gjgm
Well-Known Member
never understood this, but how do you relate a diesel engines output to an equivalent petrol. So a D3 at 160hp is sort of equivalent to what size/output petrol?, and 200 hp diesel etc...
petrol/diesel horsepower
- Thread starter gjgm
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KevB
Well-Known Member
The new D3 launched at the show is 190bhp!! So 79bhp per 1000cc. /forums/images/graemlins/smile.gif
The Mercruiser 5.0 ltr MPI is 260bhp so 52bhp per 1000cc /forums/images/graemlins/frown.gif
The Mercruiser 5.0 ltr MPI is 260bhp so 52bhp per 1000cc /forums/images/graemlins/frown.gif
duncan
Well-Known Member
an absolutely facinating topic which will no doubt see a plethora of technical explanation.............
however I believe the usable performance differential is basically down to the diesels ability to work harder over a wider proportion of is't rev range than the equivelently (HP) rated petrol. ie it's got a flatter torque curve..........
I don't think there are any appropriate rules relating them however as you would need to consider how the HP is being achieved in both cases - a small lightweight diesel reving more will get a higher HP rating but posssible at the cost of lower rev torque; fine if it's designed for lightweight applications like speedboats but stick it in a big SD hull and it would probably be a dog and for a larger fully planning hull it may never be able to rev sufficiently at speeds below the resistance hump for that hull to actually develop it's power at all - (so you stick a tiny pitched prop on to get on the plane and loose all the theoretical top end performance). Put another way a F1 engine would struggle to get a 25ft cuddy on the plan via a submerged propellor drive system despite 900hp!)
however I believe the usable performance differential is basically down to the diesels ability to work harder over a wider proportion of is't rev range than the equivelently (HP) rated petrol. ie it's got a flatter torque curve..........
I don't think there are any appropriate rules relating them however as you would need to consider how the HP is being achieved in both cases - a small lightweight diesel reving more will get a higher HP rating but posssible at the cost of lower rev torque; fine if it's designed for lightweight applications like speedboats but stick it in a big SD hull and it would probably be a dog and for a larger fully planning hull it may never be able to rev sufficiently at speeds below the resistance hump for that hull to actually develop it's power at all - (so you stick a tiny pitched prop on to get on the plane and loose all the theoretical top end performance). Put another way a F1 engine would struggle to get a 25ft cuddy on the plan via a submerged propellor drive system despite 900hp!)
PCUK
Well-Known Member
Horse power is horse power. Whether it is produced by petrol, diesel or electricity. Stated horse power ratings are generally the maximum that the engine can produce.
A diesel engine will usually produce it's maximum power at lower revs than a petrol but given the correct propeller and assuming the same weight of engine the boat's maximum speed will be the same regardless of engine type.
Now acceleration - that's a different matter!
A diesel engine will usually produce it's maximum power at lower revs than a petrol but given the correct propeller and assuming the same weight of engine the boat's maximum speed will be the same regardless of engine type.
Now acceleration - that's a different matter!
D
Deleted User YDKXO
Guest
The definition of engine power is the ability of that engine to do work (or summat like that) so, in theory, 2 engines of similar power and weight will push a boat along at the same speed. If the diesel engine has less power than the petrol engine, then the diesel boat will have a slower top speed and thats before you take account of the fact that the diesel engine is usually heavier
But power is nothing without torque. Its not technically correct but you could say that an engine with higher torque will accelerate the boat quicker and diesel engines generally have higher torque especially in the mid range.
So to compare a 160hp turbo diesel against a 200hp petrol, the diesel engine will probably have v low torque at low speed building quickly to a high torque in the mid range but tailing off at higher rpm. The petrol engine (assuming its not turbocharged) will have a torque curve that rises more linearly over the rev range. There are a lot of other factors involved but what you would expect is that the petrol engined boat will be quicker off the mark but as the torque of the diesel engine rises (as the turbo kicks in), the diesel engined boat may overtake the petrol boat but at the top end the petrol boat will go on to higher speed
Basically, its not technically correct but if you think power = speed and torque = acceleration, its a fair analogy
But power is nothing without torque. Its not technically correct but you could say that an engine with higher torque will accelerate the boat quicker and diesel engines generally have higher torque especially in the mid range.
So to compare a 160hp turbo diesel against a 200hp petrol, the diesel engine will probably have v low torque at low speed building quickly to a high torque in the mid range but tailing off at higher rpm. The petrol engine (assuming its not turbocharged) will have a torque curve that rises more linearly over the rev range. There are a lot of other factors involved but what you would expect is that the petrol engined boat will be quicker off the mark but as the torque of the diesel engine rises (as the turbo kicks in), the diesel engined boat may overtake the petrol boat but at the top end the petrol boat will go on to higher speed
Basically, its not technically correct but if you think power = speed and torque = acceleration, its a fair analogy
duncan
Well-Known Member
don't disagree with anything there but would add that the 'kicker' in marine circles is the lack of gearing and it's this that generally gives the diesel an edge in many application because of the factors you have outlined.