The bulbous bow
What is it, and why ?
Authored by: Patrick J. Bray - Naval Architect
Update by: Mr. Dan
Vyselaar andMr. Ion Livas
Brought to you by www.dieselduck.net, comments to [email protected]
What is a bulbous bow?
In the late 1950s research was undertaken to reduce the drag on large commercial cargo ships. Many different ideas were tried and continue to be tried today in the ongoing development of the science of Naval Architecture. With model testing and advanced knowledge of hydrodynamics, the bulbous bow was formulated typically giving a 5% reduction in fuel consumption over a narrow range of speed and draft. This was significant for a large ship crossing vast oceans, at a time when the cost of fuel was rising. Unfortunately, this was not enough to make it worth while for smaller yachts racing around the bay. Also, the narrow range of displacement speed was not in keeping with the yachtsman's need for speed on the water. As the market for displacement long range cruisers opened up, innovative builders began to look for answers to their consumers questions. The bulbous bow stood out as a prime solution.
Although available in many shapes and sizes, generally the bulb looks like a section of large diameter pipe with a domed end sticking out of the bow of the boat, underwater. Side bulbs, bilge bulbs, and even stern bulbs have been tried but the most consistent results have been achieved with bow bulbs. Today, to see a large ship without a bulbous bow is a rare sight indeed. Their results have been proven over countless thousands of deep ocean miles in all kinds of weather by all kinds of vessels.
How big is big and how small is small?
With the success of larger ships, smaller and smaller vessels started looking for the same results. Here on the West Coast where the fishing fleets run great distances over open ocean from California to Alaska, the desire for economy spurred optimization of the bulbous bow. The large ocean trawlers and packers were first, followed by most of the fishing fleet.
In the late 1980s the University of British Columbia, utilizing the B. C. Research Ocean Engineering Center, initiated model test work on 60 foot fishing seiners. Since that time, B.C. Research has become the center of bulbous bow technology, doing model test work for yacht builders and commercial yards alike as well as for designers and researchers. Results have shown that the continuous displacement speeds run for days at a time, coupled with the limited change of draft, make long range motor yachts ideal for this application. Vessels as small as our 47 foot Karvi have had bulbs fitted. Below this size the bulb's effectiveness seems to lessen. Over 45 feet the results are adequate and closer to the 60 foot size real gains are being achieved. This may be due to the lack of detailed research conducted on vessels under 45 feet or just because of the stockier hull forms required in these really small ships. It is possible that the stocky hull form commonly used is not compatible with the characteristics of the bulb. For maximum benefit model testing is still required to fine tune the proportions to the hull form. We have developed our hull form with a specific bulb configurations to maximize specific features and benefit fully.
So what's in it for you?
The benefit of a modern day bulbous bow will reduce your fuel consumption 12% to 15% giving you the equivalent greater range, or a slightly higher speed for the same power applied, whichever you choose to use. The greatest amount of benefit will be at the high end of the semi-displacement speed range, reducing as your speed decreases. At higher speeds wave making resistance accounts for the greater portion of the drag, and the slower you go proportionally more of the resistance is taken up by wetted surface drag. At low speed (around 6 knots and lower) the bulb will even cause an increase in drag because of it's greater wetted surface area. Luckily at that low speed the added power consumption is negligible and generally little time is spent in this speed range.
In addition, you will find increased sea keeping ability due to dampening of the pitching motion. When charging into head seas there is the chance of slamming the bulb on the troughs, but this is limited to a very narrow range of wave train and heading. A slight change in direction and/or speed will cure this ill-effect.
How does it work?
Although much is known about the bulb, much of it's functions are still in dispute. To say that on a hydrodynamic level, the destructive interference of the primary and secondary wave trains causes an overall reduction in drag which is beneficial to the vessels resistance characteristics would be true. A bit of a mouth full, but true! Or on a more physical level, that the water coursing over the top of the bulb is exerting a downward pressure that is keeping the stern from squatting, thereby allowing flatter trim, causing the vessel to run with less resistance. Still others would argue the finer points of laminar flow, with no clear conclusions. In any case, it is a fact that bulbs do work and in some cases reduce resistance as much as 25%.
Their proportions are derived from the features and dimensions of the vessel itself. The diameter (volume) is a direct result of the hull midship area. The length is determined by the stem profile, as the farther forward the bulb extends the more leverage it has but is generally kept shorter than the bow overhang. The section shape may be a modified ellipse to reduce pounding in head seas. The vertical placement is calculated so the bulb is just below the surface where it will create a wave in front of the ship interfering with the natural wave train of the vessel, creating a wave hollow where a crest should be. In this way the vessel will run flatter and the overall wave height will be reduced. The vessel will pitch less which will cause less disturbance in the water and the passage of the vessel through the water will be achieved with less fuss and muss. Any time a vessel can be moved through the water with less waves and overall disturbance to the surface less power has been transmitted to the water to create those waves. To be able to move a vessel through the water with no perceptible notice of its passage is the (as yet unattained) ultimate goal of all Naval Architects and the ultimate in power savings.
If you plan on doing some very long passages at a constant speed then you should have a bulbous bow. What is your next step? Contact a few brokers and boat dealers, ask about the efficiency of their vessel, ask about a bulbous bow. There are trawler yachts and then there are little ships designed to efficiently and economically take you anywhere you want to go on the face of the earth in safety and comfort. Research and become knowledgeable about vessels in your market then spend your money wisely on a thoroughly designed vessel. Just add large amounts of ocean water and minimal amounts of power and many happy, inexpensive years of cruising lie before you. Bon voyage!
I received an interesting email from Mr. Dan
Vyselaarregarding the "Bulbous Bow" piece, as it appears above. He brought up interesting points and I have put here below, for your information and discussion.
Martin's Marine Engineering Page - www.dieselduck.net
To Whom It May Concern:
Subject: Bulbous bow description..
I would like to start by saying that I find your webpage very useful and thorough. I am a naval architect, currently working at UBC as a teaching assistant while pursuing a masters degree. I’ve found several of my students referencing your webpage in their lab reports, and I feel they are gaining a lot from this, which is excellent. However, I have read the section on bulbous bows, and find myself disagreeing with a few of your points on the operation of the bulbous bows.
The first objection is perhaps a minor one, on the claim that “you will find increased sea keeping ability due to dampening of the pitching motion”, I disagree with this statement, for two reasons: The first being that the bulb will in general not contribute to the longitudinal stability of the vessel, since this is dependant on the moment of inertia at the waterline, and the bulb should not contribute to this term, or in other words, it will not have an effect on the period of pitch motions. The second reason being that while there will be a contribution of the bulbous bow to the viscous damping and added mass of the motion of the vessel, the literature suggests that this is negligible. In particular, Principles of Naval Architecture, volume II – Resistance, Propulsion and Vibration, in chapter 8.10- Effect of Bulbous Bow on Resistance reads ”The evidence seems to be that bulbs have little effect on pitching …” and while it does say there may be some beneficial reduction in such motions, seems to give clear indication that any effect is small. I’m sure it is entirely possible that in the case of a small vessel with a large bulb there may be an appreciable effect of a bulb, but it seems to be somewhat misleading to state that there is a definite, appreciable dampening of ship motions.
The other statement that I disagree with is the one on how a bulbous bow operates in “layman’s terms” : “Or on a more physical level, that the water coursing over the top of the bulb is exerting a downward pressure that is keeping the stern from squatting, thereby allowing flatter trim, causing the vessel to run with less resistance” The two objections I have with this are as follows:
Firstly, since the bow is immersed in the fluid, it should experience only a net positive buoyancy force from the hydrostatic aspect of the water. Since the bulbous bulb will displace fluid upwards and over it, there will be an acceleration of fluid for the water to get around the bulb. This acceleration would presumably be most pronounced at the top, since there is no free stream condition here. In a manner somewhat analogous to a shallow water flow, I would then expect the water to be flowing fastest on top of the bulb, and slower over the bottom of the bulb. This would mean if there were a dynamic force from the bulb, I would expect it to be an upwards force, although I would suspect it to be quite small. The other objection I have is the last part of the statement, which says “allowing flatter trim, causing the vessel to run with less resistance.” It has been pointed out by Taylor (1943), that large trim changes are symptoms rather than causes of high resistance, and that changes in the at-rest trim of large displacement craft produce only very small changes in resistance.
Anyways, I welcome any feedback or further discussion on this topic, I mentioned it because it seems the wording of this document has somewhat mislead some of my students, and as a result, I’ve docked them a few marks on some of their comments!
Thursday, December 02, 2004
In August 2007, I received the email below from Mr Ion Livas offering further insight on the "Bulbous Bow" from his experiences; I have put here, for your information and discussion.
Martin's Marine Engineering Page - www.dieselduck.net
I wish to commend you on your explanations of the attributes of the Bulbous Bow and its effects. I would also like to offer three points, that might be of additional help.
1 - Volume compensation. When the added volume of a large Bulbous Bow, is used to reduce the volume of the forward part of the hull, while maintaining the same block-coefficient, a finer angle of entry can be achieved. The resultant effect is that the wave-making resistance is reduced. This will result in a higher speed for the same power, or a power saving for the same speed.
2 - Speed. Using the extra 'virtual length' of the ship, occasioned by the added length of the Bulbous Bow, we have an increase of speed for the same V/√L, by the amount of the length of the bulb beyond the forward perpendicular. When the extra length is 5%, the increase of speed is √1.05, equaling an additional 2.47%. For a 15-knot cargo ship, the increase is 0.37 knots. For a 1,000 ft ship and a V/√L of 1, this increase in speed is 0.78 knots. For a Trans-Atlantic Liner it would have meant an increase of speed from 31.62 to 32.4 knots! Enough to win the Blue Riband in bygone times. For a fast Container Ship Crossing the Pacific, it corresponds to a substantial saving in sailing time!
3 - Shape. When the shape of the Bulbous Bow is that of an upside down tear-drop, rather than cylindrical, the pounding effect is reduced and the tendency to keep the bow in the water to reduce squatting is enhanced.
The Volume compensation and the V/√L relationship explained above, will give a fast ship traveling at V/√L = 1, a substantial increase in speed. For a 15 knot 500 ft. Cargo Ship, the speed increase will be about half a knot and the sea-keeping properties will improve when the Bulbous Bow is tear-drop shaped.
The above conclusions were arrived at from my personal involvement in the comprehensive studies conducted at the Paris Tank, as well as the Madrid Tank under professor Acevedo in 1964. The Studies were conducted with several cylindrical and tear-drop shaped long and very long Bulbous Bows fitted to a C2 Cargo Ship. The Sponsor of these tests was Todd's Shipyards of New York, under the direction of the late Ralph Anselmi.
Ion Livas B.Sc.
Naval Architect & Marine Engineer
86 Filonos St.
18536 Piraeus, GREECE
Mr. Patrick Bray, author of the original article (above), sent in these additional comments in August 2008...
We have now retrofitted bulbs to over 40 vessels from 40 - 160 ft. All have worked great and the owner's are more than pleased. We have also model tested close to a dozen new design (which have been built) and have the following to report. Both retrofits and new boats show a least 10% drop in fuel consumption ( more often 12%) or approx. 3/4 knot gain in speed. The pitching is reduced by almost half. On the retrofits it is the reduction in pitching that is first noticed as it is so dramatic a reduction. Our bulb designs are now fitted on all vessels built by INACE in Brazil and Aleutian Yachts and Real Ships in the US. Our bulbs work between 8 and 20 knots with the greatest benefit at 10 - 16 knots (depending on the size of the vessel). We have also fitted bulbs (successfully) to reduce a very large bow wave.
Why do bulbs work? I have written articles on it and presented technical papers on it in the US and Europe. There are many people who do not understand them and there are many who are designing them who do not have a clue how they work. It is easy to do a badly designed bulb and apply it to the wrong application.
Patrick J. Bray