lustyd
Well-Known Member
No, different forces will apply to a wave than to a boat. The point being that it's a breaking force in one direction once you follow the system all the way through. It's probably not much but it would be a difference.
Impact of tide on RPM
- Thread starter ip485
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JumbleDuck
Well-Known Member
A particularly energetic bee can fly at about 8m/s. If it flies into the windscreen when the car is doing 27m/s (60mph) it decelerates from 35m/s to 27m/s, which is a kinetic energy loss of 248 J/kg. If the car wasn't moving it would decelerate from 8m/s to 0m/s, which is a kinetic energy loss of 32J/kg. It therefore follows that the bee sustains almost eight times as much damage while the car is moving. Your Granny's consideration did her credit.
(I used to use this example with engineering students. Readers are invited to spot the mistake.)
Gary Fox
N/A
The kinetic energy loss of the bee is the same in each case, from the bee's POV. Assuming the bee is already in the car.
ip485
Well-Known Member
Exactly. And I have done just this and for me the effect is significant.
Laminar Flow
Well-Known Member
Tidal flow occurs when there is a height gradient. Constricting the flow causes a damming effect which increases the gradient and, respectively, the speed of the current, even in the Channel, which I have sailed up and down a couple of times. No matter, whenever you are traveling against a current, either tidal or in a river, you are going uphill.
Gary Fox
N/A
Yes I can see you're right. Even travelling down a river against the incoming flood tide, the boat would be getting higher above chart datum.
But I am struggling to see why my channel example wouldn't work, with a fast enough boat. You could travel against the flood, and end up closer to chart datum. I would have to work out a passage plan to prove it.
Roberto
Well-Known Member
No, strange as it may seem tidal stream does not necessarily reflect differences in tidal height, there can be an opposing current while going "downhill", or vice versa, in particular in inlets. Search for "progressive" and "standing" tidal waves 
Gary Fox
N/A
Yes I seem to remember the tide turns at Dover while it is still rising.No, strange as it may seem tidal stream does not necessarily reflect differences in tidal height, there can be an opposing current while going "downhill", or vice versa, in particular in inlets. Search for "progressive" and "standing" tidal waves
Gary Fox
N/A
We are now falling away from springs, but tonight's HW is predicted to be higher than it was at lunchtime. (Newport IoW). How is this possible? It's not a random wind or air pressure effect, because it's in all the tide tables.
JumbleDuck
Well-Known Member
You get that effect in rivers as well. The tide starts flowing out at Kirkcudbright while it is still rising, because the river is supplying water faster than the tide is removing it. By high water there can be 3 kt past the pontoons, which sometime surprises people.
Laminar Flow
Well-Known Member
Regional anomalies or phenomena do not alter the initial premise, I think.No, strange as it may seem tidal stream does not necessarily reflect differences in tidal height, there can be an opposing current while going "downhill", or vice versa, in particular in inlets. Search for "progressive" and "standing" tidal waves
Roberto
Well-Known Member
That's because of non-semidiurnal harmonic components influence: with purely semidiurnal components, HW (and opposingly LW) would move with decreasing/increasing heights cycle by cycle; non-semidiurnal components deform the whole curve up and down by different amounts at different times, one can then have for example a decreasing range (away from springs) given by a t+1 HW higher than HW at previous cycle t, together with a t+1 LW higher than LW at time t by an amount greater than the HW (t / t+1) difference, hence the lower range. Hopefully I have not messed up all the "t"s
Gary Fox
N/A
No that's brilliant thanks. The extra 5cm allowed me back onto my mooring just now!That's because of non-semidiurnal harmonic components influence: with purely semidiurnal components, HW (and opposingly LW) would move with decreasing/increasing heights cycle by cycle; non-semidiurnal components deform the whole curve up and down by different amounts at different times, one can then have for example a decreasing range (away from springs) given by a t+1 HW higher than HW at previous cycle t, together with a t+1 LW higher than LW at time t by an amount greater than the HW (t / t+1) difference, hence the lower range. Hopefully I have not messed up all the "t"s
goldchip
New Member
I think it depends on the wind, the prop speed will be higher if the hull is moving through the water. As far as the prop is concerned the water is a fixed frame of reference, but wind and sea conditions make a difference to the speed of the hull through the water.
oldgit
Well-Known Member
Does the type of anchor make any difference ?
Adios
...
The engine will need a higher RPM to achieve the same speed through the water if the boat is going with the tide.
Assuming no wind. Boat going 5 knots with 3 knots tide is pushing through the air at 8 knots. Same boat going against the tide is only pushing through the air at 2 knots. Less air drag = less power required.
Assuming no wind. Boat going 5 knots with 3 knots tide is pushing through the air at 8 knots. Same boat going against the tide is only pushing through the air at 2 knots. Less air drag = less power required.
Daydream believer
Well-Known Member
The RPM counter is electronically controlled. Once the engine has charged the batteries the electricity passing through the system might ( does it?) change. Has one considered that the RPM counter could be altering due to fluctuations in charging of the batteries & it has nothing whatsoever to do with what has been discussed. IE has the RPM meter been calibrated at all states? . If the data ain't right then you are wasting your time. How often were the readings taken? Once, twice dozen times?
The OP states later in the thread that he "Always sees a small drop in RPM" But conditions will not always be the same. Wave motion will be different, Wind will be different. Tidal flow & depth will vary..So unless the data ie the revs drop, is taken over the same conditions it is impossible to isolate a single cause. I cannot accept that a single reading in a certain condition can prove a particular cause. One has to take several readings & i doubt that the OP has motored up & down the same stretch of water a dozen times in succesion for that purpose ( Sad if he has) What was the effect on speed through the water?
We can all talk about( say) bow wave , but that changes if the conditions change. Likewise if the wave motion changes, then that cannot be isolated as the cause because it is mixed with other factors. Therefore, none of the theories suggested can be rejected outright & none can be even accepted if the means of recording the data ie the instrument- is wrong to start with
Only tank testing will solve the issue.
The OP states later in the thread that he "Always sees a small drop in RPM" But conditions will not always be the same. Wave motion will be different, Wind will be different. Tidal flow & depth will vary..So unless the data ie the revs drop, is taken over the same conditions it is impossible to isolate a single cause. I cannot accept that a single reading in a certain condition can prove a particular cause. One has to take several readings & i doubt that the OP has motored up & down the same stretch of water a dozen times in succesion for that purpose ( Sad if he has) What was the effect on speed through the water?
We can all talk about( say) bow wave , but that changes if the conditions change. Likewise if the wave motion changes, then that cannot be isolated as the cause because it is mixed with other factors. Therefore, none of the theories suggested can be rejected outright & none can be even accepted if the means of recording the data ie the instrument- is wrong to start with
Only tank testing will solve the issue.
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Arcady
Well-Known Member
Exactly so. And there, boys and girls, is the most likely answer to the OP’s original observations.
ip485
Well-Known Member
Athomson I do like your explanation. I wondered from the outset whether we are all to fixed on the boat moving through a homogeneous media (the water) when it is more complex because there are two very different media involved and perhaps the water is less homogeneous that we give credit.
I also agree my observations are wholly inadequate (scientifically). However they are a little uncanny in their consistency. I must motor upper and down the same stretch sometime!
I also agree my observations are wholly inadequate (scientifically). However they are a little uncanny in their consistency. I must motor upper and down the same stretch sometime!
Daydream believer
Well-Known Member
If an engine is at full throttle & his throttle lever is hard over & revs at 3500 rpm under load I would assume that the revs are being kept there by the govenor ( or whatever a diesel engine has to stop over revving) So if the load is lighter why does it still not rev at 3500 & stay at that all the time. Has the OP got an engine problem coming. does he need a service?