Overload Shaft Horsepower

Propulsion systems, machinery, turbines, boilers, propellers, fuel consumption, etc.
ADeblois
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Joined: Fri Feb 22, 2013 7:04 am

Overload Shaft Horsepower

Postby ADeblois » Sun Feb 24, 2013 6:51 am

First off, what exactly is "overload" shaft horsepower?

Second, why would you want to force the engines above their "rated" horsepower for any length of time? Wouldn't it damage the engines meaning more repair?!?!?

Byron Angel
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Re: Overload Shaft Horsepower

Postby Byron Angel » Sun Feb 24, 2013 2:34 pm

It is a complicated question.

Rated power of any power plant might be defined as that at which it can be continuously operated over a lenthy period of time, usually at an economical fuel consumption rate. That power level is usually specificied in the design phase of the vessel. But powerplants, like almost all other machinery, are to one degree or another built with a reserve of durability as insurance for satisfactory operation over the long term.

Overload power can be described as the maximum power (hence maximum tactical speed) achievable by the powerplant, without consideration of limiting factors. Limiting factors might be ultimate overheatng of bearings in a triple expansion engine, or the need to shut down boilers for grate cleaning in coal-fired ships, or over-heating of a bearing in a propeller shaft in any ship. I don't pretend to be a marine engineering expert; there are probably numerous other limiting factors that might come into play. In any case, overload/maximum power was never intended to be used for anything more than relatively short sprints. For example, triple-expansion powered ships, as a rule of thumb, were limited to perhaps 4 hours at overload/maximum power.

It is also necessary to understand that "overload" power can in many respects be a creature of bureaucratic definition. If you compare the designed power versus overload power ratio of WW1 British battlecruisers to that of German battlecruisers, you will note that "overload" was deemed to be 110pct of rated power in British practice , but something on the order of 140pct in German practice.

FWIW

B

ADeblois
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Re: Overload Shaft Horsepower

Postby ADeblois » Sun Feb 24, 2013 5:14 pm

Thanks for the answer Byron Angel. Your answer is well written and makes complete sense. Although I sense "overload" power would be applicable only to warships, and not so much to merchant ships? The one exception I can think of is the passenger liner/troopship SS United States, which had a steam turbine power-plant rated at 200,000 SHP with a potential "overload" power equivalent to 240,000 SHP. (120% power rating) On her sea trials in June 1952 she made 241,785 SHP (officially) at a trial displacement of 39,900 long tons (~40,000 long tons) which gave her a speed of 38.32 knots.

Byron Angel
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Re: Overload Shaft Horsepower

Postby Byron Angel » Sun Feb 24, 2013 8:08 pm

Having spent some time in the international freight forwarding business, I would observe that steamship lines in the oceanic cargo trades are always and ever, first and foremost interested in fuel economy. Every parameter of cargo ship design is oriented toward achieving the lowest possible cost per ton/mile. On the other hand, the vessels of the old trans-oceanic passenger trade differed. Speed of crossing, to deliver the passengers to their destination as quickly as possible, was paramount. So large ships with tremendous power were the order of the day. Compare the dimensions of Titanic to those of Dreadnought as an example. Such high-speed passenger ships proved to be hugely valuable as troop carriers in wartime. The SS United States, for example, could carry upwards of a division's worth of troops across the Atlantic in a single 6 day voyage. In addition, their great speed made them practically invulnerable to submarine attack. In consequence, governments generously subsidized the construction and peactime operation of such ships in exchange for gaining their use in wartime. Such subsidization programs also granted the goverments influence over their design - another reason why these passenger ships were built with such huge powerplants.

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dunmunro
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Re: Overload Shaft Horsepower

Postby dunmunro » Thu Feb 28, 2013 9:39 pm

Most navies designed some degree of overload potential into their steam power plants, so that if one (or more) boilers were damaged and forced to shut down, then the remaining boilers could be forced to burn more fuel and so produce more steam to make up the difference. Turbines could be also be overloaded and most naval turbines had extra steam inlet valves that could admit extra steam in the secondary stages of the turbine. Additionally many turbines were specified to be able to handle more steam volume than their nominal full throttle figures.

Some examples:

HMS Prince of Wales nominal full power = 110,000shp at 230rpm but PoW produced 134000 shp at ~239rpm prior to intercepting Bismarck.

Renown had a nominal full power rating of 120,000shp but was reported to have produced 160,000shp on at least two occasions (apparently leading to reprimands from their Lordships in the Admiralty).

Special boiler trials were made in (I class destroyers) Icarus and Imogen with a forced draught of 9 to 10 inches. Imogen ran one boiler on 20 per cent overload, 16,000 S.H.P. for three hours. but had trouble with the brickwork. After repairs she ran on the same boiler for two hours at 30 per cent overload, 17,000 S.H.P. without trouble, reaching a speed of 24 knots. Then followed a trial on two boilers at 34,000 S.H.P. In Icarus after the first 20 per cent over load trial the boiler back had to be stiffened up and repairs carrieded out in the dockyard.
(British Destroyers p.319)

Note how the firebrick lining of the boiler furnace was damaged by the excess heat. The I class destroyers had 3 boilers and normal full power was 34,000shp from all three boilers.


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