Formulae for Gurus


To kick off with, I found it intrigueing to see just how many advertisments are written about saving electricity by using “manufacturer-x’s” power saving device.  Consumers should be aware that as a household electricity subscriber you do not fall under the same tariff block as the industrial sector therefore it is more than likely your electricity usage is gauged on wattage i.e. true power as opposed to apparent power or VA.  VA rating can be changed by adding an inductive or capacitive network onto the phase or phases of your source – this corrects any lagging or leading current. As domestic power is a true power measurement, the power factor cannot be corrected.  If you want to read and understand a very decent article about power factor correction then go to this link: and then the continuation:


What I like about this website is that it gets straight to the point. As one of the senior forum members wrote:  

Finally, when it comes to any circuit, analog or digital, always remember that performance will ONLY be as good as the Power Supply“.  This is actually so true of computer enthusiasts the world over spending so much money on their board, graphics card and CPU that they place little significance to the PSU.  And no, most aren’t design engineers, but they can be rattled by dodgy salespeople with cheap power supplies in stock.

Another good read is the website Daycounter.Com.  They use commonly used formulae in all their pages and one which I found to be very useful is that to build coils of known diameter and length: Note that this for an air core inductor.  Inductance measuring tools are also indispensable. And they are not expensive.  The website has a whole host of calculators: which is very useful to the hobbyist. The Brown & Sharpe or more commonly A.W.G. wire gauges are also listed under the “calculators” page.  Quick word of caution, AWG is not the same as SWG, which is a British measurement. AWG is used exclusively in the electrical industry whilst SWG is a gauge to measure ferrous metals and non-electrical applications.  Solder wire is often given in mm or SWG. SWG = Standard Wire gauge, AWG = American Wire Gauge and BWG = Birmingham Wire Gauge. has the conversion table at which readers can transfer directly to a spreadsheet or program of sorts.


One of the many formulae that changed the world: Resonance.


Moving on to parallel and series LCR (inductances, capacitors and resistances) there are many permutations to get to resonance but none so easy to learn as that above.   The website LearnAbout-Electronics: is a hive of information.  There are many brilliant websites out there covering the same topic but I enjoyed their usage of bright colours to get the heart soaring as you build your first loudspeaker cross-over or coil-gun. (see Barry’s Coilgun Designs: – very informative and some great links).

The cream of the crop in online calculators come from the webside Calculator Edge:  Now what makes this website so interesting is that the user can practically do any calculation known to man. Always remember that most calculations pertaining to engineering are done in a perfect world scenario otherwise it will state the obvious changes required e.g. measuring horsepower is 33000lbs, one foot in one minute. Temperature, ambient or otherwise almost always plays a vital role but what about the drive chain. Unfortunately the drive chain is a necessary evil and the efficiency is never 100% due to friction losses etc hence measurement taken at the wheel being a better measurement such as on a dyno.


Now here is an interesting one:  How does one measure the power of a ship’s engine. I mean large marine vessels utilise huge multi-cylinder engines capable of producing many thousands of horsepower. For example the Maersk line Emma Maersk is a 109 000 HP mammoth. You can’t just put this on a dyno, or can you? One such way is via the torsion meter which essentially measures the turning force at the ships propeller shaft.  But to digress, let’s cheat a little bit and go back to our 33000lbs formula, how was this derived?  Go to wiki and read up on horsepower and James Watt, Scottish genius. I love the Scots, my grandfather was one. . Interesting to note that deriving horsepower is therefore not a very accurate calculation. SAE powers are also a bit ambiguous, very much like the powers we rate our audio amplifiers in these days. Before I continue, another very useful formula is:  Horsepower = Torque (lbs.ft) x RPM / 5250. Bruce Augenstein wrote an excellent document at where he dispels the myths and lays out the truths behind torque and horsepower. Read it.

Bollard Pull: Used in tugboats and smaller vessels – this is a surefire way of checking that pulling power, see Wiki: Larger ships of course use a multitude of measuring devices or sensors to determine torque, RPM, fuel flow etc. Torque and Rotational Speed are two of the most important measurements and whilst torque is measured using bonded strain gauges, rotational speed in RPM is measured through electrical pulses.

 I found this website very enlightening,especially coming from the Marine industry – lots of facts I was not even aware of:  I would really love interested readers from the marine industry, especially engineers to send me more information on how and when they went through sea trials and had the power measured of the vessel they crewed.  I left the shipping industry in 1988 and with technological changes I’m going to be overwhelmed by recent trends.  Having worked in the electronics sector for the last 30 years, now in the computer industry, I see myself more as a bystander than a hands on person. It was my passion, which has now taken second place to management – sadly.

Moving on to something completely different, thrust versus horsepower conversion. Can this be done? In real terms there are lots of factors to consider, one being that Power = Force X Velocity (Force*Distance/Time) and Pa = Ta V / 325 (Pa = Propulsive Power measured in HP, Ta measured in Thrust (lbs), and V is measured in knots or 1852m per hour). Let’s look at a jet fighter for instance where one looks at the following parameters to take off and fly:  Weight, Thrust (the important one by Newton = mass*acceleration), Drag and Lift.  A jet engine is a strange beast because it utilises more power to turn the compressors than the useful output of the engine. But the thrust can be enormous, again mass*acceleration. However I did find a very good article on the Aerospace pages: which covers this topic. Overall this is a very intimidating subject, one left best to the engineers out there but yet, a very, very interesting one.  I’m also interested to know how they can work out the Kilowatt rating of a gas turbine on the power grid. We have three 120MW gas turbines close to where we stay, rather much like an auxiliary backup.  

To move on to something completely different I’d like to cover the website. Actually I won’t cover it, I found it excellent. We all want to learn some thin new every day, this site does it for me.

As an aside to the contents of this page I find I don’t have the time to update this website nearly as often as I want, the last post was a long while back. Although the intention was to get as many readers as possible, we just don’t have the time to generate new material. An old friend contacted me recently through this website and I thought I had better do some updating mainly because I felt I was cheating myself.Cool – I pay money every month to host these pages and would really like to start generating new content – so I’ll start by asking readers of this page to please give me ideas. We are after all a parts exchange website – why it doesn’t work I don’t know. Time is after all valuable, my real job takes up 13 hours of my day, drained by the emotional strain of dealing with day to day problems created by the very people we sometimes employ. You can contact me via the webmaster@ ‘domain’.com address. I would really like also to get some valuable information regarding the maritime industry in 2011.

Thank you for reading this article.