. But remember, changing ferry angle is nothing more than making a turn, and when you follow a curving course when turning, whether the skeg is up or down makes a huge difference. How your boat goes through the water during the *change* of ferry angle is exactly the same as what it does when you make a turn on still water, as far as the boat's movement relative to the water around it. Changing ferry angle looks a lot different and the experience feels a lot different from a regular turn because on the river you are always using fixed objects as reference points. But what your boat does in the water is the same in both cases.
I think so and I already anser the quest
"“And what if I build up some speed at this new ferry angle then stop paddling. where will my boat go?”"
Your orientation won’t change, skeg or none. If you’re pointed 45degrees you’ll remain pointed 45degrees or if you’re sitting cockeyed you might turn but the turn will be irrespective of the current direction. Once forward progress ceases then you’ll drift aimlessly.
Your example only applies when you have just entered the fast current (like that brief period popping out from behind a bridge cason) and the current is still moving from one side to the other faster than the boat.
I think you’re perception is colored by whitewater bias. Entering a side current from an eddy, yes there is a substantial lateral acceleration which will turn the boat if the acceleration is not uniform over the entire hull. Also, in very shallow fast water the end with the deeper draft will drift downstream slower because of the rapid decrease in current velocity with depth. Ex: If you’re a tad bow heavy you’ll always (with no wind or wind with current, wind against current will blow the shallow end back upstream) point upstream when dead drifting. With a skeg you might achieve a neutral drift. This type of effect is not seen in deep water as the current velocity is pretty uniform in the top meter or so of the water column.
Chesapeake Bay, New Point Comfort to Cape Charles. Ebbing spring tide. Current 1-3kts southbound. Can’t see land. Desired course west to east. You would not believe how easy it is to end up WAY south of your desired destination in that situation because you have no way to “feel” the boat drifting southward. The compass says 90degrees the entire time and the boat is not trying to turn down current, up current, or otherwise. It might be trying to turn into the wind. Or if you’re paddling slowly and being over-taken by a following sea the boat will tend to “wallow” in the trough and Yaw back into the wave as the bow hangs in the trough and the stearn is advanced by the wave. But the current will give you no clue that your heading of 90degrees is not taking you to Cape Charles but to Kiptopeake instead, until you go ripping by a channel marker sideways.
This is the same as an airplane in a 90kt cross wind. Without sight references on the ground or a gps the pilot would never know the cross wind was there unless he suddenly flew into the cross wind and was turned upside down by the sudden lateral accelleration. Once the plane is in the cross wind there is no lateral windstress/associated acceleration so nothing to turn it and nothing to let the pilot know that he is moving sideways.
The same applies to a vessel moving through the water. Of course being on the boundary between two fluids of vastly different density presents that problem of weather cocking since the boat is entrained in the movement of the water but is subject to lateral stresses/pressure gradients in a cross wind.
“”“Now deploy the skeg and do the same friggin’ course. Now where does my boat go? same place? I don’t think so.”""
I do.
intuitive physics
You’re driving rapidly in a circle and your buddy opens his door and is flung out. Which direction does he travel relative to the circle?
You’ve bought a big corny helium baloon for your sweety on valentines day. Its string is tied to the seat belt on the passenger side so the baloon is floating next to you. When you slam on breaks, which direction does the baloon move?
so then in YHO
hull shape then has no bearing at all on maintainence of ferry angle?
I would have the same ferry angle results from a CD GT or a Prijon Yukon Expedition?
yeah right.
steve
ps. what is whitewater bias? It’s water in motion. period.
Answer this question then
What is exerting the turning moment on the hull when imbedded in the flow (excluding wind stress and waves)?
Ferry angle is determined by how fast you can go. So in a way hull shape can effect the necessary ferry angle because the slow boat needs a steeper angle but it doesn’t affect you’re ability to maintain that angle (ignoreing weather-cocking to the wind, broaching tendency in waves).
Moving water isn’t just moving water. A three knot surface current with a depth of 1meter behaves much differently than the same current in 10meters.
Additionally, in whitewater you’re constantly crossing currents of very different velocities. Each time you enter a new current the hull is subject to lateral acceleration. You “show your bottom” to the new current until you’re in it and turned to your ferry angle, then you can let it run flat and boogie to the next shore. How the hull reponds to that is very dependent on hull shape. But in the event that you’re imbedded in the flow, say crossing the bay (20miles) or crossing a flat section of the James above Richmond (300m). Once you’ve set your ferry angle the boat will continue to go where you pointed it.
Scenario: Coleman bridge. 1/4mile from Cason to Cason. Ripping ebb tide (left to right with me facing north). To get from one to the next in a straight line I need to point 15-20degrees left of the target. Once I set course I let the boat run flat (unless it’s windy but we’re just looking at the effect of current here). Were I to lean to the right the boat would continue to turn upstream. Were I to lean left the boat would turn down stream. Same applies on my surf ski. Once I set course crossing under the bridge, no ruddering is necessary to hold course. One of my usual workout routes crosses the river at the bridge so I’ve ferried the current there countless times. When there’s no wind I can let the boat run totally flat once my course is set.
nothing
~>What is exerting the turning moment on the hull when imbedded in the flow (excluding wind stress and waves)?