Maximum radius on spray rails

In a different context I recently tried to find more about 'spray rail science'. I found that many consider this topic as sort personal secret knowledge and so I could collect only little information.
Science usually is combined with tests and experiments. My life lasting past time experiment is fast boating; and always I saw poor performance with large radii (be that at the transom or at the chine/s). So I learned what everyone else knows anyway
One can put this into numbers and formulas now, but I see the tendency for the optimum heading to 'as sharp as possible' - is that not enough?
Not being a scientist myself I sometimes see engineers/designers who put so much attention to (some) details that it is forgotten that someone has to physically make it in an appropriate manner, and that the product in use later will not allow to make full use of a theoretical advantage.
A design must serve the purpose. So in this present case: would a 5m workboat not be vulnerable with a too extreme radius-approach? If it were a 5m racing boat the case of course would be different.
I understand the original question as a request of where to find a backing statement to justify the chosen detail. It then looks like most replies here are off topic

 

A good suggestion to investigate what in north american civil engineering is sometimes refered to as "hydraulics". I had a look in the textbook from my college course on the subject and found little which was applicable.

 

Yeah, it should be, also because it's rather intuitive. If you ever tried to pour some milk from one glass to another, you'll know the difference a rounding of the glass edge can make, in terms of how much milk will get into the other glass and how much will be spilt on the kitchen table/floor.

 

There was a fast skiff hauled and parked on the hardtop so I grabbed a steel ruler and did some eye balling. The average chine radius was 3.2 mm. The transom trailing edge was essentially square.... perhaps .5 mm. The lift angle of the strakes was perpendicular ,90 degrees , possibly a few degrees down. Many lifting strakes, pads , trailing edges and all manners of angles on the bottom. The chine radius in the area of the travel lift straps was eroded.

 

I don't think "as sharp as possible" is enough. It's OK for racing boats, that can live with the extra work and care, but for other applications it would be nice to know what is the penalty of different radiuses at different speeds, scales and hull forms. Just like it is important to understand the extra drag of hull roughness or fouling.

 

Well, not quite the same thing but something that can give an indication - an example from Mark's Standard Handbook of Mechanical Engineering.
Consider a flow of a fluid through an orifice of area A. The mass flow through the orifice is given by equation

Q = C A sqrt( P )​

where C is a coefficient, and P is the pressure difference acting on the fluid. If the fluid is contained in a tank with a free-surface at a level H above the orifice, the P equals the hydrostatic pressure, 2 g H. The coefficient C (coefficient of flow contraction) is given by this table, for two cases - a sharp orifice and a rounded one:


What can be learned from that table is that, for the same orifice area A and pressure difference P acting on a fluid, the flow across a sharp orifice is just 62% (0.61/0.98) of the flow passing through a rounded orifice.

Put in other terms, to get the same flow through the two orifices, a sharp one needs a pressure differential which is 2.6 times the pressure differential of a rounded one.
It means that the sharp orifice has a much higher drag than a rounded one. This is the key phrase here, the indication we need.

Now, how does this relate to planing hull chines or spray rails? This drawing will hopefully be clear enough:


Not quite the same thing, but a useful analogy, imho. The main difference is that orifice flow is constrained by axisymmetry, and hence flowlines cannot escape sideways like they would in case of spray rails. Anyways, what we can see is that by rotating the edge 90° anti-clockwise the drag becomes a lift.

The previous key phrase hence becomes: the sharp orifice (now became a chine) has a higher lift. And how do you create lift? By deflecting the waterflow sideways and/or downwards. It implies that the sharp strake (rail) is more efficient in deflecting the waterflow (or keeping it away from the hull), which is what we need to know.

Although this example is not exactly what the OP needed, and doesn't give values for different rounding radius (which, I guess, can be found in textbooks about hydraulics), at least it gives an empirically measured argument to back the claim we already know - keep it as sharp as you practically can.

Cheers

 

Just got an idea... Looks like this forum is pretty known by NA students, who come here from time to time and ask for ideas for a subject of their thesis.

How about creating a thread with a wish-list of scarcely available technical investigations of interest for the community of boaters and designers here present? Interested parties would enter a subject which they find interesting and scarcely documented, and then students and universities could pick a subject of their interest and perform a thesis work about it. It would be so nice (is it still possible in time of pay-per-read sites like Jstor, Ieeexplore and similar?) if they publicly posted the results of that work, so that everyone could benefit from the thesis research.

Two subjects that come to my mind right now (others will follow) would be:
- the influence of chine and transom edge-radius on lift and drag characteristics of a vessel
- the influence of round and hard-chine cross-section shapes on lift and drag characteristics of similar vessels.
The others might continue the list.

Cheers

 

daiquiri, I don't think this anology is really relevant here. In the orfice case there is no velocity in the tank except very close to the orfice. There is just suction, which is sperical in nature. Thus the flow comes to the orfice at different angles and needs to turn around the edge of the orfice. That's why you can greatly reduce the suction pressure drop by rounding it. You can find different inlet pressure drop coefficients for a sharp pipe starting inside a tank (up to 1.5, if far from the wall), at the wall (0.5) and well rounded at the wall (close to zero).

In the case of a spray rail or chine there is no suction and there is substancial flow velocity and it's all along the hull. The milk pouring was much closer to it.

 

That confirms the figures posted here: http://www.boatdesign.net/forums/hy...imum-radius-spray-rails-40920.html#post508363

A lot of this “stuff” does exist. But it is very esoteric. Such as this nice work:



Originally started by Bobleff in 1881 on cross flow of a planning plate (which is req'd for separation of flow at the chines)…and then to the classic work by Von Karman in 1929 (impact of bodies on water), Wagner in 1932 (cambered plates in 2D flow) and to Fabula in 1957 (upwash velocity on reflection bodies)…all before Savitsky etc.

 

I have to disagree. There is necessarily a suction component in case of a rounded chine, or otherwise you wouldn't have a flow turning around it and eventually wet the hull walls.
Whenever a flow follows a circular path with a radius R and tangential velocity Vt there must be a pressure gradient acting on it, equal to:

dP/dR = rho (Vt^2) / R​

It comes from Navier-Stokes equations. Assuming as reference pressure the one acting at R=infinite (far-field), the equation tells you that the pressure will necessarily have to decrease towards the center of the circular path (decreasing R and hence negative dR). So the lowest pressure will be at the (ideal) wall of a radiused chine, where the radius of the flow path is minimum.
And, finally, it means that there is a suction force acting on a chine edge. Which is the physical reason for the decrease of lift when a chine is rounded.

Cheers

 

@Ad Hoc,
Didn't notice that you have mentioned similar things in your previous posts. Cheers!

 

Yes there is suction from the hull surface when surface starts to turn away from the flow, but I think there is much more analogy to an end of a pipe than to an orfice in a vessel or an inlet to a pipe. The latter two are very much identical and the difference you see in the C-coefficient is due to the pipe inlet differences, which make a difference how the flow is accelarated from zero to outlet velocity and all veocitie are caused by the pressure differece at the orfice. At the chine or spray rail there is already a velocity, which not caused by the pressure difference at that location.

 

You are going into details here, when the orifice example served to get just an indication of what happens when the edge is respectively sharp or rounded - as clearly stated in that post.

And, strictly speaking, the statement "all velocities are caused by the pressure difference at the orifice" is wrong. The moving force of a fluid going through an orifice is not a pressure difference at the orifice. It is the pressure difference across the whole volume of liquid, from the point where, say, a pushing piston acts, to the orifice. Or from the elevated free-surface (if it is a gravity-driven flow) to the orifice. The pressure difference across the orifice cross-area is zero, since it is a fluid area and not a rigid surface which could bear a finite pressure difference. Not an actuator disc, in other words. Just like the fluid area around a chine cannot bear a pressure difference - hence the analogy.

Besides that, although the flow at some point far from the orifice does have a near-zero velocity, the near-field flow around the orifice has a finite velocity, so will behave differently when it arrives to a sharp edge, rather than to a rounded one. That is again a situation vaguely similar to the near-field flow around a chine, which allows us to get some indications.

Finally, may I remind you one more time that the sense of the orifice example was to find a known and measured flow analogy, which would share a similar or same physical mechanism and hence can give an indication of what is happening in case of a flow over a sharp/rounded chine. If we start analyzing details, we'll find a dozen of differences - but the physical bone of the problem is similar in both cases.

Cheers

 

Yes and No
If someone finds a formula or more data - ok; why not?
On the other hand, there is already the basic enlightenment of what is best. And any science will only confirm what we already know.

I already know that my car runs best with circular wheels
(longitudinal, not transversal :!
I am not really interested how round my wheels should be.

The formula in question will of course help to make decisions when a compromise must be found. But the matter (to me) appears more like another Faust / Wagner / Goethe