Fuel burn at hull speed

[clipper4730]

Hi all

Just trying to get a good estimate on average fuel burn at hull speed or most economical speed, which I assume would be hull speed. The yacht we are leaning toward is a 53EDMY or a 53 yachtfish. So any data on those two Would be great but any data on anything over 50 would help convertibles included. Also if you have any data on the burn from a 20kw gennie on a medium load that will also prove helpful.

Thanks in advance

William

[MikeP]

Our 53 MY at hull speed - around 9.5 Knots gets 1NM per gallon. But the slower you go the more economical the fuel burn. But I can't see idling along at 5k for days...

Don't know about a 20KW genny but our 16KWNL genny under heavy load burns one gallon in 64 minutes. I checked this with a graduated beaker with all four air conds running all 4 burners on the stove on high, oven on broil and microwave running. TV and stereo were also on.) Fridge was running normally as well as whatever other small normal loads - lights, for example.

[OBXTucker]

Are there any "temporary" fuel management gauges available to determine burn rate?

While I have a reasonable "guess" of what I'm burning, it would be nice to find the sweet spot for the best fuel efficiency. I can't see spending the money to install a full blown system, especially since I'm only buring 8-10 gallons per hour (engines and gensets).

[REBrueckner]

You'll find Mike's post representative of most Hatt MY from about 48 to 58 feet. I believe my Onan 12KW 1972 gen was supposed to burn about 1gph....I would suppose newer gennys are likely more efficient.

If your boat has DD TI's, 8 or 9 knots is likely slower than you'll want to go....as TI combustion pressures and temps are lower than naturals at lower RPM's ....

[Pascal]

i usually figure about 1GPH for the gennies, between 12 and 20KW.

for the mains, on the 15'10 beam 50 something, figure about 10gph at 9 to 9.3 kts. what is very important to realize is that as soon as you exceed that, even by just 1kts, your fuel burn goes thru the roof.

i've never run a DD powered Hatt with flow meters, but on larger boats with electronic displays, a single knot above hull speed, say from 10 to 11kts will results in almost twice the fuel burn... (35gph vs 19gph).

the wider beam boats will burn a little more than the 1510s, about 1 to 2 GPH

if you run in shallow water, under 9 to 10', your speed will drop by 0.5 to 1 kts depending on depth.

[Fanfare]

As you approach hull speed, in knots, (1.34 x {square root of the waterline}) the fuel rate goes up like the tip of a ski. Cutting back just one or two tenths of a knot should give the best mileage. Also, using your engine's alternators via an inverter will save generator fuel. Unless you are using a lot of 110/220v for AC or something the inverter should be able to keep up. At low power demand most of a generator's fuel is used to hold 1800 or 3600 rpm to maintain 60 Hz current, not to make electricity.

[MikeP]

"if you run in shallow water, under 9 to 10', your speed will drop by 0.5 to 1 kts depending on depth."

That is important to know! I once thought the boat had a problem when it slowed down at the same throttle setting when we went from 25ft of water to 8 ft. I later learned that this is normal.

ALSO you can have vibration in shallow water that disappears in deeper water. When we go up the Chester River at 1400 RPM, we hit a point where there is suddenly a vertical vibration in the boat. THe first time THAT happened, we turned around to go back to the marina because I thought we had a big problem. After a minute or two the vibration stopped. So we figured we had grabbed/shed a crab line and headed back toward Chestertown. The vibration started again. We kept going and, in deeper water, it stopped. Now, when we get that vibration up/down the Chester in the same spot, we ignore it.

[GJH]

My 20kw Onan, vintage 1991, burns .75 GPH at medium load. I validated this when we lived on a mooring for a few months, and ran the genset off the aft tank and the engines off the front.

The 1.34 x sq.rt of waterline calculation is for pure displacement boats. These planing hulls run a tad higher. Looking at the bow and stern waves, I am about 10 knots vs the 9.5 my waterline would suggest. At around 8 knots , I get about 1 statute mile per gallon, with a clean bottom and running gear (which are a very important part of the equation!). At 10 knots we are around .8 MPH. 12 knots? I don't wanna know!

The inverter/alternator is not "free" electric power. The alternator puts a load on your engine, requiring horsepower, and in turn, the inverter is not 100% efficient. All you really accomplish is reduced wear and tear on the generator and a little more quiet. My inverter bank is not fed by an alternator. Accomplishing that is on The List, someday, but after 3 1/2 years of cruising, it still hasn't made it to the top.

[bobby]

"if you run in shallow water, under 9 to 10', your speed will drop by 0.5 to 1 kts depending on depth." how does depth affect speed?

[Fanfare]

George:

I know it's not "free" electric power off the main engine alternators. But I'm running the main engines anyway for propulsion. So I think of it as "cheaper" electric power. If I am not cooking, heating hot water or running the AC then I am using very little power. Cyclic loads would be my refrigerator and ice maker. Static loads under way are two 100 watt bulbs in each engine room. Also two 12v 50 watt bulbs there as well. I used to run two 50 watt 12 v bulbs in the generator room when under way, but now I don't, I just look in with my flashlight on my 30 minute engine checks. My newish generator has a sound shield so there is not much to see anyway, especially when the generator is turned off. We travel 99+% in the daytime, so seldom any other lighting.

Running my 12.5 Kw generator for 5-600 watts doesn't seem efficient nor good for the generator. It has about a 20 hp diesel engine. Running it heats up the entire generator room to close to 180 degrees. This heat then warms the galley and forward stateroom above, sometimes requiring air conditioning to be turned on once we stop and these are occupied. The main engines are each 550 hp and I think another two or three hp for this amount of electricity just gets lost in the rounding, as does the inefficiency of the inverter. They, too, produce a bit more heat from generating electricity, but they are hot anyway once cooling water flow ceases on shutdown.

I have read of the newest types of generators which do not have to run at a constant multiple of 60 Hz, but can electronically control frequency and run at lower, more efficient variable speeds while producing less then their maximum power. These might change my thinking.