Fuel consumption again

[ulyden]

Last 30 year since direct injection came most diesel engines have the same specific consumption. Most having 230-250g/kWh at WOT and 210-220 at 80% speed. Some are better and some worse.

That means the boat consumption is dependent on the hull shape and weight. If you have two equal boats and one engine is operating at WOT and the other is operating at 80% speed or optimum the maximum difference might be 20-30g/kWh in worst case.

This is about 13% at maximum. But if the engines runs at 80% speed the power output are about 55-60% so compare you has to go to a much larger engine.

If you compare kad 42 and KAD300 the power difference is 25%. So with a normal propeller curve 2.7 the KAD300 have 230HP at 90% speed.

So you end up comparing 80% and 90% speed so the difference is small.

If you look at these windy tests you spot all this. The D6 is a heavier engine and boat weight is larger. So even if specific consumption on the larger D6 is lower the weight is larger and you end up with the same consumption.

You also see that when KAD300 reach WOT at 3900rpm and 39 knots the consumption is 12% higher than D6 350 running at 90% speed and about 75% power.

http://www.windy.no/downloads/28_GHIBLI_ KAD300DP_Ggearratio_1_68_1.pdf

http://www.windy.no/downloads/28_GHIBLI_ D6_350_PDH_gear_ratio_1_69_1.pdf

So the answer is yea the KAD300 will be slightly more economical at same cursing speed but the difference will be minimal and remember that 1knot difference in boat speed is making the same difference as 7g/kWh in specific fuel consumption at 30 knots.

So you have to be 100% sure about the boat's antifouling and weight before comparing.

 

[EME]

Windy Data

is all here on their site

 

[z1ppy]

Thanks everyone very interesting replies.

Still in two minds as to which direction to go and of course, its dependant on many other factors aside from choosing the engines when buying second hand!

i am pretty sure the KAD 42 boat is not the one for us given its done 750 hours and is 4 years older than the Kad300 version.

Would like to find a D6 360 version but that means in increase in budget which i cant make my mind up on yet!

thanks again.

 

[Deleted User YDKXO]

I agree Ben's description of the graph. However Ben, I think once a boat is fully planing the graph is remarkably flat. I have full on real time fuel measurement on current boat and real time divide-by-speed functionality, so I can see instantaous litres per mile on the dash*, and between say 22 and 27kts your graph line is almost flat. Not quite flat of course, but so close to flat as it makes no difference

My current boat is the first boat I've had with fuel consumption displayed (in lph) on the engine monitoring system and I've also been surprised how flat the nmpg fuel consumption curve is at planing speeds. Between 17kts and WOT (31kts), there's just over 10% decrease in nmpg on my boat which is not really enough to make running at slower planing speeds worthwile in cost terms. All this assumes that the data displayed by the monitoring system is correct of course!
What is surprising is the difference in nmpg at displacement speeds though. At minimum in gear (idle) speed, about 7kts in our case, the figures indicate that we are doing about 4mpg. Increase that to 8.5kts and the nmpg drops to about 2mpg and to about 1.5nmpg at 10kts. Again, assuming that the figures are correct, what this seems to indicate is that if you really want to save fuel, you have to cruise at minimum engine rpm which is stating the obvious I suppose but I didn't expect the difference to be so marked

 

[CX54WEK]

Thanks everyone very interesting replies.

Still in two minds as to which direction to go and of course, its dependant on many other factors aside from choosing the engines when buying second hand!

i am pretty sure the KAD 42 boat is not the one for us given its done 750 hours and is 4 years older than the Kad300 version.

Would like to find a D6 360 version but that means in increase in budget which i cant make my mind up on yet!

thanks again.

I wouldnt be overly worried about 750 hours on a Kad engine.

Our KAD32 (i know its not the same engine) is going on for 1000 hours now and never misses a beat, doesnt smoke and is super reliable. Provided they are well maintained they just run on and on.