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Old 03-24-2019, 07:41 AM   #1
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Best guess gph

Alrighty then....not sure where to post this question but will start here. Would anyone out there have a pretty good idea of gph on a 23000lb. boat running twin 3116 cats doing about 8 to 10 knots. Already read on older post that 8kw gen is about 1/2 to 1 gph with the high end being fully loaded. Will only be refrigerator running while underway.
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Old 03-24-2019, 07:48 AM   #2
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Give more of a description of the boat, especially the length so hull speed can be figured. There can be a vast difference in the burn rate at 8kts and 10kts as for a boat at say 40', at 10kts it will be trying to climb its bow wave and that takes a lot more power.

A typical trawler type boat at 40' at 8kts with 3116 and gennie I'd guess 4-5gph.
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Old 03-24-2019, 07:49 AM   #3
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Greetings,
Mr. E. Perhaps I am misunderstanding. Can't help you with mains usage but will you be running only a refrigerator with your genny while underway?
If so, might you be better to purchase a cheap inverter to run the fridge rather than underloading your genny? Just a thought...
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Old 03-24-2019, 07:52 AM   #4
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Hi,

Hi, I speculated about 8kn about 4gal / h and 10kn about 9 gal / h.


Do you know your boat LWL

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Old 03-24-2019, 08:27 AM   #5
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There used to be a company who made real time fuel usage. I had one installed on my N46. Worked pretty good.

Ah ha, I thought I forgot the company's name but after 3 minutes of digging around shabby memory, it popped to the surface..... Flow Scan

The URL is
http://www.floscan.com/html/blue/recreationalmarine.php
Guessing at your fuel usage is not the smartest thing in the world but, it is a start. Sort of like the manufacture's fuel usage chart and the builder's fuel usage chart.

The easiest example is, if you are cruising north in the Gulf Stream, you will burn less fuel at the same RPMs than if you are cruising south in the Gulf Stream.
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Old 03-24-2019, 08:29 AM   #6
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To expand a bit on Ski's post above, 10 kts isn't a good speed for a typical 40' twin engine trawler. Even though no semi displacement hull truly planes, at 10 kts it will be mushing though the water trying to climb over its bow wave and will be lugging its engine at that speed.


With twin Cat 3116s a 40' trawler should be able to cruise nicely at 15+ kts. That speed will be kinder to the engine than 10 kts but I would guess 15 gph at 15 kts.


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Old 03-24-2019, 10:02 AM   #7
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Ok, thanks. Most replies are about what we come up with. 8knots feels good on my boat. We have puttered up and down ICW here a few times seeing what speed feels good and listening to engines. If you listen,they will tell you what they want to run. We in the planning stages for our trip from east side of Florida back to Texas. Again thanks for replies.
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Old 03-24-2019, 10:19 AM   #8
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Ernie - please tell me what horsepower and rating of the 3116 you have. I can look up the fuel consumption by RPM, please advise your cruise RPM also.
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Old 03-24-2019, 10:45 AM   #9
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Quote:
Originally Posted by Ernie View Post
Alrighty then....not sure where to post this question but will start here. Would anyone out there have a pretty good idea of gph on a 23000lb. boat running twin 3116 cats doing about 8 to 10 knots. Already read on older post that 8kw gen is about 1/2 to 1 gph with the high end being fully loaded. Will only be refrigerator running while underway.
Quote:
Originally Posted by Ernie View Post
Ok, thanks. Most replies are about what we come up with. 8knots feels good on my boat. We have puttered up and down ICW here a few times seeing what speed feels good and listening to engines. If you listen,they will tell you what they want to run. We in the planning stages for our trip from east side of Florida back to Texas. Again thanks for replies.

From a Yachtworld listing, LWL for a Cruisers 3950 is 29.9'. (Seems low, to me; would have guessed closer to 36-38' or so.)

Anyway, best fuel economy for 30' LWL would likely be at approx 5.5 kts. Above 7.3 kts would most likely be climbing bow wave, and fuel consumption goes quickly north from there.

-Chris
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Old 03-24-2019, 02:14 PM   #10
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I know exactly how much fuel will be used, more than I can afford.
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Old 03-26-2019, 11:14 AM   #11
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[I semi displacement hull truly planes][/I]

Shallow V hulls will truly plane and they are considered semi-displacement.
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Old 03-26-2019, 12:12 PM   #12
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[I semi displacement hull truly planes][/I]

Shallow V hulls will truly plane and they are considered semi-displacement.
If they're designed to plane how can they be semi-displacement?
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Old 03-26-2019, 12:21 PM   #13
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Quote:
Originally Posted by OldDan1943 View Post
There used to be a company who made real time fuel usage. I had one installed on my N46. Worked pretty good.

Ah ha, I thought I forgot the company's name but after 3 minutes of digging around shabby memory, it popped to the surface..... Flow Scan

The URL is
FloScan Instrument Co. Inc.
Guessing at your fuel usage is not the smartest thing in the world but, it is a start. Sort of like the manufacture's fuel usage chart and the builder's fuel usage chart.

The easiest example is, if you are cruising north in the Gulf Stream, you will burn less fuel at the same RPMs than if you are cruising south in the Gulf Stream.

Floscan closed their doors about a year ago....
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Old 03-26-2019, 03:04 PM   #14
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Floscan closed their doors about a year ago....
Ah then, I was correct.
Their site is still up but the company's down.
There may be some units still on the company's shelf or perhaps the installers still have some in inventory?
If you want one, I guess you will have to go hunting.
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Old 03-26-2019, 04:07 PM   #15
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Hmmm...plowing water?

Quote:
Originally Posted by Ernie View Post
Alrighty then....not sure where to post this question but will start here. Would anyone out there have a pretty good idea of gph on a 23000lb. boat running twin 3116 cats doing about 8 to 10 knots. Already read on older post that 8kw gen is about 1/2 to 1 gph with the high end being fully loaded. Will only be refrigerator running while underway.

I agree with David and others...8-10kts is too fast for your boat -- or too slow. A Cruisers 3950 aft cabin is built on a planing hull, which is why they gave it 600hp. Your hull will begin 'plowing water' above about 7kts, and continue plowing until you are up on plane, somewhere above 16kts.

My quick guess is ~8gph at 8kts and 13gph (or more) at ~10kts. Your wake (and fuel burn) will be huuuugggge at 10kts. Running angle between 8kts and 10kts will approach 4-5 degrees (which means most of your fuel is being used to make wake, as opposed to making way). You will actually burn less fuel (better miles-per-gallon) and make a smaller wake by getting the boat up on plane.

Your Cruisers is a go-fast boat that needs to stay as close to horizontal as possible, this means going very slow or going very fast. You won't be happy with the performance of a planing hull running at what's politely called the 'transition' speed (plowing water). Handling is goofy, running angles are high, the crew is uncomfortable. Miserable for you, your wallet, and anyone in the path of your wake.... ;-)
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Old 03-26-2019, 06:54 PM   #16
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Have you asked the question over at the Cruiser Yachts Owners Club Forum, www.cruisersyachtsownersclub.com ? I would bet that someone could give you the most economical rpm's to run your boat at both a slow and a fast cruise.

Another alternative might be to peruse the threads over at clubsearay.com While hull shapes might be a bit different, I bet there is a lot of similarity in overall weight and sizes between the cruisers yachts and sea rays. While this wouldn't be an accurate number, its probably close.

Jim
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Old 03-26-2019, 07:45 PM   #17
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Have you asked the question over at the Cruiser Yachts Owners Club Forum, www.cruisersyachtsownersclub.com ? I would bet that someone could give you the most economical rpm's to run your boat at both a slow and a fast cruise.

Another alternative might be to peruse the threads over at clubsearay.com While hull shapes might be a bit different, I bet there is a lot of similarity in overall weight and sizes between the cruisers yachts and sea rays. While this wouldn't be an accurate number, its probably close.

Jim
I am not sure if I can explain this. Watch your wake. You are supposedly at hull speed when your can make your wake first come together.
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Old 03-26-2019, 08:09 PM   #18
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Have you asked the question over at the Cruiser Yachts Owners Club Forum, www.cruisersyachtsownersclub.com ? I would bet that someone could give you the most economical rpm's to run your boat at both a slow and a fast cruise.

Another alternative might be to peruse the threads over at clubsearay.com While hull shapes might be a bit different, I bet there is a lot of similarity in overall weight and sizes between the cruisers yachts and sea rays. While this wouldn't be an accurate number, its probably close.

Jim
Fule usage is greatly influenced by all the creature comforts you load.
An empty boat always gets better GPH.
Construct your own fuel verses rpm or speed
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Old 03-26-2019, 09:48 PM   #19
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Fule usage is greatly influenced by all the creature comforts you load.
An empty boat always gets better GPH.
Construct your own fuel verses rpm or speed

Absolutely true.

My guess, however, is that the OP doesn't have floscan or something similiar, or he would have done this already.

Jim
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Old 04-01-2019, 01:13 PM   #20
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My GB 36, at 7 kts, with the generator running is about 2.5/hr. Without generator is between 1.75 and 2/ hr depending on conditions.
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