# Nhp



## A.D.FROST (Sep 1, 2008)

(net horse power)What is it?This is usually stamped/quoted on the rubber stamp in your discharge book along with the ships name etc.Why


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## david freeman (Jan 26, 2006)

*I believe*



A.D.FROST said:


> (net horse power)What is it?This is usually stamped/quoted on the rubber stamp in your discharge book along with the ships name etc.Why


The NHP was the basis of an engineers certificate of competancy. Qualify sea time for a 2/e or c/e certificate had to be completed on ships of over a certain total main engine horse power NHP as registered_ Not the agregate of auxilary plant and main propuulsion plant.
The 3/e and 4/e Chiefs endorsement is a new fangled arrangement for ships with lesser horse power, and basically restricted geographical limits- the old home trade limits crudely, and may not count as watch keeping time, only as a J/E for deep sea worldwide trading vessels, when doing ticket titime for the superior C/e and 
2/e's tickets. Note I am quoting the British regs not others, when I quote home trade limits? so things may differ around the globe. .


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## Chillytoes (Dec 9, 2006)

NHP actually refers to an archaic term - Nominal Horsepower. 
To quote one definition, 'A value of engine power calculated from engine dimensions and steam pressure; it bears no real relationship to the actual power developed by an engine but was an early method by which engines were compared.' 
There are various formulas for NHP, some incorporating such values as 'circular inches' of the cylinders, etc. The term continued to be used as a guide for the inspection of engines by a surveyor and the charges associated with the inspection. 
Although originally used in association with steam reciprocating engines, there can be found equally irrelevant examples of NHP for diesel engines.


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## Graham Wallace (May 6, 2006)

Chillytoes said:


> NHP actually refers to an archaic term - Nominal Horsepower.
> To quote one definition, 'A value of engine power calculated from engine dimensions and steam pressure; it bears no real relationship to the actual power developed by an engine but was an early method by which engines were compared.'
> There are various formulas for NHP, some incorporating such values as 'circular inches' of the cylinders, etc. The term continued to be used as a guide for the inspection of engines by a surveyor and the charges associated with the inspection.
> Although originally used in association with steam reciprocating engines, there can be found equally irrelevant examples of NHP for diesel engines.


My Goodness , I remember learning that formula 1955/56 when an Eng App.

NHP = 'PLAN' divided by 33000(?). P-Pressure psi, L- length of stroke in Feet!.A area of bore, sq inches. N- rpm
If not quite correct is pretty close (I hope), after all it was 60 years ago.

For my Second's Steam ticket orals in 1962 was asked ME steam recip questions, never knew how I passed, never sailed on one!

Graham


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## Chillytoes (Dec 9, 2006)

Graham, almost but no cigar!
The formula you quote is for 'Indicated Horsepower', not Nominal HP. 
In the IHP formula, P is mean effective pressure (determined from your indicator cards); L is length of stroke; A is piston area and N is number of power strokes/minute. The 33000 is to handle the mix of feet and inches, as you note, whereas the metric formula, since it is all in metres, is similar but with no divisor. 
When calculating the Indicated Horsepower of an engine, you also have to take into account the number of cylinders and if it is double acting.
In relation to Nominal Horsepower, one simple equation (and they could be quite complicated!) is (D x S)/X; where D = cylinder diameter; S = piston stroke and X = 15 for compound engines, 12.5 for triple expansion engines and 10.5 for quadruple expansion. This illustrates that for NHP, the pressure in the cylinder is not the average throughout the stroke, but only the initial pressure. IHP, a measure of the power developed by the engine before allowance for friction, etc was deducted, and could be anything up to 5 times NHP.


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## Bill Morrison (May 25, 2013)

This is a very intriguing formula. If you try looking it up in text books there is no mention of it. I have tried the web and the best I found was this. www.paxmanhistory.org.uk/nhp-clefn.htn It would seem only James Watt knew how it works.


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## Chillytoes (Dec 9, 2006)

Bill,
If you are interested, you can look up Seaton's 'A Manual of Marine Engineering', where you'll find a couple of pages about NHP. My copy is 7th edition of 1888. I also have a small book called 'Definitions and Formulae for Students (Marine Engineering)' by E. Wood, printed in 1939 which has 7 different formulas for NHP. All ancient history, I'm afraid.


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## Bill Morrison (May 25, 2013)

Hi Chillytoes. Thank you for the information you have obtained. I went to the Gutenburg Project which is a great site for downloading old out of copyright books of all subjects. Under Mechanical Engineering and Machinery. Technology (bookshelf). I could not find Seaton's publication but he was mentioned in a reference in another publication. The work by John Bourne is there with reference to N.H.P. which when you read it, it would seem to be irrelevant to our understanding of engine power. The dictionary meaning of nominal is given as : in name only, very small in comparison with real worth: so making a comparison is not practical. The other publications in this section are worth a brows if you are interested the history of Heat Engines.

Regards
Bill


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## Graham Wallace (May 6, 2006)

Well, you live and learn, hopefully at my age more of the former and not so much of the latter. My memory is failing after all.

I'll have to dig out my old 'McGibbons', see if I can find it there. 

Graham


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## David Campbell (Mar 11, 2008)

I am in NZ visiting our daughter and today we went to the N.Z National Maritime Museum Auckland and spent a few very interesting hours there.. One of the exhibits was a model of the Canterbury Steam Shipping Company, SS Breeze.
On its plaque , it stated that the HP of the Engine was 84 NHP 500 IHP.

You can view the Ship here.
www.nzmaritime.co.nz/breeze.htm


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## Chillytoes (Dec 9, 2006)

Bill,
I had forgotten my copy of Bourne's "Handbook of the Steam Engine" (1865) which has a bit about NHP, along the lines we have been discussing. I probably forgot about the book as it is in such a bad state and waiting for re-backing. Another source is "Rudimentary Treatise on Marine Engines and Steam Vessels...." by Robert Murray (1853). Even at that early stage he notes the distinction between NHP and IHP to the effect that the former 'fixed and nominal, by which engines are bought and sold; the other fluctuating, though real, as shown by the indicator.'
In the NHP formula, the pressure was always 7 psi, as determined by Watt and this figure was used by manufacturers on the Thames, but on the Clyde they used 7.5 psi, so any value of the system in comparing the sizes of engines was often lost. 
There's no doubt that some of this old stuff can be very interesting while a degree of tedium has also to be waded through.


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## Graham Wallace (May 6, 2006)

I finally found the box that all my remaining Marine Engineering books have been stored in......after all the sea might call me after 60 years

NHP, Nothing in Southerns , nothing in McGibbons Orals , but there was something in McGibbons "Marine Engineers Pocket Book"reprint 1950

I hope I managed to copy both NHP and IHP pages and attached them below ( after a great deal of struggling to resize them)

That's me exhausted. I have reached my limit of "Peter's Principle"

Graham


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## Jim S (Jan 21, 2006)

As has been said Nominal Horse Power has no direct correlation with actual horse power either Brake Horse Power or Shaft Horse Power.
The following examples from my discharge book ;- 
Brocklebank's Magdapur NHP 1226 SHP 6800, - Mangla NHP 1294.64 
SHP 7250
For Maipura NHP is given as 1380 unfortunately there is no SHP given but I am pretty sure Maipura was less powerful than Mangla.
All these ships were steam turbine driven.


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## Chillytoes (Dec 9, 2006)

Here's another formula for NHP(oil engines). Not sure where I got it but found it in one of my note books from years ago.
NHP (oil engines)
N = 0.45I(D - 1.18)(L + D)
where I = Number of cylinders
D = Cylinder diameter in inches
L = Stroke in inches


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## Chillytoes (Dec 9, 2006)

And still more formulas....


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## Chillytoes (Dec 9, 2006)

And more....


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## Chillytoes (Dec 9, 2006)

The last one, I promise!


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## Bill Morrison (May 25, 2013)

*More NHP*



Chillytoes said:


> And more....


You beat me to it with one. I was searching Lloyds 1898 list for a sailing ship and in the lead in pages found this, which seemed out of place.

Bill


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## Chillytoes (Dec 9, 2006)

Just to close the discussion, I found this abstract in an old IMarE transactions...

*The Meaning of N.H.P. in Lloyd’s Register*
“The Motor Ship”, September, 1936.
Although in Lloyd’s register the power of all vessels is listed therein, as regards ships fitted with steam or Diesel machinery, is given in terms of N.H.P., or “nominal horse power”, the figures do not represent the machinery output as it is usually understood by engineers. For this reason we have taken exception to the retention of this symbol in the Register, without, however, discussing the reasons why Lloyd’s find it necessary to use it. Briefly, then, N.H.P. in the Register represents a figure which is a measure of the amount of work entailed in the surveyor’s inspection of the machinery of a ship. It affords a basis for computing the fees to be charged for the survey and not the actual power of the engine; or anything like it, for that matter.
Lloyd’s have fixed the following formula: -
(N.H.P.) = C x N x D²
¯¯¯¯¯¯¯¯¯
S/D + 50 where
N = No. of power cylinders
D = cylinder diameter in inches
S = piston stroke in inches (half the combined stroke for opposed piston engines)
C = 5 for four-stroke single-acting
10 for four- stroke double acting
9 for two-stroke single-acting
18 for two-stroke double-acting
16 for opposed piston machinery
The resulting figures give a “nominal” power which is quite probably not even one-quarter of the real output.


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## david freeman (Jan 26, 2006)

absolutely intriguing, I will go to the foot of our stairs for inspiration. Good job we had the French and continentals teach us the metric system, both in fact and monery wise 10 units the pound, franc or ducthmark- spelling here!. I am but a simple engineer what goes up comes down and what goes around comes around?


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