Raggie seeks fuel consumption advice

[iangrant]

Can anyone help please, My engine is a Volvo MD31A 92 HP Turbo Diesel driving a standard three bladed prop through a conventional prop shaft and bearing. The boat is displacement (Obviously) and the brass plate in the cabin says 16 tons. I reckon she is using about 5 litres an hour cruising at 6 kts. is that right?

Thanks

Ian

 

[Duncan_Hamble]

Could be right. How long is the boat? What is the maximum speed you can get?

As a rule of thumb a diesel will use 5 gallons per hour per 100hp. So, flat out it will be using 4.5 gallons per hour.

So, one gallon per hour for 6 knots could be possible. 20hp
Two gallons per hour for 7 or 7.5 knots
Four gallons per hour for 8 or 8.5 knots
Five gallons per hour for 9 knots?

 

[iangrant]

Duncan, thanks for that.
She is 42' long and (therefore) the hull speed is about 8 kts..
Engine is happy at about 3200 rpm at about 5.5 to 6 knots. Above that it seems to take lots of engine effort and fuel to go that bit faster..

Ian

 

[tome]

The brass plate is prob Registered Tonnage. The Enderlein HR42 is 11.3 tonnes displacement, but with all the crap you've put on board it's probably a bit more. Say 16

 

[Dave_Snelson]

Are you trying to rub it in good sir?? A galleon an hour? - I wish!!

Joking aside, it does sound about right to me. You are also correct in that it takes the engine a lot more effort to get just that extra knot - and it ain't worth trying for either. The reason being is that on a displacement boat you have two waves, a bow wave and a stern wave, when you really put some horse into propelling a displacement hull, it makes an effort to overtake its bow wave - without the possibility of success. All you do is burn more fuel as it attempts to ride up the wave. So... take it easy (Mebbe a lesson in there for us mobo-ers! /forums/images/graemlins/wink.gif )

 

[Deleted User YDKXO]

Duncan Hamble is about right in his calculation. A typical diesel will use about 0.25kg of fuel per KwHr so 5 litres per hour does correspond to about about 20kw, OK thats about 27hp. However that doesnt correspond with your statement that you get 6knots @ 3200rpm because @3200rpm, according to the power curve, that engine is developing about 80hp at which you should be using about 15 litres per hour
Is your figure of 3200rpm correct?

 

[iangrant]

I think it may be more like 2800, she has been out of the water for too long!!! Since before Christmas!!!

Thanks anyway

Ian

 

[iangrant]

Oi Tome you cheeky sod - just 'cause your old tub couldn't carry it..let alone you wouldn't understand it anyway.

Ian

 

[tome]

What's a de-salivation plant anyway? /forums/images/graemlins/smile.gif

 

[iangrant]

See what I mean -

Anyway Julie on a Friday...thats enought to stop any salivation.

 

[tome]

I thought it might be something for your gorgeous missus. I could see her undressing me with her fluttering eyes. When did you say you were going away?

 

[iangrant]

Oh you can have a go in June, be warned she is however, an expensive package.....she don't like beating to windward, and the fridge has to make enough ice for the vodka intake..

Ian