Savitsky Power Prediction

[Chris Krumm]

I have a copy of the Savitsky method applet published by Arneson (somewhere) . If I remember correctly, it is a very "short form" and simplified Savitsky power prediction method - assumes thrust line passes through CG, etc. Dingos' work in Exel exposes the calculations and formulas, which is really cool for gaining an underdstanding of what is behind "the answer."

Chris Krumm

[rxcomposite]

Yes, the program was very simple. Keep up the good work chris. We are all in search of "what happens if?". Dingos work is great. I have to find my copy of SNAME's publication on planing craft to decipher the formulas.

[DBDMarine]

Hi all
We make a new stern drive leg and are looking for a application that we can use to not only predict vessel speeds, gearing prop diameter and pitch.
Since the leg doesnt contain a gearbox, the drive uses a ZF 8 degree downangle box, which attaches to the drive.
Now what way would I do this is there no strut, rudder, shaft etc but planing hull?
Im trying to figure this out, looks like this is very close
Can you give me any hints please
Regards
Darren Finch
DBD Marine
www.dbdmarine.com

[atahawaii]

There is a Savitsky spreadsheet available on the web at www.hawaii-marine.com/templates. It does not use Excel's Solver or macros or Visual Basic. It does the iterations automatically and evaluates several speeds. It does both the Short and Long Forms of Savitsky's methods.

[micheler]

Hi all,
I am a new member of the forum. I found the spreadsheet of Dingo really interesting. I'm interested in making a slight modification on it in order to input manually the resistance forces that I obtain from a CFD planing computation.
The procedure may be the following:
1. determine the position of the boat with the Dingo's spreadsheet
2. compute the drag and lift forces acting on the hull via a CFD free-surface computation (with sink&trim from step 1.)
3. insert the CFD-computed drag estimates at the various speeds in the spreadsheet and re-run the computation uptating sink&trim
4. return to step 2. until convergence...

I have to say that I want to make CFD computations only of the hydrodynamic part computing viscous and wave resistance, but without simulating spray formation (almost impossible to get reliable spray-induced-drag up to date). So the results I get are only a part of the total drag/lift.
Dingo could you suggest me where I should input my CFD drag/lift forces? I think the "Berekenings" sheet, but where? How the spray formation and aerodynamic forces are taken into account?

Thanks
Michele.

[Dingo]

Micheler,

I will look into where putting the CFD results should go.

Lines 78 to 83 is where the input for the air drag due to superstructure is placed. This can be modified for lift also but then the total moments must then be recalculated (add lift due to s/str in line 13).

Please note that my e-mail address has changed.

It has also come to my attention that the calculations when using a trim-tab might be slightly off. I will look into it and re-post the spreadsheet if incorrect and as soon as I have time. Sorry for any problems.

[Tim B]

Be careful using CFD, there may be a lot of instances with multi-phase flow. You also have a trim question to get right.

Dingo, you don't have a copy of the SNAME Savitsky paper do you?

Tim B.

[Dingo]

Tim B,

I do have a copy of Savitsky's 1964 paper at SNAME, but it is not the only paper that I used for my spreadsheet. All references are quoted in the spreadsheet in the 'Berekenings' section.

[Tim B]

Ok, I'll have a look at it sometime, I'd really like to add a Savitsky method to LMH at some point, but it will need to be written in C/C++. It also infers that LMH should be finished... yeah, Christmas perhaps.

Cheers,

Tim B.

[micheler]

Thanks for your replies.
I'm trying to understand the "Berekenings" section. But I haven't the Savitsky paper so I have some questions.
I'm especially interested in the first part of the section, where the Savitsky method is applied for computing the planing hull resistance (up to row #43).
For computing the resistance, it seems that the Schoenherr '32 + the ATTC Ca of 0.0004 are applied on the Sf area.
What does the Sf (=lambda * B^2 / cos (beta_e) ) area exactly represents?
Why the Df force is computed with the 0.97 factor instead of the more usual 0.5? What it accounts for? Is it a correction for spray formation? Or is it merely a units conversion factor?

After computing all force components, it is applied an M-factor. What does this correction stands for?

Thanks
Michele.

[subbu]

Hi,

Its great to hear from u that "Dingos' work in Exel exposes the calculations and formulas, which is really cool for gaining an underdstanding of what is behind "the answer." ".

I would like to have that copy, can u send it to me

Subbu

[patrik111]

Hello All,

Is the planing equations applicable to rounded bilged hulls? (i.e. modern round bottomed beach-cats).

The question could also be read:
Are the equations valid for long, skinny and rounded hulls?
If not, at what are the input limits for the equations?

Best Regards

Patrik

Best Regards

[patrik111]

Could deadrise for instance be approximated as some sort of average over the curved surfaces?

Anyone have some sort of hint??

[rxcomposite]

Originally Posted by patrik111

Could deadrise for instance be approximated as some sort of average over the curved surfaces?

Anyone have some sort of hint??

Yes. The rulebook says that if it is a "S" curve deadrise should be measured from the keel intersection to the centerline to the outer edge of the chine.

If the keel has a fairly rounded section, it should be measured from the point tangent to the radius to the edge of the chine.

If you have a reverse chine, the edge of the chine pointing downward is still the edge.

[Willallison]

I've run Dingo's program on a number of occaisions since he 1st (very kindly) made it available to us all.
I don't doubt the validity of the calculations, but perhaps someone might be able to give me a definitive means of interpereting them....
For instance, I ran the numbers thru for a 40 footer powered by a single duoprop sterndive (hence the zero entries for rudder, strut etc...) with the following results:

Inputs

Hull Beam B 10.53 feet
VCG VCG 3.31 feet = 1.009 metres
Displacement D 14,265 lbf = 6,471 kg
Deadrise @ Transom b T 9.3 °
Deadrise @ Amidships b )0( 15 °
Distance to Amidships L )0( 14.400 feet = 4.389 metres
f 3.60 feet = 1.097 metres
e 0.00 °
q 2.182 °

Minimum Speed Vmin 6.5 kn = 11.0 feet/s This is the minimum speed valid for this analysis
Maximum Speed Vmax 141.8 kn = 239.3 feet/s This is the maximum speed valid for this analysis

Number Number of Propellers N 1

Trim Tab Chord cF 1 feet = 0.305 metres
Span Ratio s 0.32 ( <= 1 )
Deflection Angle d 0 °

Rudder Chord crudder 0.00 feet = 0.000 metres
Thickness t 0.00 feet = 0.000 metres
Area Arudder 0.00 feet2 = 0.000 m2
Centrepoint xc 0.00 feet from transom = 0.000 metres (+ve fwd)
yc 0.00 feet from baseline = 0.000 metres (+ve up)

Shaft Diameter of Shaft Fshaft 0.00 feet = 0.000 metres
Length of Shaft & Hub l 0.00 feet = 0.000 metres
Centrepoint xc 0.00 feet from transom = 0.000 metres (+ve fwd)
yc 0.00 feet from baseline = 0.000 metres (+ve up)

Strut Chord cstrut 0.00 feet = 0.000 metres
Thickness t 0.00 feet = 0.000 metres
Area Astrut 0.00 feet2 = 0.000 m2 ****
Centrepoint xc 0.00 feet from transom = 0.000 metres (+ve fwd)
yc 0.00 feet from baseline = 0.000 metres (+ve up)

Output V LCG t D T Peffective h
[kn] [ft] [metres] [°] [lbf] [kN] [lbf] [kN] [ehp] [ekW] [ft] [metres]
6 10.5 3.2 4.9135 786 3.5 790 3.5 14 11 2.71 0.826
7.5 10.5 3.2 5.4956 1,061 4.7 1,069 4.8 24 18 2.80 0.853
10 10.5 3.2 6.7204 2,011 8.9 2,029 9.0 62 46 2.90 0.884
12.5 10.5 3.2 7.4953 2,437 10.8 2,465 11.0 93 70 2.77 0.844
13 10.5 3.2 7.518 2,465 11.0 2,493 11.1 98 73 2.72 0.829
14 10.5 3.2 7.4427 2,478 11.0 2,505 11.1 106 79 2.59 0.789
15 10.5 3.2 7.2444 2,451 10.9 2,477 11.0 113 84 2.45 0.747
17.5 10.5 3.2 6.4822 2,313 10.3 2,333 10.4 124 93 2.11 0.643
20 10.5 3.2 5.6434 2,175 9.7 2,190 9.7 133 100 1.82 0.555
25 10.5 3.2 4.2328 2,034 9.1 2,043 9.1 156 116 1.40 0.427
30 10.5 3.2 3.2351 2,067 9.2 2,073 9.2 190 142 1.13 0.344
35 10.5 3.2 2.5363 2,227 9.9 2,232 9.9 239 179 0.94 0.287
40 10.5 3.2 2.0349 2,483 11.0 2,487 11.1 305 227 0.82 0.250
45 10.5 3.2 1.6647 2,815 12.5 2,818 12.5 389 290 0.72 0.219
50 10.5 3.2 1.384 3,210 14.3 3,213 14.3 493 368 0.65 0.198

...hmmm I see that the table doesn't line up all that well, but the long and the short of it is that at my target max speed of 25 knots, the spreadsheet gives a required effective hp of 156hp.
So, the question - how to relate this to SHP or even better, a published engine / propeller curve....?