Boat weight question
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Ski in NC
Technical Guru
I'm skeptical that the KK can make 7.7mph on a total burn of 2.5gph with twins. Could be true, but "show me" comes to mind.
Kind of like a kid saying his f150 truck gets 25mpg. BS flags pop up.
Kind of like a kid saying his f150 truck gets 25mpg. BS flags pop up.
eyschulman
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True that the best economy is one to two k below Hull speed but I personally prefer to travel at or just above hull speed valuing my vacation time more than the difference in $s. Considering the overall cost of my boat and maintenance the difference in cost between 7K and 10K is hardly significant. If my boat were a commercial boat or one that put on thousands of hours a year or had to have the range to cross an ocean I might do differently. I think many cursing people know the difference in fuel burn and opt for the mid range for other reasons.
eyschulman
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when we see a boat with a long WL narrow beam sharp entry big prop and relatively light build you are looking at efficiency. The modern trend in Trawler cottage types is in the opposite direction and no surprise it takes more power to move the cottage. Back when engines were considerably less powerful long low and narrow was the norm and efficiency came with the package.
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Closest data point from sea trial immediately after refit is below. Displ. at trials was 60,000lb, bottom paint had just been done etc. Note that I had Naiad's installed also, their drag will have taken some fraction of a knot off the performance.
1500 rpm, 4.0 gph (total) 7.9 kn.
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But Ski, it's only 6.7 kn and the boat has 52.3" LWL !
The JD's aren't that far above idle. That's the fuel consumption I have with higher rated versions of the same engine at that rpm.
It's like the truck is coasting downhill.
Last edited:
FF
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But Ski, it's only 6.7 kn and the boat has 52.3" LWL !
The Sq RT of 52.3 is 7.23.
Efficient cruising is done at the sq rt times .9 to 1.15 depending on the hull.
7.23 times .9 is 6.5K , drag is mostly skin friction , little from wave making,
Seems about right.
The Sq RT of 52.3 is 7.23.
Efficient cruising is done at the sq rt times .9 to 1.15 depending on the hull.
7.23 times .9 is 6.5K , drag is mostly skin friction , little from wave making,
Seems about right.
Lost Horizons
Senior Member
Once at a boat show I asked a Beneteau sales rep why fuel consumption of the ST50 is higher than that of ST52 according to the Boat Tests, which Beneteau was quoting in their marketing materials. The sales rep appeared to be surprised, started checking with his iPad, but could not find the answer. Took my business card and promised to get back to me. That was 3 years ago and I never heard from him since.
Major efficiency factors affecting fuel economy are, in descending order of importance, propeller efficiency, hull efficiency, and engine/transmission efficiency. Since hull and engine efficiencies should be relatively close between ST50 and ST52, the IPS propellers of the 50 must be responsible for high fuel consumption. If the Boat Tests numbers are correct.
Major efficiency factors affecting fuel economy are, in descending order of importance, propeller efficiency, hull efficiency, and engine/transmission efficiency. Since hull and engine efficiencies should be relatively close between ST50 and ST52, the IPS propellers of the 50 must be responsible for high fuel consumption. If the Boat Tests numbers are correct.
sunchaser
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Many side by side comparisons using identical hulls and comparing IPS to straight shafts have been reported. Hands down the IPS are more efficient. My personal favorite is the Sabre 36 IPS. In this case the engine location provided a lot more usable interior space with a very unique layout.
Then there are the IPS unique issues to consider.
Lost Horizons
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Integrated Propulsion System. A saildrive-like thingy.
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With forward-facing dual counter-rotating propellers, sorta like my duo-prop sterndrive only turned around backwards.
More efficient than inboards for speedy boats traveling on plane, partly because the propshafts are more parallel to the water surface. Not sure they're more efficient at displacement or at speeds in the teens.
The ST 52 was a completely different boat with straight drives, not IPS and with more lwl than the 50.
Lost Horizons
Senior Member
I know.
IPS is Volvo's name for their Pod systems of propulsion. Mercury has a pod system named Zeus which is somewhat comparable using Cummins engines.
It has gained a good bit of popularity in 40-60' boats based on better fuel economy and greater speed with smaller engines as well as handling characteristics. Typically in planing boats and not in full displacement.
Volvo Penta IPS - Volvo Penta IPS : Volvo Penta
eyschulman
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I think IPS popularity beyond the major advertising blitz is based on in order. #1 Joy stick high maneuverability factor. #2 builders can install in ways that free up more internal living space. #3 Speed which is always a major factor in new boat sales. #4 Claimed economy AT Higher cursing range. One interesting test was preformed on three boats from one builder(Hunts) and the results were interesting and would not in any way give a clear reason to chose IPS drives over inboard and IO drives on the basis of performance criteria..
bluemainship
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IPS love crab trap ropes
FF
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"It has gained a good bit of popularity in 40-60' boats based on better fuel economy and greater speed with smaller engines as well as handling characteristics. Typically in planing boats and not in full displacement."
I believe the reason IPS is big with builders is far less experienced labor can be used during the boats assembly.
No skilled engine adjustments or shaft hassles.
And with mixed metals there will be plenty of work for aftermarket repairs (after the warentee expires).
Most of the "popularity" seems to be from magazine writers that get a ride on a new boat , and have space to fill, and advertisers to worship.
I believe the reason IPS is big with builders is far less experienced labor can be used during the boats assembly.
No skilled engine adjustments or shaft hassles.
And with mixed metals there will be plenty of work for aftermarket repairs (after the warentee expires).
Most of the "popularity" seems to be from magazine writers that get a ride on a new boat , and have space to fill, and advertisers to worship.
Conall63
Senior Member
I have a Deere 6068TFM for my single screw set up. My boat has a fairly short LWL@ 40', with a guestimated weight of 65-70K, and a heavy displacement to length ratio of around 450. I tend to cruise in the 1500+ range keeping below hull speed, and see about 7 knots with a burn of 2.1 per hour.
Recently, on my 230 nautical mile gulf crossing, we averaged about 2.8 NMPG. The bottom hasn't been cleaned in five months.
I know this isn't really apples to apples, but it's the same/similar engine in a heavy cruising boat.
Conall
Recently, on my 230 nautical mile gulf crossing, we averaged about 2.8 NMPG. The bottom hasn't been cleaned in five months.
I know this isn't really apples to apples, but it's the same/similar engine in a heavy cruising boat.
Conall
eyschulman
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My SD boat at 46ft WL with twin JD 6068s at 7.4k burns about 3gal/Hr not much different then your boat. the light 33,000lb and slightly longer waterline are factors. This is also an example of twins not burning much more than a single to push a boat.
Some other observations:
From this website:
Approximate Diesel Generator Fuel Consumption Chart
If you compare 20kw vs 40kw. At full load 20kw uses 1.6 GPH, while the 40kw at 1/2 load uses 2.3GPH. That's a whooping 40% difference for the same payload. Isn't that an indication that smaller engine might play a role here? However it seems at higher capacity the smaller engine does not show much advantage.
From this website:
Approximate Diesel Generator Fuel Consumption Chart
If you compare 20kw vs 40kw. At full load 20kw uses 1.6 GPH, while the 40kw at 1/2 load uses 2.3GPH. That's a whooping 40% difference for the same payload. Isn't that an indication that smaller engine might play a role here? However it seems at higher capacity the smaller engine does not show much advantage.
eyschulman
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It is hard to say what is going on there. In general one gallon of fuel burnt gets about 20 +/_ Hp. no mater the size of the engine. If it takes X amount of HP at the prop or props to push a boat to Y speed the fuel burn should be close. At idle and lower speeds there may well be an inefficiency of a larger displacement engine vs. a small one. This may not be as much an issue with electronically controlled common rail engines which adapt fuel burn to load much better than mechanical systems. Generators in general do not like being run at lower loads and may well be designed accordingly. That larger motor may be wasting fuel at a lower load a common rail sx can fix that and because of pollution issues may soon be all you can buy.
twistedtree
Guru
I think the moral of the story is to not believe anything you read on BoatTest.com
I don't see anything wrong with their numbers.
FF
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"If you compare 20kw vs 40kw. At full load 20kw uses 1.6 GPH, while the 40kw at 1/2 load uses 2.3GPH. That's a whooping 40% difference for the same payload. Isn't that an indication that smaller engine might play a role here?"
Same for propulsion engines , oversize will frequently be out of the efficient BMEP, so 20-40% more fuel burn is easy.
Old TT get away with it as most have car rared HP , not cont. duty rating of an industrial engine .
Simplest for a NA engine is to figure it takes 3 cubic inches to make a HP .
So divide the engine displacement by 3 and figure that is the efficient HP , then observe what percentage of the HP your fuel burn is requiring.
The further you are from the 3=1 HP the further you are from an efficient BMEP.
Same for propulsion engines , oversize will frequently be out of the efficient BMEP, so 20-40% more fuel burn is easy.
Old TT get away with it as most have car rared HP , not cont. duty rating of an industrial engine .
Simplest for a NA engine is to figure it takes 3 cubic inches to make a HP .
So divide the engine displacement by 3 and figure that is the efficient HP , then observe what percentage of the HP your fuel burn is requiring.
The further you are from the 3=1 HP the further you are from an efficient BMEP.
