Since the algorithms are designed for specific hull types, they will be most accurate when certain conditions are satisfied. These conditions are:
§ Speed
The hullshape is very important in determining whether a particular method is applicable to a particular design. A thorough knowledge of the resistance prediction method is required. See the Bibliography section on page 52 to find the relevant documents on each of the methods available in Hullspeed.
The resistance prediction algorithms are useful only within certain speed ranges; these limits are:
|
Algorithm: |
Low - speed limit |
High - speed limit |
|
Savitsky (pre-planing) |
Fnv = 1.0 |
Fnv = 2.0 |
|
Savitsky (planing) |
Fnb = 1.0 |
None, see note below |
|
Lahtiharju (round bilge) |
Fnv = 1.5 |
Fnv = 3.8 |
|
Lahtiharju (hard chine) |
Fnv = 1.5 |
Fnv = 5.0 |
|
Holtrop |
0.0 |
FnL = 0.80 |
|
Van Oortmerssen |
0.0 |
FnL = 0.50 |
|
Series 60 |
Fnv = 0.282 |
Fnv = 0.677 |
|
Delft |
0.0 |
FnL = 0.75 |
|
Compton |
FnL = 0.1 |
FnL = 0.6 |
|
Fung |
FnL = 0.134 |
FnL = 0.908 |
|
Slender Body |
0.0 |
Up to FnL ≈ 1.0 depending on slenderness ratio |
For some algorithms, Hullspeed will calculate the resistance only for speeds within the limits indicated above. For the other algorithms, Hullspeed will calculate the resistance for any speed. The user should be aware that the accuracy of the algorithms is expected to decrease beyond the limits outlined above.
Note regarding speed ranges. Some of the formulae (Savitsky planing, Lahtiharju and Holtrop) are able to calculate the vessel resistance for any speed. However, the regression equations were derived from resistance data within specified speed ranges and these are noted in the table above. The Savitsky (planing) formula was derived from theory based on the planing behaviour of a prismatic hull; whilst there is no theoretical upper speed limit, results for speeds above approximately Fnv = 6.0 to 7.0 should be treated with caution.
Fnb - Beam Froude number
Fnv - Volume Froude number
FnL - Length Froude number, see Glossary for definition of these Froude numbers.
The resistance prediction algorithms are useful only within certain limits of hull dimension. These limits are:
|
Algorithm: |
Requirement: |
||||
|
Savitsky |
3.07 |
< |
L/V1/3 |
< |
12.4 |
|
|
3.7 |
< |
ie |
< |
28.6 |
|
|
2.52 |
< |
L/B |
< |
18.26 |
|
|
1.7 |
< |
B/T |
< |
9.8 |
|
|
0 |
< |
At/Ax |
< |
1 |
|
|
-0.016 |
< |
LCG/L |
< |
0.0656 |
|
|
|
|
|
|
|
|
Lahtiharju |
4.47 |
< |
L/V1/3 |
< |
8.30 |
|
(Round Bilge) |
0.68 |
< |
B3/V |
< |
7.76 |
|
|
3.33 |
< |
L/B |
< |
8.21 |
|
|
1.72 |
< |
B/T |
< |
10.21 |
|
|
0.16 |
< |
At/Ax |
< |
0.82 |
|
|
0.57 |
< |
Cm |
< |
0.89 |
|
|
|
|
|
|
|
|
Lahtiharju |
4.49 |
< |
L/V1/3 |
< |
6.81 |
|
(Hard Chine) |
2.73 |
< |
L/B |
< |
5.43 |
|
|
3.75 |
< |
B/T |
< |
7.54 |
|
|
0.43 |
< |
At/Ax |
< |
0.995 |
|
|
|
|
|
|
|
|
Holtrop |
0.55 |
< |
Cp |
< |
0.85 |
|
|
3.9 |
< |
L/B |
< |
15 |
|
|
2.1 |
< |
B/T |
< |
4.0 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
van Oortmerssen |
8 |
< |
L |
< |
80 |
|
|
3 |
< |
L/B |
< |
6.2 |
|
|
0.5 |
< |
Cp |
< |
0.73 |
|
|
-7 |
< |
100 LCG / L |
< |
2.8 |
|
|
5 |
< |
V |
< |
3000 |
|
|
1.9 |
< |
B/T |
< |
4.0 |
|
|
0.70 |
< |
Cm |
< |
0.97 |
|
|
10 |
< |
ie |
< |
46 |
|
|
|
|
|
|
|
|
Series 60 |
0.6 |
< |
Cb |
< |
0.8 |
|
|
5.5 |
< |
L/B |
< |
8.5 |
|
|
2.5 |
< |
B/T |
< |
3.5 |
|
|
-2.48% |
< |
LCB |
< |
3.51% |
|
|
|
|
|
|
|
|
Delft |
2.76 |
< |
L/B |
< |
5.00 |
|
|
2.46 |
< |
B/T |
< |
19.32 |
|
|
4.34 |
< |
L/V1/3 |
< |
8.50 |
|
|
-6.0% |
< |
LCB |
< |
0.0% |
|
|
0.52 |
< |
Cp |
< |
0.60 |
|
|
|
|
|
|
|
|
Compton |
|
|
|
|
|
|
|
-0.13 |
< |
LCG/L |
< |
-0.02 |
|
|
4.0 |
< |
L/B |
< |
5.2 |
|
|
0.00368 |
< |
V/L^3 |
< |
0.00525 |
|
|
|
|
|
|
|
|
Fung |
|
|
|
|
|
|
|
0.00057 |
< |
V/L^3 |
< |
0.01257 |
|
|
1.696 |
< |
B/T |
< |
10.204 |
|
|
0.526 |
< |
Cp |
< |
0.774 |
|
|
0.556 |
< |
Cx |
< |
0.994 |
|
|
14.324o |
< |
ie |
< |
23.673o |
|
|
2.52 |
< |
L/B |
< |
17.935 |
|
|
0.662 |
< |
Cwp |
< |
0.841 |
|
|
|
|
|
|
|
|
Slender Body |
|
|
|
|
|
|
|
≈ 4 or 5[1] |
< |
L/V1/3 |
< |
no limit |
Where:
|
L |
Length on the waterline |
|
B |
Beam on the waterline |
|
T |
Draft of hull |
|
At |
Transom sectional area |
|
Ax |
Maximum sectional area |
|
V |
Displaced volume |
|
Cm |
Midship sectional area coefficient |
|
Cp |
Prismatic coefficient |
|
Cwp |
Water plane area coefficient |
|
Cx |
Maximum sectional area coefficient |
|
ie |
Half angle of entrance |
|
LCB |
Longitudinal centre of buoyancy, measured from midships, positive is forward. |
|
LCG |
Longitudinal centre of gravity, measured from Midships, positive is forward. |
|
Deadrise |
Mean deadrise, or deadrise at 50% Lwl. |
|
wsa |
Wetted area of hull. |
|
Bt |
Transom beam at waterline |
|
Tt |
Transom draft |
|
Abulb |
Bulb transverse area. |
Hullspeed will allow calculations beyond these limits; however, the user should be aware that the accuracy of the algorithms is expected to decrease beyond the limits outlined above. In some cases, the algorithm may become very sensitive to parameters outside the specified range.