RPM / Speed / GPH for 53MY

[Nonchalant1]

Somewhere a few weeks ago, we had been discussing this on a thread and Mike or somebody asked "are you sure?" about 2 gal/NM at cruise speeds. I said I'd write down what my boat was doing these days, so I finally did. I'm on a river where there are few waves, and little current, but I tool readings first downstream and then upstream. Speed is COG from my GPS. RPMs are from my FloScans (very accurate), GPH from FloScans also (some may dispute accuracy, but they're right on when it comes to fuel used vs. fuel actually required to fill the tank). So here it is:

This is a 1978 53MY with 8V71TIs, 450-480 gal fuel, 250 gal water onboard.
RPMs Knots GPH Gal/NM
0640 - 6.3 - 1.8 - 0.3
1060 - 8.7 - 8.0 - 0.9
1100 - 9.1 - 9.0 - 1.0
1200 - 9.3 - 10.4 - 1.1
1400 - 10.0 16.2 - 1.6
1600 - 10.8 23.4 - 2.2
1700 - 10.9 27.0 - 2.5
1920 - 15.5 34.2 - 2.2
2200 - 18.3 44.4 - 2.4

At 1700 RPMs I'm still pushing a mountain of water and it's the highest Gal/NM. I plane by going to 2200 and then backing down. Below 1900, it tends to fall off plane with any wind or waves. Since I tend to cruise at either 1400 or 1950 I probably do cruise at about 2 gal/NM.

Doug Shuman

[Angela]

Doug,

Thanks for the stats. I am trying to budget fuel for a charteryacht business, and needed to come up with a realistic figure for one week's worth of engine fuel (and genset fuel) while island hopping around the BVIs, estimating my total mileage at about 60-65 nm/week. I had way over-estimated fuel by nearly $1,000/week! I guess that's the good news. The bad news, however, is that if fuel prices keep rising, I'll be right back to where I started. That makes it hard to compete with the sailing yachts.

Thanks again for taking the time to be so diligent in responding to my question.

Ang

[Pascal]

Doug

very good info. i notice a big RPM difference with the Ns, i guess it makes sense but it's the first time i see the numbers... with the naturals, it takes 1500rpm to get 9.3kts.

i wonder why you would run at 1400 rpm? you're burning 50% more fuel to get ONE extra knot! is it to keep the engines warm?

if others have detailed data like that, please post, I'd like to compile these in the FAQ

[Nonchalant1]

Doug,

Thanks for the stats. I am trying to budget fuel for a charteryacht business, and needed to come up with a realistic figure for one week's worth of engine fuel (and genset fuel) while island hopping around the BVIs, estimating my total mileage at about 60-65 nm/week. I had way over-estimated fuel by nearly $1,000/week! I guess that's the good news. The bad news, however, is that if fuel prices keep rising, I'll be right back to where I started. That makes it hard to compete with the sailing yachts.

Thanks again for taking the time to be so diligent in responding to my question.

Ang

Angela,

Tim runs the "McGregor II" (1972 58YF) down there from island to island at low speeds. His rates are way above sailboats because it's such a better experience. Look at the idle speed fuel use numbers above. You'll get great fuel economy at 6 knots (probably .5 gal per NM), and about 1 gal per mile at 9.1 knots. If you're only going 6 miles to Tortola or Virgin Gorda, who cares if it takes an hour vs. 30 minutes?

Doug

[Angela]

Doug,

I am familiar with the McGregor and I very much agree that the cruise from island to island should be a slow leisurely cruise in order to give the guests some boat cruising time like the sailboaters get. All of the power yachts are much more expensive than the sailing yachts simply due to the consumption of fuel instead of "free wind" for propulsion, but you get 24-hour air conditioning, if you want it, with the power vessels. I'm going to need the AC, so a sailboater, I'm not! The sailing yachts aren't feeling the crunch like the power vessels are. Rate increases are only allowed four times per year, the next being November 1. I'm curious to see what the power vessels in the fleet do. I've noticed already that some are tacking on a "fuel surcharge" in order to still be able to make the run each week (without doing so at a loss) before they are allowed to increase their rates at the end of the month. As soon as I finish the plan for my typical route, and then measure the distances on the chart, I'll plug my numbers into your fuel consumption rates for the final results. I suspect I'll be using far more fuel in the generator each week, running the AC, washer/dryer, dish washer, galley equipment, watermaker, etc., than I will through the engines. I think 6 to 8 knots will be just plenty.

But .... didn't I read/hear somewhere that with turbos, it's not good to run them under a certain RPM? Something about shortening their life span if they are not run at the higher RPMs, or am I mistaken?

Ang

[Nonchalant1]

Ang,

You can run turbo motors slow as long as you run them for 15-20 minutes every day or two to heat them up to higher operating temps and prevent carbon buildup in the piston rings. 15-20 minutes at 1900 RPMs moves you 5 NM and uses 12 gal of fuel. Put that in your plans and give your guests a little more wind in their hair.

Did'ja get a turbo boat? If you're that worried about fuel, maybe get a boat with 8V71Ns. They last longer but won't plane the boat. You do have to run naturals to heat them up occasionally too. My chart won't work for naturals.

Doug

[MikeP]

I have frequently read the advice that turbo DD motors need to be run at higher speeds periodically to avoid fouling. But I don't understand the logic or any reason that should be the case. Assuming the injectors are operating correctly and the engine is properly tuned, engine temperature, not RPM should be the governing factor as to whether any fouling will occur. Turbos have no direct influence on whether an engine will foul or not.

I can understand the advice if, at low cruise RPM, the engines do not reach their proper temp range. But if that is the case, a hotter Tstat will put the engine into a more efficient temp range.

Am I missing something here that is particular to a DD turbo motor?

[Angela]

Doug,

I don't have a turbo boat ..... yet ..... A buyer just came along for my Cruiser and I'm about to be boatless next week (and homeless). So, I'm scrambling and life is about to suck .... without a boat. It wasn't supposed to work out this way -- I was supposed to have the Hat first, and sell the Cruiser after that so I'd never be "without a boat", but that plan went afoul when a buyer came along who really wants the Cruiser.....so, I have to let her go. I've been shopping for a 58MY for a while now. I chose Hatteras because of Hat's quality and the fact that the older ones are built like tanks and have beautiful wooden interiors. My best friend has a 1968 41' convertible, restored and repowered, and I just love the quality, stability, and reliability. Every 58MY I've seen has turbos which is why I asked specifically about the turbos. I came very close to being in one in August, but rejected the boat after survey when the boat didn't appraise to contract price and made a new offer to which the owner said "no", so I'm still on the hunt. I'm looking for the four stateroom, galley up model. I chose the 58MY because I need the four staterooms, and need to stay under 60' in order to be permitted to use the mooring buoys in the BVIs. So, I'm pretty confident I'm going to end up with turbos.

Ang

[Angela]

Somewhere a few weeks ago, we had been discussing this on a thread and Mike or somebody asked "are you sure?" about 2 gal/NM at cruise speeds. I said I'd write down what my boat was doing these days, so I finally did. I'm on a river where there are few waves, and little current, but I tool readings first downstream and then upstream. Speed is COG from my GPS. RPMs are from my FloScans (very accurate), GPH from FloScans also (some may dispute accuracy, but they're right on when it comes to fuel used vs. fuel actually required to fill the tank). So here it is:

This is a 1978 53MY with 8V71TIs, 450-480 gal fuel, 250 gal water onboard.
RPMs Knots GPH Gal/NM
0640 - 6.3 - 1.8 - 0.3
1060 - 8.7 - 8.0 - 0.9
1100 - 9.1 - 9.0 - 1.0
1200 - 9.3 - 10.4 - 1.1
1400 - 10.0 16.2 - 1.6
1600 - 10.8 23.4 - 2.2
1700 - 10.9 27.0 - 2.5
1920 - 15.5 34.2 - 2.2
2200 - 18.3 44.4 - 2.4

At 1700 RPMs I'm still pushing a mountain of water and it's the highest Gal/NM. I plane by going to 2200 and then backing down. Below 1900, it tends to fall off plane with any wind or waves. Since I tend to cruise at either 1400 or 1950 I probably do cruise at about 2 gal/NM.

Doug Shuman

Doug,

This may seem like a silly question, and forgive me if it is, but are your gal/NM above for both engines combined, or for each engine?

Ang

[Nonchalant1]

These are all for the total boat. Both engines combined.

Doug