Resistance factors, planing hull at low speed

One of definitions of Hull speed I know is "speed, when length of transverse wave is equal to length of hull on waterline".

[/QUOTE]Show me any real NA who makes living on designing skiffs; we will laugh together [/QUOTE]

As far as I know, 18ft skiffs are now a one-design class, so no one could make a living from designing them.

There are designers who make a living designing VO70 for example. Bruce Farr in his time was involved in 18ft skiffs. He even mention it somewhere on his website.

Chronologically, these Australian 18ft skiffs were first boats to routinely plane upwind and sail faster as the wind downwind on the water (landyachts and iceyachts notwithstanding).

Bethwaite did not design 18footers himself (my mistake, it was his son's designs) he designed and built high performance boats he call H(igh)S(speed)P(project), with similar performance (he report 4kn boatspeed in 2kn wind sometimes). He did many research on foils and sails, and was closely involved in development of Tasar class. Remarkable point is, that Tasar, without spinnaker and trapeze, sail at similar (sometimes higher) actual speeds, as spinnaker and trapeze boats like flying Dutchman.

 

I am following everything thats being discussed here. Just to make sure I am not lost. is the series 62 hull (PNA) being discussed the stepless planing hull?

 

Yes, but this 'hull speed' is not applied for planing or semi-planing hulls. I.e. for boats capable to go over the hump this speed does not exist.

There are 3-5 yacht designers in the world who make living on sailboat designs (at least they claim so). Morelly/Melvin (designers of trimaran - last AC winner) make living on designs of workboats - windfarming catamarans...

I know those dinghy designers; we clear mess after them sometimes You don't need to tell me how good they are.

 

62 series is for stepless planing hulls, with blunt bows and narrowed transoms.

 

In the paper daiquiri posted here, Blount himself has plots of Series 62 resistance going down to FnL of ~0.1, Figures 1 and 2.

The data in the table at the end starts at FnL of 0.4 but the scope of the paper is FnL of 0.4 and greater.

 

What is wrong with published data? If there is resistance number for certain set of hull parameters, published by the author, what a reason is not to use it? Could you post hard evidence that series 62 hulls were not towed at displacement speed? In the graphs I refer to, there is pretty clear set of hull proportions mentioned, displacement stated, and friction calculation method stated. It is stated in surrounding text, that these are the graphs for total resistance of full size hulls. I compare it to full size hulls of same displacement, but of different design, calculating resistance with software, officially supported by Delft University. In the worst case, if friction is calculated differently, it could make ~2% difference in total resistance at most. Is it not enough? With this kind of approach I could suspect all sorts of bad things even for data I collect myself.
Deftship (not Freeship in this case) is just a tool, easy to use, and happened to be at hand at the moment. Hullspeed (maxsurf extension I use for the purpose at work) show same levels of resistance differences.

P.S. Do I have to post my diplomas, CV, recommendations from all the clients I did work to?
I do not think this forum is proper place to wave diplomas like battle flags.
I think this forum is place for exchange of (more or less) valuable technical information. In this respect boatdesign.net served me well so far.

 

Click here for the pdf of Mercier and Savitsky paper: http://handle.dtic.mil/100.2/AD764958
You will find the explanation of the ratio At/Ax and it's usage in the calculation, as well as a discussion of results concerning this parameter.
Cheers

 

Actually their interest was from 0.3 to 1.0 (see paper).
But they tabulate series 62 from FnL=0.4 only. Why?

We do not need to guess; we look at parameters of series as published and use it correct way.

 

In the Blount & McGrath paper "hull speed" is noted on Figure 1 as FnL = 0.4 even though planning hulls operating at higher FnL are being discussed. (Copy is attached, thanks to daiquiri for posting it above.)

"Hull speed" is a useful concept as an indicator of the speed where the hydrodynamic modes transitions form displacement to planning. Of course that transition does not occur abruptly.

 

This speed exist as beginning of the hump In any paper on ship hydrodynamics on the subject of high speed displacement, semi displacement speeds, "hull speed" (more often mentioned as Froude Number 0.4) is always mentioned as the end of displacement sailing and start of semi-planing regime. Where is it stated, that hull speed do not exist for planing boats or FN=0.4 has no significance for them?

 

I do not doubt Delft results for sailboats. The problem is that You are using Series 62 beyond its limits (known problem for amateurs). Pls refer to PNA for series 62 limits.

You can post Your diplomas if You want. I just told You that I have some sailboat prediction experience behind.

 

In the PBB #128 interview Blount discusses the Series 62 tests. In the third column on page 23 he says:

"We selected different speeds in the displacement range. And in the semi-displacement range. And in the planing range. We wanted to get the best documentation of trends in those speed ranges. Just running at equal speeds wouldn't allow us to get as much detail. in fact when you look at the speed that we did run, they're more concentrated in the planing range."

 

First, use of these terms depends on school and specialization. I use FnV for anything semi-planing and planing; not FnL.

Second: 'loosely defined term hull speed is sometimes used for this Froude number range, in which the fast increase of resistance of most displacement ships can prevent further speed increase even with larger power'. Larsson, Raven 'Ship resistance and flow', SNAME, 2010. p.34.

That was not me

 

Sorry for being so stubborn, but my question was "why I could not use data, as published by the author?". If author find it possible to publish data starting from Fn=0.25, it is hard for me to see the reason not to use it.

I do not want to post diplomas. I do not ask anyone else to post diplomas.
In fact, I find it irritating to call someone amateur before/instead presenting own arguments or listening to arguments presented.

 

Why didn't they include those displacement results in series then? Accuracy of measurements? Or?