Gary Fox
N/A
Yes possibly, if it is on a gradient, or accelerating or braking
And it is more difficult to walk back from the bar when carrying drinks.
Impact of tide on RPM
- Thread starter ip485
- Start date
And it is more difficult to walk back from the bar when carrying drinks.
lustyd
Well-Known Member
No, You're assuming that the boat is the only thing acting on the bow wave and it's not. In one direction the bow wave is acted on by the boat plus the tide in the same direction, in the other the boat and tide are opposed. The extra drag could account for a difference since motoring at a given speed against tide the bow wave would also have to overcome the force of water coming at it, which would cause a braking effect on the wave and subsequently the boat. It might not be a huge effect, but it would be different.
SimonKNZ
Well-Known Member
That's incorrect. If the water is moving uniformly, then the speed and direction of movement has no effect on the formation of the bow wave
ip485
Well-Known Member
I really like this. This is the sort of effect that might have an influence which I had not considered.
lustyd
Well-Known Member
It isn't moving uniformly once you create a bow wave, there's a bow wave in it which it acts upon
SimonKNZ
Well-Known Member
My mistake for assuming the tidal stream was not on a gradient, or accelerating or brakingYes possibly, if it is on a gradient, or accelerating or braking
And it is more difficult to walk back from the bar when carrying drinks.
Gary Fox
N/A
How can the tide 'act on' a bow wave? It's the same piece of water. I give up.
SimonKNZ
Well-Known Member
You've lost me there. The fact remains that anything moving on the surface of the water - whether this is a boat or wave - has no way of "knowing" that the water is moving, and therefore does not behave differently due to that movement.It isn't moving uniformly once you create a bow wave, there's a bow wave in it which it acts upon
TernVI
Well-Known Member
If the water is shallow, the bow wave might be affected, and a strong tide will produce some chop which slows the boat.
If the water is turbulent due to tide interacting with the seabed, that might slow the boat.
If the water is deep enough that there are no wave or turbulence effects then I think the apparent wind has more effect on boat speed.
If the water is turbulent due to tide interacting with the seabed, that might slow the boat.
If the water is deep enough that there are no wave or turbulence effects then I think the apparent wind has more effect on boat speed.
lustyd
Well-Known Member
No, the bow wave has energy, otherwise it would not be a wave. If that energy is travelling against the flow it will experience a deceleration. If it's with the flow the energy loss will be reduced. Since the boat is the thing feeding energy to the wave, it will also experience those effects.
lustyd
Well-Known Member
Bow waves have a direction, as does the flow. Normal physics applies from there onwards
SimonKNZ
Well-Known Member
Again wrong. The moving body of water forms an inertial frame of reference (Google that if you don't understand what it is, but it's basic Newtonian physics) and so its speed and direction of movement has no effect on anything moving on it. The bow wave has no way of "knowing" whether it is moving with or against the flow
prv
Well-Known Member
JumbleDuck
Well-Known Member
If the tidal stream is strong and the water isn't deep, the water at the surface will be moving faster than the water further down (just like wind shear in flying) and that might have interesting effects. Going downstream the hull would be pushed a bit faster than the prop and going upstream the hull would effectively be pushed back against the prop. Bingo, small change in revs.
I've no idea if this would actually be much of an effect and in any case I wouldn't expect it to matter if the depth of water was much more than double the draft of the boat.
JumbleDuck
Well-Known Member
Energy flux does, though, and the energy carried by waves does have a direction of motion.
ip485
Well-Known Member
Could ground effect have any impact. If the boat is moving in shallow water, and with a substantial long keel creating significant interaction with the ground beneath.
lustyd
Well-Known Member
It does in wave form!
ip485
Well-Known Member
When I have noticed this it is in in relatively shallow channels, a few metres to 6 or 7 metres beneath the keel at most and strong tidal flows (3 to 5 knots).
JumbleDuck
Well-Known Member
That's an interesting point, which also depends on water depth. In shallow water (wave amplitude similar to depth) it's the amplitude which determines speed and in deep water it's wavelength which determines speed. Which is why tsunamis build up and waves break.
However, I'm not convinced that it's relevant here and bow waves are normally quite small and therefore in "deep" water. Might be a secondary effect of water shear, though.
ip485
Well-Known Member
To be fair the IP is also a bit of a lump (as all the long keel haters will remind us with pleasure 
). It is around 23 tons in old money and a big lump of a very long and wide keel!!
). It is around 23 tons in old money and a big lump of a very long and wide keel!!
JumbleDuck
Well-Known Member
A Sheikh with racing interests commissioned a group of experts to study what made the best racehorse. After a month or so the mechanical engineer reported that thin legs worked best, the statistician reported that brown ones were faster, the geneticist advised careful attention to breeding and the physiologist recommended high lung capacity.
There was no word from the physicist and eventually, several months later, the Sheikh sent a message asking what was going on. "I'm sorry I have no results for you", came the reply, "but the case of a perfectly spherical horse is still proving of great interest."