bow flare

To err is human. But it takes a computer to really screw things up...

A lot of people don't seem to understand that while a computer can do your calculating, it can't do your thinking.

You know.... GIGO and all that. A computer works with what you give it.

 

Thank you! I did read this thread thoroughly and now the one you've listed. I was looking for a formula that details what Bob Perry says about cutting through a wave vs lifting on top. Thanks again for all of you guys' insight!

 

Be careful about hinging a design paradigm on a generalization. Nothing in naval architecture can be generalized on it’s own. All aspects of the hullform must be assessed for the range of sea states, and headings and speeds.

The NA study is called a series of RAO’s (or response amplitude operators) which describe vessel motion wrt heading speed and wave encounter. Software such as seakeeper can give a good illustration too which is a good computer application.

A craft that’s fast to windward in a chop in sheltered or semi sheltered waters is not necessarily fast at sea, either to windward or downwind.

In a sea state that tends to induce pitching in a certain craft, speed suffers considerably as a result. By decreasing buoyancy fwd you can reduce pitching to windward in some sea states. But the danger is a boat that plunges in larger waves that can be slow and dangerous and very hard to control.

A pitching study of the design should be undertaken early. Unfortunately a lot of yacht designers tend to ignore and are often completely ignorant of dynamic longitudinal stability. If they start promoting novel hullforms then you really need to consider the limitations of the feature not just it's advantages.

 

We had offices of people with sophisticated mechanical calculators doing iterative calculations. Mistakes were common. The processing time was crippling and the work was boring.

Modern PC based packages let your mind soar when you design. Feedback is fast precise and accurate. It's lead to reliability engineering that's made your world safer more comfortable and high tech ( compared with your youth)
People tend to be ignorant of the incredible engineering advances and the complexity behind the technology we take for granted. Very little of it could have been developed and refined so well without computer based design.
[edit add ] And of course people trained well enough to use those tools.

 

An earlier discussion concerned implementing flare with a ply skin. If the bow shape is convex in plan view and the stations are concave then the shape is some form of saddle. Paths through a saddle exist which are either straight or are at least geometrically simpler, allowing the shape to be implemented with less difficulty as Tunnels was saying in post #32. However, I have a couple of plywood canoes which - viewed from forward - appear to have concave sections in places. This is not unusual, and suggests that a flared can be "faked" . . .

 

I'm not knocking computer. Between myself, my wife and my older son, we own three desktop models and three laptops. As you point out, when properly used they can let you get on with what you're doing and keep your mind on your objective, instead of getting bogged down in the process instead.

But they aren't always a substitute for good judgment.

 

I too use a computer a lot. It takes care of the tedious and - if I'm toying with a boat idea for example - handles the mind-numbing calculations that do not involve creativity. I'm still working on the good judgment part though . . .

When computers arrived in the workplace there were many examples of poor judgment. Folk became obsessed with precision in all things even when the calculation was based on an estimate. I found a gross error for the value of Young's modulus in a structural calculation. The mech guys were were convinced a computer couldn't give a wrong result, and didn't want to listen. GIGO strikes again!

 

And relevant to this thread (and the fad of wave piercing bows) is the use of computers to design small craft with novel features without deriving the transfer functions (RAO's).
Lateral calm water stability is well understood, but you'll see longitudinal dynamic stability completely ignored because the designer has never heard of it ! ( and it requires expensive software or a wave tank test).
Yet for an offshore craft a pitching study of the design should be undertaken early unless you are designing a standard proven hullform.

It is even indicative of the induced pitching moment ( waterplane, CG couple) by dropping a properly weighted model into still water. Or even in the PC hydrostatics package to trim the model by say 15 degrees. Considering whether the pressure centroid- CG couple is restoring neutral or undesirable.

As I said before unfortunately a lot of small craft designers do not seem to be aware of dynamic longitudinal stability, let alone running a study of it for the prospective client. Marketing hype abounds and some designs are obviously going to be dangerous on some headings/sea states. This is where some of the yacht design packages could provide some very good feedback to the low budget and perhaps not so well informed designer.

Cats are a good example, already with slender hulls and a pitchpoling vulnerability, yet I saw a 'designer' recently promoting low buoyancy 'wave piercing' bow shapes, on a relatively small Cat for supposed offshore usage; the complete antithesis of a wholesome design.

 

- a submarine designer perhaps?

 

All of which discussion brings us to the one, money no object, vessel I drool over, since first I laid eyes on it. There's no bow flare, but elsewhere there's flair & panache beyond compare.

http://www.gunboat.com/launched/gunboat-66

Cordially,

Perry