GPH vs Miles help??

[OldDan1943]

The general consensus seems to be, 'start out with the factory/published chart and then modify for your boat, noting rpm, speed of boat, wind speed (for or against), current speed (for or against) and wave direction and height (for or against).
You have now created your very own, more accurate graph or chart. Refine as you gather more real world data.
Remember, things do change..... ie, the weight of your boat will change as the fuel and water loading changes

 

[CharlieO.]

I appreciate the o.p.s questions since I have recently become owner of a similar boat.

From what I've come up with for planning purposes is 6.5 knots at 2 gph.

 

[OldDan1943]

I appreciate the o.p.s questions since I have recently become owner of a similar boat.

From what I've come up with for planning purposes is 6.5 knots at 2 gph.

I think you will be safe with that estimate provide you tack on about 1gph when you run the generator

 

[psneeld]

So, if I understand this correctly, my range is…

600 gal - 10% = 540
540\2 gal hr = 270 @ 10 mph
270*10 = 2700 mile range

Not bad for a couple of 375 hp 3208’s ��

First, where did you get 2 gph for a Cat 3208 370hp?

If anything its 2 gph per engine and with 2 engines....I think it would be....

540/4 = 135 @ 10 mph or 1350 miles range...

But I will bet it's much less....closer to what Peter posted in post 90.

 

[BandB]

We have detailed performance charts on our boats, covering all speeds, based on fuel flow and verified by actual experience. Still we'd never cut it closer than 10% and even that is very protected. For instance if we've got a long distance, we'll conserve fuel even further the first half so we're certain to be safe the second half of the route. Also, we conserve when we don't know how long we might be held up waiting for locks.

Wind, current, waves, all conditions make a lot of difference. That's why tests are based on two way averages but even those can't be depended on as they're taken in calm conditions. However, the two way runs often differ as much as 2 knots.

We've seen boaters who love to use averages. Well, you don't get average conditions. The one time you're close is when conditions will be rough and you'll come up short.

Also, we've seen those hesitant to carry drums with them. They have 500 gallons of fuel but can extend that 20% with two 55 gallon plastic drums and be safe. Yet, professionals running long trips get the drums and use them first, before even going to the regular tanks.

I guess running out isn't a concern where you have a quick tow coming. But it is in other places and is to me, anywhere. Until you have proven data, don't cut it close. I also hear people so sure how much fuel their tanks hold. Yet, are you really sure? Is that 200 gallon tank really 200 gallons or 190? I know many examples of tanks not being the size quoted. Plus others where you can't actually get to all the fuel.

 

[backinblue]

I don't think it's a matter of having a nearby tow, I've never thought about that when thinking about fuel capacity. It's really only an issue for your intended cruise plans. If you need to travel hundreds of miles without refueling, then of course it is critical. Otherwise, you just need to be sure you have more than enough fuel for your intended trip and also whether there are places where you can re-fuel.

 

[psneeld]

I really hope you are not referring to me for " suggesting averages" and not applying it with caution against a particular voyage.

Better knowing your average as a relative new owner to a vessel than a singular or factory set of numbers. That average is a starting point not a final answer to anything.

If you think you need to calculate max range index extreme conditions...you can calculate that to a close degree or log a coup,e thousand miles under sever to extreme conditions and make up your charts then.... only but the few here log those kinds of miles.

 

[tiltrider1]

So, if I understand this correctly, my range is…

600 gal - 10% = 540
540\2 gal hr = 270 @ 10 mph
270*10 = 2700 mile range

Not bad for a couple of 375 hp 3208’s ?

I’m making some assumptions here. I starting with a 42’ boat running twin 3208’s running at 8.5kts. We are pushing a bow wave at this speed, 7kt would be considerably more efficient. I’m thinking we need 70hp to do this. 3208’s make 19 hp per gallon per hour so we would be burning 3.7 gallons an hour. This is giving no consideration for real world conditions.

I’m just doing this off the top of my head, not claiming great accuracy here but my guesses are usually in the ball park.

 

[Riff Raff]

3208’s

Had a Tollycraft 44. Twin 3208’s @375 hp each. 1200 rpm between 8-8.5 depending on bottom cleanliness. Between 4-4.3 gph total for both. No generator. Boat over propped to put bit of load on engines. Ran up to 1750ish at end day to clean turbo. Don’t remember fuel usage at 1750 but well over 10.

 

[mvweebles]

We have detailed performance charts on our boats, covering all speeds, based on fuel flow and verified by actual experience. Still we'd never cut it closer than 10% and even that is very protected. For instance if we've got a long distance, we'll conserve fuel even further the first half so we're certain to be safe the second half of the route. Also, we conserve when we don't know how long we might be held up waiting for locks.



Wind, current, waves, all conditions make a lot of difference. That's why tests are based on two way averages but even those can't be depended on as they're taken in calm conditions. However, the two way runs often differ as much as 2 knots.



We've seen boaters who love to use averages. Well, you don't get average conditions. The one time you're close is when conditions will be rough and you'll come up short.



Also, we've seen those hesitant to carry drums with them. They have 500 gallons of fuel but can extend that 20% with two 55 gallon plastic drums and be safe. Yet, professionals running long trips get the drums and use them first, before even going to the regular tanks.



I guess running out isn't a concern where you have a quick tow coming. But it is in other places and is to me, anywhere. Until you have proven data, don't cut it close. I also hear people so sure how much fuel their tanks hold. Yet, are you really sure? Is that 200 gallon tank really 200 gallons or 190? I know many examples of tanks not being the size quoted. Plus others where you can't actually get to all the fuel.

When PAE prepped for their circumnavigation with an early N40, they essentially did as BandB suggests. They built a calibrated day-tank so they could accurately monitor fuel usage. And they ran the boat from Dana Point to Seattle and back, 1000 nms each way. Even with that, they ran at lower speed the first half of the trip to Hawaii, the longest leg of a circumnavigation at around 2300 nms. The N40 carried around 950 gals of diesel as I recall, and arrived with less than 100 if memory serves so they burned around 850-875 gals for the trip.

Point being PAE knew that boat inside and out even before it was delivered from the builder. They had Lugger/ADE support and knew the engine very well too. And they still went to heroic measures for fuel burn.

That said, you gotta start somewhere. Nothing wrong with averages as long as you update as more data is derived.

Peter

 

[backinblue]

I really hope you are not referring to me for " suggesting averages" and not applying it with caution against a particular voyage.

Better knowing your average as a relative new owner to a vessel than a singular or factory set of numbers. That average is a starting point not a final answer to anything.

If you think you need to calculate max range index extreme conditions...you can calculate that to a close degree or log a coup,e thousand miles under sever to extreme conditions and make up your charts then.... only but the few here log those kinds of miles.

I was referring to B&B saying mileage is not important if you have a tow nearby. I disagree and think it's more about what your plans are. It's not really important if your fuel onboard will take you 500 miles or 600 miles if you are only planning on going out for a 100 miles over the course of a week

 

[SteveK]

Combined approach speed of 17 kts means they will meet in 58.82352941176471 hours at which time the slower vessel would have travelled 470.5882352941176 nm and the faster one 529.4117647058824 nm. Since neither vessel is likely to be capable of instantaneously reaching cruising speed or instantaneously stopping, I'd truncate to 470 and 529 NM with the last mile for manoeuvring.
And if a fly flew at 15.7 kts from bridge to bridge, out and back repeatedly, starting when the vessels started and stopping when they met, how far would the fly have flown - assuming that, unlike boats, flies can start, stop, and reverse direction instantaneously.

Your math is precise for two moving objects, except you missed the hint of post 15 and the OP posts.
The 8 knot boat will only travel 400 miles and then wait for the other boat to arrive with more fuel.

 

[RedRascal]

I appreciate the o.p.s questions since I have recently become owner of a similar boat.

From what I've come up with for planning purposes is 6.5 knots at 2 gph.

One factor that will increase or decrease your fuel burn at 6.5 knots is the prop. Some boats are over propped and can't reach the engines max rpm. This means you will run at a lower rpm at 6.5 knots vs. being propped to hit the max rpms for the engine. The lower rpms gets you less engine noise and lower fuel burn however you won't be able to push the boat as fast as a boat with a propped matched to max engine rpms. Maybe an over propped CHB 34 does 8 knots and 34 with a prop matched to max engine RPMs does 9 knots. Both are super inefficient speeds for that boat.

Do you know if you can reach max RPMs(2,600 I believe) with your boat? There are many factors that will impact max RPMs like a clean bottom and fuel system. Assuming you don't have any issues can you reach max RPMs? If not you may be over propped and will get a little better fuel burn.

 

[OldDan1943]

On the east coast of the US, if you wish to extend your mileage going north, slide into the Gulf Stream and throttle back.

 

[OldDan1943]

Per average, there are 3 or 4 types of average. Pretty much everyone chooses the 'mean average'. That is best explained as putting one hand on a hot burner and the other hand in the freezer. Some place in your body is the 'mean temperature.'

 

[backinblue]

OD, the gulf stream is pretty far offshore from my part of the east coast but you make a good point about currents in general. I boat mostly in Long Island Sound and there is an East-West tidal current that can help or hurt you. It's worth timing the tides if you are traveling significant distances east or west. Same thing with going up or down rivers as tidal currents can reach surprisingly far inland.

 

[backinblue]

Per average, there are 3 or 4 types of average. Pretty much everyone chooses the 'mean average'. That is best explained as putting one hand on a hot burner and the other hand in the freezer. Some place in your body is the 'mean temperature.'

See Dan? Your math brain is not as worn out as you thought. Although you are correct and there can be a big difference between mean and median, your burner/freezer example sounds more like an example of median than mean to me. But not everyone understands the difference and you often hear things like "the median price of homes in FL is $500K" w/o giving much thought to what that really "means". lol

 

[tiltrider1]

The lower rpms gets you less engine noise and lower fuel burn however you won't be able to push the boat as fast as a boat with a propped matched to max engine rpms. Maybe an over propped CHB 34 does 8 knots and 34 with a prop matched to max engine RPMs does 9 knots. Both are super inefficient speeds for that boat.

Lower RPMs might in theory get you better fuel economy but you won’t see it in the real world with a diesel. It takes A given amount of Torque to move the boat a given speed. By changing the prop pitch, You increase or decrease the number of RPMs needed. As the RPMs increase you actually need less HP per piston stroke meaning less fuel is used per stroke but you need more strokes. In the end the same amount of fuel is used. Now lowering the RPMs will definitely lower noise and vibration. Because diesel’s make so much torque at lower RPMs you can prop a boat to use that excess torque on the low end but as was so rightly stated the prop’s absorption rate will eventually exceed the diesels ability to make RPM’s and you will give up top speed.

 

[OldDan1943]

See Dan? Your math brain is not as worn out as you thought. Although you are correct and there can be a big difference between mean and median, your burner/freezer example sounds more like an example of median than mean to me. But not everyone understands the difference and you often hear things like "the median price of homes in FL is $500K" w/o giving much thought to what that really "means". lol

Yes, mean, mode or median?

Ah but if the time in the freezer and the time on the hot burner I do believe it is the mean. If more time is spent in the freezer, it is a weighted average or a ???
Same way with the median price for houses, if more low price homes are sold than high price home are sold it drags average down. It is a weighted average or a ????

Yes, I am brain dead.

 

[Jeff F]

Lower RPMs might in theory get you better fuel economy but you won’t see it in the real world with a diesel.

Agreed. I recently switched props and went from being slightly underpropped to slightly overpropped. At my normal cruising speeds it dropped the engine speed by about 250 rpm.

I have instrumentation that shows fuel burn and % load. When I compare performance between the two configurations the fuel burn for any given boat speed is very similar. As near as I can tell the MPG numbers haven't changed meaningfully.

 

[OldDan1943]

Jeff, those % load, fuel burn and Rpm are interesting concepts and variables.
Think of what we could do with a variable pitch prop.

 

[BandB]

I was referring to B&B saying mileage is not important if you have a tow nearby. I disagree and think it's more about what your plans are. It's not really important if your fuel onboard will take you 500 miles or 600 miles if you are only planning on going out for a 100 miles over the course of a week

Wifey B: B was being sarcastic. The other B responding now

 

[RedRascal]

Lower RPMs might in theory get you better fuel economy but you won’t see it in the real world with a diesel. It takes A given amount of Torque to move the boat a given speed. By changing the prop pitch, You increase or decrease the number of RPMs needed. As the RPMs increase you actually need less HP per piston stroke meaning less fuel is used per stroke but you need more strokes. In the end the same amount of fuel is used. Now lowering the RPMs will definitely lower noise and vibration. Because diesel’s make so much torque at lower RPMs you can prop a boat to use that excess torque on the low end but as was so rightly stated the prop’s absorption rate will eventually exceed the diesels ability to make RPM’s and you will give up top speed.

Thoughtful response, thanks for sharing more detail.

 

[RedRascal]

Agreed. I recently switched props and went from being slightly underpropped to slightly overpropped. At my normal cruising speeds it dropped the engine speed by about 250 rpm.

I have instrumentation that shows fuel burn and % load. When I compare performance between the two configurations the fuel burn for any given boat speed is very similar. As near as I can tell the MPG numbers haven't changed meaningfully.

Good insight. Also sounds like any possible reduction in prop slip with the RPM reduction didn't translate into fuel savings. Glad you could weigh in on this with data coming from instruments.

Why did you choose to over prop? I am assuming you were looking gain something.

 

[Riff Raff]

Food for thought

One reason I over propped is not as much to save fuel as to load engine more at low rpm’s to allow engine to get and stay up to operating temperature. For example, 3208’s at trawler speeds are totally loafing along. Was worried was not getting cylinder walls to temp. So over propping allows engine to operate properly, not save fuel. Current boat has properly sized power plant and prop is set to wat.

 

[Jeff F]

Good insight. Also sounds like any possible reduction in prop slip with the RPM reduction didn't translate into fuel savings. Glad you could weigh in on this with data coming from instruments.



Why did you choose to over prop? I am assuming you were looking gain something.

It was a bit of an afterthought. My boat was repowered a few years ago and they put on a new prop, 24x23 three blade. I had the original 24x29 four blade sitting around. Nice Nibral prop, but way too much pitch.

I whacked the prop on a mooring block last summer, and when having it repaired started thinking about carrying a spare prop. The prop guys were willing to depitch the original prop enough to get it in the ballpark, so we did that. Then I installed it as an experiment, knowing I could revert back to the 3 blade if needed.

I'm happy with the change. All my operation is in the midrange, where increasing the load isn't a bad thing, and reducing RPMs gives a little less noise. For my purposes it's a net positive.

 

[FF]

"So over propping allows engine to operate properly, not save fuel"

Propping the vessel to have the engine operate in a very efficient zone at low RPM is not "over proping " for most displacement cruisers.

A sport fish or water skiing boat needs full HP for a short time , cruisers only if attempting to push mud or back off a sand bar.

The slower running engine is usually more efficient as the % of load is higher , and the slower speed allows more combustion expansion .

Operating a properly propped vessel may require a lower RPM red line to keep from overloading at WOT.

 

[backinblue]

I've never heard that over-propping to reduce RPMs and save fuel is a good thing. Intuitively it sounds right, but I always understood engines to have an RPM range where they are most efficient and it's not a linear rise in fuel consumption from low RPM to high. If I look at the output curves fir my Yanmar (3800 RPM max), it is most efficient around 2600-2700 RPM. That apparently is where the engine makes max power per gal of fuel. It is less efficient above and bleow that range. Wouldn't you ideally want to run at that most efficient rpm? I was once told that most engines are most efficient at approx 80% of max RPM which isn't too far off in this example.

 

[psneeld]

Not an engine guy here, but I thought those curves are based on the " right prop"... change the prop and you might shift the "efficient rpm" curves a bit.

But as it was said, the fuel burn doesn't change much as the injector pump just meters the fuel as appropriate for the load which is not a bit higher at a lower rpm.

 

[Vandeusen]

Not wanting to fuel up often that's all.. trying to know how far can I go before having to head to a marina for fuel.. just preplanning things and trying to learn the Trawler... For my Silverton it's easy I average 1 gallon a mile.. now that we moved into the trawler, have to learn over again and Silverton was gas trawler is deisel