# Maximum radius on spray rails

Discussion in 'Hydrodynamics and Aerodynamics' started by Olav, Dec 12, 2011.

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### Olavnaval architect

I'm in need of info about the maximum radius allowed on the edge of a spray rail, hull chine or transom edge. I'm aware that this edge should be as sharp as possible for effectiveness but in real life there are practical limits to this, concerning both manufacturing issues (think FRP construction in a female mould) and susceptibility to damage.

Once I used to have a paper that mentioned some investigations on that topic, but unfortunately I can't find it anymore. Said investigation cited was carried out by an American, I think, since all numbers were in imperial units. As far as I can remember, there was something like the maximum radius as a function of speed, Fn or similar or maybe the loss of performance when exceeding a certain radius.

Maybe someone has an idea to point me in the right direction or knows of any useful source on that topic.

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### yipsterdesigner

All I know those chine rails are scarce or not documented at all
so I stay posted for doc's on those here with you

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### michael pierzgaSenior Member

For what its worth I just climbed out from under the bottom of a custom built ,high speed , exhaust pipes the size of sewer pipes, alloy , 18m motorboat. I estimate the radius at 8 mm. perhaps 6mm

For practical reasons its difficult to have sharp edges. for instance Anti fouling paint will not achieve film thickness on the sharp edge and as a result the bottom will foul at the sharp edges.

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### WillallisonSenior Member

8mm!?! Michael, I think you might need a prescription lens in your scuba mask! 1 - 2mm would be more typical.... but sorry Olav, I've not come across research that says anyting other than "as sharp as possible", as you've already said...

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### Olavnaval architect

Thanks for the replies so far!

Of course I tried to collect some data whenever given the opportunity at a marina or boatyard; the boats I was able to measure had radii of 3-4 mm (it's a bit hard to measure; you have to guess where the tangents of the radius are).

I should also add that I'm primary interested in small craft, since the project I'm doing at the moment is a 5 m planing rescue boat and I have to provide the boatbuilder with some numbers (mould construction is about to start right after christmas). I guess small boats should have sharper chines than larger craft, that's why 6-8 mm on an 18 m powerboat doesn't sound wrong to me. I measured a 12 m boat with about 6 mm radius.

When worse comes to worse I'll go with 3 mm radius, but I'd like to back up my numbers with some hard science. The trouble is - as I wrote in my opening post - that I know such investigation exists; I just can't find it anymore...

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### Ad HocNaval Architect

So long as the spray rail is at an angle promotes a downward deflection of the water, (most have a nominal 5 degree downward angle) and sufficient 'width' (ie not a radius, or semicircle, but an actual straight flat surface) then it shouldn't really matter. Since once the water has changed direction from going "upwards" and then redirected "downwards" (along the flat surface to create the spray rail at 5 degree down, say), a radius at the end of the rail wont suddenly force the water back up around the end once it starts its new downward direction.

A sharp end or a radius end the pressure difference at the end is the same. The water has already changed its vector to a negative angle. Assuming the spray rail is clear of the water, ie above. The likelihood of the water creeping around a radius would suggest that water would be "pulled", but by what? There is only atmospheric pressure above and gravity below. Only the vessels motions would promote a further redirection "upward" direction of the water.

Although it is analogous to an orifice of pipe or opening with a flow of water. Where a sharp edge gives a Cd of 0.62 and a rounded edge gives a Cd of 0.97. But the Cd is related to an actual discharge, i.e. a fixed area. This doesn't occur on a spray rail..as it is the whole length of the rail and open at one end, not closed, like a classic mass flow problem with a given head/pressure.

Although common sense tends to indicate, as noted a sharp as possible...just seems right!

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### WillallisonSenior Member

Yes - fair point John... I was primarily thinking about the trailing edge of the transom / bottom... but for a down-turned chine I can see your point....but again, everything I have ever read indicates this should be as sharp as possible to prevent re-attachment to the hull

Last edited: Dec 15, 2011
8. ### tunnelsPrevious Member

keep them sharp !! and square !! must shed water as fast and cleanly as possible !!

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### JoakimSenior Member

Put your finger beside the waterflow from a faucet in such a way it just touches (fingertip pointing downwards and nail away from the water). Note the clear deflection towards your finger, not away from it. Put your finger deeper into the flow. Note that you can't deflect the flow much away from the finger as it follows the radius of your fingertip.

What force did that? It is the low pressure caused by Bernoulli effect causing suction from the hull.

Now turn your finger so that your nail is the trailing edge. Note that the flow does not defect towards the finger any more and you can much more easily deflect it away from the finger.

Better jet use a piece of plastic etc. with a flat surface and sharp (and blunt) edge.

Savitsky suggests using sharp >90 degrees for the trailing edge:http://www.stevens.edu/ses/cms/file...f_Whisker_Spray_in_Performance_Prediction.pdf

The bigger the radius the more spray will deflect upwards and it is not impossible for the flow to reattach to the hull. The defection adds resistance and if the spray is reattached there will be added friction losses as well.

I can't really say what is the maximum radius and that would also depend on the size of the spray rail and thickness of the spray column, but The study by Clement in 1964 showed that 1/4" radius in the full scale nearly negated the effect of spray rails.

Here are some videos about the effect:

Last edited: Dec 14, 2011
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### LandlubberSenior Member

"keep them sharp !! and square !! must shed water as fast and cleanly as possible !!"

...that is correct, on our race boats we used to fill in any radius, filed them down to edges that you could cut with and you can GUARANTEE an extra knot of speed (transoms), so even though as Gonzo says, paint cannot stick to a sharp edge,it depends on application. A race boat is not antifouled anyhow, a boat living in the water will need to compromise, and a small radius(<3mm) would be needed, both for painting purposes as well as moulding.

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### JoakimSenior Member

Here it is: http://dome.mit.edu/handle/1721.3/49089?show=full

They measured the resistance of an 1/6 model of a 68' boat with 1 mm and 0.5 mm radius and found a clear difference in resistance. The total resistance increased 1-2% due to larger radius at displacement froude numbers 1.5-4.

They were able to decrease resistance with spray rails 6% at best, thus the effect will be much bigger for faster and lighter boats, which have much bigger portion of whisker spray drag.

It may also be that 0.5 mm was not sharp enough for the 11' scale and 6-8 mm is certainly way too much for any scale.

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### Ad HocNaval Architect

You are confusing flow in a pipe with that in steady open stream – totally different. In a steady flow of fluid, you cannot get crosswise flow. In a pipe you have three different forms of energy acting all at once which is not the same as on a spray rail open to the atmosphere on a hull that is planing.

Your analogy of finger and pipe is incorrect. Since the fluid exiting a pipe is subject to potential head (potential energy), pressure head (energy per unit weight) and velocity head (kinetic energy). All you have done is change the direction of the fluid between the exit and the location immediate in front of the exit, ie, where your finger is located, since the presence of the finger creates a pressure differential from the side without the finger to that where there is the finger and alters the flow.

Bernoulli’s theorem states that the total energy of each particle of a body of fluid is the same provided that no energy enters or leaves the system at any point. If water is “escaping around a radius of a spray rail then at the extreme particles of the fluid at the boundary is not subject to the crosswise limits because the crosswise flow of the water particles at the ‘boundary’ can go forward with the main body of fluid or now has the option of attaching itself to the surface of the radius which is away from the main stream into the “open” air and escape “around” a radius” (or not); thus the extreme particle of the fluid at the boundary can become detached from the main body of fluid. In other words it is leaving the system.

But as previously noted above this is not a fixed mass of fluid in an orifice situation as in a pipe flow.

You are confusing spray, fine particles of fluid, with a change in energy of the fluid itself. Since there is no influence to cause water to change direction once it has been redirected by a spray rail downwards, other than noted, the extreme particles at the boundary, or simply put, fine spray. This is the slowing down of the water particles relative to the main stream of the fluid will become detached either onto the radius or the atmosphere (cuase by friction with the air) and create spray. Just as spray is also caused as the main body of water approaches the water surface, owing to different velocity profiles.

You are confusing whisper drag with the effect of a spray rail with its size, angle location etc on the hulls surface. The purpose of the spray rail is to separate the flow cleanly. If you do not do this, it is not an effective spray rail. The angle that is best is nominally around 5-10 degrees down. If too high say 30-40 degrees you get splash back. If you place too far aft or fwd or too many etc, these all have differing effects and on different dead rise hulls too.

But if you think a spray rail of say 100mm wide with a radius of 1mm at its end is going to be significantly different from that with one of say 10mm rad, when set at an angle of say 10 degrees down, then you have misunderstood what the spray rail is doing and looking at numbers without context.

13. ### tunnelsPrevious Member

Some time back i worked for inflateable boat company , they always had problems with water climbing up the sides of the tubes and even fitted fabric spray rails of sorts to the outside edge of the tube .
I was given the job of developing a fibreglass ridged botttom boat using the same tubes ,so i stepped the hull first with a wide strake then with a stepped section 75 mm tall around the outer exsposed part of the glass hull . This way at speed the hull was able to lift the tubes well above the waters surface completely and so reduce huge amounts of drag !! The boost in speed was quite noticeable !! the outer edges were kept sharp and square over the totall distances! An addition to the original boat i widthened the two 35mm wide strakes out to 120 mm wide to produce even greater lift
and reduced the wetted area even more ! now we were smokin so to speak!

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### DCockeySenior Member

Ad Hock, are you claiming the radius of the spray rail doesn't matter? How about the radius of the aft portion of chines on a planing hull? Edge of the transom?

Last edited: Dec 14, 2011

15. ### tunnelsPrevious Member

Spent hours sharpening edges to get them perfectly sharp and square !!,chines all the way !,keel in the bottom of the vee transom even to the extent of building it out 5 mm and having a ever so slight reverse angle to shed water even quicker .

Try holding a tea spoon in the flow of water from a tap with the back of the spoon in the water flow it is actually drawn into the water flow and the water pulls even harder if the flow is stronger !!so multiply all this and its significant . For tracking in a rought sea the keel being sharp noticably helps !!

Do the same in the forward sections of yachts hulls as well, from the very front of keel forward into the bow forefoot and part way up the stem !!.

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