Passing above tickover

[Scholar Gypsy]

I think the confusion between us is:

 

i) in all the examples I am talking about above, the boat is in a "normal" canal, ie with a negligible flow when there are no boats around. That zero flow changes when a boat passes a given point, as 15 tonnes or whatever of water moves from one side of that point (in front of the boat) to the other (behind the boat) The point I was making is that if one runs aground - perhaps because one is over-revving the prop - then the net flow past the boat will be zero.

 

ii) I was only making a statement about the average speed of the water that flows past the boat as it moves along the canal. As I said earlier, the actual flow of water is much more complex than that - that's the point you are making.

[Loafer]

I think the confusion between us is:

 

i) in all the examples I am ..etc

 

 

Scholar, I don't think either of us are actually confused. You sound like you know what YOU are talking about, with laboratory conditions and flow meters installed.

 

I'm saying that I can pass a moored boat in two different ways. (More ways are available)

 

The first way is (say) is to pass said boat at 2mph with no power on, having been doing 3-4mph beforehand, but gone to idle, 50 yards before I reached him, so I'm doing 2mph when I pass him, OUT OF GEAR...

 

The second way is to approach at 1mph, then strangely put half power on with 50 yards to go, and still pass that same boat at the same 2mph, but this time with power on and pushing.

 

The first way will not affect the moored boat, other than a minor turbulence, but the second way will lean him over - because I'm pushing HIS water behind me with a lot of gallons per sec, and lowering the water level alongside both of us, because I'm throwing it all behind me with some force.

 

I'm STILL going past him at 2mph in both examples- but with a very great difference between the two passages, all down to how much water my PROP is pushing. It's a big prop too.

 

I've seen it happen!

 

Maybe it wouldn't if I had an egg whisk, spinning at 1500 rpm*, I don't know, but it sure makes a difference with a 24" prop at 600rpm*.

 

I have to go to bed now, according to the nurse, but I'll be sure to hook up back here when I'm allowed to plug the pooter back in!

 

Night night.

 

 

*Engine rev counter readings, not propeller rpm.

 

ETA i'm not actually sure of my actual prop diameter, but I'm a 29" draft with a max rpm of 1200 at full power. 600rpm is approx half-throttle. Idle is 300rpm.

Edited July 26, 2015 by Loafer

[Alan Saunders]

I couldn't disagree more. It is the speed of the boat over the ground which gives rise to the "draw". If the cross sectional area of the canal is small compared with that of the boat there is a large flow of water in the direction opposite to that in which the boat is travelling.

 

N

A boat named 'Theodora', with a distinctive fore-hold, ghosted past my inadequate, loose mooring on pins between Honey Street and Stanton St Bernard last week. Unusually, I did not hear it coming nor did my boat move before or after 'Theodora' passed, I just happened to be looking out of the window.

 

Alan