Speed Thrills II - Max Speed of the Iowa Class Battleships

By Tony DiGiulian
Updated 08 November 1999

Since I did my original essay on "Fastest Battleships" awhile ago, I've heard at least twenty claims that the Iowa class battleships could achieve even higher speeds than what I have in the essay. In about ten of those cases, I've gone to the trouble of tracking down the supposed source of the claim. In every case that I've investigated, there either proved to be no such source, the book in question made no such claim, the person was conveniently not available, or (my personal favorite) someone mistook a mile-per-hour value for a NAUTICAL-mile-per-hour value (the Iowa's designed top speed of 32.5 knots is 37 MPH).

In the hope of reducing the number of such claims that I get in the future, I decided to spend a little time determining:

"How fast could the Iowa class battleships really go?"

The Iowa's power/speed curves are reportedly classified and are not available for calculations of this nature. And, unfortunately, the two incidents that I am aware of where an Iowa class battleship made around 35 knots both occurred in relatively shallow water immediately following a refit. As such, the speeds and SHP attained are not really applicable to their deep water performance. So, I have had to fall back upon the results of the powered and unpowered model basin tests and calculations performed in the 1940s (deep sigh). Not nearly as satisfactory a source as I would like, but, still, perhaps illustrative enough of these ship's performance.

There is some data on these tests and calculations available in several publications, including "US Battleships" by Friedman, "US Battleships 1935-1992" by Garzke and Dulin and "Iowa Class Battleships" by Sumrall. In addition, there are several speed/power/displacement figures given for these ships in these three particular publications. I have used this data in standard computations to derive predicted performance.

From the model testing, the Navy calculated that at a Trial Displacement of 53,900 tons (which is about 2,000 tons less than their 1988 fit) a speed of 32.5 knots @ 212KSHP (Designed SHP) could be accomplished.

The Iowa's were constructed so as to permit a "designed overload" of 20%. This means that they could generate 20% over their designed power rating of 212KSHP without fear of damaging the engines. Based upon this and the results of the model testing, the Navy theorized that a lightly loaded Iowa at 51,000 tons could reach 35.4 knots at 254KSHP (Please note: This is generally accepted as the maximum credible deep-water speed for these ships).

Now, the Navy also estimated that every 1,000 tons less in displacement would give an additional 0.25 knots of speed. If I can "cheat" by saying that I've offloaded every last piece of unnecessary gear to reduce my displacement - after all, who needs those silly Harpoons, anyway? This is a big-gun battleship, not a DDG - then I can reduce those numbers a bit. The above mentioned reference sources have a good deal of equipment weight data which I have used for these estimates. Semi-reasonable offloads (my definition, I'm not going to cut away the foremast just yet) could reduce the displacement down to 48,900 tons (see the Appendix below for details). This is 5,000 tons less than Trial Displacement, which would imply a maximum speed of 33.75 knots @ 212KSHP. My calculations seem to show that this displacement would equate to about 36 knots @ 254KSHP.

Now, could these ships go faster? If we accept that we might just damage those engines, then it seems plausible to me that 10% above the overload SHP figure would be attainable (Note: This is known as "forcing the engines" and is not recommended for those aspiring to the higher ranks). So, let's work with 280KSHP.

A problem with this number is that it implies 70KSHP/prop. In the real world of 1940's prop design, this means that cavitation is going to happen in a big way. For that reason, I would guesstimate that 25% of this additional power is simply wasted in generating air bubbles around the blades. That being the case, then at a Trial Displacement of 53,900 tons, a tad over 34 knots would appear to be a top number. For a Light Ship Displacement of 51,000 tons, 36 knots would seem to be possible. However, there's another, real-world, problem with this much power. Even if the boilers could generate this much steam, trying to channel it into the engines may be a problem. The engines are probably not designed to handle these kinds of power inputs. Since I've never inspected the boiler/engine rooms myself (they were sealed on my one visit to the USS Wisconsin) nor have I been able to question the engineering people involved, I cannot make a judgment in this area.

Are there other ways to get these ships to go faster? If you want to "cheat" even further, you can imagine that you give the bathers along the Delaware coast a real thrill and operate the speed trials much closer to shore than is customary in order to get a bottom effect advantage. You can also get a slight increase in speed if you fantasize that the Atlantic puts the proverbial millpond to shame on the day the trials are performed. I will leave those kinds of concepts in the misty world where they belong.

Just for comparison purposes, I have dug up some of my notes on other nation's fast capital ships for which speed/SHP curves are not classified - as they both happen to be war losses. Namely, the German KM Scharnhorst and the British HMS Hood. The question is how much power would it take to drive these ships to 37 knots.

Scharnhorst at a displacement of 38,950 metric tons (38,335 tons) (source: "German Capital Ships" by Whitley):

10KSHP = 14 knots
20KSHP = 18 knots
40KSHP = 22 knots
80KSHP = 26 knots
160KSHP = 30 knots

Note that every doubling of SHP translates into an additional 4 knots of speed. As a very simple analysis, if I simply use this factor and ignore the loss of efficiency from cavitation effects (as we're getting into the 60KSHP/prop range) and ignore the steeper hull resistance calculations needed for speeds above 30 knots, then this would imply that it would have taken 320KSHP for the Scharnhorst to make 34 knots and about 500KSHP for her to make 37 knots. Quite a force job there, if I may say so. In actuality, 31 knots was about the top end for these ships, as in real life you simply can't ignore physics (except in high school).

OK, you say, but that's for a ship considerably shorter and weighing in at about 20K tons less than the Iowa. Fair enough, so, how about looking at a ship a little closer in size, like the mighty HMS Hood?

Trials run in the 1920s at 44,600-45,000 tons (source: "AS: HMS Hood" by Roberts):

8,735 SHP = 13.2 knots
14,020 SHP = 15.8 knots
24,720 SHP = 19.1 knots
40,780 SHP = 22.0 knots
69,010 SHP = 25.7 knots
112,480 SHP = 28.4 knots
150,220 SHP = 31.9 knots

Note that it takes about a tripling of horsepower at 15.8 knots to add six knots but it only takes about a doubling of horsepower at 25.7 knots to add six knots. In fact, when plotting this via a curvefit program, the power required rises at a steady rate, but then suddenly drops off at the end. Interesting.

Sorry for the wandering, it's the engineer in me. Getting back on track:

Using these numbers as a basis, then it would imply (again, ignoring all the usual suspects) that if we could only get those feisty engine room chiefs to give us a miserly 250-300KSHP, then we could sip our sherry on the bridge with a 37 knot gale keeping away the sea gull droppings. Is that too much to ask of those Jolly Jack Tars and superior British engineering technology? Sorry, but in my inexpert opinion, I'm 'fraid that it is.

Note: The forgoing was meant to be a very rough look at a serious subject and I've tried to keep the concepts and mathematics simple. In reality, a good deal of calculus and a great deal more time than I'm willing to spend is needed to do these speed/power calculations properly. My apologies if I have offended anyone's sense of propriety.

And, as long as I'm at it, let me just say that I am not a nautical engineer. I am simply a fairly well-read amateur with reference books and calculator at hand. I cheerfully admit that my power numbers may be 10, 15 or even 20% off in either direction. If someone wants to perform a more rigorous mathematical study of this subject, then I would be happy to discuss publishing it here at our Technical Board.

A Postscript:

The Iowa's were unquestionably the fastest and quite possibly the most powerful battleships ever to put to sea. To argue whether these fine ships could make 33, 35 or even 37 knots doesn't really make that much of a difference, they could have run down - or run out of fuel - any other capital ship ever built - and quite a few of the fanciful ones, as well. And, having run them down, faced no worse than even odds in a one-on-one battle with them. As battleships go, the Iowa's were fairly close to an ideal.

For further information, may I suggest a visit to the Warships1 website showing the current status of these ships:

Status of the Iowas

For a look at what the Iowa's could do today, please visit the United States' Naval Fire Support Association (USNFSA).

USNFSA Website

Appendix I:

What were my "semi-reasonable" off-loads?

The USS New Jersey's weight table of 1943 showed a Full Load Displacement of 58,132 tons. 45,155 tons of this is basic structure and is not subject to reduction per my definition. Offloading parts of the remainder:

70% of fuel oil: 5,659 tons (in reality, this would not be practical)
80% of munitions: 2,074 tons
36% of crew: 104 tons (The Navy estimated 250 lbs. per crewman, incl. equipment)
33% of Reserve Feed Water: 164 tons
80% of all stores: 1,179 tons
100% of aeronautics: 52 tons

Total Reductions: 9,232 tons

"Semi-reasonable" Light Ship Displacement: 48,900 tons

Appendix II:

Other maximum speeds achieved.

1) When the Iowa and New Jersey attempted to run down the fleeing Japanese destroyer IJN Nowaki near Truk in February 1944, both ships reached 32.5 knots with the throttles wide open, according to the Iowa's pitometer log.  With clean bottoms, they probably could have gone a bit over a knot faster.  There is no mention of what was the SHP or the displacement on that day.

2) During her Korean tour, the Iowa's Captain William Smedberg remembered getting his ship over 33 knots on at least one occasion.  There is no mention of what was the SHP or the displacement on that day.

3) Before her Vietnam deployment the New Jersey obtained 35.2 knots at 207 RPM during machinery trials.  There is no mention of what was the SHP or the displacement on that day.

4) In 1985 the Iowa slightly exceeded 32 knots at 205 RPM.  There is no mention of what was the SHP or the displacement on that day.


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