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Thread: Power Cat 18MPG Interesting?

  1. #41
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    Default Re: Power Cat 18MPG Interesting?

    Alan, you are right of course. But I think my measurements are close enough. Some tank top ups were done after pretty much no wind and no current and the fuel consumption figures are pretty spot on if not scientifically accurate. Either way those figures are what I use for extended motoring and have shown to be pretty close.

    Peter, even from the Galapagos to the Marquesas would need to average over 8K to do it in 15 days. Anyway, thats OK. The Pacific goes all the way from the Americas to Australia.

  2. #42

    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by dennisail View Post
    Alan, you are right of course. But I think my measurements are close enough. Some tank top ups were done after pretty much no wind and no current and the fuel consumption figures are pretty spot on if not scientifically accurate. Either way those figures are what I use for extended motoring and have shown to be pretty close.

    Peter, even from the Galapagos to the Marquesas would need to average over 8K to do it in 15 days. Anyway, thats OK. The Pacific goes all the way from the Americas to Australia.
    I was interested in the differences between the various fuel consumption figures.

    Fuel consumption for diesel engines is pretty consistent at an avergage of about 200 g/hp.hr. so ignoring the efficiency of the engines themselves I looked at the energy consumed.

    A mid range density for diesel is about 850 grams / litre although this can vary from 820 to 950 grams / litre. A further complication is the use of, or dosing with biodiesel, the calorific value of which is around 36kJ/g with a density of about 880 grams / litre compared to 43kJ/g for mineral based diesel.

    Most of, if not all of, the diesel on dennisails route is likely to have been mineral based, assuming this was the case. 1.7 l/h at 5 knots equates to 43 kJ/g x 850 g/l x 1.7 l/h = 62135kJ/h = 23.1hp

    Comparing this to my figures, all of my recordings were made in SE Asia where all of the diesel is bio (typically 37kJ/g), 1.9 l/h at 5 knots equates to 37 kJ/g x 880 g/l x 1.9 l/h = 61864kJ/h = 23.0hp

    It is remarkable how close these are and I wonder how closely the wetted surface of dennisails heavier monohull compares to that of my lighter catamaran?

    Obviously none of this is defacto and the accuracy of dennisail's and my measurements would probably not stand up to proper scientific scrutiny even some of my math could be suspect.

    But is does show how difficult it is to compare boats.

  3. #43
    Join Date
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    Default Re: Power Cat 18MPG Interesting?

    Thank-you Peter. I found that fascinating. And thought provoking.

    Mike
    Nothing works on an old boat, except the skipper.

  4. #44

    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by Peter View Post
    I was interested in the differences between the various fuel consumption figures.

    Fuel consumption for diesel engines is pretty consistent at an avergage of about 200 g/hp.hr. so ignoring the efficiency of the engines themselves I looked at the energy consumed.

    A mid range density for diesel is about 850 grams / litre although this can vary from 820 to 950 grams / litre. A further complication is the use of, or dosing with biodiesel, the calorific value of which is around 36kJ/g with a density of about 880 grams / litre compared to 43kJ/g for mineral based diesel.

    Most of, if not all of, the diesel on dennisails route is likely to have been mineral based, assuming this was the case. 1.7 l/h at 5 knots equates to 43 kJ/g x 850 g/l x 1.7 l/h = 62135kJ/h = 23.1hp

    Comparing this to my figures, all of my recordings were made in SE Asia where all of the diesel is bio (typically 37kJ/g), 1.9 l/h at 5 knots equates to 37 kJ/g x 880 g/l x 1.9 l/h = 61864kJ/h = 23.0hp

    It is remarkable how close these are and I wonder how closely the wetted surface of dennisails heavier monohull compares to that of my lighter catamaran?

    Obviously none of this is defacto and the accuracy of dennisail's and my measurements would probably not stand up to proper scientific scrutiny even some of my math could be suspect.

    But is does show how difficult it is to compare boats.
    When we launched our boat we had no rig, so we motored everywhere. The Honda outboards used an average of 2 litres/hour per motor. Usually we'd motor on one engine at around 6 knots, running the one engine at around 12-15 hp. (At a guess, not full throttle and revs at around 4000, rev limit being 6000)

    Again, though, that 2 litres/hour average would have included some heavier and faster running, and also anchoring/ idling atc.

    I'd say motoring we get pretty close to 2 - 2 1/2 nm/litre, so 8 - 10 mpg.

    A boat that could get 18mpg at good cruising speeds would be interesting to see.

  5. #45
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    Default Re: Power Cat 18MPG Interesting?

    Peter, your maths is interesting. However one thing to note is my revs are 1500-1700 which allows a max of just over 20hp according to the capability of the engine if it were at full throttle at these revs. But the prop is only capable of absorbing around 6hp at this RPM. So the engine is really only putting out around 6hp to push the boat at 5k. Either way it seems our fuel usage is similar when you compensate for the bio fuel. Your boat is lighter, so I guess that makes up for difference of cat vs mono wetted surface area.

    At 8K I would be using around 9.3 liters per hour. Do you have any data on what you might expect at 8K? This would be interesting to see as at 8K I am close to hull speed and making some large waves. I wonder how your cat would go here?



    Note the graph does not even go as low as the revs I used. Its probably not that good for the motor to run it at revs this low. But I would give the engine a blast every now and then to clear out carbon etc.

    Alan 2L/h sounds very good at 6K even for diesel standards. That is less than my boat would use at 6K and its even more impressive when you consider diesel VS petrol outboard consumption. Goes to show that weight really makes a huge difference.
    Last edited by dennisail; 13th January 2014 at 07:31 AM.

  6. #46

    Default Re: Power Cat 18MPG Interesting?

    Weight does make a big difference. Windage does too - putting the rig on hurt our motoring efficiency a bit. But there are compensations.

  7. #47
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    Default Re: Power Cat 18MPG Interesting?

    But there are compensations.
    Like actually being able to sail

  8. #48

    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by victor View Post
    As I was reading the latest posts regarding Power Cats, I thought you guys maybe interested in some of the specs and features of this "Concept" power cat.
    LOA 36 feet, BOA 14 feet, Draft 14 inches
    Cruising Speed 18-20 Kn, MPG 18.1
    Granted this is a tease there are more exciting features and specs to publish and of course open water tests by January-March 2014.
    Let me know what you all think.
    The time is neigh as they say. Test results yet?

  9. #49

    Default Re: Power Cat 18MPG Interesting?

    Quote Originally Posted by victor View Post
    Granted this is a tease there are more exciting features and specs to publish and of course open water tests by January-March 2014.
    Let me know what you all think.
    Any news on this yet Victor?. Build progress, tests or is it a DoDo?

  10. #50

    Default Re: Power Cat 18MPG Interesting?

    Interesting article on power cat efficiency here from Gold Coast Yachts

    http://www.allatsea.net/trends-power...el-efficiency/

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