The Offshore Voyaging Reference Site

Understanding Power and Torque

My favourite class in high school was physics, and it helped that it was relatively easy for me, but when we got to learning about torque I suddenly hit a subject that I really couldn’t understand, at least if there was any motion to the parts. 

During college and grad school I finally managed to wrap my head around torque, but it wasn’t until I got my first job and had to work with it regularly that I really got it.

This two-part series is intended to help people understand power and torque and how to apply it as a sailor. 

If this subject makes your head spin like it did mine, the good news is that, unless you want to design a lot of rotating parts, we don’t absolutely need to understand torque, particularly to go voyaging, other than to use a torque wrench properly.

But at the same time, it is a somewhat frequent topic in conversations, often with incorrect use, so it warrants demystifying (Part 1) and then going through when and how to use it (Part 2).

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Rob Gill

Interesting morning read thanks Eric…two cups of tea needed. Looking forward to Part 2, but first a question as your have challenged my memory and understanding, from long ago.

When I was a youngster, we would retrieve our heavy day boat moored in a river running off the Bristol Channel with a trailer at the end of each season. The plan was always to get this done on high tide, the problem was as the tide came in the sand could go soft, and the car wheels easily bog down. And many a vehicle was swallowed by a rising tide (up to 12 metres).

We had an old 4WD Landrover with large off-road tyres, but even this could get its wheels bogged down, and we would have to deflate the tyres to get out of the holes they dug. As we got older, we got to drive the car and were instructed that once the boat was on the trailer, to start off in second gear (not first gear) to keep the wheels from spinning – “same power, but more torque”.

So if I now understand the lesson above, yes the engine would have relatively more torque, but where the wheels met the sand, there would be less torque? And so would the reason this worked practise that there was actually less torque at the wheels for a given amount of engine power applied, and so less likely to spin the wheels?

Damn, I need another cup.

Rob Gill

Thanks Eric, yes the Landrover was a manual and the clutch took longer to engage from memory, and the take off was smoother if we used more accelerator. I hadn’t ever considered the benefits for this “hack” on the engine side of the gearbox.

William Murdoch

Great job. Marketing departments and magazine product reviewers stand back…Truth is here. All should read carefully.

Now the comments.

I’m in the USA, also an engineer. BS MS and PE. I can work in either of what used to be called Imperial Units and now seems to be called US Customary Units, or I can also work in SI. It is hard to explain one system’s foundation on a force (lb-force) and the other’s on a mass (kilogram). Confusions run wild. Just try to explain g[sub]c to an European. I’ve tried. It’s hopeless.

Cross product and dot products… you are indeed brave. Ft-lb, lb-ft, or whatever, it can be either. My wife is an English major. I would never attempt it. You did.

I would have emphasized that a force times a distance is work and a force times a velocity is power.

I also would have run out the units to produce the 9550 and 5252 conversion constants.

Great job. I love the psi on the square inches of piston head gives (knowing the gearing and wheel diameter) torque on the wheels and thus force against the asphalt. I pity Road & Track.

I’m waiting for propeller curves.

David Eberhard

We too also used second gear to avoid spending the wheels on dirt and gravel roads while pulling our travel trailer as a kid.

Now I understand the old saying the horsepower gives you, speed, and torque gives you acceleration.

Thank you, Eric

Iain Dell

I always try to educate myself on the fundamentals of any system, as that enables a much clearer understanding of what’s really happening when I press that button or pull that lever. Eric’s explanations are perhaps the clearest I’ve ever come across and are very much appreciated – many thanks. He should author a book: ‘Physics for Dummies Sailors’….

Rene Blei

Thank you Eric and John
First a reply on John’s comment : Most successful entrepreneurs never saw the inside of a university, however, university students with high marks, usually make good employees.
We all know of high torque engines as opposed to race engines. The high torque ones have a longer stroke, which gives them a longer arm on the crankshaft. Race engines have their power in the higher rpm range and as such require many gears in the transmission. The bore is often larger than the stroke.
Did you know that a rotating tire at the moment when it touches the pavement
it stands still, but the top of the tire has reached twice its “speed”. This becomes more clear when observing a tracked vehicle.
Life is a learning process and for those it is not, means having 20 years the same year experience.

Rene Blei

Thank you Eric for explaining it one step further and taking the time to answer and for the many hours it took to write your well written article !
You use one word John has also used numerous times…
It always fascinates me when when watching an idling diesel engine without a valve cover and see those valves, opening and closing in a split of a second and letting enough air into the cylinders and producing that massive power. Before Rudolf Diesel came along, sailors also used that same air to fill their sails, but unlike the diesel, that air stayed clean. However you can’t put a sail on a super tanker and expect to go anywhere.
Another trade-off.
Talking about torque, we are told electro-motors produce a lot of torque especially at low rpm. Also its efficiency, 90%plus compared to the 35% of a diesel. Too bad we can’t use those more efficiently for mobile purposes (yet), as batteries leave behind a large carbon footprint, compared with the BTU it produces with that in a gallon of diesel, taking into account the weight difference, and again we come to another trade-off.
Diesels have become extremely reliable and also extremely important in today’s economy………….until, all of a sudden, the clicking sound of those valves becomes silent and all turns dark and a few seconds later a billion $$ bridge collapses. “Luckily” it happened at midnite, not midday and 6 “anodes” lost their lives. Many more would have lost their lives, if it wasn’t for those 6 construction workers, but because of their presence, police cars were taking pictures of speeding cars in this construction zone and as soon as the Mayday call came in, they managed to stop all traffic as we watched the last truck making it safely across with only seconds to spare
Not unlike another “anode” who sacrificed his life many years ago, to save countless others.
Blame my parents for giving me the name they did, meaning “born-again”.
Now I better quit, before the boss kicks me off the ship.

Stein Varjord

Hi Rene,
I’m going off topic, but a couple of notes to your nice comment:

– The carbon footprint of batteries. With lithium, it has been significant, and still is so for the chemistries used in consumer electronics. Even worse is the use of various rare earth minerals with highly questionable sourcing. LiFePo4 has taken over in new EVs recently, and is the only smart lithium alternative in boats (at the moment). That uses none of the rare earth minerals, less lithium and has a dramatically lower footprint, as well as at least twice the life expectancy. Also, recycling the materials is easier and getting better by the day.

Then there are lead acid batteries. They are far from outdated. They get close to 100% recycled most places and have a dramatically smaller footprint. No matter what chemistry, batteries have a far smaller footprint than any combustion engine and its supply chain.

– Sails on super tankers. If boats, cars and more used sails or electric power etc, we wouldn’t need any super tankers, so their unsuitability for sails is totally fine. 🙂

– Diesels have become extremely reliable. You mention an example of that being less reliable than we usually think it is. I have another: I work with tourist roundtrip boats on the Amsterdam canals. Today they’re all electric. About 6 years ago I participated in repowering 3 boats from diesel to electric. They had been run many years before the repower and now after it.

These boats usually run 7 days a week 365 days a year and typically about 8 to 15 hours per day, with varying skipper competence and care. Really heavy use. The diesel configuration had a biweekly service interval at a mechanic and would still stop working during the day about once a month. In the electric configuration, they get service on the power train once a year, just because they’re up for general service anyway, and have not failed in action even once, after many years of extremely much heavier use than any private boat will ever see.

The total running costs, also including a new battery bank after about 5 years (flooded lead acid), is close to 90% lower than the diesel configuration. With lithium it would be even a bit lower cost, due to a much longer service life, but more complex, probably more vulnerable and outside of the competence of the companies. Their main reason for moving to lithium, which some are, is to get shorter charging times so they can run the boats more hours per day. There’s about 600 such commercial boats here now.

There’s no doubt whatsoever that battery electric boats will gradually take over most of leisure boating quite soon. In Amsterdam centre, combustion engines will be illegal from next year. One can apply for a transitional permission for newer engines until 2030, but then it’s final.

Also worth mentioning that EVs had 92% of all new car sales in Norway last year. Next year they expect close to 100%. Fossil cars have no bans or such. It’s just that most people have tried an EV and just prefer it. If that’s possible with the nature, climate and widely dispersed small population of Norway, it proves no country has an excuse. Same thing will certainly happen with boats. I think it won’t be primarily because of legislation, but because it’s just better. Diesels etc will soon be seen as similar to steam engines.

There are several developments that have to happen first. The hardest change will be the one in our heads. 🙂 We need to think a bit differently, initially.

Stein Varjord

Hi Eric,
Good points. I won’t drag this out much further here, but to confirm your words: The tourist canalboats I mention have motors rated from 10kW on the smaller ones up to 60kW on the bigger ones. Most of them have 20kW. This is plenty powerful enough for this use and feels a lot more punchy than the kW indicates, since the response is so immediate. Very nice for manoeuvres around the narrow canal corners.

However, the power drain from the batteries while at normal cruising speed, just under 4 knots (just over max allowed speed, 6km/h), is typically around 2 kW for the 20kW versions and 4-5kW for boats of 18 meters (60 feet) and 50 metric tonnes. These boats have far from perfect hulls. The big ones are often over 100 years old. It’s just the effect of a very efficient propulsion system and, more importantly; low speed. Increase the speed by 1 knot, and the power drain usually doubles, or more.

This means electric is brilliant for motor sailing through “zero wind” belts. Let it run on 1kW or less and we’ll be able to use the wind that is there much better.

Rene Blei

Hi Stein, good to hear from you again.
Thank you for your comment.
My remark to put a sail on a supertanker was obviously the wrong choice of ship.
Appreciate your comment on lead acid batteries, as I prefer to be old school and as such am also a vintage car collector and most of them still with carburettors, but realise for you to hear that, may give you a heart attack.
Holland is probably the ideal country for EV, for the same reason CND is not. Sales here, after the subsidies have stopped, are dropping, too expensive, and our winters are too cold and distances too long. Hertz Rental got rid of them all, also too expensive to operate and too many break-downs.
For your A’dam canal boats e-motors are no doubt the ideal way to operate those.
As for Norway, your home country, my compliments on the way they had the discipline to built-up that massive oil fund, instead of wasting it all by putting it into general revenue and squander it. However, your winters may well be similar to ours?? So why are the sales figures of EV so different in Norway and CND? That Trillion $$ oil fund may well have something to do with it.
You state EV are just better, and yes, I agree, but only if it fits the circumstances. For Canada diesels will have a bright future, probably in combination with hydrogen to achieve a complete combustion. The high compression ratio of a diesel is too high for hydrogen.
As for our climate, is mainly influenced by the SUN and managed by the SON, so out of our hands. Volcanic activity and forest fires alone, have largely nullified the trillions spent on Climate Change. However, technology is in our hands and who knows what tomorrow will bring. People in general are apprehensive re: changes, babies on the other hand, welcome it !