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Old 09-17-2022, 08:03 AM   #1
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Cat 3208 375HP question

hi all,
New to this exciting boating adventure (or at least these "BIG" boats) we are looking at a 88 IG 36 with a single 375 HP cat with 3700 hrs. i have searched hours to try and find a spread sheet or actual captain that has an accurate (or close to) fuel consumption rating for this boat or similar boat at cruising speed 7-7.5 knots. guessing from what i have found so far to be around 1.8-2.0 GPH . is this correct or close to correct? trust the current owners but would like a 2nd or even 3rd response. thanks in advance for your time.
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Old 09-17-2022, 08:25 AM   #2
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I think that 2 gph is a little low particularly since the Cat is a big engine running slowly at those speeds. So maybe 2.5-3 gph. My Mainship 34T with a 370 hp Yanmar (but still lots smaller displacement than the Cat) would use 2.5 gph at that speed but is a lighter hull.

But why are you obsessing over low speed fuel consumption. Every semi displacement trawler in the 36' range will have similar fuel consumption at that speed. Big engines like the 375 hp Cat will do a bit worse because you have higher parasitic loads with a big engine. A smaller engine like the Lehman 120 may do better and a modern common rail injected small engine will do better still.

But better is probably only 0.5 gph less than the boat you are considering. Unless you are going to cruise a thousand hours a year, the cost difference is trivial in the grand scheme of things.

David
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Old 09-17-2022, 08:50 AM   #3
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2 - 2.5 gph at 7 kts should be in the ballpark. 7.5 kts is a bit fast for an efficient slow cruise on that boat (6.5 kts would even be a little better than 7).
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Old 09-17-2022, 08:54 AM   #4
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I own a Mainship 350 with a Cat 3114 300 HP running at 6-8 Knots overall i use less then 2GPH
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Old 09-17-2022, 09:22 AM   #5
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In my 44k displacement twin 3208s (375) at 8+ knots is 4ish gals per min. including generator run-time. Great engine.
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Old 09-17-2022, 10:56 AM   #6
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Accurate speed is hard to measure. Running at 2 gallons per hour is very due able. Now will that translate into 6kts or 8kts. Lot of that depends on sea state, currents, wind, and how clean the bottom is.

Few people actually lie about fuel consumption. Unfortunately, most are giving best guesses as they don’t have any truly accurate way to measure fuel consumption. When working with numbers like 7 or 7.5 the difference is nearly 10%.
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Old 09-17-2022, 01:13 PM   #7
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A typical Diesel engine uses in the area of .35 to .4 lbs of fuel per brake horsepower hour. Your engine at full power 2800rpm would use a
About 19 gallons an hour. Using 2 gal/hr suggests your using about 40 hp. Given your 36 ft length would drive your boat in the 6 knot range with a clean bottom and prop. Hull speed is as about 7.5 knots with a 36ft waterline
At this point the bow starts to rise / stern drops and you need more hp. Some where in that range you would use in the area of 100hp or about 5gal/hr. The 6kn speed would be in the 1200rpm range and my guess the 7.5 knot would be about 1500. The engines should be run up to 2000+rpm each day for 5 to 10 minutes burn off carbon build up. Shows up as black exhaust. I have the same engines in my gb49. I add a fuel additive which helps keeping down the carbon. After an 6to8 hour run there is less than 30 seconds of black exhaust at 2200+rpm. The 1200rpm is about 8kns about 2+ miles per gal on the gb49 total for both engines. It’s easy to get 1 mile per gal running @10kns 1700rpm.
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