Entry Angle. What is the explanation?

From general knowledge, we know that fine entry angles are good for windward work.
But what are the dynamics, behind this?
I suspect, it is something to do with the ability of the hull to bite into the
water and resist sideways forces, and thereby help do some of the work the keel has to do.
Does it have anything to do with the bluntness of the hull shape?
Or does it have anything to do with the distribution of incoming water between windward side and leeward side?
When the boat is upright, i can see that it is a measurable angle determined by the shape of the bow section. But when the boat is well over her ear, which is when it counts most, what importance does the shape of the bow profile remain to play? As the oncoming water probably meets a totally different profile determined by the mid sections?
I also find it difficult to visualise what entry angle there is to speak of for modern generation skimming dish race boats.
Any scientific explanation is most wellcome.

 

The angle of entry tends to have a significant contribution to the pressure field around the hull. This is some pretty involved hydrodynamics and the characteristic equations are complex, Probably best to just accept that by reducing this angle the form resistance of the hull tends to be reduced.
Tests on various hull-forms support this. In practical applications it has its tradeoffs from reduced volume fwd.

Heeling is significant.
When the boat heels the water is still separated by the bow, so a narrow entrance angle is still an advantage however the flow/pressure field becomes asymmetric, resistance certainly rises but maybe not as fast as you think. For example the increase in form resistance for angles of heel
1, 5, 10, 15, 20, 25, 30 is 1%, 2%, 4%, 7%, 13%, 25% This is for one particular hull-form, but the trend is illuminating. The stiffer the vessel the better it will perform in all aspects of drive and resistance.

Leeway is another asymmetric flow producer. The sails, their balance, trim and shape are just as important to overall performance.



Thats my 5 minutes worth I hope it helps you

 

Half angle of entrance

From: http://www.hydrocompinc.com/knowledge/techexchange/tex0303.htm

"What is the best entry angle for a boat's operating speed? Entry angle (referred to as the half angle of entrance) is a significant player in vessel wave-making drag. In simple momentum terms, think of it as the cause of how quickly the mass of the water needs to get moved out of the way. A high angle also can also lead to longer flow paths and subsequently higher local water velocities (a' la Bernoulli), leading to higher viscous (frictional) drag.

Small is not always better, however. From a design standpoint, you would likely have to increase vessel length to achieve a very small angle, with a corresponding increase in the overall wetted surface. Shape with reverse curvature of the waterplane (as you might find with very small entrance angles) can increase what some refer to as the "shoulder wave", actually leading to more drag. So you always have to compromise when searching for the "best" entrance angle for minimum total drag.

A quick literature search did not quickly find anything like a curve of "best" angle versus speed. This is not a surprise, as there are so many inter-relating factors, and it would be difficult to pull out this one variable.

Measurement of this angle is somewhat subjective - it is not the measurement right at the bow. For example, you can take a bow shape, tug the stem forward and get a very acute local angle - without any measurable effect on drag. (The opposite example is with a small radius at the bow, such as from a small conical development. The measured geometric angle right at the bow is 90 degrees, but of course this is not its effective entrance angle.) You need to measure the angle some distance away from the centerline to avoid the local stem shape. We recommend drawing a line parallel to the centerline 10% of the maximum waterline beam (i.e., 20% of the half-breadth) off center, then take the tangent of the waterplane at that point. This gives a repeatable, quantifiable, measurement and avoids all of the small insignificant local shape right at the bow."


A paper on the matter:
Zardi, V., A Study on the Influence of Angle of Entrance on the Added Resistance in Waves and Seakeeping Performance of Sailing Yachts, 2000

Cheers

 

And it was published in...?

 

The latest TP52's have gone for a fuller and less concave bow entry than before. This has helped spread the volume distribution more evenly through the hull, which means that as the boat heels, there isnt so much of a change in trim and balance, and this allows them to carry deeper (more powerful) sails. The foils can be a tad smaller too as they arent being jacked around so much by the changes in trim, and not so much rudder is needed to keep things on course. The differences are pretty minute tho...

 

A looong thread on the topic of hull balance in the model yacht world can be found at:

http://www.rcsailing.net/forum1/showthread.php?t=3436

I delivered a paper on this topic at the last Chesapeake Sailing Yacht Symposium. PM me if you'd like a PDF emailed to you.

Cheers,

Earl

 

University of Southampton, School of Engineering Sicencies.


Earl Boebert,
I'd appreciate very much a copy of your paper.
You can mail it at: g.gefaell<at>mundo-r<dot>com
Thanks in advance.

Cheers.

 

Too fine and you lose hull balance, the bow goes down when she heels and she wants to broach, getting extremely difficult to control downwind. I prefer 27 degrees off the centreline for good hull balance.
Brent