It ain't necessarily so
by John Curnow, Global Editor, Powerboat.World 10 Jun 2021 09:01 UTC
Bulbous bow on the starboard hull of the Aquila 44 powercat © John Curnow
Was a time when you either had to be really big, or just plain slow to have one. Perhaps even with one foot planted firmly in both camps. It was also quite amazing just how long it took the US Navy to deliver the Ford Class carriers, when they are really a Nimitz Class with a bulbous bow added on. What is with that?
Now in what seems like a trillion years ago, I had my first experience with bulbous bows on a more approachably-sized craft, and it could march into the low to mid 20s. Hardly the norm, or so I thought at the time, but alas it just meant the era of the Fast Displacement hull form had arrived. That vessel was the Aquila 44, which I drove in 2016 and reported on in Lots to Love.
Right. Seems I am not alone in considering all of this, either. Avid reader and friend of Powerboat.World, Pat Bray in British Columbia, has also been deliberating it all. Only difference is, he's a Naval Architect, and he's been kind enough to provide us with some very brilliant intel on it all, and thank you very much, Pat, who you can find at Bray Yacht Design And Research Ltd. So take it away, Pat...
The Maths Lab
And that's math, not meth. We're cooking data here, not drugs. Taking their cue from fish boats and ocean freighters, long-range cruisers are capturing the advantages of the bulb, and there are many. Sceptics may question these claims, but solid proof is in the 'numbers'; the data not only from model test results, but from retrofitting bulbs to numerous types and sizes of yachts, with definite and measurable advantages.
Let me introduce you to some real-life success stories. The owner of a Symbol 72 found himself challenged by an overweight vessel with an excessive bow wave. Unable to achieve the designed speed and performance, a bulbous bow was recommended. Upon retrofitting a custom bulb designed specifically for the hull shape of his yacht, he experienced a much-reduced bow wave, leading to increased speed and fuel efficiency: 3.5 knots increase in speed, in fact. This owner then went on to commission a new state of the art yacht with an optimized hull form incorporating a fuel-efficient bulb.
The delivery trip on Amnesia IV from Vancouver, B.C. to San Diego, CA took 7 half days with an average speed of 9.6 knots over the ground. With generator run time included we averaged 3 litres per mile (6.25 US gal/hr). Even in 40+ knot winds and really big seas the boat handled very well.
Just what is a bulbous bow?
In the late 1930s research was undertaken to reduce the drag on large ships. With model testing and advancing knowledge of hydrodynamics, the commercial 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, and in the 1950s bulbs were commonly used on commercial vessels.
At that time, this was not sufficiently worthwhile for smaller, non-displacement yachts, cruising local waters. However, once the market for long-range cruisers opened up in the late 1990s, innovative yacht designers began to look for ways to reduce fuel consumption. The bulbous bow stood out as a potential solution. Since then, technology has allowed for refinement of the bow bulb to give even more advantages.
In these model test photos, you can see the effect that the bulb has on the bow wave. The bow wave is pushed forward and flattened over the bulb, followed by a longer, lower wave, and also a reduction in the midship hollow. Overall, the wave pattern is flatter, meaning less resistance for the hull to move through the water.
How does it work then?
Although much is known about the bulb, many of its functions are still in dispute. On a hydrodynamic level, the destructive interference of the primary and secondary wave trains causes an overall reduction in drag that is beneficial to the vessel's resistance characteristics. 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. In any case, it is a fact that bulbs do work, and do their best work above hull speed and achieve a resistance reduction of up to 23%.
Bulb proportions are critical, and derived from the features and dimensions of the vessel itself. The bulb must be designed for the specific hull it is to be fitted to. 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, thereby reducing resistance.
How much does size matter?
Here on the West Coast where the fishing fleets run great distances over open ocean from California to Alaska, the desire for economy spurred on the optimisation of the bulbous bow.
In the late 1980s the University of British Columbia, utilising the B.C. Research Ocean Engineering Center, initiated model test work on 60-foot fishing seiners. Results then showed that running at continuous displacement and semi-displacement speeds for days at a time, coupled with the limited change of draft, then also made long-range motor yachts ideal for this application.
Even more current research demonstrates the results are significant in vessels over 40 feet, and real gains are being achieved above the 60-foot bracket. This difference may be due to the stockier hull forms required in the really small ships, which possibly are not quite as compatible with the characteristics that make the bulb work so well in the longer vessels. However, a correctly proportioned bulb can still provide advantages even in the smaller vessels.
In the more that sixty retrofits that we have engineered and collected data on, all of them report a minimum 0.74 knot increase in speed, or 12% drop in fuel consumption, along with a 50% reduction in pitching motion.
Pros and cons...
The benefit of a high-tech bulbous bow is that it will reduce your fuel consumption 10% or more, giving you the equivalent greater range, or higher speed, whichever way you choose to apply it. The greatest amount of benefit will be at the higher 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 may cause a slight increase in drag because of its greater wetted surface area, however at that low speed the added power consumption is negligible.
Generally little time is spent in this low-speed range, so the effect is not noticeable. In addition, you will find increased sea-keeping ability due to the 50% dampening of the pitching motion. One boat we tested could go two knots faster with the same degree of comfort in waves as it did at the slower speed without the bulb, and this seems to be the average result.
When charging into head seas there is a small chance of slamming the bulb on the troughs, but this is limited to a very narrow range of wave train and headings. A slight change in direction and/or speed will cure this ill effect. The increased comfort level (from pitch reduction) is the biggest bonus noticed (and appreciated) by owners of all the retrofitted yachts.
When anchoring off the bow ensure that the anchoring arrangement clears the bulb or as an alternative you could consider installing an anchoring bowsprit to ensure the anchor falls well away, especially in conditions where swinging occurs; or use anchor pockets port and starboard.
Overall, the pros far outweigh the cons, so enjoy.
At 1650 RPM the vessel previously did 11.5 knots; after the refit she does 16.8 knots. The trim angle at the old speed was 5 degrees and is now 2.8 degrees. Nautical miles per gallon went from 0.15 to 0.28, an increase of an almost unbelievable 186%. The fuel burn was reduced from 76.5 gallons/hour to 60; and all this at 16.8 knots. The bow spray is significantly lowered and the motion in a seaway is just as dramatically improved, with a major reduction in pitching motion.
So who can, and who can't?
Any vessel over 40 feet, regardless of size, material, or age can be retrofitted with a bulb. An often-asked question is; will the cost exceed the benefit? Generally the benefit outweighs the cost, especially if the boat is to be used for extensive cruising, it will provide a clear advantage.
Bulbs have been built out of steel, aluminium, fibreglass, and wood. For an existing boat they are attached externally to the bow, in effect creating a bumper, which if damaged does not breach the watertight integrity of the hull.
Once properly designed, any reputable shipyard can create and install a bulb in a short period of time with the vessel out of the water. Factors that can complicate the installation are bow thrusters that occur within the outline of the bulb, and transducers and thru-hulls that have to be moved or extended to reach the new line of the vessel skin, so be sure those are taken into account.
The bulbous bow above is performing perfectly. It has dramatically reduced "hobby-horsing" and we are now only burning 22.5 gph at 8.3 knots which has increased our cruising range from around 3600 NM to 4000 NM.
In new construction, the benefits of the bulb can be designed in from the start. It will be an integral part of the hull, with a watertight bulkhead separating it from the interior spaces. Successful new builds have proven the effectiveness and efficiency of this technology.
The real deal about bulbous bows
- They are even more effective in reducing resistance once you get above hull speed. People think of bulbs only for displacement hulls where in fact a bulb starts to work most effectively once you approach hull speed, and reach their maximum benefit above that.
- Reduce your trim angle by as much as 1 degree, improving visibility.
- Reduce your pitching motion significantly, increasing comfort and improving seakeeping.
- Lower the height of your bow wave, reducing water on deck and environmental wave train impact.
- Increase your range under power from 12 - 15%.
- Reduce your fuel consumption up to 23% depending on the vessel speed.
- In a new build vessel you can fit an engine with 20% less horsepower, have fuel tanks 12-15% smaller, and still reach your contract/designed speed and range.
- In a new build it will reduce your Gross Tonnage by reducing hull volume because the smaller fuel tank requirement makes for a lighter boat.
- Work very effectively as a light icebreaker bow in new ice.
Takeaways
- If your craft is over 40 feet in length, and does not go faster than 24 knots, then a bulb could be for you.
- If you plan on doing some long passages, then a bulb could be for you.
- If you want a more comfortable motion through the sea, then a bulb could be for you.
What is your next step?
Contact a Naval Architect who is knowledgeable about bulb technology. Be prepared to give them serious technical data on your vessel, or if you don't have that data, understand you will have the cost to re-establish that information. If you are willing to go with a bulb of relatively standard proportions and form, no model testing should be necessary.
If you are starting design and construction of a new vessel and want it to have a bulb, check with your Naval Architect. If they do not specialise in bulbs, consider having a specialist design the bulb to work correctly with your hull's shape and size. This will ensure the best possible benefits.
Ultimately, just add large amounts of ocean water and minimal amounts of power for many happy, and less expensive years of cruising lie before you. Bon voyage!
Thanks Pat!
Short takes
Reading that Lionheart Capital and John H. Ruiz bought Cigarette was enough to sit up and pay attention. Their current craft range is priced from USD 600K to 3.5M, but I got to pondering the future some more, and wondered if question was whether it is not all so James Bond and no longer has a home in the PC world?
I mean turbine powered cats obliterated them for speed. The more user-friendly outboard powered centre consoles pretty much took care of the remains. True, they have gone that very route themselves, but being skinny and not having the lumpy, straight through exhaust V8s just isn't the same thing, now is it?
Ultimately I remain interested to see what new money can do with the brand most synonymous with the era, and maybe they can share it all with Barbara Broccoli when they work it out. A good one to watch, me thinks...
Maritimo S600
The new M600/S600 pair are identical from the aft deck BBQ/seating console all the way for'ard to the stem of the brilliant M55/S55, including the engine room (just longer prop shafts). The big difference is the longer running surface out aft, which is really best described by seeing the transom rise up from the trailing edge of the optional extended adventure deck you see here on our test boat. That's huge!
It also allows for an even shallower shaft angle than the M55/S55's brilliant 9.1 degrees, and it is all done without the need for tunnels. Then there's the added buoyancy it provides for, the fighting chair and fish wells you can opt to have installed, along with a full wedge transom to back down hard on the big ones so they just can't get away, and lastly the option to go up to a pair of Scania's V8 1200s - the kings of the power to weight title fight. In those you would have to think it would be over 35 knots WOT and genuine low 30 knot cruise.
Personally I can see the single level Maritimo S600 with a half tower (I reckon we need to look out for a super-sexy version BTW), and the V8s getting the nod from serious fishos on both sides of the Pacific. Given the walk around decks, you could almost argue that it is one giant sized enclosed centre console...
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John Curnow
Global Editor, Powerboat.World