I have calculated the hydrodynamic resistance of a hull with 3 methods :
1 - manually, using the method exposed by paper "Sailing yacht performance in calm water and in waves" by Gerritsma, Keuning & Onnink, at the 12th international symposium on yacht design and construction, HISWA, november 1992
2 - By Freeship
3 - By programming myself in java the same method (thanks to FreeShip code for help :)
Methods1 and 3 give the same result. Method 2 gives another result on the form resistance. So, is there a bug ?
Here are the results for method 2 (freeShip) and for method 3 (my program in java). Note that I did the maximum to present the results in the same way. I hope the javascript form on this forum won't cause trouble in data presentation.
Woops, _I_ did the bug. Once checked and rechecked I found that, for evaluating Freeship resistance calculation, I took the real value of LCB for my test design, i.e. -2.46 %. For manual calculation and for testing my own java program, I took 0%. This because I had a bug in my hydrostatic module (declaring two times the Lcb variable). Slaps...
I fixed that and now my java program gives the same results than Freeship, the small differences on the last digits being acceptable for two different programming languages.
I apologize for the trouble, and I can confirm that Freeship resistance calculation, having been reprogrammed in java, checked against the original 1992 paper, gives correct results.
Fun to see that such a small correction on LCB, i.e. less than 30 cm on a 12 m yacht, gives such a difference (more than 10% on form resistance).
Best regards
Jean-Pierre
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Hello
I have calculated the hydrodynamic resistance of a hull with 3 methods :
1 - manually, using the method exposed by paper "Sailing yacht performance in calm water and in waves" by Gerritsma, Keuning & Onnink, at the 12th international symposium on yacht design and construction, HISWA, november 1992
2 - By Freeship
3 - By programming myself in java the same method (thanks to FreeShip code for help :)
Methods1 and 3 give the same result. Method 2 gives another result on the form resistance. So, is there a bug ?
Here are the results for method 2 (freeShip) and for method 3 (my program in java). Note that I did the maximum to present the results in the same way. I hope the javascript form on this forum won't cause trouble in data presentation.
Method 2 : FreeShip
----------------------------------------------------------------------------------
Input variables
----------------------------------------------------------------------------------
General
Start speed : 0.00 [kn]
End speed : 20.00 [kn]
Speed step : 1.00 [kn]
Water density : 1.025 [t/m3]
Water viscosity : 1.1867*10-6 [m2/s]
Hull
Length waterline : 10.550 [m]
Beam waterline : 3.020 [m]
Draft hull : 0.548 [m]
Total draft : 1.400 [m]
Wetted area : 26.48 [m2]
Waterplane area : 24.07 [m2]
Displacement : 7.280 [m3]
LCB : -2.464 [%]
Cp : 0.5840
Keel
Average chordlength : 0.000 [m]
Wetted area : 0.00 [m2]
Rudder
Average chordlength : 0.00 [m]
Wetted area : 0.00 [m2]
Calculated variables
----------------------------------------------------------------------------------
Cwp = 0.7555
Cm = 0.7140
Am = 1.18 [m2]
Lwl/Bwl = 3.493
Bwl/Tc = 5.511
Lwl/Displ^3 = 5.443
Tc/T = 0.391
Aw/Displ^0.67 = 6.408
Final calculations of resistance
+-------+-------+-------+---------+---------+-----------+---------+---------+---------+
| Speed | Speed | Speed | R_f | R_r | R_T | Power | Cp opt. | LCB opt |
| [kn] | [m/s] | [Fn] | [N] | [N] | [N] | [kW] | [-] | [%] |
+-------+-------+-------+---------+---------+-----------+---------+---------+---------+
| 0.00 | 0.00 | 0.000 | 0.0 | 0.0 | 0.0 | 0.00 | 0.000 | 0.000 |
| 1.00 | 0.51 | 0.051 | 13.3 | 4.7 | 18.0 | 0.01 | 0.000 | 0.000 |
| 2.00 | 1.03 | 0.101 | 46.6 | 9.4 | 56.1 | 0.06 | 0.000 | 0.000 |
| 3.00 | 1.54 | 0.152 | 97.7 | 24.7 | 122.4 | 0.19 | 0.000 | 0.000 |
| 4.00 | 2.06 | 0.202 | 165.2 | 73.1 | 238.3 | 0.49 | 0.000 | 0.000 |
| 5.00 | 2.57 | 0.253 | 248.6 | 174.4 | 423.1 | 1.09 | 0.000 | 0.000 |
| 6.00 | 3.09 | 0.303 | 347.4 | 350.6 | 698.0 | 2.15 | 0.000 | 0.000 |
| 7.00 | 3.60 | 0.354 | 461.1 | 678.3 | 1139.3 | 4.10 | 0.000 | 0.000 |
| 8.00 | 4.12 | 0.405 | 589.4 | 1652.4 | 2241.8 | 9.23 | 0.000 | 0.000 |
| 9.00 | 4.63 | 0.455 | 732.0 | ------- | --------- | ------- | ------- | ------- |
| 10.00 | 5.14 | 0.506 | 888.8 | 521.0 | 1409.8 | 7.25 | 0.000 | 0.000 |
| 11.00 | 5.66 | 0.556 | 1059.4 | 720.6 | 1780.0 | 10.07 | 0.000 | 0.000 |
| 12.00 | 6.17 | 0.607 | 1243.8 | 741.0 | 1984.7 | 12.25 | 0.000 | 0.000 |
| 13.00 | 6.69 | 0.657 | 1441.7 | 787.4 | 2229.1 | 14.91 | 0.000 | 0.000 |
| 14.00 | 7.20 | 0.708 | 1653.0 | 843.5 | 2496.6 | 17.98 | 0.000 | 0.000 |
| 15.00 | 7.72 | 0.759 | 1877.7 | ------- | --------- | ------- | ------- | ------- |
| 16.00 | 8.23 | 0.809 | 2115.4 | ------- | --------- | ------- | ------- | ------- |
| 17.00 | 8.75 | 0.860 | 2366.2 | ------- | --------- | ------- | ------- | ------- |
| 18.00 | 9.26 | 0.910 | 2630.0 | ------- | --------- | ------- | ------- | ------- |
| 19.00 | 9.77 | 0.961 | 2906.6 | ------- | --------- | ------- | ------- | ------- |
| 20.00 | 10.29 | 1.011 | 3195.9 | ------- | --------- | ------- | ------- | ------- |
+-------+-------+-------+---------+---------+-----------+---------+---------+---------+
Method 2 : My java software
----------------------------------------------------------------------------------
Input variables
----------------------------------------------------------------------------------
General
Start speed : 00,00 [kn]
End speed : 20,00 [kn]
Speed step : 01,00 [kn]
Water density : 001,025 DensityStr
Water viscosity : 01,1867*10-6 [m2/s]
Hull
Length waterline : 10,550 m
Beam waterline : 03,02 m
Draft hull : 0,548 m
Total draft : 1,400 m
Wetted area : 26,48 m2
Waterplane area : 24,07 m2
Displacement : 7,280 m3
LCB : 0,000 [%]
Cp : 0,5840
Keel
Average chordlength : 00,00 m
Wetted area : 00,00 m2
Rudder
Average chordlength : 00,00 m
Wetted area : 00,00 m2
Calculated variables
----------------------------------------------------------------------------------
Cwp = 0,7555
Cm = 0,7140
Am = 01,18 m2
Lwl/Bwl = 3,493
Bwl/Tc = 5,511
Lwl/Displ^3 = 5,443
Tc/T = 0,391
Aw/Displ^0.67 = 06,408
Final calculations of resistance
+-------+-------+-------+---------+---------+-----------+---------+---------+---------+
| Speed | Speed | Speed | R_f | R_r | R_T | Power | Cp opt. | LCB opt |
| [kn] | [m/s] | [Fn] | [N] | [N] | [N] | [kW] | [-] | [%] |
+-------+-------+-------+---------+---------+-----------+---------+---------+---------+
| 00,00 | 00,00 | 0,000 | 00000,0 | 00000,0 | 0000000,0 | 0000,00 | 000,000 | 000,000 |
| 01,00 | 00,51 | 0,051 | 00013,3 | 00004,5 | 0000017,8 | 0000,01 | 000,000 | 000,000 |
| 02,00 | 01,03 | 0,101 | 00046,6 | 00009,0 | 0000055,7 | 0000,06 | 000,000 | 000,000 |
| 03,00 | 01,54 | 0,152 | 00097,7 | 00026,1 | 0000123,7 | 0000,19 | 000,000 | 000,000 |
| 04,00 | 02,06 | 0,202 | 00165,2 | 00078,9 | 0000244,1 | 0000,50 | 000,000 | 000,000 |
| 05,00 | 02,57 | 0,253 | 00248,6 | 00191,4 | 0000440,0 | 0001,13 | 000,000 | 000,000 |
| 06,00 | 03,09 | 0,303 | 00347,4 | 00389,4 | 0000736,8 | 0002,27 | 000,000 | 000,000 |
| 07,00 | 03,60 | 0,354 | 00461,1 | 00811,3 | 0001272,3 | 0004,58 | 000,000 | 000,000 |
| 08,00 | 04,12 | 0,405 | 00589,4 | 01895,6 | 0002485,0 | 0010,23 | 000,000 | 000,000 |
| 09,00 | 04,63 | 0,455 | 00732,0 | ------- | --------- | ------- | ------- | ------- |
| 10,00 | 05,14 | 0,506 | 00888,8 | 00561,5 | 0001450,2 | 0007,46 | 000,000 | 000,000 |
| 11,00 | 05,66 | 0,556 | 01059,4 | 00764,3 | 0001823,7 | 0010,32 | 000,000 | 000,000 |
| 12,00 | 06,17 | 0,607 | 01243,8 | 00785,3 | 0002029,0 | 0012,53 | 000,000 | 000,000 |
| 13,00 | 06,69 | 0,657 | 01441,7 | 00824,1 | 0002265,8 | 0015,15 | 000,000 | 000,000 |
| 14,00 | 07,20 | 0,708 | 01653,0 | 00877,5 | 0002530,5 | 0018,23 | 000,000 | 000,000 |
| 15,00 | 07,72 | 0,759 | 01877,7 | ------- | --------- | ------- | ------- | ------- |
| 16,00 | 08,23 | 0,809 | 02115,4 | ------- | --------- | ------- | ------- | ------- |
| 17,00 | 08,75 | 0,860 | 02366,2 | ------- | --------- | ------- | ------- | ------- |
| 18,00 | 09,26 | 0,910 | 02630,0 | ------- | --------- | ------- | ------- | ------- |
| 19,00 | 09,77 | 0,961 | 02906,6 | ------- | --------- | ------- | ------- | ------- |
| 20,00 | 10,29 | 1,011 | 03195,9 | ------- | --------- | ------- | ------- | ------- |
+-------+-------+-------+---------+---------+-----------+---------+---------+---------+
Woops, _I_ did the bug. Once checked and rechecked I found that, for evaluating Freeship resistance calculation, I took the real value of LCB for my test design, i.e. -2.46 %. For manual calculation and for testing my own java program, I took 0%. This because I had a bug in my hydrostatic module (declaring two times the Lcb variable). Slaps...
I fixed that and now my java program gives the same results than Freeship, the small differences on the last digits being acceptable for two different programming languages.
I apologize for the trouble, and I can confirm that Freeship resistance calculation, having been reprogrammed in java, checked against the original 1992 paper, gives correct results.
Fun to see that such a small correction on LCB, i.e. less than 30 cm on a 12 m yacht, gives such a difference (more than 10% on form resistance).
Best regards
Jean-Pierre