Pointless question

irish snowblower

I was thinking about all this flooding and fast flowing water and wondered if a boat with a planing hull was tied up at a jetty and the water was flowing quick enough would it plane

fergal.b

I'd say so the same way a kite will takeoff when the air is moving under it





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irish snowblower

Makes sense

Steve

More chance the jetty would be gone first though.

Wolfsberg

I think it would, in theory, if the water was moving fast enough. The physics are similar for flow jets I would imagine....

http://youtu.be/l_PDA0oqaFc

Wolfsberg

I think it would, in theory, if the water was moving fast enough. The physics are similar for flow jets I would imagine....

http://youtu.be/l_PDA0oqaFc

pedroeibar1

Wolfsberg wrote: »

I think it would, in theory, if the water was moving fast enough. The physics are similar for flow jets I would imagine....

http://youtu.be/l_PDA0oqaFc

That does not work IMO because the hydrodynamics are very different. The surfer has control by exerting changing / appropriate mechanical (weight) forces on areas of the board to make it respond to his commands. A moored boat cannot do that. Even if the boat were tied to a concrete quay wall rather than a jetty I doubt it would plane. The hydraulic force of the water would vary as the angle of the boat to that force changes. The bow line would be fully extended thereby holding the bow down – the line would not be long enough to allow the bow up, which is necessary to plane. I’d guess the varying forces of the current and boat’s response movements would eventually bash it to pieces on the quay wall (if it was not “towed” under first.

Ever notice how all the photos posted of craft in danger on the Shannon never are moored properly – use of springs seems to be unknown by a large number of boaters.

Wolfsberg

pedroeibar1 wrote: »

Wolfsberg wrote: »

I think it would, in theory, if the water was moving fast enough. The physics are similar for flow jets I would imagine....

http://youtu.be/l_PDA0oqaFc

That does not work IMO because the hydrodynamics are very different. The surfer has control by exerting changing / appropriate mechanical (weight) forces on areas of the board to make it respond to his commands. A moored boat cannot do that. Even if the boat were tied to a concrete quay wall rather than a jetty I doubt it would plane. The hydraulic force of the water would vary as the angle of the boat to that force changes. The bow line would be fully extended thereby holding the bow down the line would not be long enough to allow the bow up, which is necessary to plane. I d guess the varying forces of the current and boat s response movements would eventually bash it to pieces on the quay wall (if it was not towed under first.

Ever notice how all the photos posted of craft in danger on the Shannon never are moored properly use of springs seems to be unknown by a large number of boaters.

Add your reply here.
I'll take your word for it!

Steve

pedroeibar1 wrote: »

That does not work IMO because the hydrodynamics are very different. The surfer has control by exerting changing / appropriate mechanical (weight) forces on areas of the board to make it respond to his commands. A moored boat cannot do that. Even if the boat were tied to a concrete quay wall rather than a jetty I doubt it would plane. The hydraulic force of the water would vary as the angle of the boat to that force changes. The bow line would be fully extended thereby holding the bow down – the line would not be long enough to allow the bow up, which is necessary to plane. I’d guess the varying forces of the current and boat’s response movements would eventually bash it to pieces on the quay wall (if it was not “towed” under first.

Ever notice how all the photos posted of craft in danger on the Shannon never are moored properly – use of springs seems to be unknown by a large number of boaters.

I would disagree, a planing hull (with the correct shape) needs to be bow down - as opposed to climbing an ever increasing bow wave in displacement mode - in order to plane.
I don't know a lot about power / planing engined boats apart from driving the odd rib but trimming forward and levelling the boat gives massive speed gains once you get over the hump.

You still need to be doing 12-15 knots to plane - no jetty or marina would survive that... a mooring?? don't know but wouldn't like to be on it.

pedroeibar1

Jeez. He’s just been appointed Mod and he is being contrary!:D

Let’s assume we are dealing with a planning hull, not displacement or semi-displacement (no river would run fast enough to cause the latter two to plane.) Planing hull means there is no real bow-wave, no stern wave, no cavity mid-wavelength on the hull and no hump.

You are correct on weight displacement (why raggies move crew about to maximise trim*) but you then contradict your argument in saying that the bow needs to be down – Bow down is never a good position, it makes steering difficult and in all boats it increases the speed required to get on a plane. The ideal is to keep the hull level. That is why Fergal and his iron-donkey mates have trim tabs. Add speed, bows lift too much, stern gets buried, boat wont ride properly. Answer = trimtabs.
During WW2 the Italians had stepped hull MTBs, on each of which the boat would rise in accordance with speed – it was a fuel saving thing, but they were pigs to handle. Each "step" was at a slightly different angle. Baglietto was the main designer, he went on to do mega yachts.

In a moored situation the water pressure will push the boat back to the utmost tension of the bow rope. As we are talking river flow, tidal drop is not in issue so the vertical angle is narrow and the forces on the bows are more likely to cause it to bury and probably will veer uncontrollably from one side to the other and will not lift on a plane (and probably beat itself to death on th equay wall.)
That's my take on it.:)


*Like most skippers I have moved crew around just to annoy them and stop them getting too comfortable. As the late great Denis Doyle used to say “When someone is telling a story the boat stops to listen”.

HeidiHeidi

pedroeibar1 wrote: »

Jeez. He’s just been appointed Mod and he is being contrary!:D

*Like most skippers I have moved crew around just to annoy them and stop them getting too comfortable. As the late great Denis Doyle used to say “When someone is telling a story the boat stops to listen”.

Both of these made me LOL :D

Steve

pedroeibar1 wrote: »

Jeez. He’s just been appointed Mod and he is being contrary!:D

I've actually had mod powers here for a while - just recently made it official..

And yeah, I'm a grumpy old 'contrary' git and make no excuses..

mmm, methinks the only way we can solve this is to find a river flowing at 16kts and moor a rib in it.. or maybe drive a rib bow down at the same speed and see does it plane?? what do you think?

pedroeibar1

Steve wrote: »

And yeah, I'm a grumpy old 'contrary' git and make no excuses..

Youv'e met your match, 'cept I'd bet I'm older and more contrary! :P

Steve wrote: »

mm, methinks the only way we can solve this is to find a river flowing at 16kts and moor a rib in it.. or maybe drive a rib bow down at the same speed and see does it plane?? what do you think?

Perhaps, not sure how though, as I would not know anyone who drives a rib.;)