1. EXPERIMENTAL AND NUMERICAL STUDIES ON TRIM EFFECTS George Tzabiras
Presentation at SNAME Greek Section
November 13th, 2014
EXPERIMENTAL AND NUMERICAL STUDIES
ON TRIM EFFECTS
George Tzabiras
Laboratory for Ship and Marine Hydrodynamics (LSMH)
School of Naval Architecture and Marine Engineering
NATIONAL TECHNICAL UNIVERSITY OF ATHENS

2. Trim optimization is directly related to the resistance minimization and, therefore, to the overall efficiency of a ship. There are various options to face the problem.
The scope of the present work is to study the influence of trim on the hydrodynamic performance of various types ships. The studies are based on model experiments carried out in the Towing Tank of NTUA as well as on CFD calculations by methods developed at LSMH-NTUA.
CRUCIAL ISSUES
Can we trust simple resistance tests (including dynamic sinkage and trim) or we should face the real problem of self-propulsion?
The optimum trim is realizable?
Can we measure accurately the benefits of trim optimization
What happens in rough seas?
Trim influence is associated
to bow and stern flow conditions

3. Stern separation about
Bulb influence on bow wave
(trim by bow)
Stern separation about
immersed transom

4. Extrapolation of self-propulsion experiments (second scale problem)
PART I : TOWING TANK TESTS The problem of scaling Equal Froude numbers (V/(gL)1/2 ) at model and full scale but substantially different Reynolds numbers (VL/ν)
Froude Hypothesis (first scale problem)
Extrapolation of self-propulsion experiments (second scale problem)

5. The towing tank of LSMH,NTUA 90mx4.5mx3m

6. TOWING TANK TETS ON SIX MODELS IN ORDER TO
IVESTIGATE THE TRIM INFLUENCE ON THE RESISTANCE
BULK CARRIER
PASSENGER SHIP SINGLE-SCREW
ROPAX FERRY TWIN-SCREW
SEMI-SWATH
SAILING YACHT
FISHING VESSEL
Convention for “theoretical” trim angles
(+) trim by bow
(-) trim by stern

7. BULK-CARRIER (L=183m, Δ=37,000t)
Single screw, bulbous bow, wetted transom Full Load (T=10.15m) and Heavy Ballast (T=7.25m)conditions
Model scale 1:35

8. Ship main particulars

9. Model resistance vs. Froude no. at T=10.15
Experiments in random sea-state show the same trends !!!

10. Resistance differences % at model scale (-) corresponds to “gain”

11. Dynamic trim and heave (sinkage) at model scale

12. EHP vs. Froude no. (ship, T=10.15m))

13. EHP differences % vs. Froude no. (ship)

14. Heavy Ballast Condition Ship T=7.25m

15. Model Resistance vs. Froude no.

16. Resistance differences % vs. Froude no. (model)

17. Total trim and heave (sinkage) vs. Froude no. (model)

18. EHP vs. Froude number (ship, T=7.25m)

19. EHP differences % vs. Froude no. (ship)

20. Single screw, bulbous bow, wetted transom Scale 1:15
Passenger Ship
Single screw, bulbous bow, wetted transom Scale 1:15
Main particulars
Lines plan
A A2 A A4 A5
Bulb immersion YES YES NO YES YES
Transom immers. YES YES YES NO NO

21. Bow wave at low speed
Bow wave at high speed

22. Model resistance vs. Froude number

23. Resistance differences % (model)

24. Total trim angle and heave vs. Froude number

25. EHP vs. Froude number (ship)

26. EHP differences % vs. Froude number (ship)

27. ROPAX PASSENGER-FERRY (Twin-screw, bulbous bow, wetted transom) Scale 1:35

28. Resistance vs. Froude number (model)

29. Resistance differences % vs. Froude no.(model)

30. Trim by stern
Trim by bow

31. Heave, dynamic and total trim angle

32. EHP vs. Froude no. (ship)

33. EHP differences % vs. Froude no. (ship)

34. SEMI-SWATH Passenger-ferry L=64m T=3.3m Scale 1:12

35. Model resistance vs. Froude no.

36. Resistance differences % vs. Froude no. (model)

37. Dynamic trim and heave vs. Froude no.

38. EHP vs. Froude no. (ship)

39. EHP differences % vs. Froude no. (ship)

40. Sailing yacht L=15m, T=0.25m, Scale=1:4

41. Resistance differences % vs. Froude no. (model)

42. Total trim angle vs. Froude no.

43. EHP differences % at full scale

44. Traditional fishing vessel PERAMA L=19.3m T=2.2m Scale=1:10

45. Model resistance vs. Froude no.

46. Resistance differences % vs. Froude no. at model scale

47. Total trim angle and heave vs. Froude no.

48. Full scale EHP and EHP differences %

49. CFD shows the effect of scale and the propeller operation on stern separation about the traditional fishing vessel “PERAMA” Usual towing tank extrapolation (Froude) appears questionable

50. Self-propulsion parameters at two model speeds

51. PART II: COMPUTATIONAL FLUID DYNAMICS (CFD) advantages: fast and less expensive, no scale effects shortcomings: discretisation and modeling errors, difficult to simulate exactly the propeller
CFD is based on the transformation of the Navier-Stokes differential equations to a set of non-linear algebraic equations that can be solved using high performance computers.
Values for different variables are obtained on grid nodes.

52. CFD shows that the scale effect on the formation of waves about a ship is practically meaningless. Therefore, the geometrical similarity is at least fulfilled when performing towing tank tests

53. CFD shows the propeller effect on the stern wave formation Whenever this effect is strong, SHP is influenced noticeably

54. CFD compares the formation of waves at steady forward speed between the potential and viscous flow solutions

55. TEST CASE : CHEMICAL PRODUCT CARRIER DWT=20,000mt L=150m, B=23
TEST CASE : CHEMICAL PRODUCT CARRIER DWT=20,000mt L=150m, B=23.20m, D=13m, Prop. D=4.25m (CPP), SHP:6000KW Tested Conditions at Vs=14 Knots Full load FL1 (trimmed) Sea-trials FL2 (zero trim) Full load FL3 (zero trim) Heavy Ballast BL1 (trimmed) Heavy Ballast BL2 (zero trim)

56. CFD calculations following a hybrid method (Free-surface by potential flow and N-S underneath) Propeller model : actuator disk Full load (FL1,FL3) and sea-trials (FL2) conditions
δEHP(1-2)=14%
δEHP((3-1)=-1.6%
δSHP(1-2)=18%
δSHP((3-1)=-0.13%

57. Ballast condition at 14 Knots Trimmed and zero trim conditions
δEHP(2-1)=2.16%
δSHP(2-1)=-4.75%
(opposite trend than EHP)

58. Wave formation at the speed of 14 knots with and without trim Heavy ballast condition

59. Computations at full-load condition with trim at the reduced speed of 10knots (“low steaming”)
Noticeable result
EHP(14)/EHP(10)≈
SHP(14)/SHP(10)

60. Wave patterns at the speeds of 14 and 10 Knots Full load condition

61. Predictions at “low steaming” cases in still and “rough” conditions (full load condition with trim)
With the established SHP of 1900 KW
the ship will achieve 8.7 Knots in “rough” conditions
or, equivalently, will lose 1.3 Knots.

62. CONCLUSIONS
The effect of trim on the hydrodynamic resistance depends on the ship type (hull geometry), the speed and the loading condition.
Unless there is strong evidence that EHP is directly related to SHP, self-propulsion tests are required to find the real influence of trim on the fuel consumption.
Experiments in towing tanks are accurate at model scale and may also provide safe information for full scale ships as regards trim effects.
CFD comprises a fast and substantially less expensive tool for trim optimization, but the accuracy of results suffers due to numerical uncertainties which may be of the same order as the expected benefit.
References
G.D. Tzabiras, “Resistance and Self-propulsion simulations for a Series 60, CB=0.6 hull at model and full scale”, Ship Technology Research, 51, 2004, pp
G. Tzabiras and K. Kontogiannis, “An integrated method for predicting the hydrodynamic performance of low-cB ships”, Computer-Aided Design Journal, 42, 2010, pp
M. Iakovatos, D. Liarokapis and G. Tzabiras, “Experimental investigation of the trim influence on the resistance characteristics of six ship models”, IMAM-2013 Int. Conference, La Coruna, 2013, pp
G. Tzabiras and K. Psaras, “Numerical simulation of self-propulsion characteristics of a product carrier at various speeds”, to be presented at HIPER14 Int. Conference