Savitsky Power Prediction

[Dingo]

Danialsan, Hoe gaat 't?

I entered the numbers that you gave and I obtained different results. I noticed that you are using a trim tab of 1 ft chord and no rudder (?). The top speed that Savitsky will predict for this boat is 45 kts (trim > 15 deg at 50 kts) and will require 262 effective kW (or 352 ehp). This is the power required to tow the boat, not what the engine power is. The actual brake power depends on the propeller and transmission losses, which as a basic rule is 60% and 97% efficient respectively.

Also, you used an LCG of 10.5 ft which is very far forward for a planing craft. I re-ran it for an LCG of 8 ft (2.438 metres) and got a lower effective power of 207 ekW (277 ehp) for 50 kts, or a brake power of 355 ekW (476 bhp). Note that your deadrise is 27 degrees; high for a fast planing boat. I would suggest lowering it to 16 degrees unless you are going to use it in rough open seas.

For the others, and yourself, here is an updated version of my planing spreadsheet. It includes an addition for wind resistance due to superstructures and above water hull; the theory for the rest remains unchanged.

Dingo

[Danielsan]

Hi Dingo, Met mij goed en met U?

Thx for the reply

Did some more precise measurements, as I didn't have the drawings on my other PC

and came to the next results:
(I am a bit disapointed, as I want to build that boat from scratch and for the lowest budget possible, but now I would need a high power engine? So I can't fall back on my 6l +150HP Perkins long block)

How do I have to translate this to prop size/pitch rpm, any idea?

Length of Waterline LWL 24.34 feet
Beam B 9.180 feet
VCG VCG 1.02 feet = 0.311 metres
Displacement D 6,700 lbf = 3,039 kg
Deadrise @ Transom b T 16.61 °
Deadrise @ Amidships b )0( 19.89 °
Distance to Amidships L )0( 12.172 feet = 3.710 metres
q 1.372 °
Angle of Thrust Line e 9.25 °
f 3.32 feet = 1.012 metres

Minimum Speed Vmin 6.1 kn = 10.3 feet/s This is the minimum speed valid for this analysis
Maximum Speed Vmax 132.4 kn = 223.4 feet/s This is the maximum speed valid for this analysis

Length Overall LOA 24.34 feet = 7.420 metres
Maximum Beam Bmax 9.81 feet = 2.990 metres
Moulded Depth of Hull Z 4.95 feet = 1.509 metres
Height of House HSS 1.30 feet = 0.396 metres
Breadth of House BSS 8.00 feet = 2.438 metres
Frontal Area of House ASS 10.4 feet2 = 0.966 m2

Number of Propellers N 1

Chord cF 1.2 feet = 0.366 metres
Span Ratio s 0 ( <= 1 )
Deflection Angle d 5 °

Chord crudder 1.60 feet = 0.488 metres
Thickness t 0.10 feet = 0.030 metres
Area Arudder 2.59 feet2 = 0.241 m2
Centrepoint xc -0.20 feet from transom = -0.061 metres (+ve fwd)
yc 0.24 feet from baseline = 0.073 metres (+ve up)

Diameter of Shaft Fshaft 0.10 feet = 0.030 metres
Length of Shaft & Hub l 3.00 feet = 0.914 metres
Centrepoint xc 0.20 feet from transom = 0.061 metres (+ve fwd)
yc 0.80 feet from baseline = 0.244 metres (+ve up)

Chord cstrut 0.00 feet = 0.000 metres
Thickness t 8.00 feet = > 15° metres
Area Astrut 0.75 feet2 = > 15° m2 ****
Centrepoint xc 3.50 feet from transom = 1.067 metres (+ve fwd)
yc 2.00 feet from baseline = 0.610 metres (+ve up)

V LCG t D T Peffective h tcr Comments
[kn] [ft] [metres] [°] [lbf] [kN] [lbf] [kN] [ehp] [ekW] [ft] [metres] Lew.[°] Angeli [°]
6 7.45 2.271 7.8498 509 2.3 535 2.4 9 7 2.66 0.811 > 15° > 15°
7.5 7.45 2.271 9.6101 959 4.3 1,017 4.5 22 16 2.75 0.838 > 15° > 15°
10 7.45 2.271 10.8676 1,370 6.1 1,466 6.5 42 31 2.52 0.768 14.97 > 15°
12.5 7.45 2.271 9.6235 1,346 6.0 1,428 6.4 52 39 2.10 0.640 10.33 11.44
13 7.45 2.271 9.2981 1,325 5.9 1,403 6.2 53 39 2.02 0.616 9.70 10.85
14 7.45 2.271 8.6521 1,281 5.7 1,352 6.0 55 41 1.88 0.573 8.62 9.81
15 7.45 2.271 8.0358 1,240 5.5 1,303 5.8 57 43 1.76 0.536 7.74 8.94
17.5 7.45 2.271 6.6964 1,163 5.2 1,214 5.4 62 47 1.51 0.460 6.14 7.28
20 7.45 2.271 5.6434 1,130 5.0 1,173 5.2 69 52 1.32 0.402 5.08 6.11
25 7.45 2.271 4.1786 1,171 5.2 1,207 5.4 90 67 1.07 0.326 3.78 4.59
30 7.45 2.271 3.2527 1,312 5.8 1,348 6.0 121 90 0.91 0.277 3.04 3.66
35 7.45 2.271 2.6356 1,522 6.8 1,559 6.9 163 122 0.80 0.244 2.58 3.04
40 7.45 2.271 2.2053 1,782 7.9 1,823 8.1 219 163 0.73 0.223 2.26 2.60
45 7.45 2.271 1.8939 2,081 9.3 2,127 9.5 287 215 0.68 0.207 2.04 2.27
50 7.45 2.271 1.6621 2,412 10.7 2,462 11.0 370 276 0.64 0.195 1.87 2.03

[ABoatGuy]

You may also want to search SNAME for Donald Blounts papers. He has a few which apply a correction to the hump area for a better fit to reality.

aboatguy

[gonzo]

Without doing the numbers, I am sure there must be some error. The HP seems too low. Also, how are you going to build a boat of such low displacement to be structurally adequate for 50 Knots?

[Danielsan]

Originally Posted by gonzo

Without doing the numbers, I am sure there must be some error. The HP seems too low. Also, how are you going to build a boat of such low displacement to be structurally adequate for 50 Knots?

Good Question.

I do find it has low dispacement or maybe not low displacement, but more low draft? the one goes with the other I know...

On the other hand I did find some (fast)planning boats that where 10% bigger, with around 3600 Kg of displacement

On the other hand (damn I only have two of them) it does not have to do 50kt, if it goes by 30kt and it keeps a float I will be verry happy.

I am also a bit concerned about the low draft, displacement as if I would go out on our North sea, it can be kind of rough.

Anu sugestions, all welcome...

Daniel Peeters

Dingo,
Thanks a lot for making the spreadsheet available. Its superb.

Rob

[Ilan Voyager]

I have just downloaded the excell sheet by Dingo (nice work!) The FP does not seems to be too low; it seems that it's the HP at the PROPELLER (I have bot looked in detail in the sheet cells) which about 50 %

Efficiency gear box: 95 % (new, when old ...oooh)
Efficiency shaft, bearings, joints: 95 % if everything turns round.
Efficiency of the propeller; that hurts badly:
9.6m diameter propeller 79 tons of bronze 85 to 90 % efficiency at 105 RPM, but this propeller is not common on small power boats for evident reasons.
Common 13 to 15 inches propeller turning at 2000 RPM...in the very best case 65 % efficiency. 45% of the HP is used to make foam in the water.

So 0.65x0.95x0.95: 0.58 or 58 % of efficiency.

Take the given HP and multiply by 1/0.58 or 1.724 and I think you'll have the true horses. I'll compare the excell sheet with some empiric formulae I have and I'll give you the results.

[Danielsan]

Thx for the comments,

So in the best case I will need to have around 300 Hp to get my boat at 30-35kt?

Do I get this right? What is the influence of torque, there is a diff. between Diesel and Gas engines. Won't I need to have less Hp because using a dieselengine?

Greetings,

Daniel Peeters

[martok13]

When i apply this excel sheet to my catamaran..... i get all #VALUE!

Length of Waterline LWL 66.48 feet
Beam B 14.000 feet
VCG VCG 6 feet = 1.829 metres 95396.64 210311.4325
Displacement D 210,311 lbf = 95,397 kg
Deadrise @ Transom b T 17 °
Deadrise @ Amidships b )0( 21.5 °
Distance to Amidships L )0( 33.239 feet = 10.131 metres
q 1.064 °
Angle of Thrust Line e 3.00 °
f 3.00 feet = 0.914 metres

Minimum Speed Vmin 6.0 kn = 12.7 feet/s This is the minimum speed valid for this analysis
Maximum Speed Vmax 132.4 kn = 275.9 feet/s This is the maximum speed valid for this analysis

Length Overall LOA 78.00 feet = 23.774 metres
Maximum Beam Bmax 27.50 feet = 8.382 metres
Moulded Depth of Hull Z 12.00 feet = 3.658 metres
Height of House HSS 14.00 feet = 4.267 metres
Breadth of House BSS 27.50 feet = 8.382 metres
Frontal Area of House ASS 385 feet2 = 35.768 m2

Number of Propellers N 1

Chord cF 0 feet = 0.000 metres
Span Ratio s 0 ( <= 1 )
Deflection Angle d 0 °

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)

Diameter of Shaft Fshaft 0.00 feet = 0.000 metres
Length of Shaft & Hub l 15.00 feet = 4.572 metres
Centrepoint xc 5.50 feet from transom = 1.676 metres (+ve fwd)
yc 0.00 feet from baseline = 0.000 metres (+ve up)

Chord cstrut 0.00 feet = 0.000 metres
Thickness t 8.00 feet = > 15° metres
Area Astrut 0.75 feet2 = > 15° m2 ****
Centrepoint xc 3.50 feet from transom = 1.067 metres (+ve fwd)
yc 2.00 feet from baseline = 0.610 metres (+ve up)

[Dingo]

Marktok13, The reason that you have VALUE in all the boxes is because the trim is above 15 degrees at all speeds. This theory is valid only for trim angles below 15 degrees.

Hi there,
I am most interested in learning a bit more about the "Sea Keeping" and "Roll stability" aspects of Dingo's spreadsheet. ie what the numbers really mean .

So, my questions:

Are the following papers possibly available on-line anywhere:

Lewandowski, Edward M., "Prediction of the Dynamic Roll Stability of Hard-Chine Planing Craft", Journal of Ship Research, juni 1996

Savitsky & Brown, "Procedures for Hydrodynamic Evaluation of Planing Hulls in Smooth and Rough Water", Marine Technology, Okt. 1976


Dingo, you suggested e-mailing you for translation issues, I figured you have already given your time in the spreadsheet, I could get off my bum and translate it myself.
So, can anyone direct me to a suitable on-line Afrikaans/English translator?
(I'v had a look and there's not to much out there)

Thanks again for the spreadsheet,

Rob

[nevd]

Rob,

Lewandowski, Edward M., "Prediction of the Dynamic Roll Stability of Hard-Chine Planing Craft", Journal of Ship Research, juni 1996 is on the SNAME CD. It is highly mathematical.

Let me know if you find somewhere to get the Savitsky paper as I am also interested.

nevd

[mistral]

for Dingo

i've put savitsky calculus routine into a mathcad spreadsheet as you've done with excel;the surprise came when i saw that given the same input parameters, outputs are dramatically different in terms of power required, although similar in terms of trim angle. How could it be possibile?? i used savitsky formulations as they're explained in Larsson & Eliasson text,
other question: i don't find any parameters in your spreadsheet involving propeller efficiency. Am i wrong??
If you're interested i may post you the mathcad file to understand differencies between them;
other few questions:
have you compared your results with some commercial software like hullspeed???
have you tested them with any real boat??

thanx
Mistral

[Dingo]

Mistral, Please note that the calculations from Planing.xls give the effective power, not the required engine power. Hence there are no propeller efficiency results used since I am only interested in the actual drag (or thrust vector). Regarding propeller efficiency, I use another program that I wrote based to the B-series to determine the efficiencies of the prop for a given diameter, pitch etc. and then use the various losses for the final installed power included. The trim is worked out using mostly Savitsky's theory based on equilbrium of force vectors. I am not familiar with the Larsson & Eliasson text. Also, I do not have MathCad so unless you can post the calculations in a text format, I would not be able to debug them.
I have compared my results with other programs that other engineers use and they compare favourably. One that I have directly compared with is NavCad, and the results corresponded well with thenm, as well as various waterjet manufacturer's curves for hull resistance.

To all, I have received some e-mails stating that they have trouble running the spreadsheet Planing.xls. Note that macros must be turned on. It is not password protected, but be sure that the Visual Basic editor can display the macros, otherwise you will get wrong results or #VALUE in various cells. Check also that anti-virus software will allow the running of the macros, or that your firewall allows them through when downloading. (Note, if the trim is greater than 15 degrees, the theory will not be valid). Also be careful when using it for catamarans since I have not yet adapted it for cat use yet (I've just moved house and jobs, so I've been busy).

Dingo

[rxcomposite]

Sorry but you are reinventing the wheel.I've seen a savitsky based program by arneson drive.Its tailored to arneson but it is a simple filling in the data program. if you want the complete series of formula, a publication (i dont know if it is still current) by SNAME about prediction of performance on planing craft.Yes, it can be programmed into excell but it is not simple.
Rxcomposite