Impact of tide on RPM

[lustyd]

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.

I think it is relevant. Aside from the wind, the bow wave is the only other force acting on the boat, and we all know how big that effect is normally! Any force acting on the bow wave in the direction of the boat would cause a greater than expected result, just as bow waves themselves do. At low speeds with no bow wave there is probably no difference at all, as you approach hull speed that effect will increase. OP was talking about max revs, so we can assume the bow wave is responsible for boat speed in this scenario and therefore completely in control of speed through the water, and therefore will have an effect on RPM if something acts on it, such as tide.

 

[Gary Fox]

I think it is relevant. Aside from the wind, the bow wave is the only other force acting on the boat, and we all know how big that effect is normally! Any force acting on the bow wave in the direction of the boat would cause a greater than expected result, just as bow waves themselves do. At low speeds with no bow wave there is probably no difference at all, as you approach hull speed that effect will increase. OP was talking about max revs, so we can assume the bow wave is responsible for boat speed in this scenario and therefore completely in control of speed through the water, and therefore will have an effect on RPM if something acts on it, such as tide.

The tide can't 'act on' the bow wave, they are all part of the same piece of water.

 

[tillergirl]

Drag (physics)

 

[lustyd]

The tide can't 'act on' the bow wave, they are all part of the same piece of water.

Yes they are, but the bow wave is moving through that water and has its own direction and force.

 

[lustyd]

Drag (physics)

drag wouldn't apply here as the boat and water move together

 

[JumbleDuck]

The tide can't 'act on' the bow wave, they are all part of the same piece of water.

Tidal flow and surface waves definitely can interact ... that's why wind-over-tide can be nasty.

 

[Chae_73]

drag wouldn't apply here as the boat and water move together

only if the boat is simply drifting with the tide.

 

[lustyd]

only if the boat is simply drifting with the tide.

Since the boat at rest is drifting with the tide, the energy to make it move at a given speed to overcome drag alone is the same in all directions.

Edit to add - assuming the boat is moving forwards in those directions!

 

[SimonKNZ]

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."

An engineer believes the theory is an approximation to the real world
A physicist believes the real world is an approximation to the theory
A mathematician doesn't realise there is a connection.

I'm an engineer with a MSc in maths, not sure what that makes my position!

 

[Chae_73]

Since the boat at rest is drifting with the tide, the energy to make it move at a given speed to overcome drag alone is the same in all directions.

ah yes, see what you mean; agreed

 

[Gary Fox]

Tidal flow and surface waves definitely can interact ... that's why wind-over-tide can be nasty.

But that wasn't the original question.

 

[lustyd]

No but it does affect the original question, unless as suggested we're discussing spherical chickens in a vacuum

 

[Laminar Flow]

If we set aside the concept of tide as an effect of the moon's gravity for a moment and investigate why water flows in a river it becomes apparent that the water flowing downhill does so because of the potential energy stored in it is being converted to kinetic energy. Tidal energy has the same effect but due to gravitational force. A boat traveling against a tide or a river is essentially traveling uphill i.e.. This requires some energy, no matter how shallow the incline of the plane i.e. the steepness of the slope or, relatively, the speed of the tidal flow.
In this sense IP 85's observation that his boat travels slower through the water against the tide than with it, when it is benefiting from "potential" energy is correct. This is apart from the effects of waves or turbulence in the water caused by the flow of course.

 

[RichardS]

Since the boat at rest is drifting with the tide, the energy to make it move at a given speed to overcome drag alone is the same in all directions.

But surely if you apply that logic to the bow wave being created by the boat then it means that the drag on the bow wave is also the same in all directions for a given STW?

Richard

 

[Soco2]

Wow! Some real BS been spoken on this forum.
Good luck coming up with your crazy answers?‍

 

[JumbleDuck]

But that wasn't the original question.

Not directly, no, but it may be relevant,

 

[Slowboat35]

My Gran once got terribly worked up because a bee flew into the car and she was upset to think that we were forcing it to fly at 60mph which was obviously much faster than was good for it...

Was it cruel not to slow down to a more manageable 20mph before it had a seizure?

What do people think about that?

 

[Beneteau381]

I did just ask that.

These were the answers I was expecting.

However, there is a reason for it. My max, rpm is 3.500, but when motoring against a really fast tidal stream I always see a small drop in rpm with the throttle fully open.

Sometimes you need to take a reality check and consider why something is occuring.

If you are tied to a pontoon and try to get full chat on your engines in gear, you wont get it. The boat needs to be moving through the water to get flow to make the prop work. The tide WILL have some effect, given my example, I suspect that the prop will be more efficient?

 

[JumbleDuck]

Wow! Some real BS been spoken on this forum.
Good luck coming up with your crazy answers?‍

Thank you for your valuable and well-thought-out contribution.

 

[Gary Fox]

If we set aside the concept of tide as an effect of the moon's gravity for a moment and investigate why water flows in a river it becomes apparent that the water flowing downhill does so because of the potential energy stored in it is being converted to kinetic energy. Tidal energy has the same effect but due to gravitational force. A boat traveling against a tide or a river is essentially traveling uphill i.e.. This requires some energy, no matter how shallow the incline of the plane i.e. the steepness of the slope or, relatively, the speed of the tidal flow.
In this sense IP 85's observation that his boat travels slower through the water against the tide than with it, when it is benefiting from "potential" energy is correct. This is apart from the effects of waves or turbulence in the water caused by the flow of course.

It could be either uphill or downhill. A boat travelling down-channel, heading west against the flood, would be travelling downhill, as the water would be getting deeper in Kent, and shallower in Cornwall. If the boat did a U-turn, it would be travelling uphill, with the tidal stream.
However it would be travelling at the same speed through the water, and bow wave and rpm would be identical, travelling in either direction.
Apologies to foreigners if you don't know the channel area.