Tidal Effect on Hulls

[alec]

Does hull shape have an effect on tidal flow ?

i.e. would a long keel hull be harder to push against a foul tide than say a fin and skeg ?

Or does the tide have an equal effect on all bottoms, deep draft or low draft ?

I gave my view to a friend buying a boat but am not sure my fluid dynamics is up to the mark.


Regards,


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[Oldhand]

The answer to your question, as asked, is no. Floating objects do not affect the tidal flow of large water masses.

What you seem to be asking is nothing to do with tidal flow but to do with a hull's drag through the water. The less drag the hull creates when moving through the water, the less energy is required to sustain that movement. Hull drag varies due to many things but assuming there is a smooth hull and water surface, the wetted area and the hull form will be the parameters which cause the greatest varition in drag, assuming waterline-length speed is not exceeded. For low drag and thus ease of propulsion through the water, in simple terms you want minimum wetted surface area and a hull form of low "frontal area" with smooth curves without abrupt changes.



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[alec]

Many thanks for the response. I will pass you’re comments on.

As the long keeled boat has a larger wetted surface, would the tide ‘grab’ more of it than say the fin and skeg boat ?

Regards,


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[oldharry]

As Oldhand said, the tide doesnt make any difference. I think you may be confusing 'tide' with 'wetted surface area'. The larger the wetted surface area the more drag there will be - and this is constant regardless of tidal conditions and the level of drag is a function of the speed of the boat through the water, not the movement of the water itself in relation to the seabed which is what 'tide' is. The only variable is the smoothness of the underwater hull finish - which is why racers spend hours polishing their hulls. A badly fouled boat with loads of weed and barnacles will be significantly slower than it was when it was clean for example.

But the tidal flows in the sea surrounding a boat are the same whatever shape the hull is. A 2 knot head tide or current will slow any boat down by 2 knots whether its a 17foot dayboat or the QM2


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[AndrewB]

Tidal Effect on Hulls: Lee-bowing.

While I agree in general that the tide is like a moving walkway which affects all objects similarly, hull shape may matter with regard to the ability to lee-bow. Is it not true that a yacht that is very close-winded may gain a further advantage if it can get the tide onto its lee bow, bringing the apparent wind further aft?

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[charles_reed]

I think the last two posters

may have misunderstood your question - to re-pose it "which anchored boat will be the first to demonstrate the change in tidal stream direction, a long-keeled boat or a fin & skeg design".

The answer would be the one with the greatest wetted area - for two similar sized boats the heavier, which is probably the long-keeler.

This leads on to a much more definitive answer, fin/skeg tend to be more likely to be wind-borne than long-keelers. This has to do with the ratio of wetted area/windage and is likely to be of far greater importance in real life as mixing fin-keelers with long keelers in a low tide, high wind situation can lead to considerable underwriter grief.



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[MarkV]

This talk of "wetted surface area" affecting drag has reminded me of a debate I once got into about friction between solid objects, in this case friction is largly independant of the surface area of contact between the two surfaces. Is the "Wetted surface area" analagous to the surface area in the case? Common sence would sugest not but I thought I would ask. Also wouldn't the hydro dynamic effects have an equally major part to play, so a long keel that naturally want to go in a straight line on a well balanced boat might have less drag than a fin keel on a poorly balanced boad with loads of weather helm.

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[AliM]

To answer the bit about frction between solid objects - that is independent of area in contact because when you go down to a microscopic level, the two surfaces are rough and only touch at a few points. It is the adhesion between those few points that determines the coefficient of friction.
When it is a liquid/solid surface, then the liquid forms a boundary layer on the surface and its the viscosity of the liquid in that layer that determines the drag. That does depend on surface area, so the long-keeled boat will have more drag than the racing boat.
None of this, as others have said, has anything to do with tidal drift. Think of it as just affecting your speed through the water when anything (motor or sails or windage or even your anchor) tries to move your boat relative to the water (which itself is moving because of the tide).

Whew!

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[alec]

Re: I think the last two posters

Thanks everyone, I am enjoying the responses.

I am still confused about the tide speed and it’s effect on boats..

Let’s say you are looking at a broad creek of 50 metres width. The depth near the sides is say 5 metres, then, working towards the middle, 10 metres, 15 metres, then say 20 metres in the middle. Ignoring back eddies, when you look at the creek from the side it is common to see detritus right across the creek running at the same speed ( on the surface). But we know that when sailing against the tide it is faster ( less current) in the shallower bits.

Do water particles in this situation only move at the same speed at the same specific depth and the surface speed is a poor indicator of tide speed ?

Regards,


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[MarkV]

Re: I think the last two posters

I believe that the speed of the water should be fasted in the middle where it is furthest away from the bank/bed. The water near the bank/bed will experiance the friction/drag (think of the water being still and the river bank and bed moving, and therefore dragging some water along with it). Also a lot of rivers have fairly flat beds so that at the surface the distance to the bank/bed is similar over a wide area therefore I supose the water would be traveling at a similar speed, it would not be until you get close to the bank that you experiance any differences. Any land that sticks out into the river may also cause back eddies so in some places the water will actually flow in the "wrong" direction.

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[SteveGorst]

Re: I think the last two posters

It could be the detritus is being blown by the wind. My experience of canoeing where you are only immersed in the water by a couple of inches is that there is definately less current towards the edges. This is due to the drag that the bottom exerts on the water slowing the water down. The deeper the water the less influence this drag has on the water at the surface. It still has an effect on the water near the bottom though.

At a depth of 5m in a slow moving creek the effect on the surface water would be less than a depth of 2m in a fast flowing river.



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[charles_reed]

well done, Alison

Now explain the effects of turbulence on a liquid/solid interface.

If you want to read it up you'll find it in C A Marchaj "Sailing Theory and Practice" (pp235, 243,290) with all the relevant formulae. Engineering Dept should have a copy if it's not in the main library.

Your supervisor should be proud of you.

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[Evadne]

Re: Tidal Effect on Hulls: Lee-bowing.

Yes, but that's because the wind is not affected by the tidal flow, for all practical purposes, so by lee-bowing you can get an additional displacement to windward, courtesy of the tide. As it's the water and not the boat that's doing this, all boats on a given course will be affected by the same amount.

It reminds me of the problems a colleague had, trying to explain to an otherwise fairly learned audience, that you don't increase the drag on a vessel when you crab across the tide, because your course through the water is still dead ahead.

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[oldharry]

Re: I think the last two posters

Dont think so Charles: the question was 'which is harder to push against a foul tide' - I understand the question here to be whether a boat with a larger keel would be more difficult to push through ther water against a foul tide. I agree entirely that moored boats will swing on the turn of the tide at different rates - see it most days at East Head with quite 'interesting' consequences!

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