How Much Diesel is Enough?

[TacomaSailor]

Originally Posted by Kenomac

Mark,

'Curious... then why does my Yanmar 100hp turbo diesel only consume 1 gallon per hour at 1800-2000rpms? Enough to drive our 25 ton boat at 6 knots in relatively calm seas <15 knot winds.
What am I doing wrong?

Propellor Curve = At low RPM very little of the diesel power is put into the water moving the boat. The amount of POSSIBLE diesel power transferred from the engine to the water increases as a cubic function of the percentage of max RPM (for your particular engine/propeller combination). Fuel consumption is a function of the power pushed through the propellor NOT the power the engine could produce at that RPM.


A Yanmar 4JH-UTE max HP = 100 at 3600 RPM
ASSUMING the correct prop (diameter and pitch) is used - then following are some points on the propellor curve (KM4A transmission) as found in the Yanmar Factory Service Manual:

- Engine RPM
- Engine HP possible at that RPM at output shaft before transmission
- actual power put into water that can be used to move the boat
- Specific Fuel Consumption (grams diesel per Horsepower)
- Percentage of possible engine power transferred to water

1800 RPM
53 HP at output shaft
6 HP at prop
227 grams diesel / HP
11%

2000
62
12
196
19%

2200
70
18
189
26%

2400
81
30
181
37%

2600
85
37
178
44%

2800
91
46
175
51%

3000
95
55
174
58%

3200
97
65
174
67%

3400
98
78
175
80%

3600
100
92
177
92%

[SeaSon]

There is only one time you'll ever worry you have too much fuel on board.

When the boat's on fire.

[Kenomac]

Originally Posted by TacomaSailor

Propellor Curve = At low RPM very little of the diesel power is put into the water moving the boat. The amount of POSSIBLE diesel power transferred from the engine to the water increases as a cubic function of the percentage of max RPM (for your particular engine/propeller combination). Fuel consumption is a function of the power pushed through the propellor NOT the power the engine could produce at that RPM.


A Yanmar 4JH-UTE max HP = 100 at 3600 RPM
ASSUMING the correct prop (diameter and pitch) is used - then following are some points on the propellor curve (KM4A transmission) as found in the Yanmar Factory Service Manual:

- Engine RPM
- Engine HP possible at that RPM at output shaft before transmission
- actual power put into water that can be used to move the boat
- Specific Fuel Consumption (grams diesel per Horsepower)
- Percentage of possible engine power transferred to water

1800 RPM
53 HP at output shaft
6 HP at prop
227 grams diesel / HP
11%

2000
62
12
196
19%

2200
70
18
189
26%

2400
81
30
181
37%

2600
85
37
178
44%

2800
91
46
175
51%

3000
95
55
174
58%

3200
97
65
174
67%

3400
98
78
175
80%

3600
100
92
177
92%

Thanks for the explanation, your math skills are impressive.

Ken

[mainesail]

Hi, You really need to assess your needs for speed etc. Many people have very satisfactory cruises without ANY auxiliary power-of whom the Pardeys are best known. IMHO too many cruisers want all the goodies that keep them, they hope, from any uncomfortable situations. Discomfort is a part of the package, and to my mind, a part of being real and alive. So think hard about your needs and true priorities. Fair Winds..

[sailnow2011]

The way I look at it....
I need to have enough fuel to go half way plus 10% of whatever trip[ I am making. We have 140 gallons, enough to go 1200 miles. If I were to go 3000 miles I would carry some jugs. We are also adding another 40 gallons of diesel tankage, and going to carry a 18 gallon rolling fill station on the stern, and 8 jerry cans. Total diesel range would be 2000 miles or so, and there is no passage we could not do, that if the rig went down at the half way point, we could not motor home.

[Cheechako]

Originally Posted by Kenomac

Why in the world would you leave your fuel tanks empty all the time? If you're worried about the fuel going bad, then simply install a good fuel polishing system. Is the diesel quality that bad where you sail?

I left the two extra tanks (I had 4 total) empty after filling them once. I just decided I didn't need them, used that fuel and didn't refill. Why fill, let it get old, device a polishing system for 4 tanks, two of which I don't need?

[Adelie]

Originally Posted by Kenomac

Mark,

'Curious... then why does my Yanmar 100hp turbo diesel only consume 1 gallon per hour at 1800-2000rpms? Enough to drive our 25 ton boat at 6 knots in relatively calm seas <15 knot winds.

What am I doing wrong?

Because at that speed you are only using about 18hp. WOT you would use close to 5gal/hr


Sent from my iPhone using Cruisers Sailing Forum

[jreiter190]

Originally Posted by mainesail

Hi, You really need to assess your needs for speed etc. Many people have very satisfactory cruises without ANY auxiliary power-of whom the Pardeys are best known. IMHO too many cruisers want all the goodies that keep them, they hope, from any uncomfortable situations. Discomfort is a part of the package, and to my mind, a part of being real and alive. So think hard about your needs and true priorities. Fair Winds..

Ask the Pardeys how many tows they accepted. I know for a fact they were towed thru the Canal and have run across others who claim to have either towed them or witnessed them being towed.Not taking anything away from a good shipwright and sailor. The last Isaw of them, they were hanging stuff off docks in San Pedro for Cruising World. Now, Slocum was a real windjammer, and I'll bet he took tows when they were available. I sailed all over the Bahamas without an engine for a year and took a tow a couple times. Every time a Clipper made port, a steam tug towed her to the wharf. Guess this off topic, sorry. How much fuel is enough for a sailboat? Enough to make you feel like you have enough.

[buzzstar]

Simply put: huh? Practically put, what is the typical consumption rate of the identified vessel with the specific engine, absent that information, very similar vessels with that engine model, or as a last resort, the engine manufacturer's claims. What is the approximate range, plus a a cushion? Does that meet your comfort level, sailing plans, and sailing ability Don't forget, depending on your routes, you may use extra fuel in places where replacement is not available (e.g., passes to some atolls, and running from weather). Me, I like to sail, and use fuel for emergencies and necessities such as passes and weather. I am almost willing to wait for wind when it comes to destinations, but it is ALWAYS the captain's preferences that count.

[nhschneider]

Earlier this year, we had considered joining a fleet of boats sailing from the Maldives, through the Gulf of Aden and into the Red Sea, armed with 'Ship Security Officers'. One of the requirements to join this fleet was the ability to motor at 6 knots for 1,100 miles without refueling. I have been in touch with several of the boats that completed the passage (all arrived safely) and they did, in fact, motor almost the entire way. Our two 90-gallon fuel tanks along with our very economical Kubota engine would have come very close to allowing us to motor that entire distance.

in John Rousmaniere's book, The Annapolis Book of Seamanship, he emphasizes being 'fore-armed'. This means being prepared for what might happen without being frozen by analysis paralysis. To me, having adequate fuel supplies for what could happen is part of being 'fore-armed.'

Frankly, I find it troubling when some suggest that you 'should do this' or 'don't do that'. Sailing and cruising is a very personal choice and I recommend that you do what you find pleases you. If you dislike discomfort, I suggest you find a boat, route, or time of year that will make it more comfortable.

Fair winds and calm seas.

[hpeer]

I think the bigger underlying issue is..."What is your schedule?"

I've had to do longish passages either motoring or motor sailing because I had to get somewhere. So I went. Crossed the Gulf of Main Shelbourne to Marblehead twice. 44 hours in dead calm in the big boat. 57(?) hours, unfavorable wind, motor sailing, in the small boat....13 hp WOT.

Had I much more time things could/would have been different.

But they weren't, so it's what I did.

[Steadman Uhlich]

Originally Posted by sailnow2011

The way I look at it....
I need to have enough fuel to go half way plus 10% of whatever trip[ I am making. We have 140 gallons, enough to go 1200 miles. If I were to go 3000 miles I would carry some jugs. We are also adding another 40 gallons of diesel tankage, and going to carry a 18 gallon rolling fill station on the stern, and 8 jerry cans. Total diesel range would be 2000 miles or so, and there is no passage we could not do, that if the rig went down at the half way point, we could not motor home.

Hello.
What follows is written in a friendly tone of voice.

Your post up above has some interesting points in it that caused me to pause, and write.

The first point about having enough fuel to do half the distance of the passage caught my attention. At first glance it sound like an easy or reasonable rule of thumb.

But...

You mention the amount of fuel, but not the type or size of boat.

As I see it, the size of the boat is important.

By that I mean that having hundreds of gallons of fuel onboard may be more easily carried on larger sailboats, but not so easily on smaller boats.
--------

I made a Pacfic offshore nonstop passage, about 2,500 miles, on a boat whose engine was not running. We had plenty of fuel, but could not use it. Good thing we had sails.

[secrabtree]

[QUOTE=blackcrow;
------------------------------------------------------------

correct rule of thumb for diesel engines is 1 gal per 20 horsepower per hour,
[/QUOTE]

Something is wrong with my boat. I have a 1971 Cal 39
with a Perkins 4-108 having, I believe 48 horsepower. Using your rule of thumb it should burn 2.4 gph. But mine only burns about .67 gph.

[Jim Cate]

Sam, that high burn rate is if you were running the engine flat out, generating the full 48 HP. You likely are not doing that at your .67 g/h usage. Doesn't take much power to drive a slippery hull at speeds less than ~80% of hull speed, goes up very quickly for those last few knots.

Jim

[TacomaSailor]

Originally Posted by secrabtree

Something is wrong with my boat. I have a 1971 Cal 39
with a Perkins 4-108 having, I believe 48 horsepower. Using your rule of thumb it should burn 2.4 gph. But mine only burns about .67 gph.

Again - Prop Curve and HP requirements to move hull thru water determine fuel consumption. The full load HP at a given RPM is not what determines the fuel consumption.

Using the Cal 39-2 specs on Sailboatdata.com. Following is info from a Perkins 4-108 propeller curve on the old Perkins Web Site but I increased the weight by 2,000 pounds to account for fuel, water, beer, and people.

The boat needs 10 HP at the prop to move at 6.4 knots with a clean bottom in still water.

2300 RPM is 10 HP thru the prop. Using a BSFC of 0.39 pounds / HP / hour results in 0.55 GPH at 6.3 knots

22 HP is needed to move at 7.3 knots and results in a fuel consumption of 1.2 GPH at 2900 RPM.