Two hulls, which has least drag?

John

That’s the interesting part. Once you understand the L/D ratio’s implications, you realise that hull shape, i.e. transverse sections etc, has actually very little influence. Let me explain further.

Here is another graph show the variation of resistance with length-displacement (L/D) ratio:



As you can see, same trend.

If you now look at the influence of B/T ratio shown here:



As you can see, basically no real appreciable difference.

This too has been tried. As shown by the series of different sectional shapes, all with the same L/D ratio:



Each has their “sweet spot” at various speed (or Fn), but overall, no real applicable difference. These models ranged from semi-circular to a wide zero transom immersion stern, to a pure canoe etc. Even a ‘crude’ model is shown. This, same L/D ratio, was simply a ‘test’. It is a hull made from a square block of wood, with just the bow, cut at an angle to be ‘pointy, just like a child would make with a saw, that’s it. Thus even a very crude hull, is not that much difference. The greater the L/D ratio the more these factors begin to converge in the sense that shape has very little influence.

As a final note, you should begin with separating the resistance part of your investigation with the seakeeping. The two are not always mutually inclusive of each other. For example, a SWATH posses the superior seakeeping, but its resistance is the worse! Seakeeping is terribly complex too, but there are some “obvious” drivers.

 

Validation is a problem for CFD and especially with very complicated
scenarios. They are also very expensive.
That's why I suggested using free programs to get first estimates.

Delftship used to have an attractive animation of a ship pitching in waves
on their front page. There many others on Youtube and the web sites of
commercial CFD vendors.

AdHoc has shown you the influence of L/D^{1/3} on resistance.
You can use Michlet to investigate that for yourself. I'd be very surprised if
there was a significant difference in resistance for the two hulls.

SMP will give you a reasonable estimate of the added resistance in head seas.
It will also give you the pitching behaviour of the two candidate hulls, so you
can make a decision on that criterion.

Mind you, that will require a fair bit of time and effort on your behalf. Neither
Michlet nor SMP are easy for beginners. But then neither are CFD programs.
Simple linear models have the great advantage that their results are
easier to understand, and faults can more easily be explained. Inexperienced
users of CFD will have no idea why their code might be giving unusual results.

Good luck!

 

But will cost much, much less money than making the two models with CNC tools, constructing and assembling the test rig and performing the test in the lake or wherever, just to get very uncertain results.

 

Thanks for posting the graph above.

What shape hulls were used to generate the data in that graph? What was the L/D? How much did the wetted surface area vary with B/T?

 

The results are for the NPL series. See:
Molland, A.F., Wellicome, J.F. and Couser, P.R.,
"Resistance experiments on a systematic series of high speed displacement
catamaran forms: variation of length-displacement ratio and breadth-draught
ratio",
University of Southampton, Ship Science Report 71, 1994.

There are many other reports for this series, starting with Bailey's work:
Bailey, D., "The NPL high speed round bilge displacement hull series",
Maritime Technology Mono-graph, No. 4, RINA, 1976.

 

It's interesting that several folks initially assumed John Perry was asking about the effect of L/D on drag when he was actually asking about what happens to drag when L/D is held constant and B/T and L/B vary.

"Conventional wisdom" seems to be that long, narrow boats have lower resistance, which is not infrequently interpreted as meaning reducing the beam will lower resistance. But the graph Ad Hoc posted indicates that making the beam narrower while maintaining the same displacment and length will have negligable effect on resistance. So is the conventional wisdom wrong? Not if the decrease in beam is accompanied by an increase in length and/or decrease in displacement.

Another example that when there are multiple input variables which are not all independent the sensitivity to changes in any one input variable can depend on what is held constant.

 

The original NPL hull series data (and results using regression equations) are
plotted in several useful ways in the attached.

Are you hinting at AR effects on wings? I'm not sure if we nutted that out
to your (and others') satisfaction. I did put together some data and a
program where it could be investigated fairly systematically.

 

AR effects on the induced drag of wings are a good example of different sensitivities depending on what is held constant. But I've seen many other examples over the years.

Much of the disagreements about AR and induced drag in a past thread apparently were based on different uses of the term "induced drag". If someone wants to peruse that topic perhaps either a new thread or reviving the old one would be in order rather than hijacking this one.

 

Leo has already given you to source data for that graph. Below answers your other questions:

 

I think you've answered your own question about starting another thread, or visit the thread Leo mentioned.

 

Thank you.

 

I didn't ask a question about starting another thread. Rather I made a suggestion.

Perhaps there will be a response to the comment above alleging that my use of language is not clear, etc. If so I'll leave it the readers to decide who is confused, etc based on what was written.

I'm very familar with the thread about aspect ratio which Leo seems to have alluded to since I started it and was probably the primary contributor to it. It can be found here. http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/myth-aspect-ratio-36836.html

 

John,

I find your problem interesting - if you send me the models, I could try and run them when I have time - flat water & head waves.

 

When you do, it would be interesting to compare the results with the formula in the ORC VPP (eqn. 83, page 65).

 

John Perry,

Check your private massages box.