Fuel consumption again

[z1ppy]

Evening.

I know there was a thread recently on fuel consumption but can any one add any real knowledge to my zippy logic?

Situation is

Identical boats. 36 ft planing motorboats

One had Twin kad 42 one had twin kad 300. Both on ourdrives and identical to all intents other than engines.

My thinking is the kad 300 will use less fuel than the kad42 at a given speed?

Example. Both boats are doing 40 kts

The kad42 will be at pretty much wot to achieve. The kad300 will not be at wot therefore using less fuel?.

Telling me I am stupid is acceptable as long as you explain why!!!

Thanks
Confused zippy!!!

 

[burgundyben]

I'm not sure any difference would be due to one being at part load and the other at WOT, more that some engines would achieve different levels of efficiency at different outputs.

I think you have to look at the amount of work being done, in the case you gave the amount of work is the same, so any delta in consumption is due to different efficiency at that point.

Lateboater is the man for this, he'll go on about BMEP and allsorts.

As was noted on here some years ago, there are no free knots, the faster you go, the more it costs. Pity.

 

[[2068]]

You need to look at the fuel consumption curves for each boat/engine to get the answers you are looking for, and it's not straightforward.

What I can say is that two boats at their optimum cruising speed, will use about the same amount of fuel to get from A to B if fitted with Kad42 or Kad300, if both are correctly propped.

Where it could go wrong is if the Kad42 boat is underpowered, and the engines optimum cruise speed (say 3000rpm) means the boat is barely on the plane and the hull is not working as efficiently as it could.

The Kad300 has the 24v head, so probably a bit better breathing at higher rpm for high speed cruising if the hull is happy to do that. So long as those pesky valves are adjusted perfectly.

 

[[2068]]

As was noted on here some years ago, there are no free knots, the faster you go, the more it costs. Pity.

Per hour, yes. But per mile, so long as you don't go crazy and use full beans, a fast cruise can often end up costing exactly the same as a slower one, due to increased prop efficiency at higher speeds. Other option is to slow right down and do 6kts, which is even more efficient, but very boring.

 

[stelican]

Evening.

I know there was a thread recently on fuel consumption but can any one add any real knowledge to my zippy logic?

Situation is

Identical boats. 36 ft planing motorboats

One had Twin kad 42 one had twin kad 300. Both on ourdrives and identical to all intents other than engines.

My thinking is the kad 300 will use less fuel than the kad42 at a given speed?

Example. Both boats are doing 40 it's.

The kad42 will be at pretty much wot to achieve. The kad300 will not be at wot therefore using less fuel?.

Telling me I am stupid is acceptable as long as you explain why!!!

Thanks
Confused zippy!!!

yes the 300 will uss more fuel to develop the extra hp
both overstressed unreliable motors

 

[Warpa]

A question here...if worried about fuel consuption why buy something that the engines will stretch the wallet? I have often wondered this and seen people on a regular basis just sitting in their lovely boats with friends and family, is the fuel consumption the reason

I have also heard the same stories from two others that have dry stack storage in the more lucrative areas of town Sorry to hijack the thread so to speak, but if filling the tanks are an issue then i would say the boat is too large for said owner

 

[[2068]]

yes the 300 will uss more fuel to develop the extra hp
both overstressed unreliable motors

That's rubbish.

Initial versions of Kad42 were unreliable because of design problems, not over stress. Later versions of Kad42 and all versions of Kad43 are very reliable.

Kad300 is reliable ish, but it was at the very top of what could be done with the 3.6L block, so poor maintenance can have dramatic effects, especially if you don't adjust the valves and the post that the Y-piece thingy sits on drops off

 

[stelican]

That's rubbish.

Initial versions of Kad42 were unreliable because of design problems, not over stress. Later versions of Kad42 and all versions of Kad43 are very reliable.

Kad300 is reliable ish, but it was at the very top of what could be done with the 3.6L block, so poor maintenance can have dramatic effects, especially if you don't adjust the valves and the post that the Y-piece thingy sits on drops off

So in your opinion they are unreliable
I believe owners pushed them too hard hence piston pick up/ warped manifolds

 

[z1ppy]

Interesting comments from all. Will do some more research on both the engines thins weekend but at the moment, the all round package with the kad300 is on top.

Warpa, I guess if everyone took this view we wouldn't have many motorboats. I don't suppose many people like going to the fuel berth, it's a choice we makeq based on what we can afford. A 36 foot boat doing 40 kts is not going to be cheap on the motion lotion! The question was around if the larger engined boat would be more efficient at cruise speed than the smaller one.

Rest assured that if this works out we will be don the water a lot, regardless of cost cos I will not be able to get that grin off my face!!

 

[burgundyben]

Per hour, yes. But per mile, so long as you don't go crazy and use full beans, a fast cruise can often end up costing exactly the same as a slower one, due to increased prop efficiency at higher speeds. Other option is to slow right down and do 6kts, which is even more efficient, but very boring.

I dont think that's correct.

For every boat and engine combo you could develop a graph that shows consumption on Y axis and speed on X axis, there might be places on the graph where the curve is more or less steep, but there are no completely flat areas and no dips.

 

[[2068]]

You're right, that's a better description than "it's not straightforward".

But on Flower Power, so long as I keep her planing, fuel consumption per mile seems not to vary much unless I go bananas and exceed 3200rpm. Other hull shapes will be different.

 

[Deleted User YDKXO]

A question here...if worried about fuel consuption why buy something that the engines will stretch the wallet? I have often wondered this and seen people on a regular basis just sitting in their lovely boats with friends and family, is the fuel consumption the reason

I have also heard the same stories from two others that have dry stack storage in the more lucrative areas of town Sorry to hijack the thread so to speak, but if filling the tanks are an issue then i would say the boat is too large for said owner

You're wrong. Fuel consumption is an issue for all of us. Most of us who can afford to buy a motorboat didn't get into that position by throwing money away needlessly. I can afford to buy the fuel to run my boat but that doesn't mean I'm not interested in operating it as efficiently as possible

 

[jfm]

I agree with burgundy ben. In your question Zippy you are assuming all other things are equal. Hence both engines/props are doing an identical amount of work. Any difference in fuel consumption can only be due to differing engine thermal efficiency, which will be less than 5% for those motors

Alas the assumption isn't exactly how it is in real life. The props might be different for starters and I'd expect you'd see maybe 10% difference in fuel burn, but heck that isn't much in the grand scheme

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

*when I can find the courage to dare look...

 

[ian38_39]

Think you may see a dip on a planing boat as it goes properly onto the plane from the hump or there wouldn't be much point in a planing hull. everyone would just have SD hulls and big engines. The balance of weight and power must have something to do with it.

 

[Whitelighter]

Firstly, neither boat will hit 40knts, WOT or otherwise

Ok, firstly I read it as 36ft flybridges, so *MAYBE* the KAD300 engined one will hit 40knts, but its going to be pretty close to WOT and in a dead neutral sea. The KAD42 engined boat doesn't stand a chance.

I had 43's in a 34ft boat and spent time changing props and the best I ever saw was 38knts, and that was in perfect conditions without any clutter on board

 

[Bassplayer]

I agree with burgundy ben. In your question Zippy you are assuming all other things are equal. Hence both engines/props are doing an identical amount of work. Any difference in fuel consumption can only be due to differing engine thermal efficiency, which will be less than 5% for those motors

Alas the assumption isn't exactly how it is in real life. The props might be different for starters and I'd expect you'd see maybe 10% difference in fuel burn, but heck that isn't much in the grand scheme

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

*when I can find the courage to dare look...

Exactly as I have found on my boat. Lph is about the same from 18kts to 24kts and better than at 10kts. Good excuse for making a large wake!!

 

[rubberduck]

So therefore large wake's are environmentally friendly. Who's brave enough to post that on the scuttlebutt.

 

[burgundyben]

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

I'm a bit of a nerd with these things...it would be very interesting to draw the graph for the boat and then compare with the fuelling tables held in the ECU.

I'm surprised its that flat, certainly on the deep V like mine, graph is rather steep above 20 knots. To the point that these days we mostly trundle about at 20 knots, its kinder on the old girl too.

 

[gjgm]

Unless you are just out for a spin, LPH doesnt sound very useful to me,rather LPM as thats what is costs from A to B. Windy used to have all their boat, data on their website-maybe they still do, but as an idea of curves, this is the 32 with 300s
revs/knots/lpm
2000/16/2.25
2500/26/1.74
2800/31.4/1.66
3000/34.4/1.74
3200/37/1.84
3400/39.2/1.99
3600/40.6/2.12
3800/43.2/2.17
3950/44.6/2.42

Unless I mistyped anything!

 

[burgundyben]

Unless you are just out for a spin, LPH doesnt sound very useful to me,rather LPM as thats what is costs from A to B. Windy used to have all their boat, data on their website-maybe they still do, but as an idea of curves, this is the 32 with 300s
revs/knots/lpm
2000/16/2.25
2500/26/1.74
2800/31.4/1.66
3000/34.4/1.74
3200/37/1.84
3400/39.2/1.99
3600/40.6/2.12
3800/43.2/2.17
3950/44.6/2.42

Unless I mistyped anything!

I suspect some typographical errors!