Bulbous Bows

[chuck]

Does not compute.
At 160,000 shp you get about 27 knots, that,s it.
To achieve 30 knots you will need about 200,000 shp, and one needs a sustained/continuous 30 knots to sometimes maintain the timetable. EG leaving harbour several hours late.
The figures you give show a 25% overload. Do that on a regular basis, and after a while the engines will suffer.
Mary is rated at 212,000 shp for 30 knots, without overload. This is about right.
With the bulbous bow and her more efficient underwater hull form from bow to stern. Normandie could probably make 30 kots on 200,000 shp as opposed to 212,000 for the Mary.

What are the formulas you used to compute this?

[chuck]

If anybody thinks that a 80,000ton, 1000 foot long ship can be driven at 30knots using ONLY 140,000shp, then THEY ARE NUTS. Who wrote this rubbish?

That is 80,000 gross tons, Her displacement was only ~70,000 tons, so given her dimension (320 meters by 36 meters) the block coefficient of her hull must be considerably less than a capital ship of comparable displacement.

That her engines were rated 160,000 shp seem to have been agreed upon by several sources. Whether sustained use of overload above that maximum will damage the engines largely depends on how they defined normal maximum and overload. In other cases, such as the Fantastique class destroyers, the French gave a maximum HP requirement in their specifications, and then offer monetary bonus to the the power plant builder for each HP in excess of the specified requirement. So the stated maximum engine power is really nothing more than a contractual minimum, the actual engines having been developed by contractors incentivized to go for as high as they can. Perhaps this same process is followed with Normandie, and her true sustained maximum having been made much higher than stated maximum as a result of the contract structure.

[chuck]

Regarding earlier post about waterplane area. By my understanding of "waterplane", a bulbous bow would not effect waterplane are unless the ship is seriously trimmed down by the stern and the bulb becomes partially exposed. Waterplane area also does not effect instantaneous bending stress. What increase in waterplane area near the bow might do is reduce the change in draft resulting from each ton of weight added near the bow. To reduce bending moment forward what is needed is hull volume under water and buoyancy, not increased waterplane area.

[Guest]

Ok, finally got a few minutes to elaborate on my earlier post. This is pretty long, I apologize in advance for my long winded-ness.

I am going to try make some distinctions about what I am talking about, because the current discussion is covering several different bow forms, and I seem to have caused some confusion earlier.

First, there's the bulbous bow. I think everyone is pretty familiar with what this is: a protrusion from the forefoot of a vessel above the baseline. The purpose of this is to improve fuel efficiency by reducing the resistance of the ship moving through the water. It accomplishes this in two ways.
1.) The biggest contribution of the bulbous bow comes from the fact that it produces its own bow wave ahead of the stem of the vessel. The ship also produces a wave from its stem as it moves through the water. The net result of these two waves is less than what the ship would produce without the bulb, thereby making her move through the water more easily.
2.) To a lesser degree, the bulb breaks the surface tension of the water ahead of the ship, again making it easier for the ship to move through it.
The success of the bulbous bow is just about indisputable, nearly every commercial vessel built since the beginning of the 1980's has one, and if ever there was a group of people concerned with efficiency, it's the merchant marines of the world.
Please note that efficiency in moving through the water does not necessarily mean higher speeds, though it can. With most bulbous bowed ships (ie merchants) it's all about the costs and the fuel consumption. The performance for a ship is measured daily at sea and the ratio for the engine efficiency is called "slip." Military ships are not as concerned with fuel efficiency as they get their fuel budgets from taxpayers these days. For example, the typical US conventional warship UNREPs every three or four days. The fleet stays at sea because of the fleet auxiliaries that service them every day.

The second bow type discussed here, and the one to which I was originally referring in my guest post, is the flared forefoot (aka bulbous forefoot). There is no fore-and-aft protrusion in this case, only athwartships (breadth). For examples of this type of bow construction, North Carolina, South Dakota, and Iowa class ships have them (and not to be exclusive of other nations, it's necessary to point out that HMS Hood as well as HMS Repulse both had this type of flared forefoot), as do ships like S.S. United States and many commercial ships built from the 50's on up to the 80's, when the bulbous bow took over. The flared forefoot is a feature found on a clipper bow, with a forward raked stem and flare from the waterline up to the bulwark. This type of bow was developed to add buoyancy to the forward part of the ship to lessen the wetness when a ship pitches. I said in my previous post that this also helps to reduce stress on the structure of the ship when she pitches. This has a couple of potential benefits:
1) A drier warship means that her deck fittings and weapons take less salt spray and therefore are exposed to less corrosion. Further, weapon mounts that are in "wet" areas of the deck can be used in less-than-ideal conditions without excessive risk to the crews from being washed overboard.
2) Steel ships are extremely flexible. Overtime, this flexing will cause stress fractures which can end the life of a ship, even if there is no catastrophic failure. The more you can reduce this flexing, the longer the life of your ship.
The structure of the forefoot is not any stronger than not having it flared, but the added buoyancy it creates supports the ship forward. Flare topside has the same effect of added buoyancy once the bow plunges into the sea.
I sailed for six months as a cadet on a ship with just such a flared forefoot and clipper bow, and she kept the sea quite well, and it took serious weather to get the foredeck wet. She was built in 1960.

The third type of bow being talked about on the board is the "ram" bow, aka the plough. These are found on older (WWI and prior) battlewagons and battlecruisers of most navies of the world. The purpose of the plough was (at least on American ships, I can't speak for the British here) to help to decrease resistance of the ship moving through the water. These bows created very wet ships and the US Navy found that much of the secondary armament was so wet as to be a constant problem and by the time of the Pennsylvanias, many serving officers were hoping that the next generation of US Battleship featured the clipper bow with the flared forefoot.
These bows were not intended to actually ram ships. I can't answer that for the ships of the late nineteenth century, though, I really doubt that was ever a serious consideration. Someone made the comment that a 14kt ship could easily cover the distances of artillery range while their opponent reloaded and I have to take issue with that and would be interested in discussing a scenario in another thread, but I don't think it's in the scope of this post.

I mentioned waterplane earlier, and it seems to have caused some confusion. There is only one waterplane, and it is at the waterline of a ship. The waterplane for a vessel can change, depending on her draft, and indeed, many bulbous bows are exposed regularly on merchant ships when they are in ballast.

The waterplane reference in my first post was to illustrate how fine an entry clipper bowed ships with flared forefoots have. The ship is wider beneath the waterplane (has more area, which translates to more volume when taken in 3 dimensions) and therefore has more buoyancy than a ship that continues with the narrow forefoot. This can also be clearly seen in a half-breadth plan.

When it comes to the question of why different nations used different configurations, I can't answer those questions. The designers of the ships had their reasons, carefully researched and planned, and all we can do is look at the success or failure of the design. The US Navy designed its battleships from the 1910's on with the Pacific ocean in mind and a future war with Japan.

I'm quite happy to talk about this stuff, so I will happily respond to comments and questions.

Some references that I use, aside from sailing myself:
Friedman "US Battleships an Illustrated Design History"
Captain Peter Vecchio, USMM, LCDR MMR USNR

[Werner]

There is an interesting, missing bow. The severely cut-away bow as one would see on a German battlecruiser of WW.I. Much of the forefoot below the waterline is simply missing. I have often wondered why such a bow would exist, since the lack of buoyancy there must have caused unacceptable pitching with abrupt transitions.

Maybe you can explain that one....

[chuck]

There is an interesting, missing bow. The severely cut-away bow as one would see on a German battlecruiser of WW.I. Much of the forefoot below the waterline is simply missing. I have often wondered why such a bow would exist, since the lack of buoyancy there must have caused unacceptable pitching with abrupt transitions.

Maybe you can explain that one....

For the bow torpedo tube?

[Foeth]

1.) The biggest contribution of the bulbous bow comes from the fact that it produces its own bow wave ahead of the stem of the vessel. The ship also produces a wave from its stem as it moves through the water. The net result of these two waves is less than what the ship would produce without the bulb, thereby making her move through the water more easily.

No, the bow wave doesn't really iterfere with the stern wave, but the bow wave

2.) To a lesser degree, the bulb breaks the surface tension of the water ahead of the ship, again making it easier for the ship to move through it.

No, surface tension does absolutely zip for the resistance of a ship and the bulbous bow doesn't do anything about that to make it less than zero.

[ar]

To guest,
SEveral points.
Hood, Renown and Repulse do NOT have bulbous bows or flared forefoots. Hood appears to have one but that is in fact a continuation of the built-in bulge that carries up to the bow.

You say that the flare of the SS United States runs up tp the bulwark. NOT so, the flare goes up to a KNUCKLE. This knuckle is at the point where the black colour of the hull meets the white. VERY difficult to see in photos except at just the right angle. I tale this from the builders body plan with photo confirmation.
Flare; Too much and you get slamming which gives a much wetter ship further along the ships structure such as the bridge, and excessive strain on the hull. US ships had a greater flare than their RN counterparts. British ships tended to actually be drier in heavy weather as they slammed less in a heavy sea at the same speed. Also less strain on the hull. This came to the fore when US ships operated with the HOME FLEET in the Artic during the war.
I hope this helps a little.

[gerinc]

1.) The biggest contribution of the bulbous bow comes from the fact that it produces its own bow wave ahead of the stem of the vessel. The ship also produces a wave from its stem as it moves through the water. The net result of these two waves is less than what the ship would produce without the bulb, thereby making her move through the water more easily.

No, the bow wave doesn't really iterfere with the stern wave, but the bow wave

Foeth- What I said was s-t-e-m, not s-t-e-r-n, the stem being the bow, please see the quote above.

2.) To a lesser degree, the bulb breaks the surface tension of the water ahead of the ship, again making it easier for the ship to move through it.

No, surface tension does absolutely zip for the resistance of a ship and the bulbous bow doesn't do anything about that to make it less than zero.

I would like to see your information on this, though again, as I said in my post, surface tension is a much smaller aspect compared with the bow wave produced by the bulb.


cheers

[gerinc]

There is an interesting, missing bow. The severely cut-away bow as one would see on a German battlecruiser of WW.I. Much of the forefoot below the waterline is simply missing. I have often wondered why such a bow would exist, since the lack of buoyancy there must have caused unacceptable pitching with abrupt transitions.

Maybe you can explain that one....

I'll look into it, many Japanese ships possessed this hull form as well, and I've often wondered. Today, ships with a very rounded forefoot are for ice breaking, but they're not like knife-like as the warships were, so I don't know.

[Dan K]

All IJN capital ships prior to the Yamatos and the Shokakus had the forefoot cut away. According the the US Technical Report to Japan, report # S-01-3 Characteristics of Japanese Naval Vessels - Article 3, Surface Warship Hull Design, this feature is sometimes referred to as a Maierform. Conversely, the Yamato bowform was said to reduce "the ship's resistance" (I assue they meant water resistance) by 5-6%

[gerinc]

To guest,
SEveral points.
Hood, Renown and Repulse do NOT have bulbous bows or flared forefoots. Hood appears to have one but that is in fact a continuation of the built-in bulge that carries up to the bow.

You say that the flare of the SS United States runs up tp the bulwark. NOT so, the flare goes up to a KNUCKLE. This knuckle is at the point where the black colour of the hull meets the white. VERY difficult to see in photos except at just the right angle. I tale this from the builders body plan with photo confirmation.
Flare; Too much and you get slamming which gives a much wetter ship further along the ships structure such as the bridge, and excessive strain on the hull. US ships had a greater flare than their RN counterparts. British ships tended to actually be drier in heavy weather as they slammed less in a heavy sea at the same speed. Also less strain on the hull. This came to the fore when US ships operated with the HOME FLEET in the Artic during the war.
I hope this helps a little.

SS United States' flare does go to a knuckle, you are correct. I have seen the ship in person. The paragraph containing my reference to the United States was talking about clipper bows in general, and not specifically to hers.

With the flared forefoot, I see we're either defining things differently or it's a subjective opinion as to what qualifies as a "flared forefoot." Hood, Renown, and Repulse all have greater breadth at their forefoot than they do at their waterline. This is the definition of a flared forefoot. The the narrowest part is at the design waterline, and the bow widens down towards the baseline. Indeed, even in the American cases like the SS United States or any of the ten WWII built battleships, the forefoot only swells relative to the stem, not the run aft. No military ships had bulbous bows through WWII except the Yamato class, that I am aware of.

You're also correct that US capital ships were wet, frustratingly so at times, but it was not from plunging through the wave, rather being splashed by the ship's bow pounding the wave. However, the earlier ships (pennsylvania and earlier) went into the seas, as opposed to over them. The clipper bow was intended to fix this. A ship going through a wave takes a lot of water on her decks, and this creates a lot of weight above her metacenter which can cause stability problems if the water isn't shed very quickly, especially in heavy seas. This is also why the US ships had a very pronounced sheer running from fore to aft, to shed any water shipped aboard. Very few ships today are built without flare on their bows (topsides, I mean, not forefoot). Pounding (slamming as you call it) is best combated by changing course slightly or slowing down. Even the British ships couldn't steam at flank speed into a substantial swell. I also feel compelled to point out (all in fun, of course) that when the British ships were serving with the US Pacific Fleet, the fuel economy and endurance of HM's ships were inferior to that of the american ones. Ship design is always a trade-off.

But I don't want a tit-for-tat US/British discussion here. Clearly, as in the case of the US ships, the desired result wasn't always achieved. However, ships on both sides of the pond were built with this style of front end for long after the war, both commercially and militarily. In the US, the Navy has only recently begun using the bulbous bow with the last two Nimitz class carriers. Testing has shown that the sonar bulges on the bows of some ships (which are under the baseline) do not have any effect on efficiency, so those aren't considered bulbous bows.

Gotta run now, but I look forward to hearing your opinions further!

Cheers!

[Foeth]

I would like to see your information on this, though again, as I said in my post, surface tension is a much smaller aspect compared with the bow wave produced by the bulb.

You are flat wrong. Surface tension is not a factor in ship resistance. The bulbous bow cannot reduce what does not play any part. So, I'm not the one who should be delivering information for this claim.

I also feel compelled to point out (all in fun, of course) that when the British ships were serving with the US Pacific Fleet, the fuel economy and endurance of HM's ships were inferior to that of the american ones.

which says suprisingly little on the resistance of RN ships and added resistance due to motion. The engine part is rather important too. Plus, endurance is mostly determined by the amount of fuel you carry.

[chuck]

I'll look into it, many Japanese ships possessed this hull form as well, and I've often wondered. Today, ships with a very rounded forefoot are for ice breaking, but they're not like knife-like as the warships were, so I don't know.

All IJN capital ships prior to the Yamatos and the Shokakus had the forefoot cut away. According the the US Technical Report to Japan, report # S-01-3 Characteristics of Japanese Naval Vessels - Article 3, Surface Warship Hull Design, this feature is sometimes referred to as a Maierform. Conversely, the Yamato bowform was said to reduce "the ship's resistance" (I assue they meant water resistance) by 5-6%

If you look more closely, you would find the German and Japanese forefoot cutaways are very different.

On German capitalships of WWI era, there is a sharp knuckle in the profile of the stem just below the waterline. At the waterline the stem is sharp and near vertical, below the knuckle, the entire forefoot is cut away in a triangular fashion. More importantly, the stem below the knuckle is blunt. Also this type of cutout seems to be associated with a underwater torpedo tube located just below the knuckle. I would guess this design is prompted by the needs of an underwater torpedo tube at the bow.

The circular cutaway shape of the lower stem of the Japanese warships, Marierform as you called it, follows a large radius curve that turns smoothly from a near vertical cutwater into the horizontal keel. The stem is sharp all the way from the cutwater to where it meets the keel. I would guess the function of this circular cutaway stem is to reduce resistance and slamming in heavy seas by always presenting a sharp entry at the front of the ship even if the bow comes entirely out of water.

Also If you look at the forefoot of the so called ram bow of early British dreadnoughts, you would find that beneath the forward projection there is also a circular sweep back of the stem to meet the keel like those on Japanese warships. This circular forefoot profile in fact made its first appearance in the Japanese fleet on board transitional pre-dreadnoughts purchased from Britain right after Russo-Japanese war.

[chuck]

Hood appears to have one but that is in fact a continuation of the built-in bulge that carries up to the bow.

According to the drawings in AOTS, the bulging shape of Hood's integral torpedo blister appears to subside completely abreast the B turret and was not contiguous with the bulge near the bow. The bulging shape near the bow appears to start at around the breakwater forward of A turret.

[Lesforan]

It would apear to me that, given the ability of longer hulls to achieve greater speed on the same horsepower, that a non-protruding bulbous bow would alow for a "longer" hull surface without increasing the overall length of the underwater hull.
Maybe this is a factor in their application?

[chuck]

It would apear to me that, given the ability of longer hulls to achieve greater speed on the same horsepower, that a non-protruding bulbous bow would alow for a "longer" hull surface without increasing the overall length of the underwater hull.
Maybe this is a factor in their application?

No.

The effect of neither a protruding bulb nor a "flared forefoot" has anything to do with increasing effective length. Their effect is simply to generate a secondary bow wave that would be partially cancel out the primary bow wave of the ship.

[Lesforan]

Thanks, Chuck.

I'm still learning. This forum is a great place to do that.

[gerinc]

I would like to see your information on this, though again, as I said in my post, surface tension is a much smaller aspect compared with the bow wave produced by the bulb.

You are flat wrong. Surface tension is not a factor in ship resistance. The bulbous bow cannot reduce what does not play any part. So, I'm not the one who should be delivering information for this claim.

That may be so, that doesn't mean that I'm not interested in seeing what information you have on the subject, if you wouldn't mind.

which says suprisingly little on the resistance of RN ships and added resistance due to motion. The engine part is rather important too. Plus, endurance is mostly determined by the amount of fuel you carry.

My comment about the RN ships' endurance had nothing to do with the shape of their hulls, but was, as I said, in good fun.

cheers

[Foeth]

That may be so, that doesn't mean that I'm not interested in seeing what information you have on the subject, if you wouldn't mind.

I don't mind, but there is just no information. Nobody bothered to check that influence because anyone sufficiently knowledgeable to do such studies would know surface tension doesn't play any effect in the resistance of a ship (which is a nice way of saying that anyone who would want to determine this influence doesn't know what he's doing). Surface tension waves only play a part in really really really small waves (Say 8/12 inch or 1.71 cm) and that wave lengths contributes absolutely nothing to a ships wave resistance. This wiki page doesn't seem to be half-bad. Surface energy is depending on the curvature of the liquid-gaseous interface. This curvature is high for drops and beads of water, but quickly goes to very small values for larger surfaces. In a glass of water, you can see the edges going up slightly, but not a huge curvate in your glass.