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Old 04-28-2021, 06:24 AM   #1
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Estimated GPH

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
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Old 04-28-2021, 07:08 AM   #2
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Somewhere around 4.23145 gph.....


That's my guestimate.



If it's a displacement hull, you'd probably get a bit better and gut feeling that 8 to 10 knots is a bit too fast for hull speed. I'd use 8 knots, very slightly under hull speed.


However, it takes XXXX gallons of fuel converted to HP to push a boat thru the water at hull speed, and the number of engines, HP of engines, kind of hull and size doesn't make a HUGE difference.



Sure a smaller 15K# boat will be more efficient and a bigger 60K# will burn a bit more but bet the difference isn't quite double or so.



The absolute best way is with a fuel flow meter. They you'll know extremely close (within 1%). Second way is take a long trip at a constant power and fillup at the other end. Not easy to to, however. Third way is to keep a log over the years, keep the power "close" and you'll get a good idea, but that takes time.


I'm looking at a fuel flow meter. It would be nice to know if a very minor adjustment in power/speed produced a significant difference..... especially if you do a LOT of cruising.
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Old 04-28-2021, 08:50 AM   #3
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I don't disagree with seevee in the least.

If this will be of any help, there is a pdf link on my website (linked with permission of the author) that walks through the entire process of calculating your burn rate. http://uscgcaptain.johneasley.com/Fe...onEquation.pdf
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Old 04-28-2021, 10:37 AM   #4
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Assumption: Sounds like a Marine Trader is a semi-displacement boat.
From my years running my Nordic Tug, which is a semi-displacement hull, I found that running slightly below hull speed was the "sweet spot" for fuel consumption. We had a Cummins 6BTA, 330 hp engine and we operated it between 1200-1400 rpm most of the time. WOT is rated at 2800. This gave us a SOG of about 7-8 knots ((very close to hull speed). Over several years tracking fuel, operated that way, we achieved a fuel burn of 2.0 gph. This is not an "exact" figure and I did not even try to factor out the limited use of a diesel furnace or the limited use of our generator (we had solar so ran the gen set very infrequently).
According to the spec sheet for that engine, our experience matched almost exactly what the manufacturer listed. On the spec sheet, it was obvious that increasing the rpm even slightly above the 1400 rpm mark, would result in large, and for each 100 rpm increase an even larger, fuel burn increase, and the gain in speed was negligible.
The hull speed for your boat is approx. 8.25 knots, so operating just below that speed should give you the best gph burn and the best nMPG as well.
For example, tested speeds and fuel consumption at various rpm's for my NT 37:
1300 RPM 2.0 gph 8 knots 4.0 nmiles per gallon

1800 RPM 5.2 gph 10 knots 1.9 nmiles per gallon

2800 RPM 16.8 gph 16 knots 0.9 nmiles per gallon
Hope that helps, and it matches what the article John linked to quoting the experience of the semi-displacement Krogen Express.

For us, operating our "Tug" at just below hull speed (we had a LWL 37 feet) which equated to just below 1/2 throttle, gave us a very comfortable experience with the least fuel burn. Contrary to what some will tell you, this is not "hard on the engine". The engine will actually last more hours run like this compared to it being run at 80% load most of the time (all things being equal).
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Old 04-28-2021, 02:47 PM   #5
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We have a very similar CHB 41 with twin Lehman 120's. My guess:
8kts = 4gph
10kts = 8gph, maybe more.
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Old 04-28-2021, 02:51 PM   #6
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At 10 knots you will be around 8 to 9 GPH. If you slow down to 9 it will be around 5 to 6 most likely. The slower you go the better MPG. It is a tradeoff, time vs fuel burn. We have the same engines in our 41’ President. Fairly similar boats.
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Old 04-28-2021, 02:55 PM   #7
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4-5 GPH US
But only 3.3-4.1 GPH Canadian
We also get more litres in a gallon.

But just a WAG
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Old 04-28-2021, 04:42 PM   #8
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JB, We've completed the loop July 2019, our Bayliner 3888 has a LWL of 34.5' and weighs 23,000# loaded for the loop. We took a full year to complete the loop and travelled 6026.6 miles putting 792 hours on our 175HP Hino NA diesels. We burned 2960 US gallons averaging 3.73 gph. Our average speed was 7.6 mph or 6.6 knots. Fuel averaged $3.00 US/gal for the most of the trip.
I would suggest budgeting about $10,000.00 for fuel for the trip.
Hope this helps.

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Old 04-28-2021, 05:06 PM   #9
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Ver good numbers have been given here. I ran a GB42 with twin FL120s (not your engines, but same weight and hull for boat) and had a 29-year running average of 3.25 GPH with not a lot of generator time in the mix, but add .5 to .75 GPH for that.
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Old 04-28-2021, 06:21 PM   #10
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There is a big difference in fuel burn between 7 and 10 knots. If you insist at trying to make 10 kts then listen to socalrider and Comodave with their estimate of 8+ gallons per hour.

Speed is a very hard thing to measure in a boat. GPS knows your speed over ground but has no knowledge of your speed through the water. In the PNW many boaters are very good at running with the tide, wind to their back and traveling 50 miles in 5 hours and then claiming for all of eternity that they have a 10kt boat but forgetting that miles and knots are not the same or forgetting that they had a 1 kt average push the whole time.

Keeping track of gallons actually used is not so easy either. Many people idle in and out of the harbor and then add those hours to their travel time. A boat is very efficient at idle speed and this can greatly impact the gallons per hour number.

I find that my gallons per hour goes up when traveling up and down the coast. This is because the boat runs at 10kts for 40 hours before we idle in and fill up vs operating in the PNW were I will idle in and out 12 times in 40 hours.
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Old 04-28-2021, 06:28 PM   #11
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All good information above. You're going to really start sucking more fuel after around 8 knots.

Personally, I don't even bother with GPH any more. That's a holdover from the old days of voyage planning based on dead reckoning, when we had no reliable speed indication.

While you still need to account for wind and current, the number you really need to know is MPG, or, better still, gallons per 100 (or whatever) miles. I see no value in doing all the math to get GPH, then always having to convert to a useful value in my head.
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Old 04-28-2021, 07:26 PM   #12
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My numbers are pretty accurate over quite a bit of distance. However having done the calculations during our trip from Virginia I have never checked it again. Actually IMO it isn’t a big deal to me what the fuel burn is. It isn’t that important in the overall scheme of owning a boat.
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Old 04-28-2021, 09:29 PM   #13
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General - Summation

Interesting item about water, road and air-flight vehicles... in general... friction against a physical medium is a primary incremental multiplier of power expended [i.e. fuel used] per mile traveled.

Simply, In General: Faster a vehicle moves under power = more fuel consumed per mile traveled.

Therefore, facing the reality of physics, there is a basic perceived sweet spot regarding each vehicle's speed in direct correlation to personal time expended and fuel used during travel.

The so called "sweet spot" varies greatly regarding different types of vehicles, condition of and power sources in vehicles as well as the wishes of vehicle owners.

For most boats, especially the o' so many tried n' true mono hulls such as displacement, semi displacement and even planing designs... the sweet spot for best nmpg, while still making decent head way, seems to be from 1/2 to 1 knot below hull speed. I know it is as such for our boat. And, by reading years of posts regarding fuel-used/time-expended for other boaters... it clearly appears most boat owners feel the same.

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Old 05-03-2021, 01:23 PM   #14
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I have Cummins engines and the company provides a chart of fuel burn at various rpms. Check with your manufacturer for the same. As mentioned previously anything above hull speed is really wasteful of fuel. It is calculated by taking the square root of the waterline length and multiplying by 1.34. Assuming your WL is 39' that would yield 7.2 kts. At that speed you will burn about 3.5 gph. At 12 knots it will be 4-5X that depending on whether it is a planing or semi displacement hull. I suspect the latter so figure on 17 -18 gph. On my Mainship 43 with planing hull at hull speed of 7.5 kts I burn 3.2 gph At 12 kts barely planing it burns 16 gph. At 16 kts cruise speed it is 22 gal/ hr. Big difference.
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Old 05-03-2021, 02:35 PM   #15
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Rthomas,
My math and yours for hull speed give different results, but the general principle we both agree.
You calculated a hull speed of 7.2 knots for a 39 foot LWL, while I calculate the hull speed to be just over 8.3 knots. I do agree that for best miles per gallon burned you are best to be just below hull speed for most boats.
See my real world experience listed in post #4. One poster commented on the low fuel consumption used while slowly motoring into and out of marinas and anchorages, etc. but for me that was probably at least somewhat offset by generator use and diesel heater use which I did not track separately.
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Old 05-03-2021, 02:57 PM   #16
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Quote:
Originally Posted by Rthomas View Post
I have Cummins engines and the company provides a chart of fuel burn at various rpms. Check with your manufacturer for the same. As mentioned previously anything above hull speed is really wasteful of fuel. It is calculated by taking the square root of the waterline length and multiplying by 1.34. Assuming your WL is 39' that would yield 7.2 kts. At that speed you will burn about 3.5 gph. At 12 knots it will be 4-5X that depending on whether it is a planing or semi displacement hull. I suspect the latter so figure on 17 -18 gph. On my Mainship 43 with planing hull at hull speed of 7.5 kts I burn 3.2 gph At 12 kts barely planing it burns 16 gph. At 16 kts cruise speed it is 22 gal/ hr. Big difference.
Calculator shows: 39' WLL = 8.3683 knots hull spd.

https://www.easycalculation.com/phys...calculator.php

I'd say about 7.5 knots would enable rather efficient fuel use.
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Old 05-03-2021, 05:10 PM   #17
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Fuel costs

Quote:
Originally Posted by jbowlby View Post
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
Well, I can give you a baseline to begin with. My GB 36 Cl, single FL 120 + generator running consumes 2.5 gph at 7 kts (in normal conditions), so you will burn considerably more on a bigger boat with more HP. I would bet that boat will burn 5 gph at 10 kts, if not more.
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Old 05-03-2021, 05:29 PM   #18
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What is a 40 foot Marine trader doing with 450 hp? Get rid of those two engines and replace with two 90 hp F.L.s (or less, like a couple 50 hp yammers).

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Old 05-03-2021, 05:42 PM   #19
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Quote:
Originally Posted by Pete Meisinger View Post
What is a 40 foot Marine trader doing with 450 hp? Get rid of those two engines and replace with two 90 hp F.L.s (or less, like a couple 50 hp yammers).

pete
I'd love to replace my twin 120's with FL80's or modern Yanmars...

But I suspect that such a project would probably cost $80-100k, which is more than my boat is worth and vastly more than any potential fuel savings.
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Old 05-03-2021, 07:38 PM   #20
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Quote:
Originally Posted by socalrider View Post
I'd love to replace my twin 120's with FL80's or modern Yanmars...

But I suspect that such a project would probably cost $80-100k, which is more than my boat is worth and vastly more than any potential fuel savings.
socalrider - you are soooo correct! I've seen people put $70 to well over $100K in repowering their boats, so they could save a grand or two a year in fuel costs. Then... they sell the boat some years later at definite $$$ loss compared to if they had simply left the other power plants in state.

Different strokes... fer... Different folks!
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