Low RPM cruise

[ontherocks83]

Originally Posted by sailorboy1

My boat will use 0.6 gph to go 5 knots. It will about 0.8 gph to do 6.5 knots. But near as I can determine is that it will use 1.1 gph at 7+ knots.

Those as facts in my logs, it matters little whether I motor at 5 knots or 6.5 knots far as fuel economy goes.

Of course I might be lying.

Originally Posted by Kenomac

And your numbers are good for our 53 and our 62, right about the same. The diesels are hardly working and the burn rate comes out about even when distance covered is factored in.

just out of curiosity how are you guys getting your GPH figure? Do you have a fuel flow meter or using some other form of calculation?

[Kenomac]

Originally Posted by ontherocks83

just out of curiosity how are you guys getting your GPH figure? Do you have a fuel flow meter or using some other form of calculation?

Dip stick and credit card, along with many thousands of miles of motoring and motor-sailing. If I stay below 6 knots, it’s truly mind-boggling how far each boat will travel on a gallon of diesel.

[ontherocks83]

Originally Posted by Kenomac

Dip stick and credit card, along with many thousands of miles of motoring and motor-sailing. If I stay below 6 knots, it’s truly mind-boggling how far each boat will travel on a gallon of diesel.

Especially when you get into the size boat you are sailing. Our 30' Catalina wasn't drastically far off both of your numbers, but it also had a tiny 11hp 2 cylinder diesel.

[sailorboy1]

Originally Posted by ontherocks83

just out of curiosity how are you guys getting your GPH figure? Do you have a fuel flow meter or using some other form of calculation?

I write down in the log the engine hours and gallons on each fill up.

[SailRedemption]

Originally Posted by a64pilot

Yes, my opinion is whatever RPM your engine requires when under load to come close to being fully warmed up. If your regular cruise temp is 180f then whatever it takes to get to or close to there.
It’s not an RPM thing either, heat comes from combustion mostly, and it takes very little fuel to raise a Diesel to a high idle, so it’s not getting warmed up, you want it working, which usually means driving the boat of course.

It’s my belief that Yanmar issued the race the engine up every two hours in neutral for the majority of boats that were idling them for hours a day charging batteries. Yanmar like all manufacturers primarily is concerned with getting their product through the Warranty period, and I’d bet they were getting warranty claims on newer motors in the charter fleets, who are big customers and need taking care of, or they may switch motors on their newer boats.
It’s not meant for those motoring or motorsailing between say 1500 and 1800 RPM, or if it is, what this race it in neutral about?

Running an engine at 80% of max thinking that is good for it is nuts.
That’s right at 2900 RPM on a 3600 RPM motor.
That is getting close to how the powerboat people run theirs, and their motors don’t last nearly as long as ours, and in truth theirs are usually better built more robust motors than ours too.
Aircraft engines that have a TBO of between 1500 and 2000 hours are only run usually at 85% of max power for a maximum continuous power, that’s not suggested, that’s if you exceed this You will break it power setting.
Most that own their own airplane and do their own work only cruise at 65% of power.

No, I think it’s like every other item on a boat, size or so that your using it mostly in the middle 1/3 of its capability and you’ll get good service out of it.

I don't think RPM is a not an indicator of load. Sure the higher the RPM the more load will be on the engine. Having an EGT gauge tells you when your engine is operating at its ideal RPM and/or load. Also, with a turbo, a boost gauge will, in conjunction with the EGT, tell you more information about how your engine is loaded. You could be at 1500rmp and be hitting the right temps and boost pressures if the load on the engine is high.

Originally Posted by Dockhead

Fuel consumption in miles per gallon for a displacement hull will go up rather steeply with increasing speed, almost from 0. That's a matter of physics -- hull resistance goes up exponentially even before you get to hull speed effects.


So your experience is bizarre -- could only be some mistake in measuring or calculating, or some oddity of the propeller or engine.

Dockhead, the Yanmar 4JH4-TE (which I'm sure is VERY close to your engine) operating temp is around 700*f. That was from the tech at my local Yanmar distributor (not dealer). I believe this is also in the installation manual as well. I had my yanmar 4JH4-TE over 1000* running wide open with a fouled prob tied to the dock. So lots of load, but this isn't ideal obviously.

[Exile]

Originally Posted by Kenomac

Nyuk, Nyuk, Nyuk!: http://www.cruisersforum.com/forums/...ew-208259.html

Bummer Ken! I figured you either made it across or had some of the not unexpected new-to-you boat maintenance pains. Never figured on your crew being so miserable and interfering with your plans. Hope it's a happier thread after your attempt next May.

[a64pilot]

Originally Posted by Exile

Makes a lot of sense a64. By "fully open thermostat temp" do you mean the 180ºF that my particular boat engine is set at? (200ºF for my truck). My engine temp is closer to 190ºF when I'm running above about 60% or ~2100 rpm. Ditto on my truck (maybe 210ºF) if I'm pulling a heavy trailer up a grade.



I don't want to further sidetrack the thread but just wanted to be sure what you meant by fully open thermostat temp.



Thanks.

This is a screen shot of my manual where it says thermostat cracks at 75 to 78 c and is fully open at 90c. They open gradually and modulate water flow, they are not like an electric switch, open or closed.

[a64pilot]

Originally Posted by Dockhead

Fuel consumption in miles per gallon for a displacement hull will go up rather steeply with increasing speed, almost from 0. That's a matter of physics -- hull resistance goes up exponentially even before you get to hull speed effects.


So your experience is bizarre -- could only be some mistake in measuring or calculating, or some oddity of the propeller or engine.

I have graphed fuel consumption over speed on three different boats, all with flow meters, although the last was picked from the can bus so it was calculated fuel flow, not actual measured, it was also the only gas motor and an outboard.
They were all three planing boats and I was certain that If I looked hard enough that I could find a sweet spot, a spot just when they broke onto plane that I actually got better fuel mileage than a speed greater of less than the sweet spot.
Well what I found surprised me, that was regardless of anything else, trim tab setting etc, the faster you go, the worse the fuel mileage, yes the graph had steep spots where you burned a lot more fuel for very little increase of speed, like right before you broke onto plane, but it never dropped, there was no single speed that got better fuel mileage than a slower speed. This was on two Sportfishemen, a 36 twin engine and a 45’ twin, and my single engine outboard center console Dive boat.
They are not sailboats of course, but with all three the faster you went, the more fuel you burned over a distance, best fuel consumption was single engine, idle.
Now this was in a no current situation, current may change that of course.

[Exile]

Originally Posted by a64pilot

This is a screen shot of my manual where it says thermostat cracks at 75 to 78 c and is fully open at 90c. They open gradually and modulate water flow, they are not like an electric switch, open or closed.
Attachment 178407

Thank you. Learning all the time. I'll check my engine manual on this.

[a64pilot]

Originally Posted by Kenomac

It has more to do with the fact that our diesels really aren’t working that hard up until they try to dig a bigger hole in the ocean.

What are we now, the Three Stooges?

Think of it like the Jimmy Carter years when the knucklehead decreased the speed limit from 65mph down to 55mph to supposedly safe fuel, but all he ended up doing is slowing down commerce and pissing everyone off. After the engine gets up to speed pushing a car or truck to 65mph, it doesn’t take much to keep it there, it’s the accelleration or climbing hills that burn the fuel. It’s the same with displacement hulls.

No, that is not true either. Read up on Hypermiling, the best steady state fuel mileage will be obtained at the slowest speed that the engine will run smoothly in the highest gear, however if you really want to Hypermile, you need to pulse and glide.

[Delfin]

Originally Posted by a64pilot

I have graphed fuel consumption over speed on three different boats, all with flow meters, although the last was picked from the can bus so it was calculated fuel flow, not actual measured, it was also the only gas motor and an outboard.
They were all three planing boats and I was certain that If I looked hard enough that I could find a sweet spot, a spot just when they broke onto plane that I actually got better fuel mileage than a speed greater of less than the sweet spot.
Well what I found surprised me, that was regardless of anything else, trim tab setting etc, the faster you go, the worse the fuel mileage, yes the graph had steep spots where you burned a lot more fuel for very little increase of speed, like right before you broke onto plane, but it never dropped, there was no single speed that got better fuel mileage than a slower speed. This was on two Sportfishemen, a 36 twin engine and a 45’ twin, and my single engine outboard center console Dive boat.
They are not sailboats of course, but with all three the faster you went, the more fuel you burned over a distance, best fuel consumption was single engine, idle.
Now this was in a no current situation, current may change that of course.

Planing hulls? Totally different fish than a displacement hull, which I thought your sailboat was so that was what we were talking about. For displacement hulls, physics determines that the faster you go, the lower the nmpg. For planing hulls, there is a sweet spot but the difference in nmpg isn't great at different planing speeds. On the other end of the spectrum and another completely different fish is a hydrofoil, where the faster you go, the greater the nmpg. It all has to do with the effect of drag at different speeds for different designs.

[a64pilot]

Yes, they were planing hulls, although I graphed from idle to full RPM.
I don’t think it matters, the result is the same.
Of course a displacement hull would have used less fuel at displacement speeds.
Bottom line, to go faster requires more energy, more energy means more fuel burned, it really is that simple.
If you ever get yourself in a situation that absolute fuel range is important, slow down, the more you slow down the longer it’s going to take, but the less fuel it will take too.
It just like riding a bicycle, anybody can average 10 mph, but 20 mph is tough, 5mph is even easier.

[markpierce]

Originally Posted by SailRedemption

I don't think RPM is a not an indicator of load. ...

Rate of fuel consumption is a measure of load. Comparing fuel consumption rate to maximum rate of fuel consumption measures relative load.

[jimbunyard]

Originally Posted by sailorboy1

From my experience acturally tracking my fuel usage I've determined that the mpg between 5-6.5 knots is basically the same. And that 6.5 knots is almost exactly 2 knots below my hull speed.

It's funny that before it took 2 years of fuel tracking for me to decide that it made no sense to go 6.5 knots instead of 5 knots because I was only looking at gal/hr and would motor at 5 knots for that. Back then I thought 6.5 knots was motoring HARD I still sometimes only motor at 5 knots were not in a hurry just because it's quiet and/or that I what to charge batteries longer.

Originally Posted by sailorboy1

My boat will use 0.6 gph to go 5 knots. It will about 0.8 gph to do 6.5 knots. But near as I can determine is that it will use 1.1 gph at 7+ knots.

Those as facts in my logs, it matters little whether I motor at 5 knots or 6.5 knots far as fuel economy goes.

Of course I might be lying.

.2 gph seems rather significant to me...with a 50 gallon tank 5 knots gets you 10 extra gallons (83 miles) at the end of the tank, as compared to 6.5 knots.

Perhaps you mean it's matters little financially?

[Kenomac]

Originally Posted by jimbunyard

.2 gph seems rather significant to me...with a 50 gallon tank 5 knots gets you 10 extra gallons (83 miles) at the end of the tank, as compared to 6.5 knots.

Perhaps you mean it's matters little financially?

People keep missing his point, or the simple point that he TRAVELED FARTHER during the hour he traveled at 6.5 knots... an extra 1.5 nm was covered, so his fuel consumption was roughly the same as when he motored at 5 knots. Sailorboy was burning .2 gallons more to get 1.5nm further along. Distance traveled needs to be part of the equation, not just gph to determine fuel economy.

Our diesels work very little to maintain these cruising speeds in the 4-6 knot range, nothing in common with power boats with planing hulls. More like an 18 wheel diesel rig going down the highway at a steady 65mph on a flat road, where a relatively small amount of energy is required to keep it rolling along after it gets up to speed.