Discussion
ciaranthemurph said:
TTwiggy said:
Imagine doing a compression test...
It's actually very easy.Get engine running at full power in flat water (you may need a large piece of water for this)
Wander out to the clyinder head
Open the indicator cock to blow out any crap
Close indicator cock
Connect peak pressure gauge
Open cock
take reading
close cock
remove peak pressure gauge
Repeat on each clyinder until done
On the modern ships there is a pressure sensor which reads the clyinder pressure and there is another sensor that reads the crank position. Combine the two and you can see the pressure over a whole cycle which tells you a great deal about what is happening inside the engine
tank slapper said:
All of these are trying too hard. This is what's in our boat... 13hp from 750cc and it weighs over 200kg, but it only uses a litre of diesel an hour.
Blimey, I'm not last! I'll have a heady 39hp... if the ducks gang up they could push me backwards...Whilst I'm not a fan of petrol engines in boats I could make an exception for this beastie:
http://www.youtube.com/watch?v=JXS3GBRX1Ys
tank slapper said:
All of these are trying too hard. This is what's in our boat:
13hp from 750cc and it weighs over 200kg, but it only uses a litre of diesel an hour.
I'm pretty sure you need to put that up for an award as it may have broken the laws of physics. 13hp from 750cc and it weighs over 200kg, but it only uses a litre of diesel an hour.
I make that roughly 99.999% efficiency.
There is such a thing as a throttle, which contrary to popular belief has more than two positions. 
It's installed in a sailing yacht, which tend to have pretty efficient hulls, and so when motoring you don't need to use a great deal of power. If you go everywhere at full throttle then you obviously use more fuel than that, but you don't go very much faster you just make a larger bow wave.
It's installed in a sailing yacht, which tend to have pretty efficient hulls, and so when motoring you don't need to use a great deal of power. If you go everywhere at full throttle then you obviously use more fuel than that, but you don't go very much faster you just make a larger bow wave.
tank slapper said:
There is such a thing as a throttle, which contrary to popular belief has more than two positions.
It's installed in a sailing yacht, which tend to have pretty efficient hulls, and so when motoring you don't need to use a great deal of power. If you go everywhere at full throttle then you obviously use more fuel than that, but you don't go very much faster you just make a larger bow wave.
No No No a pretty efficient hull is one with shedloads of power on hand that gets planing and therefore reduces the volume of water displaced as she travels.It's installed in a sailing yacht, which tend to have pretty efficient hulls, and so when motoring you don't need to use a great deal of power. If you go everywhere at full throttle then you obviously use more fuel than that, but you don't go very much faster you just make a larger bow wave.
I generally 'only' burn 80-90 lph at cruise but could reduce the speed by 30% and only see a 10% reduction in fuel. Conversely I could open the taps and empty the tanks at close to 160 lph if I really wanted to. Diminishing returns both sides of optimum cruise consumption.
If I were to quote the 1100hp figure it would only be the higher consumption figure that was relevant. In all other conditions the engines are producing less power.
But I do agree that most of the Romford Navy use the throttle as an on off switch.
MOTORVATOR said:
I generally 'only' burn 80-90 lph at cruise but could reduce the speed by 30% and only see a 10% reduction in fuel. Conversely I could open the taps and empty the tanks at close to 160 lph if I really wanted to. Diminishing returns both sides of optimum cruise consumption.
I'm not sure you are right. If you plot a graph of a planning hull showing fuel burn on one axis and speed on the other, you get an exponential curve, with a section not so steep just after getting on the plane.That does not accord with your 30% speed drop for 10% fuel gain.
Huntsman said:
MOTORVATOR said:
I generally 'only' burn 80-90 lph at cruise but could reduce the speed by 30% and only see a 10% reduction in fuel. Conversely I could open the taps and empty the tanks at close to 160 lph if I really wanted to. Diminishing returns both sides of optimum cruise consumption.
I'm not sure you are right. If you plot a graph of a planning hull showing fuel burn on one axis and speed on the other, you get an exponential curve, with a section not so steep just after getting on the plane.That does not accord with your 30% speed drop for 10% fuel gain.
Huntsman said:
MOTORVATOR said:
I generally 'only' burn 80-90 lph at cruise but could reduce the speed by 30% and only see a 10% reduction in fuel. Conversely I could open the taps and empty the tanks at close to 160 lph if I really wanted to. Diminishing returns both sides of optimum cruise consumption.
I'm not sure you are right. If you plot a graph of a planning hull showing fuel burn on one axis and speed on the other, you get an exponential curve, with a section not so steep just after getting on the plane.That does not accord with your 30% speed drop for 10% fuel gain.
Obviously all deep vees are not the same, but for me I would reckon a complicated s curve arangement for fuel efficiency if we use litres per hour.
As at displacement I will go through a natural progression of speed gained against horsepower. Then as she goes into semi displacement it requires a greater proportional increase in horsepower for speed gain until finally she falls over the hump and horsepower can be reduced to maintain the same speed. Of course from then on the greater the horsepower the faster we go but with diminishing returns.
All of which would be affected by the use of tabs etc to change the hump point or change attitude on the plane and of course the point that turbos kick and improve the engine efficiency.
I think.
Simpo Two said:
Although if it was linear it would mean he could be stationary and still use 107 lph... the reciprocal of perpetual motion - so near yet so far....
Says the chap sat in the corner with clenched MOTORVATOR said:
Probably didn't explain that well Ben. The point I was making is that a minor reduction of revs and hence horsepower would result in falling just off the plane and a greater proportional reduction in speed than fuel saved if you know what I mean.
Obviously all deep vees are not the same, but for me I would reckon a complicated s curve arangement for fuel efficiency if we use litres per hour.
As at displacement I will go through a natural progression of speed gained against horsepower. Then as she goes into semi displacement it requires a greater proportional increase in horsepower for speed gain until finally she falls over the hump and horsepower can be reduced to maintain the same speed. Of course from then on the greater the horsepower the faster we go but with diminishing returns.
All of which would be affected by the use of tabs etc to change the hump point or change attitude on the plane and of course the point that turbos kick and improve the engine efficiency.
I think.
But then if you compare the energy required for a planing hull to get from A to B against a displacement hull, the displacement hull wins hands down every time. Compare something like a Predator 63 with a Daschew Offshore FPB64: The Sunseeker with 3000 litres of fuel, a cruising speed of 30ish knots and a fuel consumption of 200 litres per hour has an endurance of 15 hours and 450 miles, against the Daschew with a cruising speed of 9 knots and fuel consumption of 12 litres an hour giving it an endurance of 250 hours and 2250 miles range with the same fuel load (actually it can carry 12800L).Obviously all deep vees are not the same, but for me I would reckon a complicated s curve arangement for fuel efficiency if we use litres per hour.
As at displacement I will go through a natural progression of speed gained against horsepower. Then as she goes into semi displacement it requires a greater proportional increase in horsepower for speed gain until finally she falls over the hump and horsepower can be reduced to maintain the same speed. Of course from then on the greater the horsepower the faster we go but with diminishing returns.
All of which would be affected by the use of tabs etc to change the hump point or change attitude on the plane and of course the point that turbos kick and improve the engine efficiency.
I think.
Obviously they are designed with different things in mind, but the principle stands - the improved efficiency by planing only applies when comparing the boat against itself when not planing.
tank slapper said:
But then if you compare the energy required for a planing hull to get from A to B against a displacement hull, the displacement hull wins hands down every time. Compare something like a Predator 63 with a Daschew Offshore FPB64: The Sunseeker with 3000 litres of fuel, a cruising speed of 30ish knots and a fuel consumption of 200 litres per hour has an endurance of 15 hours and 450 miles, against the Daschew with a cruising speed of 9 knots and fuel consumption of 12 litres an hour giving it an endurance of 250 hours and 2250 miles range with the same fuel load (actually it can carry 12800L).
Obviously they are designed with different things in mind, but the principle stands - the improved efficiency by planing only applies when comparing the boat against itself when not planing.
Well yes but then it depends on how we measure efficiency. LPH is really a pretty useless figure unless we further include miles per litre as you have done above or indeed time to travel a distance.Obviously they are designed with different things in mind, but the principle stands - the improved efficiency by planing only applies when comparing the boat against itself when not planing.
In your original engine statement you were talking about a 13hp engine with a burn rate of 1lph if used wisely. At 1lph it is probably only making something like 5hp, in truth why have the 13hp if you are never going to use it?
It could also be said that the same size planing hull cruising easily against a displacement hull running very hard to get somewhere could well be more efficient both in terms of fuel use and time expended.
Push that same Dashew with the taps open to get somewhere and the range will fall through the floor to something similar to the Predator.
Throw a head current into the equation and suddenly the Pred becomes more fuel efficient rangewise.
As you say they are designed to do different things.
MOTORVATOR said:
Says the chap sat in the corner with clenched knuckles buttocks as he thought I was trying to launch him off the back of the boat.
Given the enthusiasm with which beer cans were rolling off the back into the river like little depth-charges it is as well that the vessel could only stagger up to 21kts!The extra power is there as a safety margin, as you probably know. Given a force 8 headwind, or a strong current then the extra power is sometimes necessary (although whenever there is a reasonable wind the sails can generate far more power than the engine, but sometimes sailing isn't desirable).
With that Daschew, you could probably maintain your speed in conditions it wouldn't be safe to be doing 30+ knots in a planing boat (they reckon a 15-30% increase into 30 knots and large waves), but then the Predator isn't going to be mid-atlantic with no port to run to.
With that Daschew, you could probably maintain your speed in conditions it wouldn't be safe to be doing 30+ knots in a planing boat (they reckon a 15-30% increase into 30 knots and large waves), but then the Predator isn't going to be mid-atlantic with no port to run to.
tank slapper said:
The extra power is there as a safety margin, as you probably know. Given a force 8 headwind, or a strong current then the extra power is sometimes necessary (although whenever there is a reasonable wind the sails can generate far more power than the engine, but sometimes sailing isn't desirable).
With that Daschew, you could probably maintain your speed in conditions it wouldn't be safe to be doing 30+ knots in a planing boat (they reckon a 15-30% increase into 30 knots and large waves), but then the Predator isn't going to be mid-atlantic with no port to run to.
Totally agree.With that Daschew, you could probably maintain your speed in conditions it wouldn't be safe to be doing 30+ knots in a planing boat (they reckon a 15-30% increase into 30 knots and large waves), but then the Predator isn't going to be mid-atlantic with no port to run to.
I'd still be nervous of all that glass around me in a force 8 though regardless of design.
MOTORVATOR said:
Well yes but then it depends on how we measure efficiency. LPH is really a pretty useless figure unless we further include miles per litre as you have done above or indeed time to travel a distance.
In your original engine statement you were talking about a 13hp engine with a burn rate of 1lph if used wisely. At 1lph it is probably only making something like 5hp, in truth why have the 13hp if you are never going to use it?
It could also be said that the same size planing hull cruising easily against a displacement hull running very hard to get somewhere could well be more efficient both in terms of fuel use and time expended.
Push that same Dashew with the taps open to get somewhere and the range will fall through the floor to something similar to the Predator.
Throw a head current into the equation and suddenly the Pred becomes more fuel efficient rangewise.
As you say they are designed to do different things.
But afterall, this is PH, should we not be going everywhere flat out? Making as much noise as possible and using the most fuel we can?In your original engine statement you were talking about a 13hp engine with a burn rate of 1lph if used wisely. At 1lph it is probably only making something like 5hp, in truth why have the 13hp if you are never going to use it?
It could also be said that the same size planing hull cruising easily against a displacement hull running very hard to get somewhere could well be more efficient both in terms of fuel use and time expended.
Push that same Dashew with the taps open to get somewhere and the range will fall through the floor to something similar to the Predator.
Throw a head current into the equation and suddenly the Pred becomes more fuel efficient rangewise.
As you say they are designed to do different things.
I organise a powerboat rally, last year we had just over 10,000hp out on the water.
dave-the-diver said:
13hp! 750cc!
Wasteful excess.
8.02hp from 318cc @76kg is where it's at!
David
Fantastic, thats the ticket good old Yanmar 1GM10 the stalwart of british boating and the engine of choice for the discerning gentleman.....now you are going to tell me its not a Yanmar...Wasteful excess.
8.02hp from 318cc @76kg is where it's at!
David
My old one (currently boatless due to lving in the wrong place):
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