by Bijit Sarkar, Naval Architect

Introduction

The eternal search of a naval architect – a perfect bow. Sadly, it never exists. Different bow forms are good for different types, sizes of vessels and seaways.

What does a naval architect want out of the bow he designs?

·         As low a resistance as possible in steady state calm water motion at design draft. However, for cargo ships, which spend a good percentage of their lives at lower or even light drafts, a compromise is necessary at times.	Image : Rafael Castelo Branco
·         Under wave conditions, a minimum increase in resistance.
·         Minimal pitch motion, and more importantly, pitch accelerations,	pic:http://www.rmrco.com
·         Minimum spray on deck – This is not important but inconvenient, can be easily deflected away from the ship with a flare above the load water line.	Pic: watercolour painting by author
·         Minimum green water – This is when the large wave breaks over a deck. The huge mass of water crashing on the deck is a serious challenge for deck structure forward and can snap off outfit items.	Pic: pinterest.ca
·         Slamming into waves.

 

 

 

The history of the bulbous bow

I do think that after Archimedes’ Principle, the concept of bulbous bow is the first really great thing that happened in naval architecture. All the while naval architects were tuning their designs here and there – angle of entrance, a finer shape, avoiding that shoulder and hoping for a 2% shaving off the resistance when the concept of bulbous bow gave it a quantum leap, saving a whopping 5-8%, albeit for a specific Froude number range.

The first ship designed with a bulbous bow was a German ocean – liner called SS Bremen , owned by Norddeutscher Lloyd Line . She, and her sister ship SS Europa (Later SS Liberte’) had high speed steam turbine engines and a stream lined profiles and started plied the transatlantic sea route in the late 1920s. These two ships were national symbols of pride for Germany for pre-war years of 1930s. She held the prestigious Blue Riband, an unofficial accolade given to the passenger liners in the Transatlantic route of that era with the record highest (average) speed.

_{Source : Wikipedia}

The Americans followed soon with SS President hoover and SS President Coolidge in 1931, although their bulbs were considered to be experimental, by both owners and shipyards

 

_{Source : Wikipedia}

 

Vladimir Yukevich working on the design of SS Normandie

SS Normandie , designed by Vladimir Yukevich, had a superbly designed hull with a massive bulb at the bow, resulting in a clean entry into the water flow and a highly reduced bow wave. This reduced the wave resistance considerably, allowing it to attain a trial speed of 32.2 knots   Normandie’s arch rival in the ocean passage trade, RMS Queen Mary, was of similar dimension and achieved speed of 32.84 knots at trial.   However, here are the comparison of their main Propulsions, showing that Queen Mary needed 30% more power . SS Normandie RMS Queen Mary ·Four turbo-electric, total 160,000 hp ·4 × Parsons single-reduction geared steam turbines ·4 shafts, 200,000 shp

 

Name: Queen Mary	Name: SS Normandie
Maiden voyage: 27 May 1936	Maiden voyage: 29 May 1935
British ocean liner built by John Brown & Company in Clydebank, Scotland	French ocean liner built in Saint-Nazaire, France

_{Source : Wikipedia}

 

 

Different bulb shapes in use

DIFFERENT BULB SHAPES
Delta	Oval	Nabla
Taylor Bulb Pear Shaped Bulb	Elliptical Circular Cylindrical Lens Shaped	Most common form today
“U” shaped hulls Large variations of draught Wave piercing bow	“U” or “V” shaped hulls	“V” shaped hulls Clearly defined loaded and ballasted draughts
More susceptible to slamming	Less susceptible to slamming	Less susceptible to slamming

 

The X-bow: a modern innovation

Source : Ulstein website

The first ship with X-bow design is MV Bourbon Orca, anchor handling tug supply vessel (AHTS) built for Bourbon Offshore Norway, the Norwegian division of the French Groupe Bourbon.by Ulstein (2006). She was awarded Ship of the Year 2006, both by Skipsrevyen[4] and Offshore Support Journal. In 2007, the Bourbon Orca design model was included in the London Science Museum’s display of innovative technology. The vessel’s operator claims that the design achieves higher speed and a calmer motion in head, rough seas.

As an extension of the bulb from, Ulstein came out with the concept of X-bow in the year 2005 and the first ship was commissioned in 2006 .

The bow, by incorporating a large forebody coming right out of water and sloping aft right to level of a high first exposed deck, causes a different volume distribution with a tapered fore ship shape as well as sectional angles, resulting in a wave piercing effect at small wave heights, and also reduces pitching and bow impact loads in rougher seas with bigger waves.

Major benefits of the X-BOW , as per Ulstein, are as follows

• Improved power efficiency
• Soft entry in waves
• Very low levels of noise and vibrations
• Reduced bow impact and slamming
• Reduced wave-induced vibrations
• Improved resting time
• Lower acceleration levels
• Lower pitch response due to volume
• More comfortable on board conditions
• Lower speed loss
• Reduced spray

However, X-bow is more suitable for bigger vessels in rough seas. Because of the shape, the spray runs up the hull and hence to avoid the spray hitting your bridge window, the same has to be high. A small vessel may not have those many decks and will naturally have problems. The windage area increases and also the risk of plunging deep into the sea. For a conventional bow, as she plunges deep into the sea, the displaced volume increases, increasing the momentum to reduce pitching motion and acceleration. In case of a ship with X-bow, it is the opposite.

Parameters affecting the design of a bulbous bow

A bulbous bow is defined by the following parameters, which are also shown in the diagram below.

BB	=	MAX WIDTH OF BULB
LPR	=	LENGTH OF BULB MEASURED FROM FP
ZB	=	HT. OF TIP OF BULB FROM BASELINE
ABT	=	TRANS. SECTION AREA AT F.P
ABL	=	LONG. SECTION AREA OF BULB AT CENTRELINE
VPR	=	VOLUME OF BULB FROM F.P TO TIP

 

Below are the linear and non-linear form co-efficients from which the bulb parameters are derived

LINEAR			NON-LINEAR
NAME	DESC.	FORMULA	NAME	DESC.	FORMULA
CBB	BREADTH CO-EFFN	BB/B	CABT	CROSS SECTION CO-EFFN	ABT/AMS
CLPR	LENGTH CO-EFFN	LPR/LPP	CABL	LONG SECTION CO-EFFN	ABL/AMS
CZB	HEIGHT CO-EFFN	ZB/TFP	C∇PR	VOLUMETRIC CO-EFFN	VPR/∇
B = BREADTH OF SHIP BB = BULB BREADTH AT FP LPP = LENGTH BTWEEN PERPENDICULARS LPR = PORTRUDING LENGTH OF BULB TFP = DRAFT AT FORWARD PERPENDICULAR			ABT = CROSS SECTIONAL AREA AT F.P AMS = MIDSHIP AREA ABL = LONG. CROSS SECTIONAL AREA FORD OF F.P VPR = VOLUME OF BULB FORD OF F.P

 

TYPICAL RANGE OF CO-EFFICIENTS ( KRACHT , 1970)
LINEAR			NON-LINEAR
NAME	MIN.	MAX.	NAME	MIN.	MAX.
CBB	0.170	0.200	CABT	0.064	0.122
CLPR	0.018	0.031	CABL	0.068	0.146
CZB	0.260	0.550	C∇PR	0.0011	0.00272

 

How does the bulb work?

As one can see in the diagram, a conventional bow creates a bow wave. That means a high pressure zone – much like as if the ship is pushing against a pileup of water. The energy going into this shows up as bigger waves and goes to contribute to wave resistance.

In contrast, the bulb due to its forward motion is pushing the water sideways and up , creating a crest ahead of the ship , with the trough occurring aft of hull entry, which necessarily means reducing the pressure on the hull, resulting in lower wave resistance. The higher the speed within that Froude number range , the more is the effect of the bulb

Apart from reducing the resistance at the specific Froude number range (>0.238 and < 0.563) , the effect on sea-keeping characteristics are also important. Bulbs typically are v-shaped on the bottom to minimize slamming in rough seas. The large volume so far ahead of the ship produces extra buoyancy and a high moment that counteracts pitching motion.

The following diagram is a quick selection help for selection of the type of hull and within that the zone of applicability for the bulb. The diagram is qualitative and designers should use their own discretion.

 

CFD analysis of ships with bulbous bow

Now that Naval architects have huge computing power at their disposal and with the advancement of technology in the area of computational numerical methods, the quality of ship design has advanced and refined.

While I still rank a model test to be a far more reliable tool to estimate the resistance of the ship, Iterative CFD analysis with continuous changes in the hull form can result in a far better hull form to begin the model test. To change the model a number of times physically is a time consuming and costly affair and CFD can make the process faster, better and more economic. Bearing in mind that most of this work has to be done at a pre-bid state, where a return is not always assured, CFD is definitely a useful tool .

 

References:

1. Design Optimization of the Lines of the Bulbous Bow of a Hull Based on Parametric Modeling and Computational Fluid Dynamics Calculation
   1. Weilin Luo ^i,ii and Linqiang Lan ⁱⁱ
   2. School of Mechanical Engineering and Automation, Fuzhou University, Fuzhou 350116, China;
      1. Fujian Province Key Laboratory of Structural Performances in Ship and Ocean Engineering,
      2. Fuzhou 350116, China
   3. Design & Construction of Bulbous Bow
      1. Manuel Ventura
   4. Website – Ulstein
   5. Design Principles of Ships and Marine Structures – Prof. S.C.Mishra
   6. Basic Ship Propulsion – Prof. J.P.Ghose & Prof. R.P.Gokarn

 

 

 

 

 

 

Disclaimer:

The views, information, or opinions expressed are solely those of the author and do not necessarily represent those of TheNavalArch Pte Ltd and its employees