The Limits of Bulbous Bows

At first glance, a bulbous bow seems like a ridiculous idea.  Ships are skinny and pointy to minimize the waves they generate.  We don’t want waves.  Waves take power from the ship and increase our fuel consumption.  But bulbs are designed to intentionally create large wave systems.  Why?

The key is what happens when we put a small bulb in front of a large ship bow. (Figure 1‑1)  The ship bow generates large waves.  If we can reduce or cancel those waves, we can recover some of that lost power.  Enter the small bulb.  Place a bulb (1) in front of the main bow, and it also generates its own wave system (3).  By careful engineering, we can arrange things so that the trough of bulb’s wave aligns with the crest of the main bow wave (4).  This partially cancels the main bow wave and recovers some of our lost energy (5).  Sure we pay a resistance penalty for placing that chunky bulb in front, but the benefits on the main bow are greater.

Figure 1-1: Superposition of Bulbous Bow Waves
The careful engineering is the tricky part.  First, the two wave systems must line up to cancel out.  Here’s the catch:  the wavelength of these two systems changes with the vessel speed.  They only line up at one speed.  We can design that to match any speed you want, but you only get one design speed.  All that time you spend entering harbor and slow steaming, the bulb actually makes things worse; it may even increase your bow wave.  A difference of just two knots from your design speed is enough to make the bulb useless.  The lesson here:  only install a bulb if your ship will spend the majority of its voyage at one specific speed.  This holds for the entire life of the vessel.  If fuel prices go up, slow steaming helps you less because the bulb works against you, unless you stay at the design speed.

Even with those limitations, there are many ships where a bulbous bow really will offer major fuel savings.  But only if you install the right bulb.  Bulbous bows are just as varied as ship hulls.  You cannot just slap a pipe on the front, weld a cap, and call it a success. (Figure 2‑1)  The shape, length, and width of the bulb are all carefully calibrated to match your specific hull.  Without that careful engineering, chances are good that the bulb will do more harm than good.

How to design the right bulbous bow?  We start with some hand calculations.  A 1978 paper by Mr. Kracht is one of the most comprehensive instructions for this. [1]  But that is still just a preliminary estimate.  With the advantages of modern computing, you want a CFD analysis to really optimize the bulb and ensure you have a good shape.

I know that CFD is expensive.  But you only have one chance to get bulb right.  After construction, it is permanent hull structure.  Any modifications require drydocking (money), remove the old bulb (more money), and fabricate a new one (too much money).  Better to invest in the engineering and be sure of success the first time.  The CFD analysis understands all the complexities of the interaction between a bulb and the hull.  No simplifications here.  CFD simulation is just one step removed from reality and provides excellent assurance that the bulb will be a success.

You may be tempted to reduce costs and reduce the CFD to a minimal scope.  Your minimum scope should include the following two design cases.

1. Base case: unmodified hull without a bulbous bow
2. Bulbous bow added

This will verify that the bulb actually improves your situation.  Otherwise, a CFD analysis may focus on optimizing a starting bulb design.  It heralds a 2% reduction in fuel consumption due to the optimization.  But without the base case, the CFD may completely miss that the bulb added 10% to your fuel consumption and you would be better without any bulb at all.

Bulbous bows are designed assuming calm water conditions:  no waves and no storms.  But we all know that calm water rarely exists on the ocean.  Another research paper discovered that for most storm conditions, the bulb still works and reduces fuel consumption. [2]  The one limitation for this was with especially large waves or small vessels that pitch around in normal seas.  In these cases, the bulb never gets a chance to generate a steady wave system and help you.  “As a rule, for fast ships with low block coefficient and pronounced wavemaking, the thrust increases very strongly at very low wave steepnesses while for slow ships with high block coefficient and pronounced wavebreaking, this increase in thrust is less.” [2, p. 87]

Just remember, bulbous bows are wonderful when they work, but they are very easy to get wrong.  Bad bulbs or wrong applications do worse than nothing.  They work against you and make things worse than if you had no bulb at all.  Understand the limits of a bulb so that it can work for you and not against you.