Defining Midship

[Ad Hoc]

Originally Posted by DCockey

How accurate do you find commercial vessel resistance prediction which has had much more development than that for sea kayaks?

It has been mixed. However once the “errors” were identified it has been very consistent. But you need to ask yourself are you after an absolute value or a good ball park figure?

The commercial tank testing we did, we use the same tank same set up as for all our hulls, thus minimising any systematic errors, which we noted in years gone by. Our own in-house tank testing, the same. We use the same methodology, tank etc etc…thus minimising systematic errors. The results have been very good.

But it is not just about the “prediction”. The build is very important. If you do not keep a tight control on your build, and not actually monitor it, you’ll end up with serious anomalies which at first are hard to explain by a mathematical review!

[DCockey]

Originally Posted by Leo Lazauskas

Michlet is not CFD.

It's a lot closer to CFD than the regression based formulas.

Michlet is physics based. Same as CFD.

Michlet uses simplified equations. So does all or virtually all CFD practical for marine applications.

Michlet uses an approximate boundary condition. So does most CFD used for free surface marine problems.

Michlet uses an intergral formulation. What is commonly refered to as CFD uses field equation formulations.

Many folks using and promoting CFD do not fully comprehend the approximations inherent in the various variaties of CFD.

[Tim Hall]

Thanks for the discussion guys...good stuff. Ad Hoc: thanks for the definitions there. Obviously Cwp and Cvp are no-brainers, because there's no question of where the waterplane is. Just for the record here's my perspective on all this:

1) I realize the form coefficients are more or less theoretical (i.e. the boat is what is regardless of what the numbers say). But being new to this, it's important for them to tell me something, and that I'm calculating them so they're meaningful.

2) I'm not too concerned about how accurately KAPER is going predict performance. When you go to paddle the boat, you're not going to know if real world conditions are matching up to the resistance curve you had plotted.

This whole thing came about basically because I had a hunch my prismatic was a bit beefy, KAPER seemed to confirm this, and then it called into question exactly how the coefficients should be calculated and how meaningful they are depending on how they're defined.

[Leo Lazauskas]

Originally Posted by DCockey

It's a lot closer to CFD than the regression based formulas.

Michlet is physics based. Same as CFD.

Michlet uses simplified equations. So does all or virtually all CFD practical for marine applications.

Michlet uses an approximate boundary condition. So does most CFD used for free surface marine problems.

Michlet uses an intergral formulation. What is commonly refered to as CFD uses field equation formulations.

Many folks using and promoting CFD do not fully comprehend the approximations inherent in the various variaties of CFD.

Yes, but just because a duck makes waves similar to a kayak doesn't mean that either should be referred to as a "ship".