Bulbous Bows

[Dan K]

I know (or believe) that use of such bows cuts resistance. But, what does that mean exactly - does greater efficiency translate into a potentially higher top speed, more rapid acceleration, greater fule efficinecy, and mix of all three or .............?

TIA

[chuck]

A mix of all three at higher speeds through reduction in wave drag. At lower speeds a bulbous bow actually increase drag, anf thus reduce fuel efficiency and lower acceleration obtainable at each power output.

[Seasick]

The USN is putting bulbous bows in the CVN fleet mainly to improve high speed performance for flight operations and for other reasons. The fuel thing isn't to critical for a nuke.

[Foeth]

Bulbous bows are not for efficiency, not for acceleration, but for reducing wave resistance at a given operational speed range. The bow produces a wave that interacts favourably with the wave system of the ship. Waves cancel each other out and resistance reduces. At other speeds, this wave system will increase the resistance. In effect, fuel efficiency is improved at certain conditions, and you may apply the bulb such that top speed is increased for a given power.

In increase in efficiency doesn't give you anything appreciable in speed. Cutting down resistance will.

[Dan K]

Thx for the replies.

I understand how decreasing resistance will allow for greater speed. Practically speaking, why does lengthening the hull accomplish the same thing?

[reigels]

A bulbous bow also increases boyancy at the bow which can help reduce pitching and thus keep the props in the water (or keep a level deck for flight operations).

[Foeth]

Just considered the ship as a simple body that generates roughly two wave systems: the waves from bow and the waves from the stern. When the crest of the bow coincicides with the wave crest of the stern, the total wave height increases. You have to realize that the wave height squared determines how much energy is stored in this wave; energy you have to provide for with your engines to keep this speed. So, waves adding up is bad. This is why the bulb tries to generate a wave so that its crest hits a trough; waves cancel and the resistance is decreased.

All things considered equal, when stern and bow waves interact badly, resistance increases so much that ordinary displacement ships need excessive power to gain any speed. This is why Iowa needs so much power to go only so little faster than other ships.

Now, wave lengths indicate when crests and troughs coincide, and wave lengths depend directly on ship speed. So given a ships length, longer ships will have their unfavourable interaction at higher ship speeds. The governing ratio is ship length devided by the square root on the top speed. You'll note that this ratio is pretty constant within a ships class. With this ratio, Scharnhorst is actually faster than the Iowa's.

Of course, there are many other features in hull shape that depend the total resistance and top speed, but ship lengths really adds up if you want to go fast. Catarans, for example, are extraordinary slender and have little difficulty overcoming this maximum wave resistance (called hum speed). After this point, resistance will actually go down again. This is very important if you are a planing hull or a water plane trying to take off! But the ordinary displacement hull of a battleship can not exceed this speed. This is also why I do not believe that Iowa ever sailed 35 knots, unless going down hill from Niagara falls.

A bulbous bow also increases boyancy at the bow which can help reduce pitching and thus keep the props in the water (or keep a level deck for flight operations).

That is absolutely not true.

[chuck]

Why is not true?

Bulbous bow increases the buoyancy to weight ratio of the extreme bow of the ship by adding underwater volume there. Since in a ship without bulbous bow the buoyancy there is effectively zero due to the narrowness of the cutwater, the buoyancy increase given by the bulbous bow must be greater than the weight increase caused by the bulbous bow. So this would have the effect of increasing the moment of the buoyancy force distribution about the pitching axis compared to the moment of mass distribution of the ship about the same pitching axis, thus this should act to reduce amplitude of pitching.

Also, bulbous bow does increase efficiency in the sense that it reduce the overall drag at certain speed range, and thus allow smaller installed power and lower fuel consumption while achieving the same speed.

[Timmy C]

Practically speaking, why does lengthening the hull accomplish the same thing?

I know jack all about fluid dynamics, but would I be somewhat correct to say that this is similar to why high speed vehicles have a more streamlined shape?

[chuck]

Practically speaking, why does lengthening the hull accomplish the same thing?

Because increasing hull length does not by itself reduce the bow wave resistance as bulbous bow would.

[chuck]

This is also why I do not believe that Iowa ever sailed 35 knots, unless going down hill from Niagara falls.

Hey, currents up to 15 knots are known in some Fijords. Maybe Iowa had a strong tail current.

[Foeth]

Also, bulbous bow does increase efficiency in the sense that it reduce the overall drag at certain speed range, and thus allow smaller installed power and lower fuel consumption while achieving the same speed.[/quote]

Effiency is the ratio of required power overcome by delivered power. That says absolute nothing about installed power. If I give you a 1 ton load to carry by rucksack and calculate your walking effiency to be 60%, that is worse than walking without it at an efficiency of 50%. Even though efficiency is better. With a bulbous bow, your resistance goes down at some speed points, but your propulsive efficiency may be higher or lower. Doesn't really matter. Matter of definition I guess. Of course, total installed engine power for that speed goes down. But even at that lower resistance, I would pick a lower propulsive efficiency as my total installed power is lower.

Why is not true?

I'm not saying a bulbous bow cannot be used to some buoyancy related item. But, you just don't add a bulbous bow to correct trim. If you're anything but the most incompetent naval architect, you get the trim right to a certain degree regardless of hull shape. If you need a bulbous bow to correct trim, your design was absoutely worthless to begin with!

I know jack all about fluid dynamics, but would I be somewhat correct to say that this is similar to why high speed vehicles have a more streamlined shape?

No This has nothing to do with streamlining. This particular case of fluid dynamics goes for ducks, cajacks, battleships, and icebergs! Of course, streamlining is never a bad idea, as you just have to get the total resistance down. Wave resistance is only one component, but a very important one. With a good hull shape, you can achieve a great resistance reduction. But, longer ships still have the benefit of their length. Plus, you can only go so far with streamlining. You still need your strength and space for cargo/weapons and machinery. Longer ships are typically more expensive too. However, if you want to keep the cost per unit of cargo down, bigger ships are the way to go. Container ships are getting bigger and bigger by the year.

Because increasing hull length does not by itself reduce the bow wave resistance as bulbous bow would.

That is not a trivial matter. Of course, a bulbous bow is a very cheap way to reduce resistance, most certainly cheaper than adding 20 meters to your ship. But the overall resistance characteristic would be a matter of some more extensive engineering. And then you add the bulbous bow anyway. But keep in mind that at different draughts and speeds, the bulbous bow can give you a resistance penalty.

[Dave Wooley]

The main rational of the bulbous bow is to reduce the vessels resistance to motion but under certain conditions .
CVN77

Picture Northrop Grumman

[ar]

I note at least two types of bulbous bow,
One, where the fore end of the bulb is forward of the LWL.
Two, where the bulb does Not project forward of the line of the stem at any point.
The liner United States has the latter. The bulb flares out but does not project forward.
Can someone please explain the differences between the two.

[chuck]

The difference between the two types of bulbs appears to be largely evolutionary. The type of bulb that swells out laterally but does not project forward seems to have made its first appearance around 1918. The non-protruding bulb was very common on WWII era warship in the US and German navies. The Japanese appears to have introduced the first protuberant bulb aboard the Yamato, but this type of protruding bulb did not seem to gain widespread acceptance until about 1955-1960. The non-protruding type of bulb now seems to be very rare. The protruding bulb seems to be very dominant, and the degree of protrusion seems to be increasing consistently since 1960, resulting in the almost obscene protuberances on the bow of some of the most modern ships, mostly cruise liners.

[JohnmCA72]

...
This is also why I do not believe that Iowa ever sailed 35 knots, unless going down hill from Niagara falls.
...

They may have been making turns for 35 kts...

JM

[Dan K]

My interest stems from my fascination with Junyo & Hiyo, the converted IJN carriers. They had non protruding bulbous bows, and 4 bladed props (perhaps the only IJN ships to carry a 4 bladed prop). These designs were included to maximize their performance. I was just wondering how these design features might have impacted them, also how adding length and /or power might have improved their performance. I appreciate the responses.

[Foeth]

I don't know anything about the desing process of those ships, but Yamato's bulb was chosen based on extensive model testing. So, I think it's reasonably safe to assume other ships were based on some model testing as well, taking the best candidate. The number of blades on a propeller has some influence on efficiency, but not much.

[chuck]

Junyo and Hiyo's bulbous bow and 4 bladed props may simply be a hold over from their original express passenger liner origins. Bulbous bows were universal on fast passenger liners of the era. Yamato not withstanding, IJN actually was not into bulbous bow is any big way. AFAIK, Yamato class and Shokaku were the only IJN porpose built warship to possess bulbous bow. Even Shokaku's sister Zuikaku didn't have a bulbous bow.

[Werner]

Interesting to note Musashi was built alongside Junyo at Mitsubishi in Nagasaki, and Hiyo was built at Kawasaki in Kobe alongside Yamato. Perhaps there was some cross-pollination....

A. Nani's plan drawing of the hull of Junyo shows a forefoot similar to North Carolina's. I might add that his drawing of Yamato appears more closely in agreement with current thoughts than the later Skulski's.