Mahe 36: Fuel Consumption

[jean1146]

Eleven I have 2 Volvo's 20hp on my boat.it seems to be plenty so far.burning less than a gallon an hour on both engines at 6kt.I am sure that I will get in a situation where this may not be enough.I have the standard fixed props.JC.

[jean1146]

Istyles is 3000 RPM maximum operating range? on my 20' 2200 RPM is what is rcomended by Volvo. That's about 6 to 7 Knots depending on the conditions,burning less than 1 GPH on both engines.JC.

[Scott730]

Jean
You can operate the D1-20's at 2400 to 2500 rmp and reach max torque and h.p. and still only burn 1 gallon per hour for both engines, the max rpm is 3200. Here is a spec sheet on the engines:
http://www.volvo.com/NR/rdonlyres/D9...AE6/0/D120.pdf

Scott

[jean1146]

Thanks scott. I have been cruising at 2200 RPM,but I will now try 2400.probably will gain another knot may be.JC.

[Scott730]

I would like to mention that you could operate these engine at 2800 rpm with no problem at all and get even more power out of them, but you would be burning around 1.5 gallons per hour. I routinely operate the engines at 2500 to 2700 rpm's for extended periods of time and have cruised at 7+ knots. You just burn more fuel and have a little more engine noise, but if you want to make that anchorage before dark........

[lstyles]

For the 30 hp engines:
On our trip we figured that motoring at 2700 rpm which we did for 15 hrs straight at one point (no wind) we used about 1.1 gallon/hr using both engines. Operating at 2000rpm we figured we used about 0.8 gallon per hr for both engines. Charging the batteries used about 0.7 per hr both engines.
Lori

[Therapy]

Originally Posted by lstyles

For the 30 hp engines:
On our trip we figured that motoring at 2700 rpm which we did for 15 hrs straight at one point (no wind) we used about 1.1 gallon/hr using both engines. Operating at 2000rpm we figured we used about 0.8 gallon per hr for both engines. Charging the batteries used about 0.7 per hr both engines.
Lori

I keep seeing this kind of evaluations for marine stuff and still do not get it.
I know currents etc make a difference in SOG but mpg is a more real figure to me.

If one burns a little more fuel but goes faster then the total time the engine is run and the amount of fuel burned could be the same.............in many instances.........what am I missing...........???
Besides the fact that for each setup there is an optimum that takes a long time to find.

[Eleven]

Your in the Engineering phase again which I do have some experience of.
First I would say keep a note book on engine runs and note boat speed, engine revs, wind speed and direction.
Your (nautical)miles per gallon is speed divided by gallons per hour.
Shouldn't take long to work out your economical speed and your hurry along costs per mile. That will give you a good ides of how far your boat will could go on the fuel on board and whether it will go futher if you slow down a bit.
I suspect, not tested, that most economical speed is between two thirds and three quarters of the top speed. DATA PLEASE.
Oh the wind. Obviously it makes a difference, by making a note you'll have some idea of how much difference it makes. If the difference is big your prop may not be right.

[Nordic cat]

Originally Posted by Therapy

I keep seeing this kind of evaluations for marine stuff and still do not get it.
I know currents etc make a difference in SOG but mpg is a more real figure to me.

If one burns a little more fuel but goes faster then the total time the engine is run and the amount of fuel burned could be the same.............in many instances.........what am I missing...........???
Besides the fact that for each setup there is an optimum that takes a long time to find.

Therapy; there are a couple of basics regarding fuel consumption and boat speed for boats in displacement mode, i.e. NOT planing, and Catamarans NEVER plane!

First there is hull speed, which can roughly be calculated as:

1.34 x SQRT Lwl. So for a 36 ft boat, the hullspeed will be around 1.34 x SQRT36 = 1.34 x 6 = 8 knots. This is the point where there will be a resistance "hump" that is very inefficient to go beyond when motoring.

The next "fact" is that fuel consumption for a motor/propeller on a boat in displacement mode can roughly be calculated, using: Consumption = Constant x n e3. ( n is rpm) So if you know your consumption at one speed, you can roughly calculate what it will be at another.
Basically this means that to double the revs your consumption will be 8 times higher. This is not entirely true, as alternator loads, fricton losses etc. are relatively higher at lower speeds. Also windage, waves, wind speed and wind direction play a role, and are part of the Constant for a set of defined conditions.

I hope this clears things up a bit.

Regards

Alan

[slomotion]

I think Therapy's point was simply that while gph implies some sort of reasonable progress under some defined set of circumstances, it doesn't by itself tell you anything about how far or fast you can go. It simply tells you how long you can run the engine for a given supply of fuel under those conditions. Mpg doesn't tell you anything about speed either, but it tells you exactly how far you can go for a given supply of fuel. I think gph was probably popularized in the context of big twin engine cabin cruisers with embarassingly low mpg - even then it was generally and helpfully expressed as gph @ x knots. It is certainly true that the variables of wind, current, tide, seas, etc. make an EPA style mpg rating for boats kind of silly. That doesn't mean that it's not a uselful number for a given boat on a given trip under given conditions.

[Therapy]

Originally Posted by slomotion

I think Therapy's point was simply that while gph implies some sort of reasonable progress under some defined set of circumstances, it doesn't by itself tell you anything about how far or fast you can go. It simply tells you how long you can run the engine for a given supply of fuel under those conditions. Mpg doesn't tell you anything about speed either, but it tells you exactly how far you can go for a given supply of fuel. I think gph was probably popularized in the context of big twin engine cabin cruisers with embarassingly low mpg - even then it was generally and helpfully expressed as gph @ x knots. It is certainly true that the variables of wind, current, tide, seas, etc. make an EPA style mpg rating for boats kind of silly. That doesn't mean that it's not a uselful number for a given boat on a given trip under given conditions.

Righto!

A simple number. I know there are more variables on water than on land!

When I (if ever since this week was full of bad news for the future) get a boat, one of the things I will know is how many miles per gallon I will get given a set of parameters, ie light wind, low seas, bla bla, etc.

Thanks.

[VisaView]

Hello all,

I was hoping to get actual numbers on a cruising range at @ 6 knots aboard the Mahe 36
I will be taking delivery This Sept. 24 in La Rochelle and need to know how far she'll go on 53 gallons of diesel.
Thanks,
Bill

[freetime]

Do you have the 20 or 30hp engines? Where do you live?

[VisaView]

Fuel consumption on Mahe 36
Hello all,

I failed in earlier post o mention the size of the engines, they are the 20HP Volvo's

I was hoping to get actual numbers on a cruising range at @ 6 knots aboard the Mahe 36
I will be taking delivery This Sept. 24 in La Rochelle and need to know how far she'll go on 53 gallons of diesel.
Thanks,
Bill

[DtM]

I guess you are working in US gallons.

We use 4.5 to 5 litres per hour cruising at 6 knots or so. With a 200 litre tank (which is the standard Mahe tank) that would give a safe range of 240 miles.

daniel