Resistance factors, planing hull at low speed

RX, I'll give it a look. Thanks.

 

Is it the one where they use Crouch-type coefficient to evaluate the designs of different length? It does not work

 

No. But it agrees with what you said.

 

At least for me separation is still not a basic question. I didn't ask, I said that friction calculation problems don't end at Fn=0.4. Re should be discussed, not Fn.

Yes at higher Re separation happens sooner. So what do you consider as a good LWL based Re to stop worrying about laminar effects? 3e6, 5e6, 10e6 or even 20e6? A 2 m model at Fn=0.4 has Re~4, thus it still can be mostly laminar. The same model at Fn=2 may have Re~17 or then less than half of that due to shorter LWL at speed. Still quite a big portion of the area can be laminar. Even at 10e6 in the worst case you may have a 50% error in Cf due to wrong assumption for separation.

Or is separation behavior constant and known for each TT? Many papers I read while I wrote my VPP's lacked the fine details of friction calculation at model scale. I think the actual measured data should be always published, not only the full scale extrapolation of it.

 

Sorry, Slavi, but the best reference I have is to the paper by your compatriots:

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.

I would not be surprised if some of these excellent researchers have written more on the topic.
Leo.

 

This is all clear to me.

Where number FnL=0.4 comes from? Not from me! For Series 62 we are talking about FnL=0.85 and corresponding Re at least... PermStess was trying to use low Re measurements without turbulence stimulators (re-calculated to full-scale!) to prove something, that's it. Hope now he understands where his mistake was.

 

This has got a little bit off track maybe, suffice to say if the experts aren't in agreement on the theory side, not much hope of nailing down an answer. Empirically, the total lack of deeply submerged transoms among successful displacement hulls pretty well tells me it ain't a minor source of drag. There would be some possible potential advantages, such as greater stiffness in pitch, but that seemingly would cost too much for it's limited benefit.

 

Let me repeat. I did use low speed data, recalculated to full size, AS PUBLISHED BY THE AUTHOR. All the extrapolation, friction uncertainties and the like are handled not by me, but by Mr. D. Blount. And published in not scientific publication, but in Professional Boat Builder magazine.



Now I am convinced, that author had no right to publish low speed data, recalculated to full size, because it is not reliable, not accurate and so on. He had even less right to do it, because earlier, in more scientific publication, he tabulated the results of measurements starting from higher speeds.
I am also convinced, that nobody could use resistance, recalculated to full size by anybody else (no matter how authoritative that anybody else is), to prove anything, anywhere.

Ok, let it be.

It is great relief for me, that nobody seem to question results of Yacht hull, as calculated with Delftship software, according to Delft University Yacht series.

Now I will busy myself with OWN calculations of high speed hulls at low speeds, and present the results to this forum.

 

But I said this from beginning: one can't use those results, don't use series beyond limitations.

Unfortunately all planing hulls series are reported above FnV=0.8...1.0.

 

I just look at resistance data of our powerboat design, LWL=13.2m, DSIPL=15.1t, AWS=42.7m2, design speed 50kts. For lowest tank-tested speed of 10kts we get 10.7kN of total resistance, that corresponds to FnL=0.45, FnV=1.05.

Using Delft series we get 7.3kN of total resistance by calculation; wetted area was reduced to represent round-bilge shape (calculated using DSYS standard formula, AWS=35.7m2). DeGroot series gives about 8.5kN for same wetted area.

No appendages were included.
Hope this helps.

 

I will be stubborn again. Not I use series beyond limitations. PBB magazine (presumably Mr D. Blount) published the graphs from Fn(L)=0.25.

 

No problem; but this is already above hull speed, i.e. in area, where yacht hull resistance rise very steeply with speed, i.e. it is already above speed range, yacht hulls are OPTIMIZED for.

 

OK, but if You are qualified user You should understand what is behind these results, yes?

 

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.

 

I don't think you can get the same BL for different Re. You can approximate the BL thickness of a turbulent BL by delta=0.37*x*Rex^-0.2. If you scale the model by lambda and use the same Fn, you get delta_scaled=delta*lambda^0.7. Thus the BL thickness gets proportionally thinner at scale up. Adding roughness to the whole hull would only make the difference bigger. Then again full scale hulls are typically not hydraulically smooth and models are.

I think the main issue is how to calculate the friction resistance at model scale. Using stimulators help, since with them you can use turbulent flat plate formulas. Fn<0.4 the friction resistance can be about 70% of the total resistance in model scale and you are really trying to measure all the rest, thus it is very important to be able to accurately calculate the friction part. Using whole hull roughness would make it worse by increasing the viscous resistance without adding accuracy to the friction calculation.

This problem is less dramatic for prismatic hulls, since they have much more residuary resistance at Fn<0.4. Thus the errors in friction calculation have less impact on the calculation of the residuary drag. What would be the difference in predicted full scale resistance for a prismatic hull at Fn~0.3 with and without stimulators or with too small and big enough model? 10% or 30%?