Effective Fuel

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Nick
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Effective Fuel

Post by Nick »

We’re in the Bahamas and trying to calculate our max range in the event we’re unable to buy fuel before getting back to the US. I know our boats carry 110 gallons in the aft tank and 70 in the front for a total of 180 gallons. I also know you don’t want to drain the forward tank for fear of getting air in the line to the generator. I assume you cannot fully pick up every last drop in either tank, so what have you all found to be the maximum “available fuel” and what has been your max range at what rpm?
Nick & Barb
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Re: Effective Fuel

Post by Ortolan »

Nick-

I’m sure you’ll get more exacting info from people like James Power or Dick & there may be some variation depending on exact pickup tube depth. Going by our experience, the useable fuel is quite limited.

The main tank: Once we got very low - the gauge had shown empty for over a hour & when I physically measured the fuel with a yardstick thru the top of the tank, there was only 2” of fuel. Since I assume the pickup tube is around 1” I wouldn’t have wanted to go any lower especially if not dead calm. When completely filling the tank, it took only 84 gallons, so I take that as the usable fuel.

The auxiliary tank: The transfer pickup is right at 1/4 on mine & after that the transfer pump will lose prime, however I have run the transfer pump just until you start to hear the gurgling of air, then immediately flip the pump into the other direction for a few seconds to re-prime, then off & close the valves. I have then still used the genset for a couple of hours as it’s pickup tube should be lower. When completely filling the tank, it only took 31 gallons.

So I only came up with only 115 gallons of usable fuel - quite an unpleasant surprise! If you need to get serious range, the best way would be to keep your speed extremely low - 5 or 6 knots.

We left the Bahamas a week ago & I would suggest moving ahead with a careful, safe exit as weather allows. A possible combo of many virus cases and cruisers not following protocols could make things very uncomfortable.
Russ
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Re: Effective Fuel

Post by deising »

Excellent detail from Russ. Here are some more data points from my experience.

Realizing that not all gauges may be repeatable, we nonetheless see 77-80 gals to fill the main/aft tank from E to F. I have never run it below E or physically checked the tank. I once left the valves open on the aft tank after filling both tanks to F and gravity pulled what I guessed was another 10 gals into the main tank. There was no spillage of fuel, thankfully due to the vent design.

I routinely transfer fuel from the fwd tank until the gauge reads 1/4 and dare not go further. Filling that tank to F on the gauge takes 38 gals for us. I make the assumption we have about 125 gals of useable fuel if I were to fill the main tank past the F until I heard fuel backing up the fill tube (which I rarely do).

Our 75HP engines do not push our cruising-heavy boat into the semi-planing regime without running them near full power and I refuse to do that. We are former sailors and usually quite happy with 7.5 kts near displacement speed. Over 16K miles have shown we average 3.7 nautical miles per gal staying at the low speed except for the occasional high-speed run to keep the engines/turbos "happy."
Duane Ising
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2006 PDQ MV 34 - hull 91, 75HP, 3-blade

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Re: Effective Fuel

Post by duetto »

we have the 75 hp and 3 blades. we generally cruise at 13.5 knots which equates to 3350 rpm. at this speed we have averaged 2.4 nautical mpg for 15 years. included in this number is idling into and out of harbors, docking, etc. based on 125 usable gals this would give 300 nm range.
john & diane cummings
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Re: Effective Fuel

Post by deising »

John,

It would be interesting to find out what our boats accurately weigh loaded for cruising. I suspect your much earlier hull number is a bit lighter, and perhaps you do not carry as much as we do when cruising. When we did the pre-purchase survey, it was a very light ship and we reached 16 kts at full power.

As loaded for cruising and full of water and fuel, we might be able to reach 12 kts at full power. Anything less than 12 kts and we are partially plowing through the water - very inefficient.
Duane Ising
m/v Diva Di
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Re: Effective Fuel

Post by duetto »

hi duane,

when we went for survey with minimum weight we made 18.5 knots. when we cruise to bahamas we're fully loaded fuel, water, 6 months provisions, 10.5 ' rib with 15 hp, bikes, etc. for the past 2 days we crossed florida and we ran both days at 14+.

i remember talking to pdq at a boat show and they said our boat was considerably lighter than 2004+ hulls.
john & diane cummings
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Re: Effective Fuel

Post by duetto »

hi nick,
don't know if you saw this. i have complete article if you want.

Effective 0900 tomorrow (Tue 3/24) the Bahamas is closing its borders and no foreigners may land by private vessel or any other means. Also, there is a 24/ curfew as well as many other restrictions.
Those of us already here are affected insofar as we can no longer go ashore except for very specific reasons. No word yet on availability of marine fuel or potable water.
24-HOUR LOCKDOWN: PM announces border shutdown, expanded COVID-19 emergency powers
CoronavirusMarch 23, 2020March 23, 2020 at 7:37 pm
All airports, private shall be closed to intl. flights
All seaports closed to regional and intl. seafaring and private boats
No visitor permitted to enter or disembark
All public beaches, roads closed
john & diane cummings
duetto mv34 #23

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Re: Effective Fuel

Post by AlanH »

When it comes to fuel, I have found hen filling the tanks to the point where there is some back flow in the fill tubes, there is actually room for more fuel in the tanks. I have used a gas can to put about 8-9 more gallons total in the two tanks. A word of caution though, one time there was a tiny leak at the cover for the fuel gauge sending unit and the boat sat for a couple days before we used it. Probably only leaked an ounce but it was a job to get rid of the smell.

Alan
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Re: Effective Fuel

Post by Ortolan »

I’d be curious, in a fuel emergency, could you access some of the extra fuel in the auxiliary tank? If you shut the genset valve off (so it doesn’t drain & of course not use it), remove the pickup tube of the transfer pump & add a length of rubber tubing to reach further down into the tank - then transfer another 25 gallons? I believe the bottom of the tank is “V” shaped so as long as you don’t reach down too far you won’t get any debris.
Russ
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Re: Effective Fuel

Post by Cat Daddy »

On mine, when the port engine runs out of fuel the main tank held 94 gallons. You can get a little more usable on plane because the pick ups are closer to the rear of the tank. When you slow down coming off plane there is an unpleasant surprise waiting.

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Re: Effective Fuel

Post by AMCarter3 »

Rafe, did you really mean to say 94 gallons (out of the 115 gals) remained in your main tank when your port engine quit? 94 equals almost 82% of the fuel was still in your tank. I don't get. Nothing like that happens in our boat.
Mac Carter
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Re: Effective Fuel

Post by deising »

Mac, I am sure Rafe meant that it took 94 gallons to fill the tank after the port engine quit.
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Re: Effective Fuel

Post by Cat Daddy »

Duane is correct, it took 94 to fill it. That still is a fair amount of unusable fuel.



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Re: Effective Fuel

Post by Nick »

Thanks guys! Super helpful once again. Explains a lot! Disappointing to hear our big, fat confidence inspiring 180 gallons of fuel actually translates into something much less actually available, but I had begun to suspect as much. I am embarrassed to say I’ve run the aft tank “dry” 3x’s so far as I try to dial in expectations (I was able to predict fuel fill to the gallon on our old boat). Speed is an obvious factor, but I’m finding current and sea state/wind create more variability than I’d expect. It doesn’t help that the gauge is not particularly accurate. It’s always our starboard engine that conks out first. The first time it happened I was imagining all sorts of mechanical issues until, thankfully, the port quit soon thereafter and I smiled in relief despite drifting in the ICW.....gotta be fuel. Unfortunately we’re not ex-sailors (haha) so reducing to 6 knots to extend range represents yet another Covid-19 sacrifice. BUT we’re up to the task although fuel seems still widely available here in the Bahamas. Thank you again for info.
Nick & Barb
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Re: Effective Fuel

Post by deising »

Nick,

Your comment about wind and sea state affecting your range is quite true. We like to speak in terms of average miles per gallon because we know how far away our destination lies. A better way to gauge fuel usage, however, is gallons per hour. If you are bucking heavy seas and currents for a very long passage that takes you 12 hours, say, you will know much better how much fuel is being consumed multiplying 12 times your GPH for the chosen power setting.

If you had following seas and a fair current, that same passage might only take 9 hours, and your fuel usage would be very close to 9/12ths as much. The miles per gallon would vary by the same ratio. The dilemma, of course, is that you can't always know in advance how many hours the passage will take because conditions might change.

In that vein, on my spreadsheet tracking fuel usage, I see a pretty consistent 2.0 gallons per hour for both engines. Variation occurs only because the fill-ups assume the gauges are repeatable. Sometimes my wife will note a quick jump in the gauge just before it reaches the F mark and then we have a slight overfill from the last time. The overall average over many fill-ups, however, is 2.0.

We toyed with the idea of cruising from our west coast of FL to Isla Mujeres in Mexico and then down to Belize and Guatemala. I know we would need a fuel bladder or a number of jerry jugs to make that 430nm passage with prudent reserves, though.
Duane Ising
m/v Diva Di
Punta Gorda, FL
2006 PDQ MV 34 - hull 91, 75HP, 3-blade

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