Full displacement trawler power requirement.

[Whitelighter]

Consider a 45-55ft displacement trawler.
At displacement speed what is more important? Power or torque?

I can push a 25t boat off the dock but how much power does it need to turn a prop big enough to give 8knts say

 

[BruceK]

Torque.

 

[kashurst]

Power
Power = Torque x speed

in such a boat almost any engine over 150Hp would probably work fine. (I am guesstimating the power requirement based on big sail yachts)
However as it is likely to be used for 10's of 1000's of hours, a large capacity engine turning slowly will develop the required torque at a lower speed (low rpm power) and will last for ever. You could put in a 900cc twin cylinder with a turbo charger but it would probably wear out quite quickly as it would have to run at a high rpm to generate the equivalent power and need a large ratio gearbox to give the required torque.

 

[MapisM]

At displacement speed what is more important? Power or torque?

In a sense, neither.
I'm not going to enter into the acrobatic estimates that I made on my old lady, in order to calculate the power absorption through the rpm range (mechanical engines don't have the luxury of real time fuel burn, load, etc.), also because I can't even remember them.
But the bottom line is that she's a 53' displacement trawler, 31 to 36T depending on load, and at 8kts her props demand 80hp overall, give or take.

With a single screw, which from almost every viewpoint is a better setup for a boat of that type, I would expect at least a 10% better efficiency.
Torque is essential for real (i.e. fishing) trawlers for obvious reasons, but in pleasure boats with a full displacement hull, both power and torque actually used are just a fraction of what is available.

 

[NBs]

I think cummins marine 6,7l 230hp or John dere 230hp is roughly perfect 45-55 feet FD hull trawlers engine. Good combination of engine displcement - torgue and power and good service for long life expectancy.

And with the qsb series engine you can drive +58hp/800rmp or over 24/7 without any carbon or glazing problems in the cylinders.

NBs

 

[piratos]

Consider a 45-55ft displacement trawler.
At displacement speed what is more important? Power or torque?

I can push a 25t boat off the dock but how much power does it need to turn a prop big enough to give 8knts say

Honestly that depends on who you are asking.

Here are two different programs to calculate the power needed, and they will probably come to different results :
https://www.volvopenta.com/marineleisure/en-en/products/info/engine-configurator.html
http://www.psychosnail.com/boatspeedcalculator.aspx

 

[Deleted User YDKXO]

At displacement speed what is more important? Power or torque?

In very simple terms power determines the max speed at which the boat can go (assuming the prop is correctly pitched) and torque determines how quickly the boat reaches that speed (ie acceleration)

 

[kashurst]

watch this
https://www.youtube.com/watch?v=u-MH4sf5xkY

 

[Whitelighter]

Thanks everyone that’s all very helpful

 

[NBs]

Very simple exaples this topic

https://stevedmarineconsulting.com/horsepower-and-torque/

NBs

 

[BruceK]

In a displacement boat one can forgo large amounts of power because the work done is not as great (say compared to planing) however you still require the torque because you dont have adaptive gearing through the range to use max
power efficiently.

 

[benjenbav]

In very simple terms power determines the max speed at which the boat can go (assuming the prop is correctly pitched) and torque determines how quickly the boat reaches that speed (ie acceleration)

An old one, possibly familiar to 911 turbo owners from the aircooled era:

Oversteer = hit the wall backwards
Understeer = hit the wall forwards
Power = how fast you hit the wall
Torque = how far the wall moves

 

[Deleted User YDKXO]

An old one, possibly familiar to 911 turbo owners from the aircooled era:

Oversteer = hit the wall backwards
Understeer = hit the wall forwards
Power = how fast you hit the wall
Torque = how far the wall moves

Now, now I have only one word to say to you and thats PSM which suspends the laws of physics until, er, it doesnt and you hit the wall or in my case get beached in a gravel trap in front of the pits and your mates collapse in hysterics

 

[benjenbav]

Now, now I have only one word to say to you and thats PSM which suspends the laws of physics until, er, it doesnt and you hit the wall or in my case get beached in a gravel trap in front of the pits and your mates collapse in hysterics

Well, a small secret - I've just been playing at rebuilding my suspension and honestly couldn't drive it in a straight line on the first trial. It wasn't too bad with power on or on full braking but anything else and it was difficult to predict which way you were going to spin - but spin you must. I had to drive like a granny. Back to the drawing board and cranked up some negative camber and it's both sharp turning in and solid. More like a granny on acid.

Ahem, but back to the trawler set up NBs seems to have it clearly.

 

[Deleted User YDKXO]

Well, a small secret - I've just been playing at rebuilding my suspension and honestly couldn't drive it in a straight line on the first trial. It wasn't too bad with power on or on full braking but anything else and it was difficult to predict which way you were going to spin - but spin you must. I had to drive like a granny. Back to the drawing board and cranked up some negative camber and it's both sharp turning in and solid. More like a granny on acid.
.

Here's your man http://www.centregravity.co.uk/

 

[QBhoy]

Prop pitch and the intended use will have a huge influence on such a thing.

 

[Portofino]

Now, now I have only one word to say to you and thats PSM which suspends the laws of physics until, er, it doesnt and you hit the wall or in my case get beached in a gravel trap in front of the pits and your mates collapse in hysterics

Older aircooled ones where analogue cars they did not have PASM , the A is active btw .
Modern stuff has the nannies and depending on the model , to some extent you can turn them off or reduce them .

Back to the OP , can you mention the actual engine and boat combo ?
How ever in a MCQ simple two box answer where you can only cross one box to answer the OP ,s question I would put a cross in the torque box .

 

[longjohnsilver]

With our 48' displacement trawler yacht we have 2 x 135hp lumps. At our cruising speed I reckon we're using about 40hp from each engine.

 

[NBs]

With our 48' displacement trawler yacht we have 2 x 135hp lumps. At our cruising speed I reckon we're using about 40hp from each engine.

Hi LJS

Can you estimate what is your greatest need for power when sea current, waves and wind resist your passage vs normal situation use about 80hp?

And what do you think about the weight of your Trawler loaded for use?

NBs

 

[longjohnsilver]

Hi LJS

Can you estimate what is your greatest need for power when sea current, waves and wind resist your passage vs normal situation use about 80hp?

And what do you think about the weight of your Trawler loaded for use?



NBs

Unless we want to go slightly faster we almost always cruise at 1600 rpm, regardless of wind, sea etc. But we have stabs, without them one quite a few occasions we simply would not be out at sea, far too uncomfortable.

At a guess our weight is about 26 tons.