Definition of bow entry angle

[DMacPherson]

You really need to address the objective purpose for the IE (half-entrance angle) in order to develop a way to quantify it. As you can see from the thread there are as many definitions as there are perspectives. I had a (very) young designer some years ago tell me that he always puts a tiny"clipper bow" inflection in the nose because "it reduces the IE and makes my predicted speed faster" - even though it is hydrodynamically irrelevant and just adds wetted surface. Likewise, others refuse to put a small rounding on teh nose because the IE is geometrically 90.
For a prediction, treat it as nothing more than a data variable. And to know how to measure it you therefore need to know how the author of the prediction method measured it. There are a number of prediction models that use IE, and they were the principal angle at the bow reglecting any localized shape right at the nose. And, not only does the measurement need to give a figure that is hydrodynamically relevant, it also needs to be consistent, well-behaved, and not subject to personal interpretation by different people.
So, quite a number of years ago, we proposed the following, which has been used quite successfully in the NavCad software and other applications. It is very similar to the DnV noted above, but uses B/10 instead of B/4, which is closer to the definition of the various original prediction method authors.

Half entrance angle
The angle of the waterplane entrance at the bow, neglecting local shape. The measurement of the angle between the waterplane edge and the centerline, this is often a somewhat subjective definition. It is recommended to define this using the measured tangent slope of the waterplane at a point BWL/10 (i.e., 20% of the half-breadth) off the centerline. This offers a systematic definition that will be consistent for all hulls.​

 

[laukejas]

Who would use your algorithm? When would the algorithm be used? How accurate does it need to be?

Holtrop and Mennen published a regression based method for predicting ship resistance.
http://mararchief.tudelft.nl/file/33815/
https://fenix.tecnico.ulisboa.pt/downloadFile/3779577315267/RP_Lecture8.pdf

Anyone who wants to design canoes, boats, ships or other floating things with Solidworks, and wants to see hydrostatic calculations real-time that Solidworks doesn't provide. Anything from a toy boat to an oil supertanker. As for the accuracy... The more, the better. I think that if I could measure it with 1 degree of accuracy, I'd call that good enough.

My personal theory : if you have an analytical definition of the waterline curve, let'say y(x), the x point where you can use the tangential method to determine a significant half angle entry is where the curvature 1/R is minimum (up to zero if you have a local flat just before the rounded stem end), 1/R being equal to : y" / (1 + y'^2)^(3/2).
In most cases, when approaching the stem, the curvature 1/R decreases, then a minimum (up to zero), then an increase due to the rounded of the stem. It is where there is the minimum that you can have your significant half angle. If the waterline is pointy without any roundness, that gives the very end. If you have an inflexion point (giving also a zero of the 1/R function), it is there to evaluate the entry angle.

That's a bit difficult to define with limited API tools, but I'll look into it.

You could use the definition in DnV "DNVGL-ST-0111 - Assessment of station keeping capability of dynamic positioning vessels":

View attachment 143982
View attachment 143983
which seems reasonable to me...

I like this approach very much. Easy to implement, and makes sense too.

You really need to address the objective purpose for the IE (half-entrance angle) in order to develop a way to quantify it. As you can see from the thread there are as many definitions as there are perspectives. I had a (very) young designer some years ago tell me that he always puts a tiny"clipper bow" inflection in the nose because "it reduces the IE and makes my predicted speed faster" - even though it is hydrodynamically irrelevant and just adds wetted surface. Likewise, others refuse to put a small rounding on teh nose because the IE is geometrically 90.
For a prediction, treat it as nothing more than a data variable. And to know how to measure it you therefore need to know how the author of the prediction method measured it. There are a number of prediction models that use IE, and they were the principal angle at the bow reglecting any localized shape right at the nose. And, not only does the measurement need to give a figure that is hydrodynamically relevant, it also needs to be consistent, well-behaved, and not subject to personal interpretation by different people.
So, quite a number of years ago, we proposed the following, which has been used quite successfully in the NavCad software and other applications. It is very similar to the DnV noted above, but uses B/10 instead of B/4, which is closer to the definition of the various original prediction method authors.

Half entrance angle
The angle of the waterplane entrance at the bow, neglecting local shape. The measurement of the angle between the waterplane edge and the centerline, this is often a somewhat subjective definition. It is recommended to define this using the measured tangent slope of the waterplane at a point BWL/10 (i.e., 20% of the half-breadth) off the centerline. This offers a systematic definition that will be consistent for all hulls.​

If I'm to use the DnV method, I could make the B/x to be customizable, so the user can change that number (B/4 or B/10 being the default). This is exactly what I was looking for.

Thank you all very much, I consider the problem to be solved.

 

[Dolfiman]

In most cases, when approaching the stem, the curvature 1/R decreases, then a minimum (up to zero), then an increase due to the rounded of the stem. It is where there is the minimum that you can have your significant half angle. If the waterline is pointy without any roundness, that gives the very end. If you have an inflexion point (giving also a zero of the 1/R function), it is there to evaluate the entry angle.

Here attached an illustration, with various degrees of roudness of the bow end, of my evaluation process of this angle for a sailing yacht, and again I then just compare this output value with the usual ones, e.g. as recommended by Ted Brewer in "Understanding Boat design" :
« Half angle of entrance : the angle, measured at the LWL, between the hull centerline and the actual waterline shape. Fine angles are desirable for good performance but can be overdone, creating a wet boat in a seaway. Angles below 19-20 degrees would be considered fine, 20-24 degrees is fairly usual for a cruising yacht and angles of 25 degrees and above are considered bluff bows today but were fairly common in the '60s. »

 

[skaraborgcraft]

The issue I had with the Brewer description, is that is does not state how far back from the entry the measurement angle is take. For instance, I once owned boat that had an almost hollow entry that morphed into a full hull a little further back. My confusion is that that boat and others like it, would have two very different angles, depending on the point of measurement. If there was a simple agree formula such as "10% of the LWL aft of the bow entry", I would be less confused. Without a specific agreed point of reference, 2 people could come up with different numbers for the same boat.

My interest was peaked when late 18th Century beach launched fishing boats started to go over to engines, the builders pulled in the planking at the bow below the waterline, it was subtle, but done for a reason, as engines back then were low power. I dont believe they worked with formulas, more by trail and error.