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Old 02-11-2016, 03:42 PM   #1
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Boat weight question

I am reading reviews on boattest.com

For a Kadey-Krogen 58, which weighs about 96k lbs, twin John Deere V6 158hp.
At 1250RPM the boat travels at 7.7MPH, use fuel 2.5GPH. MPG is 3.1

For a Beneteau Swift Trawler 50, which weighs 35k lbs, twin Volvo D6 435hp.
At 1500RPM the boat travels at 7.5MPH, uses fuel 4.8GPH. MPG is 1.6

So I understand the Beneteau has a much bigger engine, hence consume more fuel. But it only weighs about 1/3 of the KK!

My conclusion is: Assume I don't want a fast boat, say I am ok with 12-15 knots top speed, Assume my goal is long range cruise, to save fuel(hence increase the range), the engine choice is much much more important than the weight of the boat? This is somewhat counter-intuitive to a landlubber like me, one would think it takes much more energy to move the heavier boat?

To extend the question, boat bottom shape also plays a role in fuel consumption, but by how much? From above example it seems to me the engine choice plays an overwhelming role, pretty much shadowed other factors? Or is it the KK has a much more efficient bottom than the Beneteau?

The KK has a draft of 5'3" while the Beneteau is 3'5".
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Old 02-11-2016, 03:54 PM   #2
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Engine size can play a big role and hull shape even more so.I am not familiar with either boat, but that would be my guess.

Might want to check the KK specs again. I think 96K might be a bit high for a 58 ft boat,
The KK52 only weighs about 45K. Couldnt find any specs for thr 58
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Old 02-11-2016, 04:01 PM   #3
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I think your numbers for the KK58 fuel consumption are off. I'm guessing that what you have is per engine. Meaning that the likely figures are 5 gph and 1.5 mpg. As a point of reference, my KK54 is about 80,000lb loaded and in order to get 7.7kt I'm burning about 5 gph and about 1.6 nmpg. This is with a 1996 JD 6068T rated at 225HP - an older version of the engines in the KK58.

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Old 02-11-2016, 04:02 PM   #4
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I think your numbers for the KK58 fuel consumption are off. I'm guessing that what you have is per engine. Meaning that the likely figures are 5 gph and 1.5 mpg. As a point of reference, my KK54 is about 80,000lb loaded and in order to get 7.7kt I'm burning about 5 gph and about 1.6 nmpg. This is with a 1996 JD 6068T rated at 225HP - an older version of the engines in the KK58.

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And I'm running at about 1750 rpm to make that speed.

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Old 02-11-2016, 04:10 PM   #5
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Well, double checked the numbers are correct. KK weights 96k, according to KK website the latest version weighs 99k. According to Beneteau's website the Swift 50 weighs 35k lbs.

And the fuel consumption numbers are for both engines for both boats. boattest.com usually make the numbers comparable.

http://www.boattest.com/review/kadey-krogen/483_58
Beneteau Swift Trawler 50 (2013-) 2013 Reviews,performance,compare,price,warranty, specs,Reports,Specifications Layout, video | BoatTEST.com
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Old 02-11-2016, 04:14 PM   #6
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Ok - so I read the article and found the section on fuel consumption. I fell for the old kt vs mph problem. The figures you gave were for 7.7 mph or 6.7 kt. At that speed my tables show that I burn 2.5 gph - exactly the same as you quote for the twins in the KK58. This makes more sense. A general rule of thumb is that going to twins doesn't significantly increase your fuel consumption at a given speed. Neither does going with a larger engine.

So back to your question - I'm not sure why the big difference. In general it takes less power to move less weight. Hull shape can make a big difference though - the KK58 is definitely shaped for fuel consumption. Larger engines should not make that much difference.

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Old 02-11-2016, 04:23 PM   #7
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Tony is on the right track: engine size, displacement and hull shape, the latter two being more important.


The JDs on the KK are not that much smaller than the Volvo D6. I am not sure which JD engine is in the KK as I don't think they make a V6, so I assume it is the 4045 4.5 liter engine. The Volvo is 5.5 liters and the difference in fuel consumption to the JD is almost un measureable.


The much, much bigger effect is hull shape and displacement.


The KK is a full displacement hull. The Beneteau is semi-displacement. SD hulls can take almost twice the horsepower and therefore fuel as a full displacement hull below displacement speeds PER TON of displacement. 7.5 kts is well below the displacement speed of the boats that you cited.


But an even bigger effect is displacement. Below hull speed if the hull shape is the same, horsepower required is almost directly proportional to displacement. So the KKs 96K lbs (probably correct) is more than twice the reported (probably not correct) weight of the Beneteau.


So to summarize:


Engine displacement- negligible
Hull type- SD uses up to twice the fuel as FD
Displacement- The KK is 2+ times the weight of the B


And I agree with whoever said that the KK uses 5 gph on both engines at the reported speed. That makes everything said above line up.


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Old 02-11-2016, 04:29 PM   #8
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Just to roil the waters, so to speak;

My boat displaces 150,000 lbs with low fuel and water.
Fuel in engine room saddle tanks -- 1200 gal. imp
Water under fo'c'sle -- 1000 gal imp

Boat is 57' oal, draft 8 1/2 ft
Hull shape - see pic
Engine Cat 353 turbo -- 390 hp @ 1225 rpm

Performance as per PO;
7 to 7 1/2 knots @ 800 rpm, approx. 4 1/2 gph

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Old 02-11-2016, 04:33 PM   #9
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Quote:
Originally Posted by djmarchand View Post
...
The JDs on the KK are not that much smaller than the Volvo D6. I am not sure which JD engine is in the KK as I don't think they make a V6, so I assume it is the 4045 4.5 liter engine. The Volvo is 5.5 liters and the difference in fuel consumption to the JD is almost un measureable.
...
The engines on the KK58 are JD 6068TFM75 M1 rated at 158HP. These are 6.8 liter straight six engines.

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Old 02-11-2016, 04:43 PM   #10
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The weight might be right on. Our selene is also displacement and at 60ft LOA she's 100,000 lbs. More with full tanks.

For the record we have one 430HP engine and burn 6.4 gal/hr at 1800 RPM for 8 kts

If you're looking to cruise at 12+ kts you're looking at semi displacement or full planning boats
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Old 02-11-2016, 05:01 PM   #11
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Once the boat is up to speed, the weight becomes less of an issue, getting a heavy boat up to speed is what takes extra fuel.

The GPH rates are quoted for cruising....
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Old 02-11-2016, 05:09 PM   #12
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So the "overwhelming factor" here is actually the hull shape, not the weight, Woo, this is an eye opening point. I never would have thought that!
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Old 02-11-2016, 05:20 PM   #13
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So the "overwhelming factor" here is actually the hull shape, not the weight, Woo, this is an eye opening point. I never would have thought that!
As has been pointed out on the forum, for most people, fuel consumption is not the biggest cost in owning a boat. It is often not a primary reason for picking one boat over another. However, for long distance passage making, lower fuel consumption provides for longer range with given tankage. This can easily be a critical factor for crossing oceans.

Boats like the Krogens and the Nordhavns are correctly termed "passagemakers", partly for their range and partly for their seakeeping ability.

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Old 02-11-2016, 05:35 PM   #14
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So the "overwhelming factor" here is actually the hull shape, not the weight, Woo, this is an eye opening point. I never would have thought that!
Not entirely sure of that. The effect of weight has been well documented in many types of boats and is most accurately identified and quantitated using a given boat with more or less weight. The effect is certainly less significant at speeds below hull speed but still there. I think a lot of apples and oranges comparisons are being made with no controls. Hull shape is important but weight and waterline length are also. MY 34,000 lb. 46.5 Ft. WL SD twin with two JD 330HP (660 total) will cruise at 7K <3G/H and at hull speed of 9.2K <5G/H. What does that mean about other boats not much. But if I can remove 4,000lb of weight there would be improvement but most notable above the hull speed and the top speed. As a matter of fact when the boat was first launched in a lighter condition the performance was notably better.
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Old 02-11-2016, 07:04 PM   #15
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There are many factors involved in these two boats and the comparison. First, it hull speed. The KK58's is going to be higher than the ST50. Then you're comparing entirely different forms of propulsion, comparing straight drives to IPS. There is a mid range on the IPS where their usage suffers. Note that at 1250 RPM or so, the numbers are comparable, then at WOT they are too. If you're going to make comparisons and try to extrapolate to any conclusions, you're going to have to find more comparable boats.

The assumptions you made might be valid but you can't base them on this comparison. The information shows that at 1250 RPM they're comparable in nmpg but just going different speeds. Then there is a mid range where the IPS doesn't do so well. However, it reaches double the speed.

So hull design, waterline length (longer waterline length gives higher hull speed), method of propulsion are very important factors.
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Old 02-12-2016, 12:53 AM   #16
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OK, You guys got me all confused again. Let me try to ask the question again:
If I travel from point A to point B, using above two boats
KK: Speed: 7.7MPH, fuel consumption: 3.1MPG
BT: Speed: 7.5MPH, fuel consumption: 1.6MPG
Keep in mind both are under hull speed.
Question: Why KK, being 3 times heavier, travel slightly faster but use much less fuel?
Is it:
Length? 58' vs 55'
Engine? 2 x 158-hp John Deere 6068TFM75 M1 vs 2 x 435-hp Volvo Penta D6 IPS600
Hull shape? Full displacement vs semi
Am I missing anything?
If multiple factors all contributed to the result, is there one factor being major? Or is it evenly divided among all factors?
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Old 02-12-2016, 01:27 AM   #17
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The KK is well below hull speed. The ST is right at hull speed, might not even be below it.

Full displacement vs. planing.

Don't know where you got your lengths. The ST50 hull length is only 43'9". I don't know waterline length but somewhere probably in the 35-37' range. The waterline length of the KK is 52'3". So you're probably talking hull speeds of around 8.4 mph vs. 10.6 mph or so So you're comparing the KK at 3 mph below hull speed vs the ST at less than 1 mph below hull speed. The ST's waterline length is about 70% that of the KK.

Two entirely different propulsion systems. One straight drive, one pod. Pod does great at a lower rpm than the speed you used and at a high speed. Lousy between hull speed and planing speed.

Entirely different types of engines and gearing for very different purposes.

You're comparing speed to speed, but you've picked the best area of the KK's curve and because of the lower hull speed you've picked a lousy point of the ST curve. Lower the ST speed to 6.3 mph and you have the exact same mpg of 3.1.

You're missing the point that these just are in no way boats that can be compared. You can't pick one element. The KK is designed for economical very long range cruising and equipped accordingly. The Beneteau is designed to plane and combine a greater speed with more modest economy. You cannot make any assumptions of generalizations based on such a comparison as you have made.

I'm going to give you a parallel since you're a landlubber. A Ferrari FF by the same logic should get much better economy than a Ford Explorer, but it gets about half. That tells you nothing about weight.

There isn't one factor that determines efficiency alone. Size and design of hull, engines, gearing, propellers, form of propulsion are all factors. Weight would be at some point but not at the speeds you're talking about. Weight isn't a factor in hull speed.
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Old 02-12-2016, 01:44 AM   #18
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I'm struggling with this. I think there is something wrong with the BT data/performance.

I'll give you another data point: my OA Mk 1 has twin JD 6068 TFM75 M3 rated, each 201 HP. Same engine as the KK but different rating. My boat displaces 65,000 lb at 3/4 fluid load. A SD hull shape.

So, 6.6 kn (7.6 MPH) is achieved at 1200 rpm, 2.2 gph total and 3.0 nmpg (for statute, rounded to 3.5 MPG )

I believe the KK figures quoted.

For the BT, something is wrong somewhere........

Edit: Ok, B&B identified a bunch of things. For the French it seems a 50 is only 43, and waterline length even less. My 50 has 46.5' waterline length.
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Old 02-12-2016, 02:11 AM   #19
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There isn't one factor that determines efficiency alone. Size and design of hull, engines, gearing, propellers, form of propulsion are all factors. Weight would be at some point but not at the speeds you're talking about. Weight isn't a factor in hull speed.
Dave Gerr published a very good article in the Westlawn Masthead in 2008. Refer to pages 12-17. He advocates a better 'hull speed' calculation that includes displacement. I have not played around with his formulae to see how much effect weight has at slow speeds, I just know that my sea trial data fits his predictions quite well.
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Old 02-12-2016, 04:21 AM   #20
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Different sea trial

Quote:
Originally Posted by BandB View Post
...
Don't know where you got your lengths. The ST50 hull length is only 43'9". I don't know waterline length but somewhere probably in the 35-37' range. ...
All the references I can find show the ST50 LWL as 43'9". LOA is 49'2".

Quote:
Originally Posted by Insequent View Post
...
For the BT, something is wrong somewhere........
...
Here's a different on the water test that shows at 1270 rpm the ST50 is making 6.0 kt (6.9 mph) at 2.1 gph. That equates to 3.3 mpg (statute).

Fast-Lane Liveaboard: Testing The Beneteau 50 | | PassageMaker

That puts the ST50 much closer to the KK58 numbers.

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