Are you going any slower?

[seafox]

Yeah, I should have made the poll for launchees not yachtees. I spend bugger all on diesel. Just something I noticed with the launches recently.

I had lots come up from behind and then take ages to pass. I looked at my speed and was only doing 5 to 6knots. That was when I realised that they are going slower than normal.

Bloody great.

[Alan Wheeler]

If we go any slower we will be going backwards.

Darryl does that often. Mind you, he is slowly teaching me patience.
No I can't say I have noticed it over here. And I talked to a fizz boat owner today and asked the same question. He also said no. He figured you can't have as cheap a holiday any other way. Even a drive to CHCH and back now will cost you a hundred in fuel at minimum. Take a motel room for 120 a night and add a meal out and it hits the pocket real hard.
One interesting point I learn't today. I was going over the specs of a large Launch. She was a planning hull with twin Cummins engines, shaft driven. The fuel per hr for each engine was almost spot on the same as my boat engine usage. Then when the boat reached the plane, the fuel increased of course, but the maths worked back to my boat vs time, and both boats used about the same fuel over the distance. He just got there faster was all. So really, he had no reason to pull back on the tap, because the difference in fuel use was very little. And that is why Transport trucks use Diesel engines. They difference between empty and full is only a 20% increase in fuel consumption. But a petrol engine, the difference is 80% and can be higher.
Displacement hulls can make substantial savings however. Fuel use can rise dramaticaly just to get that extra knot.

[delmarrey]

Back in the early 80's when I had a power boat my dock buddies use to make fun of me because I'd wait for a favorable tide to go in/out the jetties. I'll bet they aren't laughing now. That ole beast would burn 25 g (94 lt) an hour at full throttle and half that at 12 kt. The fuel prices here have tripled in the pass two years.

[rebel heart]

I think the only thing I'm doing differently is running the electric heater instead of the diesel cabin heater when at our slip. But I've been gutting a lot of the engine driven gear, but more because of the non-fuel related engine costs.

[seafox]

ha, I thought of you when I saw the going backwards comment. That is what you say to us.

Originally Posted by Alan Wheeler

Darryl does that often. Mind you, he is slowly teaching me patience.
No I can't say I have noticed it over here. And I talked to a fizz boat owner today and asked the same question. He also said no. He figured you can't have as cheap a holiday any other way. Even a drive to CHCH and back now will cost you a hundred in fuel at minimum. Take a motel room for 120 a night and add a meal out and it hits the pocket real hard.
One interesting point I learn't today. I was going over the specs of a large Launch. She was a planning hull with twin Cummins engines, shaft driven. The fuel per hr for each engine was almost spot on the same as my boat engine usage. Then when the boat reached the plane, the fuel increased of course, but the maths worked back to my boat vs time, and both boats used about the same fuel over the distance. He just got there faster was all. So really, he had no reason to pull back on the tap, because the difference in fuel use was very little. And that is why Transport trucks use Diesel engines. They difference between empty and full is only a 20% increase in fuel consumption. But a petrol engine, the difference is 80% and can be higher.
Displacement hulls can make substantial savings however. Fuel use can rise dramaticaly just to get that extra knot.

[irwinsailor]

Does running a diesel engine in a sailboat at a lower rpm really save fuel? I thought that it is best to run them at the recommended rpm for the best economy. In my case that would be about 2500 rpm for my engine that is rated for 3600rpm.

[AnchorageGuy]

IS, Running at an optimum speed will save fuel but most manufacturers also don't recommend running at max RPMs for more than a certain period of time. So Basically we have our cruising RPMs with a period of time each day running a max RPMs to keep the engine happy. With our Yanmar without turbo we try for 1/2 to 3/4 hour per run or 24 hours. It seems to work for us.

[Alan Wheeler]

Does running a diesel engine in a sailboat at a lower rpm really save fuel?

It compleatly depends on the Hull type. Planing hulls will be more economical at upper RPM's. Displacement hulls require an eponential increase in power(thus fuel consumption) as you push the boat closer to theoretical hull speed.

[irwinsailor]

My Westerbeke is rated for 3600 Max rpm , the manual states that the fuel burn is 2 gallons per hour at 2500 rpm. I have found that to be about right. Would I save fuel by running my engine at .. say 1800 rpm? Would running my engine for extended periods at 1800 do any harm to it?

[AnchorageGuy]

IS, Different boats and hull configurations as well as sea state affect your fuel usage. Only a fuel flow meter and sea trials under calm conditions will give you a base but it is never going to be that exact every day. Just a guideline. Without a way to measure fuel flow on your boat it is going to be hard for you to tell other than filling the tank, running the boat at different RPMs under the same conditions and refilling the tank. Even then it is only a close estimate. Some manufacturers, but mostly for power boats, provide a fuel flow chart based on RPMs. But how much is in your tank, lockers and how much anchor chain you carry can even change all of that.

[Alan Wheeler]

Would running my engine for extended periods at 1800 do any harm to it?

Not really. Although it is usually recomended that if you run for extended times at slow RPM, you want to run the engine hard every say two hrs for a short time and then return to the slower speed again.
Fuel usage per RPM doesn't really change. "When I say Doesn't really", I mean within about 20%. It is the fuel usage over ground covered that make the really big differences. So planning huls can have wide variances of fuel usage based on loaded wait, sea states, wind etc etc. Displacement hulls don't tend to get the wide variances till you try pushing it too hard.

[MidLandOne]

Just in case not known, a number of modern fast planing powerboats I've dealt with have flat fuel consumption curves in terms of miles per gallon from the time they get on the plane right up to full speed. Of course, their fuel use when not on the plane is the highest in terms of MPG.

So, for those boats slowing down will not conserve fuel at all. But as Alan infers, lessening the deadweight carried, etc will reduce the fuel used.

Getting to yachts, what has to be kept in mind is that fuel use is directly related to power output at the time, and that is not constant for a given rpm. When the throttle is set on a diesel engine it is setting the rev's the engine is to run at and the governing will just vary the fuel flow to maintain those revs according to the power needed to maintain them. The power needed to maintain those revs will vary alot between different design boats even with the same engine in them and for a given boat according to the conditions of the day (cleanliness of bottom and prop, sea state, wind, etc).

As has been said, unless one can do trials with accurate fuel flow measurement one is pretty much in the dark. Manufacturer's fuel use curves are of minor help unless one actually knows the power the engine is developing (which can be modelled but that is not available to most of us).

We have a slowish reving engine (max around 2,900rpm) and under normal load conditions I normally run it at around 70-75% of max for continuous cruise as that gets us to around hull speed if the boat is clean and is friendly to engine life (and also not so noisy ).

I have no real idea if that is the most economical or not in terms of our fuel use for us though . But as Alan says I also suspect there is little change in fuel use for a given boat unless pushed hard.

But I would hope that if a boat has been well designed with liaison between the designer and the prop builder and in terms of engine selection then running the boat at revs which just get the boat to hull speed in moderate conditions is somewhere close to the most economical in terms of fuel use. I know that liaison took place with our own boat.

[David M]

Lets not confuse terms. Efficiency mean miles per gallon or whatever units of fuel volume or fuel weight and distance one chooses to use.

Planing hulls are not more efficient on a plane as compared to when they are in their displacement mode. At 18 knots, I get one mile per gallon. If I drop the boat down to its theoretical planing speed, I get a much better miles per gallon. If I drop the boat down to four knots, even better miles per gallon. This is not a guess, I plotted out the fuel consumption curve for myself. My efficiency curve was plotted based on the manufacturers fuel meters that came with the boat and running a distance using GPS in two different directions at near the same time....so as to zero out any current or wind effects. It was an easy conversion from gallons/hour to miles per gallon knowing speed both ways.

One point of interest is that a power boat almost on a full plane gets worse consumption than a boat that has just reached a full plane...above that speed and the consumption can get worse than when the boat is almost planing.

The fuel consumption curve for miles/gallon curve is shaped like an S laying on its side with the ends of the S extended down and up...there is a little high point in the curve at the point where the boat is almost on a plane.

A diesels specific fuel consumption, lb/(hp·h), is what it is getting in terms of the amount of horsepower out of a given volume of fuel...the maximum is generally around 70% to 80% of maximum horsepower. This is generally into its higher rpm ranges. Many people confuse this with where a boat is most efficient. No diesel has its maximum specific fuel consumption at 100% RPM. Some manufacturers do recommend running them at 100% periodically for a set period of time to reduce carbon accumulation.

A little more clarity on sizing a propeller...if your propeller is preventing a healthy engine from reaching it maximum RPM...(NOT TORQUE OR maximum specific fuel consumption)...then your prop is either pitched too high or the diameter is too large or both. There are formulas for determining pitch/blades/diameter/rpm versus horsepower. Anyone who is decent at math can do one.

[MidLandOne]

Originally Posted by David M

Lets not confuse terms. Efficiency mean miles per gallon or whatever units of fuel volume or fuel weight and distance one chooses to use.

Planing hulls are not more efficient on a plane as compared to when they are in their displacement mode. At 18 knots, I get one mile per gallon. If I drop the boat down to its theoretical planing speed, I get a much better miles per gallon. If I drop the boat down to four knots, even better miles per gallon.

That is not the case with modern high efficient hulls and drives though. I know, I have been owner's representative on board during the acceptance sea trials prior to delivery of newly constructed such vessels and I know of many other planing vessels that are the same.

I hauled some figures out for just one of those in order to give an idea - the very best fuel consumption in miles per gallon measured accurately on acceptance trials while off the plane were nearly 25% more than those when the boat was on the plane so in terms of mpg was much more inefficient off the plane. Then from the time it was over the hump onto the plane up to maximum revs (so up to maximum speed, in this case 38 knots in tanks full condition) the fuel use in miles per gallon was constant.

That is not uncommon for modern planing vessels, both commercial and pleasure.

[Alan Wheeler]

I am a little confused with what you are on about. Sure a planning hull is going to be inefficient while off the plane. And the transition point between the two is usually the most fuel hungry. Although that does depend on hull design and length. Some plane eaiser than others. But back to the point. A planning hull is desinged to be more fuel efficient at the higher speeds. A displacment hull is efficient only because it takes far less power to push it to speed. If both hull types have been designed ruffly correctly, then fuel consumption will be ruffly similar through the displacment stage and then the planning hull will start to dramaticaly increase fuel consumption. But that increase is then summed against ground it can cover. So if both hull types were to leave point A together, The planning hull will reach point B first. But fuel burn't would be ruffly twice that of the Displacement hull. I do say Ruffly. Of course, you can throw huge engines into the planning hull and push water out like a rocket sled and burn ungodly amounts of fuel and get to your fishing spot before any of the runabouts pushed the choke back in.