Resistance factors, planing hull at low speed

Behind is residual resistance coefficient for certain set of hullform parameters + friction. Sometimes form and wave components are separated. Sometimes spray making is separated.

As to reliability of Series 62 in particular, following excerpt do encourage me to put some faith in the figures:
From PBB Dec2010/Jan2011, page 23:

DB: …test plan. I think part of the
significance of Series 62, besides pre-
senting all this data, was actually the
format of presenting the data in terms
of design utility. I was involved when
we started working on that. For example:
What speeds do we run? We selected
different speed increments in the dis-
placement range. And in the semi-
displacement range. And in the planing
range. We wanted to get the best doc-
umentation 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
speeds that we did run, they’re more
concentrated in the pre-planing range
than in the planing range. Above vol-
ume Froude number 3, the increments
are pretty large. ...

My guess is, with so many data points in displacement speeds, at least trends, at least idea about magnitude of resistance should have been gained by person, who run the tests. And I do not think that all the data was simply discarded as waste of time and effort.

You did a quite good explanation, why this low speed data could have been omitted from reports :

"Yes, but this is area of practical interest for this boat; for such boat below 8kts is already of no interest (just because 8-15kts is speed limit in some rivers or marinas). Maybe lower speed performance of some interest for manoeuvring, but efficiency is not the case then. "

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Original post in this thread was

"Resistance factors, planing hull at low speed
We all know a planing hull isn't ideal for low speed operation, due to higher resistance from the hard chine interfering with smooth flow of water and the "suck" of the submerged transom. Would some know-it-all, er....expert tell me in ballpark figures which of those two is the more retarding, and by what margin ? Shapes vary of course, but think of a boat where the chine is immersed from at least midway back. I am particularly interested in a power cat in this regard, where chines will be immersed from well forward."

For some reason my understanding was there is talk about PURELY PLANING hull, without any compromise for any other speeds.

On the other side, under "low speed" my understanding was "below hull speed". As it appeared further in this thread, reason for interest in low speed operation was to reduce wake. And I know all to well from personal experience, that planing boat at or just above hull speed is extremely efficient wave-making machine. Hence again -below hull speed => at Fn(L) =~0.3.

I also did some work/advice for two boat owners here, in Lithuania. For a moment, I am familiar with boat, with 4kW/t of power, yet she is unable to reach hull speed at full throttle. And suck the fuel like crazy at Fn(L) 0.3...0.35. And one with 8kW/t of power, just capable to reach hull speed, but owner never run her above Fn(L) ~0.3, to have reasonable fuel consumption. The main reason is they have pure planing hull forms and are overweight. First of them had twice the power in earlier life. Obviously extra engine did not help, and was discarded.
While the yachts I know, with ~3.5kW per ton and folding propellers quite easily motor just above hull speed. And fuel consumption do not skyrocket.

Hence 2 times difference in resistance between pure planing and pure displacement/biased-to-early-semi-planing hulls don't look so incredible to me.

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And this issue of PBB happened to be one of so rare sources for data on pure-planer at pure displacement speed.
On the other side, when we compare all kinds of hulls optimized/intended/biased for semi-planing range, differences normally do not run so high. Quite as You demonstrate in Your calculations.
I just compared yacht and NPL, Series 63 hulls, and did not find much difference. BUT: NPL, Series 63 are round-bilge hulls, with transoms significantly above baseline, far from Series 62, clearly designed for planing regime at all cost. Hence no surprise they perform good at semi-displacement and displacement speeds.

 

There is an interesting article about towing tank testing in the January 2011 issue of Seahorse.

"Recent work has shown that studs disturb the flow least by adding minimum profile drag when they just reach beyond the BL"

It also states the unsolved problems with turbulence transition in TT, CFD and also in real scale for sailboats (and displacement boats).

It even claims that the original transition measurements by Reynolds can not be repeated due to added vibration from traffic. Has anyone more information about this?

 

Agree, for most real-life hulls is below 2, as I said before.

 

Usually resistance of turbulence stimulators is deducted from results of resistance measurements...

 

Yes it is, but that is one more source of error, since you don't accurately know the resistance of stimulators and while testing sailboats you have different heel, trim and leeway angles, thus also different stimulator resistances. In the Seahorse article the goal was to see sub 1% differences in total resistance.

 

To me, Seahorse is not proper reading on tank testing procedures
Methods for turbulence simulators exist for decades; yes there are inaccuracies but if You know better solutions (not from Seahorse) pls let us know.

 

The question how reliable and accurate (for low Fn) the resistance data recalculated to full size, published in PBB, still remain.
Would very interesting to clarify it.

Full planing hull no less real life, if not everyday used for low speeds. Although I still meet examples sometimes.

 

Not really. If You take interceptor craft, they can be designed for 40-50kts of top speed (only 5-10% of operation) but have patrolling speed of 10-20 kts (up to 80% of operation). And in this case slow speed performance is becoming critical as it effects endurance; we studied this issue in detail.

 

I did see one hull of such ship in demolition -not difficult to see the lines.
About 45-50m long, top speed 45-50 kn. Fn(L) for 50m LWL and 50kn -1.163; Just over the edge between semi planing and planing speed.
It was not pure planing craft (not similar to 62 series anyway). Very narrow (waterline entry ~7-8 degrees, eyeball measurement) and deep entrance; somewhat intricate stern shape with transom well above baseline. Chine full length, totally below WL at rest. I doubt if she did lift her forefoot out of the water at top speed.
If not chines, it was more similar to series 63, or NPL.

 

We do not deal with 45-50m boats; I was talking on 15-25m interceptor craft that we design. Those are pure planing hulls.

 

I agree with you, most Seahorse articles are not very scientific. Nor is this one, but I found it interesting and it its written by Bruce Parson, who is research director at NRC Institute for Ocean Technology. They have quite nice facilities: http://www.nrc-cnrc.gc.ca/eng/ibp/iot.html

 

Yes, but You understand such publications are mostly for marketing. I do same way sometimes

 

Blount discusses his inclusion of displacement speed data for Series 62 in the PBB article:
"In the original Series 62 paper there's a Figure 7 that shows resistance-to-weight ratio vs volume Froude number for the five models we tested at the standard condition. I've prepared a new figure with essentially the same data, and labeled it Figure 1 for this article. What I've added in the new figure are the resistance characteristics below a volume Froude number of 1, which covers the displacement speed range. In the Series 62 paper we chose not to include that data--mostly as a matter of graphic presentation, not because we were hiding anything."

Also, as has been quoted here several times Blount in the same article said "In fact, when you look at the speeds that we did run, they’re more concentrated in the pre-planing range". The statement was made by a participant in this discussion that "Generally accepted pre-planing range is FnV=1...3" with the inference that was what Blount meant by "pre-planing". In the article Blount consistently uses the term "semi-displacment" for the range between displacment and planing which he defines as FnV 1 to 3. Presumably if he meant that data was concentrated between FnV 1 to 3 he would have used semi-displacment as he did everywhere else in the article.

The usefulness of Series 62 data at FnV less than 1 given the test methodology and current state of knowledge of Re number effects and correlation is a valid question. What also needs to be kept in mind is the intended use of the data and accuracy needed for that particular use.

For determining resistance within few percent then the data may not be very useful. But in assessing whether a motorboat planing hull design has 20% higher resistance or 100% higher resistance than an efficient displacement sailboat hull at displacment the lower speed Series 62 data may be of some though limited use even with the uncertainties and inaccuracies due to laminar vs turbulent boundary layers.

Why the interest in the resistance of motorboat planing hulls at displacment speeds with immersed transoms is yet another question.

 

Yes, the main message seemed to be "don't trust CFD, use TT". As a CFD engineer (not much for marine applications), I don't quite agree with some points he made.

 

I trust all methods only if they are anchored to sea trials

There are interesting publications of professor L.Larsson on marine CFD problems; they are integrated in his recent book published in 2010 by SNAME.