What's more important - HP or Torque?

Otter

Otter

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Trying to arrange a new engine installation for next year, looking at a range of engine sizes, one is the largest version in the mid range with 58hp and 152 torques, the next is the smallest in the maxi range from the same manufacturer, it's only 12hp bigger but has 226 torques. I don't know what torques are, are they really important? It's £2190 extra and I need to know if it's worth it! The designer specified 35hp to 100hp for the CW 34, currently we have a 1977 75hp Thornycroft that in the foulest weather only occasionally goes to 70% throttle and cruises nicely at 50% throttle. How many HP's has it lost in 45 years? And will it have lots of torques.

I don't know much about Diesel engines but I'm trying to learn quickly!
 
Torque with out doubt. I have 2 older engines in two very different boats one a K3 Kelvin and the other a D3 National both have limited HP but both will pull up tree stumps. My third boat has a D4 Fordson but that is still awaiting being put in the water.
 
Power is generally what matters. You can swap torque for speed or vice versa (power = torque x speed) with a gearbox but you're stuck with the power the engine delivers.
 
Torque comes into it if you are concerned with acceleration - or more particularly where on the rev range the torque is strongest. So if you have a planing boat torque is important to lift the boat above the "hump".

However, on a displacement boat neither the amount of torque or the rev range is so important as the engine operates at virtually constant speed. The engine does not move the boat - the propeller does. Said it many times because it is misunderstood. Determine the optimum propeller size, which is a function of the displacement of the boat, its waterline length (which determines its speed potential) and the amount of space you have to swing the prop. Calculate the optimum propeller size to achieve maximum displacement speed and then calculate the shaft speed and horsepower required to turn the prop.

Typically you will be looking at shaft speeds of around 15-1800 rpm, so the common gearbox ratios are between 2:1 and 2.6:1 with engines that peak between 3000 an 3600 rpm. The slower the shaft speed the larger diameter prop you can swing, but often the ideal diameter is limited by the clearance between the shaft centre and the hull.

The 75hp engine you have is probably a bit too big and you could probably look in the 50-60hp range. However you need to do the sums (or get your chosen engine supplier to do them). If you want to find out more yourself get hold of a copy of The Propeller Handbook by Dave Gerr, or you can put your boat's data into the Propcalc programme on www.castlemarine.co.uk with your chosen engine/gearbox combination to work out the optimum propeller size and speed potential.
 
To answer your question, torque is the twisting force of the engine or its ability to turn the shaft. Horse power is the strength of the engine.
If thinking of the engine as a person and the prop shaft a manual capstan which you stick a lever in to a hole to turn as they did in the olden days, then The HP rating is the size of your muscles the torque rating is the length of your lever. The longer your lever, the easier it is to turn the capstan.
So a smaller hp with more torque may find it just as easy to move the boat as a big hp engine with low torque.
Ideally if you look at the power and torque curves on a graph then you want the maximum torque to be developed at the same rpm as the maximum power.

I hope this makes sense although I'm starting to confuse myself :)
 
cryan said:
Ideally if you look at the power and torque curves on a graph then you want the maximum torque to be developed at the same rpm as the maximum power.

I hope this makes sense although I'm starting to confuse myself :)
Click to expand...

I fear you have confused yourself.
As Jumbleduck wrote, power = torque x speed. From that it's self-evident that peak torque and peak power cannot be developed at the same rpm on any engine that achieves more than 1 rpm, which is all of them. (The one exception would be an engine artificially governed to rev no higher than its peak torque rpm, which none of the OP's options will be.)

Torque and rpm can, however, be numerically equal at the same point in the rev range, but that's just a function of the units used and will only rarely be where either of them peak. To be more specific about Jumbeduck's equation, in imperial units hp = torque x rpm/5252 (the formula in FullCircle's link). This means that power is always numerically equal to torque at 5252rpm and nowhere else. (The torque and power curves will cross at that point.) But since very few yacht engines will rev that high, it's not likely to happen. In kW/Newton-metres, that point rises to 9549rpm, which as far as most of us are concerned, only motorcycle engines can reach. So it's pretty meaningless and only useful as a quick check that, say, a car magazine hasn't got its graphs badly wrong.

What these formulae do tell us is that torque and power are not unrelated entities but are fundamentally and inextricably linked. Increase one at any point in the rev range, and you increase the other (and the inverse, to answer Lazy Kipper's question about loss of 'torques'). They also tells us that in the OP's case, the 58hp engine will be higher-revving than the 70hp lump. As well as its effect on potential top speed, that has obvious implications for gearbox ratios, propellor size and pitch and so on. Which in turn takes us back to Tranona's advice.

One other thing the OP might consider is that peak torque rpm is always the place where specific fuel consumption is least (i.e. best economy/greatest efficiency), although even that simple fact is somewhat muddied in practice by other factors.
 
Thanks for the replies. I've used the psychosnail calculator and it produced a result of 58hp with a hull speed of 7.5knts and a displacement of 13 tonnes. As we usually motor at 5.5kts I guess the extra power is for waves and head wind. The engine we're looking at produces 2.6 torques per hp, but the next one up in the range produces 3.2 torques per hp. I think from the replies so far, that ratio is far from constant so it all depends where in the rev range the biggest ratio is? As for lost hp, the boys at Top Gear are always measuring their 30 year old car buys and finding they've lost 50% of their horses. Is that more of a function of petrol engines? And will I have lost many of my 75 horses in the 35 years and 5000 hours since the engine was installed? Hours is pure guesstimate based on previous owners cruising patterns. The 'lost' is important as I'd like to know what I've got at the moment.
Thanks for the help.
 
Doubt it has lost significant amount of power. A Diesel is not like a petrol engine. The the speed lever (throttle) sets the revs and the power produced by the engine at that revs depends on the load put on it by the propeller, and then the governor provides the fuel. If you can get near to full revs in gear the engine is producing near its full power. As your calculations show, you have more power than you require so the prop may well never require full power.

The advantage of having a bit more power than you actually need is that you can achieve cruising speed at a lower RPM and the extra power is useful in adverse conditions - useful in a big heavy boat like yours. However it is defining the "bit more" that is difficult. If you can achieve hull speed at 75% of power then no point in having the extra as all you can do with it is push against the speed limitation of the hull. The ideal is to be able to achieve hull speed at near maximum revs - so you find recommendations from manufacturers of at least 90% - so on small Volvo engines for example the recommendation is 3000rpm (out of max 3200). That allows you to cruise at around 65-70% maximum (2200-2300).
 
Lazy Kipper said:
The engine we're looking at produces 2.6 torques per hp, but the next one up in the range produces 3.2 torques per hp.
Click to expand...

The is no such unit as a 'torque' as you may have gathered from earlier posts. It's pound-feet or Newton-metres. Regardless, what your figures actually show in a rather elliptical way is that the smaller engine is higher-revving than the larger one, as I said earlier. You probably didn't need to do any sums to find that out.

Lazy Kipper said:
As for lost hp, the boys at Top Gear are always measuring their 30 year old car buys and finding they've lost 50% of their horses. Is that more of a function of petrol engines? And will I have lost many of my 75 horses in the 35 years and 5000 hours since the engine was installed? Hours is pure guesstimate based on previous owners cruising patterns. The 'lost' is important as I'd like to know what I've got at the moment.
Click to expand...

The Top Gear figures aren't especially helpful, since they take no account of how well serviced (or not) the engines might be, or their general condition. I'd put money on any engine which has lost anything like 50% of its power having something fundamentally wrong with it.

Assuming an engine is as per spec in terms of valve adjustment, etc, power loss with age in an otherwise healthy engine is primarily due to degredation of the valve seat and the portion of the valve bearing on it, carbon build-up, injector and injector pump maladjustment in diesels and loss of compression. The latter is easy enough to have checked. Power losses are generally highest on highly-tuned engines, which marine diesels are not, although a sloppy old injection pump can make a huge difference. It's impossible to put a rule-of-thumb figure on this but I'd be surprised if yours, if healthy, is much more than 15% below its new values and quite possibly less.
 
Bear in mind that those winkers on Top Gear are either driving non street-legal supercars or completely shagged out unroadworthy wrecks! Unfortunately, many of our boats fall into the second category... But a diesel that starts well is generally pretty healthy.

Rob.
 
macd said:
I fear you have confused yourself.
As Jumbleduck wrote, power = torque x speed. From that it's self-evident that peak torque and peak power cannot be developed at the same rpm on any engine that achieves more than 1 rpm, which is all of them. (The one exception would be an engine artificially governed to rev no higher than its peak torque rpm, which none of the OP's options will be.)

Torque and rpm can, however, be numerically equal at the same point in the rev range, but that's just a function of the units used and will only rarely be where either of them peak. To be more specific about Jumbeduck's equation, in imperial units hp = torque x rpm/5252 (the formula in FullCircle's link). This means that power is always numerically equal to torque at 5252rpm and nowhere else. (The torque and power curves will cross at that point.) But since very few yacht engines will rev that high, it's not likely to happen. In kW/Newton-metres, that point rises to 9549rpm, which as far as most of us are concerned, only motorcycle engines can reach. So it's pretty meaningless and only useful as a quick check that, say, a car magazine hasn't got its graphs badly wrong.

What these formulae do tell us is that torque and power are not unrelated entities but are fundamentally and inextricably linked. Increase one at any point in the rev range, and you increase the other (and the inverse, to answer Lazy Kipper's question about loss of 'torques'). They also tells us that in the OP's case, the 58hp engine will be higher-revving than the 70hp lump. As well as its effect on potential top speed, that has obvious implications for gearbox ratios, propellor size and pitch and so on. Which in turn takes us back to Tranona's advice.

One other thing the OP might consider is that peak torque rpm is always the place where specific fuel consumption is least (i.e. best economy/greatest efficiency), although even that simple fact is somewhat muddied in practice by other factors.
Click to expand...

Yeah, reading that line again I have not been clear.
Yes Ideally you do want max torque and max hp at the same time but in reality thats not possible. So what your aiming to achieve is the best balance between power and torque.
Power=torque x Speed in the simplest of terms but in reality there are losses from friction and heat. This is why the power curve will drop off at the top of the rpm range.

As per the op question, what is more important? Both are important. But I suspect that the engines your looking at will have sufficient torque to enable the power output to do its job.
 
Thanks for the replies. Compression test was as near new levels as made no odds. Starts first time spring through autumn, steams but doesn't smoke hardly at all. Winter it needs to turn over for 10 seconds or so, deep cold of winter more like 15 -20 seconds as there's no pre heating. We get to 7 knots at 75% revs and the next 25% adds a whole knot!

Why are we changing? Every year we spend £1,000 on the thing, it's money wasted when £6,500 will buy us a new 58hp engine fitted that's a fraction as noisy, a third of the weight, doesn't vibrate like an old Ford tractor, doesn't smell and has a three year warranty. The hp issue is critical as the next size up is 12hp more and £2k more!
 
Lazy Kipper said:
Thanks for the replies. I've used the psychosnail calculator and it produced a result of 58hp with a hull speed of 7.5knts and a displacement of 13 tonnes. As we usually motor at 5.5kts I guess the extra power is for waves and head wind. The engine we're looking at produces 2.6 torques per hp, but the next one up in the range produces 3.2 torques per hp. I think from the replies so far, that ratio is far from constant so it all depends where in the rev range the biggest ratio is? As for lost hp, the boys at Top Gear are always measuring their 30 year old car buys and finding they've lost 50% of their horses. Is that more of a function of petrol engines? And will I have lost many of my 75 horses in the 35 years and 5000 hours since the engine was installed? Hours is pure guesstimate based on previous owners cruising patterns. The 'lost' is important as I'd like to know what I've got at the moment.
Thanks for the help.
Click to expand...

As usual when a poster asks a question like the one you have, some replies tend towards the pedantically correct in a technical sense, and risk creating more confusion than ever in the mind of a non technical questioner. But the reality is simple. To push your boat through the water requires power and that is the power of your engine less any losses on the way. So focus on power. Forget torque. As for what your engine is now developing, use the calculator thingy mentioned above in reverse. In other words, put into the calc different power figures to find out which one gives you the max speed of 5.5kn for your boat and that will be what your engine is producing at 5.5kn, or better if the calculator has built in a safety margin.

Your engine will not be developing the full power it did when new but it wont have faded to the same degree that petrol engines do.
 
I don't really understand this need for loads of torque in a marine engine. I can understand in a car because you need it for good acceleration. In a car, however, the tyres are 'stuck' to the road so when you accelerate all the torque/power is transmitted via a 'solid' linkage. In a boat the linkage is not solid, the propeller is acting in a liquid medium. If you accelerate the engine hard with loads of torque surely the propeller just slips some more (cavitates)?
 
There is nothing worse IMHO than being under 'powered' when punching into a dead nasty headsea between two breakwaters with headwind too, the odd breaking crest, and everything doing its devilworst to stop the boat dead each time the bow wedges into the next crest and defy the engine and propellors attempts to keep momentum and steerage on...

So by whatever calculations used, get that one right and I would say you have made a wise choice! Actually thats IM(scary)E, not IMHO ahem
 
Going through the same exercise at present and I will share my thoughts and learnings if it helps.

hull calculation was cica 24HP to hull speed in flat water. I wanted a Beta. 30 HP was a 3 cylinder revving higher, 35HP was a 4 cylinder with maximum power at lower RPM and more torque, 38 HP was same engine as 35 just running at higher revs to obtain more power.
I discovered my old prop was too small in diameter and too high a pitch and also wrong handed for the engine so accepted I needed to change it. Accepting that meant I could optimise the engine / prop combination to match the right gearbox / engine combination means I will get the right combination of
-grunt when minoevering in a marina
-prop wash to help when minoevering
-economy when cruising
-working the engine hard enough for it stay healthy with time

So I chose the 35 as I wanted a bigger engine that was less stressed but couldn't see the point in choosing the 38 as i would never run the engine that high in the rev range to take advantage of the last 3 horses (at a chunk more money too!)

So my advice is to find the power requirement you think you need then speak to the engine supplier and see what he thinks. look carefully at the spec of each engine and then choose.

By the way I am replacing a Volvo MD21A rated at 50 HP @ 3000 RPM. In theory I am fitting an undersized engine compared to original spec. In reality, the original was grossly oversized and I cruised at 1800 rpm and did almost 3 knots at tickover in marinas because I had the wrong prop fitted.

I suspect a previous owner had swopped the prop out in the past rather than it being wrong from new.
 
Tell me about 3knots at tickover, I don't think we can go slower than that, 700rpm is tickover and that's plenty fast enough in a marina! Ideally we'd like to add a 10hp parallel electric drive for lots of reasons but mainly silent motor sailing, saving £2k on the diesel pays for half the electric drive. In 10 years time hybrids will be mainstream, the technology has leaped ahead in the last 3 years but although I'm a natural early adopter, it's too soon for us, we'll install the new lump in a way that leaves space for an electric installation later. Another 2 years and it will be much cheaper. I first started looking 6 years ago and it was shocking how bad the offerings were, now the likes of electric yacht are delivering really well sorted solutions but that's another thread! Meanwhile the advantage of 58hp is that it's delivering it's maximum torque when we need it most but it's about 9hp shy of what we actually have at the moment...
 
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