fuel consumption

Re: fuel consumption .... 20 hp per gall etc....

Interesting table!

I tried to PM you with my data, but you're over your message limit so here it is:

Etap 30
30'
3500 kg
VP 2002
18 hp
50 l
1.13 l/hr
6 kn
2000 rpm
20 l
 
Prop absorbing etc.

I'm not up on engine calcs .. so that is why I made the above very tongue in cheek .....

If the power is ~5HP at 1/3rd throttle - then the calc. becomes extremely believable ... As I said in the post - for simplicity I used linear slide on throttle vs HP ... knowing it to be incorrect ... but don't know any other way.

Cheers ... so my 2 ltrs / hr is about right !!
 
Re: Prop absorbing etc.

I have just realised that another factor should be mentioned.

If you were in a headwind, the fuel consumption at the same engine speed would rise, just as it would fall if you were motorsailing.

Note all this approximate arithmetic is diesel engines only.

Petrol engine specific fuel consumption rises dramactically at part load.
 
Re: Prop absorbing etc.

How is that possible frank?
Throttle at same position, therefore air/fuel ratio is the same.
Metered pump fuel output at that throttle setting is the same
Load at the prop, assuming the same density of water is the same
Friction in the drivetrain system is the same.
Total revolutions at the prop is the same.

What you mean is that if you want to maintain same speed in different conditions, you need to alter throttle settings accordingly, and thus consumption will change.

Incidentally, your 45hp engine at tickover will be producing much more than 5hp - that would be stalled as soon as the alternator kicked in on high load and you engage a load through the garbox. Can work it out, but going to work now. Go look at the BMEP figures.
 
C\'mon guys ...

It's all a bit tongue in cheek anyway ...

Normal steaming conditions will resume later !!!
 
Re: Prop absorbing etc.

I said at same rpm.

The diesel engine is not throttled, but is controlled by the amout of fuel injected, and the load at that fuel setting controls the speed.

So, if the load goes up due to headwind more 'throttle' is needed to maintain engine speed, as you say, and more fuel is used.

Diesel engines do not have a constant air/fuel ratio, they have air flow that can be regarded, at any one speed, as constant, and at part load the air/fuel ratio is lean.

This is why when overloaded the exhaust can go black due to excess unburnt fuel.

I was ignoring ancilliaries, true, the alternator can use a lot of power on maximum charge.
 
Re: Prop absorbing etc.

How does the engine recognise the wind load on the hull?
The prop will churn the water at the same rate, regardless of the restraining pressure. You could tie the boat to a pontoon and get a similar fuel burn/hour because the forces only change by the rate of flow of the water. Take the prop out of the water or change its position causing cavitation then you have a change in load and thus a change in prop speed.
Engine speed adjustment is required to maintain boat speed under varying conditions, I see that, but I dont see how the throttle adjusts itself going into wind.
 
Re: Prop absorbing etc. .... load and off-load ...

I also cannot understand the change of throttle / load argument ....
The only time I can see that prop-load would alter is when going from a light load at basic prop speed into a head-sea etc. or wind that slows the boat down and prop then initially finds itself biting into water that is not clearing as fast .. I know it sounds crap ... but we are talking that few seconds etc. when boat slams or near stops into that force opposing.

I can't see that throttle setting has changed or fuel / air to the engine except due to the momentary reduction of revs as prop bites into the slower water ...

But surely at end of day .... most thinsg balance out - unless of course you enjoy an excess of punching into weather ?

The biggest effect on fuel cons. surely is that - the prevailing weather and the setting of throttle to maintain a given speed ? against the elements ...
 
Re: Prop absorbing etc. .... load and off-load ...

I agree. I think that the revs are pretty much constant dependant on the 'throttle' setting therefore the prop speed is fairly constant. I think the load on the prop changes very little, even if the boat is tied to the dock. It's just moving a constant volume of water the whole time - a bit like a jet engine. I believe that this means that the fuel consumption at a given engine speed will vary very little over time.

Now when you consider distance travelled rather than time then of course the consumption varies enormously!

If you're motoring at 4 Kn into a 4 Kn current you'll only use the same GPH but your MPG is infinite!
 
Re: Prop absorbing etc......... Ah Ha ..... jumping up and down !!!

As a boat that often does exactly that ..... I would like to jump in here and pose this ....

If engine set at throttle that under normal conditions gives 4kts ... then raise sails to set that also would give 4kts ... what is result ?

A slight increase over 4kts .... Why ?

A prop is not 100% efficient and therefore the sails will aid it to get to 4kts ... but the prop is spinning at greater than actually if prop pitch is drawn out than 4kts .. due to losses and effort ... so you gain a bit from the already 4kts ... if you see what I mean ?

So as I would expect from my boat - I would likely gain possibly a knot from the engine and sail combination at a guess.

The prop anyway un-loads much much quicker with the sails up as above ... so it's a non-question really ... it's a bit like asking a race car to say its fuel consumption and throttle setting when tail-gating another car and being pulled along compared to when it's in free air and having to drive itself ...
for same speed ... On your boat you would be able to reduce engine throttle to maintain same ... as sails are working ..

Or another way ... the sails are doing opposite to a head-sea now ... not against boat movement - but in favour of it ...

??????????? prop absorbing ......... that means it is expending the power in energy and so pushing the water ... again it will result in above ... so it has to be similar power .......... but boat now will be moving fractionally faster .. using up some of that prop excess.
 
Re: Prop absorbing etc.

frank, please give in on this. I am teasing you mate.
You undoubtedly have knowledge of diesels and so do I , as I have been designing powertrains in automotive for years. Nigel is on the case, and that is always entertaining, expect tome to come along and rubbish Nige soon.
Try working out a Turbo Diesel ascending Grossglockner Pass towing at Gross Vehicle Mass, its the clutch which fails first followed by the body. Whats the consumption? Doesn't matter, this one tells you how much weight you can pull up a slope, not consumption. Same, same never mind, as they would say in Pidgin.
 
Re: Prop absorbing etc.

Jim.

Clutch shouldn't fail, in my day they were usually designed to take c. 1.4 x max engine torque, (cars, not trucks)

However, a serious question.

Should the prop be matched to be able to meet max revs with the boat stationary, e.g. tied to a pontoon?

Engine installer's advice seems to be that it should not be able to. I rather like the idea of being able to get max power with the boat almost stationary, e.g. in a headwind.

1800 rpm in my boat gives about 4 1/2 knts in smooth water no wind.

Direct into F5/6 with waves and fully reefed main comes down to c. 1 1/2 to 2 knts, same engine speed.

Superseal 26 Nanni 14hp saildrive 13x8 folding prop.

Prop is that funny Italian thing with a blunt trailing edge, designed to make it better in reverse.

I am not sure that I like it.

Think we got thread drift.
 
Re: Prop absorbing etc.....

Before Tome accuses me of claiming to be expert ..... I posted as interest and if wrong to be "politely" corrected ... as the thread drift was really interesting ....

I couldn't resist the engine + sails one ... as that hits on pretty well most of how my boat is sailed ... being a heavy motor sailer.

So onto a question ... you both mention alongside berth and my question is regarding engine loading up and once up to speed unloading ... My understanding - correct me if wrong please - is that an engine shouldn't reach max revs or unloaded if boat is held or prevented from reaching the speed that engine is trying to achieve. A diesel should be loaded up to make it work. Reason that an engine does unload alongside is actually as the prop cavitates ...
Reason I ask is I know my engine is underpropped - as it unloads quickly ... and because the boat is small - putting a larger prop on would be an even bigger disaster in astern prop-walk. But when I run it up alongside .... I always give a jab ahead and astern before casting off to make sure all is ok ... the engine does unload quickly ... and so when underway as well.

Of course Tome will probably step in here having been disappointed in his Stable wind-up .... and accuse me of allsorts ..... /forums/images/graemlins/grin.gif /forums/images/graemlins/tongue.gif /forums/images/graemlins/tongue.gif /forums/images/graemlins/tongue.gif /forums/images/graemlins/cool.gif /forums/images/graemlins/cool.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/blush.gif
 
Re: Prop absorbing etc.....

just to add to the debate: Perkins 115HP, running at 1200 to 1300 rpm (5knots boat speed with no headwind), does consistently 4 litres per hour. Almost double that at 1800 rpm.
 
Re: Prop absorbing etc.

frank,
boat stationary does not indicate correct prop size, as turbulent flow vortices will be generated in a stationary location and impinging on the hull and other obstructions without being carried away by the flow normally generated by fore (or aft) movement of the boat.
The engine is propped correctly (as I understand it) such that max torque of engine should be reached at about 7/8 throttle in calm conditions. I had an Evolution 25 that would thrash away with a folding prop obviously hypercavitating and producing much less drive, mostly due to shaft down angle and shallowness of water around it. However, the engine did not have the torque to push a larger pitch, so at 20 knots of headwind, my boat stopped. Completely. Thats a compromise of lifting keel boats that sit in b@gger all water.

My current setup (Yanmar 3YM30)is comfortable pushing 3200RPM at just under full throttle, and will make 3450RPM max with a cloud of smoke, a dipping stern, and about 0.1kt extra. Hull speed reached, i.e well matched.
 
Re: Prop absorbing etc.

Actually there is a fair point here somewhere.... The throttle position isn't fixing the fuelling that's fixed by the governor in the fuel pump... the throttle is fixing the output rpm. I agree that output rpm won't change in a headwind.
 
Re: Prop absorbing etc.

I probably didn't make my thinking clear.

The advice is that the engine should not be able to get to max rpm with the boat held stationary e.g. tied to pontoon.

This says to me that you would not get the full opportunity to make headway if the boat were brought down to a very low speed due to headwind/waves.

I have no factual data; this is only my feelings.

Nearest I have been to this is the day after the 2005 ROI, when we were going from Hamble to Lymington, towing a dinghy.

Forecast SW 3/4

Came round Calshot and it was more than that.

Later, that wind strength found out that the halyard jammers slipped, (now replaced by Spinlock XA clutches) we motored, wind was about F6/7 and we were down to about 1/1.2 knts at 2250 rpm.

Bear in mind that this was on Lake Solent so the waves were small, hence my thinking that I would like to be able to get maximum thrust at close to zero speed through the water.
 
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