antifouling leading edges

[Phoenix of Hamble]

I keep reading posts with people referring to adding an extra coat or two to leading edges of rudders and keels....

Now, its a long while since I studied any hydrodynamics, but I seem to remember that the water flow over the leading edge is nice and smooth, yet turbulent over the trailing edges....

I have assumed over the years that the turbulence is more likely to remove eroding antifoul than smooth water flow, hence its more important to have good thickness on the trailing edges than leading ones..... and hence two coats on the trailing edge is more important than on the leading edge....

This is reinforced in my mind by seeing boats with growth hanging off the back of the keel, but not off the front when lifted...

Discuss?

 

[savageseadog]

You're right with regard to the hydrodynamics. However when painting the edges of underwater foils I think there is a tendancy for the edges to "show through" the paint.

 

[graham]

Like you I allways notice more weed off the trailing edeges of the rudder.

I usually slap an extra coat around the waterline where it gets an extra scrubbing afloat a few times each season.

 

[Bilgediver]

I think you maybe ignoring water velocity at the leading edges. Self eroding antifoulf is probably affected as much by water traveling past it and at the leading edges of the aerofoil the water may not be turbulent but is moving faster so possibly erodes more antifoul than other areas of the keel and rudder.

Like many here I apply extra antifoul around the water line , the bow,keels and rudder. Unfortunately as the recipe seems to change each year it is dfficult to run comparisons year to year. We also notice that the fouling in our marina varies from year to year. One year may be mussels and fronds of weed. Next year barnacles,next year green grass weed.

It is difficult to prove what works and what doesn t so we try our best to keep moving with the minimum of effort /forums/images/graemlins/smile.gif

 

[Bilgediver]

I think you maybe ignoring water velocity at the leading edges. Self eroding antifoulf is probably affected as much by water traveling past it and at the leading edges of the aerofoil the water may not be turbulent but is moving faster so possibly erodes more antifoul than other areas of the keel and rudder.

Like many here I apply extra antifoul around the water line , the bow,keels and rudder. Unfortunately as the recipe seems to change each year it is dfficult to run comparisons year to year. We also notice that the fouling in our marina varies from year to year. One year may be mussels and fronds of weed. Next year barnacles,next year green grass weed.

It is difficult to prove what works and what doesn t so we try our best to keep moving with the minimum of effort /forums/images/graemlins/smile.gif

Now where is that tacky lanolin for the prop!!!!! /forums/images/graemlins/smile.gif

 

[thomashoebus]

I would say antifouling will first wear off in the leading edges because of the waterflow. Where the flow is turbulent the eroding capacity of the water is much lower that is why you have also more growth on the trailing edges. I think

 

[Phoenix of Hamble]

Well thats kind of why I got round to thinking about this...

is it the speed of water flow, or the turbulence that causes the main erosion effect?

 

[Twister_Ken]

Water from a jet wash is linear flow, rather than turbulent.

What that has to do with the price of fish I'm not sure.

 

[Phoenix of Hamble]

But then you intentionally disrupt the flow by aiming it staright at a perpendicular surface... a jet wash wouldn't work so well if you aimed it to run parallel along the surface to be washed would it....?

 

[ccscott49]

When I mean leading edges, I mean all around the waterline extra on the bow and basically anywhere else until theres no antifouling left in the drums!

 

[Topcat47]

The answer differs for sailing and motor boats and in motor boats between planing and displacement craft.

In a sailing boat, and most displacement vessels, the flow of water around the hull is predominantly laminar. At the Bows, the water in touch with the hull is almost stationary as the water parts under the action of a pressure "wave" before the obstruction. This is best illustrated by watching the smoke trails in a wind tunnel as the air passes an aerofoil. The effect is not so noticeable in hydrodynamices but the same principles apply. This is also true of suspended rudders. Ruddes may be hung from the keel or a skeg. In these cases, the leading edge of the rudder is almost completely in zero flow conditions and anything that does fasten itself there will not wash off no matter how sick the toxin in the antifoul makes it, so I always add a coat here to ensure the fouling gets a good dose.

There is no sense in doubling the thickness of antifoul around the bows for erosion considerations, however, I do it (two or three coats along the leading edge) because it also helps to have a thicker paint coating when you meet floating obstructions in the water.

The reason weed grows best on the trailling edges of rudders and keels is that these are the closest to clean water. The fouling gets a good grip while you're tied up in the marina or on your mooring and the motion of the vessel has a harder job trying to scub it off under way.

Turbulance is only a problem when you come to transoms when the laminar flow across the hull has to make a sharp turn and loses touch with the hull. Even so, there is usualy a thin laminar region so once again erosion is not so much a problem. It is often hard to get the paint to adhere to sharp corners though and the paint layers tend to be thinnest where two edges meet so an extra wipe with the brush helps ensure there's enough paint there.

On Planing craft, however, the speed of the water past the hull coupled with the uncertainty of exactly where the waterline is (due to speed and trim considerations) place specific demands upon the antifoul used. In such cases, where would you re-inforce the coating anyway?

 

[Phoenix of Hamble]

Thats a useful and interesting reply... Thanks Topcat...

 

[tobble]

[ QUOTE ]
Turbulance is only a problem when you come to transoms when the laminar flow across the hull has to make a sharp turn and loses touch with the hull.

[/ QUOTE ]

A flow can become turbulent while flowing in a uniform manner, as Reynolds famously showed by emperical experiment in 1883. for flow over a flat plate, if the Reynolds number exceeds about 10^5, the flow is likely to be turbulent, (see http://en.wikipedia.org/wiki/Reynolds_number ) so for 6 knots (about 3 m/s) in sea water, the flow will be turbulent after traveling about 50 m... but turbulance can be induced by any obstruction such as a log impeller, and will occur faster if the surface is rougher... and flow certainly WILL be turbulant as it comes of a trailing edge, because it effectively has to instantaniously change the rate of shear which it cannot do; and because the flow is sub-sonic, it is likely that some disturbance will propagate "backwards" so some of the fluid in contact with the boat is turbulent, which will errode paint faster than laminar flow.

simple /forums/images/graemlins/wink.gif

 

[Topcat47]

Well I for one don't own a 50metre boat, neither do I mix with people who do so the 6 kts/50m figures are purely academic. Similarly the turbulence induced by hull protrusions is pretty small when you've a top speed of 6kts anyway and most of your sailing is sub-4. Furthermore, such turbulence is localised. at or around the obstruction and once you move away from it, laminar flow predominates again.

If you have acces to a test tank, I'm quite prepared to be proven wrong.