Estimated GPH

[mvweebles]

For budgeting purposes, you will burn around 4000 gals for the 6000nm loop

You'll average around 7.5 kts overall, and average around 5 gph thus the 4000 gal approximation. Figure $3.50-$4.00 pet gal, and you're at $12k-$15k in fuel.

Don't forget to budget for oil changes - probably three changes in the 800 engine hour run.

Have fun.

Peter

 

[Vadim]

Hello,
I'm looking for your opinion on an estimated GPH for this boat.
Cruising 8-10 Knots.
Understanding that fuel cost is only a small expense of owing a boat. We want to do the Great Loop, a 6,000 mile plus trip, and are currently building a budget.


42' Marine Trader
Dimensions
LOA: 41 ft 10 in
Beam: 13 ft 8 in
LWL: 38 ft 0 in
Maximum Draft: 3 ft 6 in
Displacement: 34000 lbs

Engines
Total Power: 450 HP

Engine 1:
Engine Brand: Ford Lehman
Year Built: 1988
Engine Model: SP225
Engine Type: Inboard
Engine/Fuel Type: Diesel
Engine Hours: 1700
Drive Type: Direct Drive
Engine Power: 225 HP
Engine 2:
Engine Brand: Ford Lehman
Year Built: 1988
Engine Model: SP225
Engine Type: Inboard
Engine/Fuel Type: Diesel
Engine Hours: 1700
Engine Power: 225 HP

Cruising Speed: 10 knots
Maximum Speed: 15 knots

I have the exact same engines, at your displacement speed which is about 8.5 knots you should have 5.5 gph consumption. You should also consider bypassing the intercooler. That will reduce the HP to about 180 from 225.
You do not need this HP to run your boat and you will protect your engine valves.
As far as the budget to do the loop, $50K is the figure you should set aside.

 

[RickyD]

One gallon per nm? Otherwise that would be one heck of an efficient setup for a 55.

I woke up this morning thinking about the boat, and this post. It suddenly occurred to me that I had written 1 gallon per hour instead of 1 gallon per mile. It should be 10 gallons per hour at 10 knots. I have Cat. 3208s. My brother has the same engines in his Offshore 47 and the one time I went on an overnight cruise we determined 1 gallon per mile using his Flow Scans.

 

[Comodave]

I'm getting the following: sqrt(39) = 6.245 * 1.34 = 8.368 kts hull speed.

Ok, that is what I get now... Don’t know how my math was working, or apparently not working, when I got 7.2.

 

[socalrider]

Ok, that is what I get now... Don’t know how my math was working, or apparently not working, when I got 7.2.

(1.34*39)^0.5 = 7.2

Order of operations error!

 

[CaptTom]

You can greatly simplify the math.

First of all, start out with a square for LWL (25, 36, 49, etc.)

Then, use 1.33 (one and one-third) for the multiplier (close enough!)

When you get your answer, interpolate based on how far you were between the LWL numbers (a 30' waterline is half-way between 25 and 36, so your hull speed is half-way between 6.65 and 8, or around 7.3 knots.)

You really don't need any more precision than that. Individual hull shapes and conditions are going to give you a half-knot variation anyway.

Fine-tune your preferred cruising speed underway. At some point more throttle doesn't result in as much of an increase in speed. And you'll see your wake building as you try to climb out of the hole. Back down from there and you've got your sweet spot.

 

[Captain Doug]

Firehoser75 just gave the same data I have on my Nordic Tug 37 with the same engines. I run mostly at 1400 and see 7.8 to 8.1 it’s and fuel burn about 2.5/hr including generator.

 

[koliver]

You can greatly simplify the math.

First of all, start out with a square for LWL (25, 36, 49, etc.)

Then, use 1.33 (one and one-third) for the multiplier (close enough!)

When you get your answer, interpolate based on how far you were between the LWL numbers (a 30' waterline is half-way between 25 and 36, so your hull speed is half-way between 6.65 and 8, or around 7.3 knots.)

You really don't need any more precision than that. Individual hull shapes and conditions are going to give you a half-knot variation anyway.

Fine-tune your preferred cruising speed underway. At some point more throttle doesn't result in as much of an increase in speed. And you'll see your wake building as you try to climb out of the hole. Back down from there and you've got your sweet spot.

Nice shortcut, Tom.

I also find that what you are trying to achieve is not the theoretical hull speed, but more of a "Sweet spot" speed, where you are not raising the bow much, and not making a big wave, but get there as quickly as you want to.

I my case, the math says my hull speed is 8.58 knots.

I can go that speed, but I find it is higher than my "sweet spot" speed, which is between 8.0 and 8.2. At hull speed, I would get there a little sooner, but my mileage would suffer. At "sweet spot" my mileage is very close to 2.0 mpg (nautical miles per US gallon). My stern wave is small enough that I rarely need to slow when going by kayakers, canoes, smaller craft., though I do if the water is otherwise flat.

I use 3.78 litres/USg, 4.54l/CanadianGal
41'LWL
2100 to 2300 rpm on TAMD41s

If my speed rises to the Hull speed, so too do the bow, the height of the stern wave and the fuel consumption.

At 7 knots or less, there is no wave, the boat is flat, RPM is about 1500. I have no idea what the mpg would be at that speed.