Resistance factors, planing hull at low speed

Why are you attacking the credibility of the Series 62 data at slower speeds?

 

The question is where the data is published and why they are not included in technical paper? Agree that PBM is not scientific publication.

I will ask Donald about this data, really interesting.

Many amateurs and 'software users' are reading this thread, don't You think so?

 

Me attacking??? Have You opened PNA Vol.2 on page 101?

 

Yes PBB is not scientific publication. But not fiction magazine either.

I am sure, many amateurs and "software users" visit here. Normally I still analyze the arguments first.

 

Hmm, yes, and I like to read it.

I do agree that in US where 90% of yacht designers do not have any formal engineering degree it is valuable source of technical information. But we have to look at more scientific sources sometimes...

Good, so we should not encourage them use systematic series beyond their limits.

 

""I am sure, many amateurs and "software users" visit here."
Good, so we should not encourage them use systematic series beyond their limits."

Of course; in this particular case, the usable lower Fn limit is questioned.

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On the other side, as I see from quick analysis I posted, with increasing L/B and L/D, difference between planing and displacement hulls at low speed becomes increasingly smaller.

 

For me it is not questioned; if one is using 62 for calculations (as presented in PNA diagram) he should stay above FnV=1.0.

Have You checked wetted surface (AWS) of planing hull and yacht hull?

Problem with DSYS series is that AWS is calculated automatically. If 'trawler' hull formally fits in DSYS it will get much smaller AWS and resistance curve will be lower then calculated using other displacement craft methods. Though this is not right way to do the calculations: one should always look at parent hull shape when choosing series.

 

From all I now and what is posted about 62 in this thread, it appears:
*models were actually tested at displacement speeds
*series was intended primary to investigate planing regime, up to very high speeds
a preliminary conclusion from this contradictory statements could be:
some data below hull speed exist, but measured at big intervals, and omitted from some publications, as being of secondary importance only.

62 Series I took as published in PBB. As stated in the PBB text, friction is already included here.
Yacht hull parameters are all actual, entered manually for each configuration calculated. I double-checked every number.

 

Where did you get this figure? Are you counting offices or persons? I can easily come up with about 20 design offices which mainly work with sailboats. Many of them have 3-5 or more yacht designers.

 

Not true, officialy series 62 starts at FnV=1 that is very beginning of semi-displacement mode (FnV=1...3).

I would say: these data are not included in any scientific publications of series.

Or was found unreliable by authors of series? Or?

Very small graphs published in popular magazine is only source of these valuable data

This was very smart to compare sailing yacht hulls with planing powerboat hulls; good exercise but nothing for reality of design. Comparison of powerboats (displacement) with powerboats (planing) is much more rational approach.

 

My numbers come from own experience and communication with colleagues.
Yes, I mean commercially successful design offices having income exclusively from development of sailboat designs.
You can list those offices here, as well as number of sailboat designs they produce per year

I agree that some designers sell stock sailboat designs to amateur boatbuilder or small yards, but this is not the case.
In our portfolio before crisis, sailboats and motorsailers comprised about 15% of all design comissions (2-4 designs per year). Now is even less; we have developed only 1 sailboat design in 2010.
If You look at designers usually known for sailboats designs, in most cases can clearly see bias to powerboats in last few years. Probably they do not advertise it too much but they do it. Because unfortunately market is there; and sailboat market keeps shrinking.

 

OK, now I am in office and looking at original paper by E.Clement 'How to use SNAME small craft data sheets for design and resistance predictions' 1963, where more detailed results of 62 parent model are presented. It is stated that models were tested without turbulence stimulators, model 4667 in particular, on Fig.5 p.2.

The results are presened for: FnV=0.00, 1.00, 1.25, 1.50.... So no points are tabulated below FnV=1.0 excerpt one for static condition.

So the conclusion is: even if they did runs at displacement speeds around FnL=0.3, such measurements would be doubtful as no proper turbulence stimulators were used and the results are subject to scale effects. Turbulence stimulators is 'a must' for such low speeds...

Moreover, talking about scale effects on transom craft there are differences between model and reality due to differences in boundary layer thickness (model can run with wetted transom while real boat can have dry transom), also in presence of propulsors.

Hope this explanation is enough to understand why results 62 should not be used beyond parameters of series, at low Fn in paricular.

 

Donald Blount has recently published plots of Series 62 data below FnL of 1.0 in the Blount and McGrath paper (to 0.1) and in PBB #128 (to 0.25).

In a previous post I quoted Blount in the PBB #128 article, p 23, third column but made a mistake when typing and misquoted Blount on where the concentration of speeds were. I've corrected that mistake and will also copy the quote below:
"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 pre-planing range than in the planing range."

Note that Blount himself says "In fact when you look at the speed that we did run, they're more concentrated in the pre-planing range than in the planing range."

On p21 of the PBB article Blount says:

"Series 62, by contrast, covers the spectrum: it starts at zero speed, which is hydrostatics; goes up through hull speed, which again is the hydrodynamics of displacement vessel; but it then carried it on through the semi-displacement realm of hydrodynamics, becaus you begin to get speed-related effects in the semi-displacement regime-that is, you have a combination of hydrostatic and hydrodynamics."

It is entirely possible that more data was collected than is generally known, and that appears to be the case here.

 

Just a couple of points...

1. Boundary layer thickness was thought to have an influence, but then Doctors et al changed their mind in the light of their more recent experiments.

Form the abstract:
"There is now strong evidence that the transom-draft Reynolds number plays a much lesser role in this phenomenon than previously thought."

A Study of Transom-Stern Ventilation
Doctors, L.J., Macfarlane, Gregor J., and Young, Richard,
International Shipbuilding Progress.

2. I agree that turbulence stimulators are necessary at low speeds on small models, but they introduce quite a lot of uncertainty themselves.
There is a very interesting paper on this topic.

Bertorello, C., Bruzzone, D., Caldarella, S., Cassella, S., Cassella, P. and Zotti, I.,
``From model scale to full size. Investigation on turbulence stimulation
in resistance model tests of high speed craft",
Proc. FAST 2003,
Ischia, Italy, Oct. 2003, pp.\ A1-1--A1-8.

It should send a chill through anyone who places too much faith in experiments on small models

All the best,
Leo.

 

Again: without turbulence stimulators data at low Fn is useless for resistance evaluation. This is probably the reason why it was not included in official results of series.

Seems You do not understand what is pre-planing and what is planing range. Generally accepted pre-planing range is FnV=1...3 (some researches specify different upper limit - 2.5 or 4.0); planing is FnV>3. So once series 62 starts at FnV=1.0 (PNA Vol.2 p.101) that is pre-planing range; so Blount's statement is 100% correct.

FYI: displacement range is FnV<1.0; is not covered by published results of series.