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Old 12-14-2018, 02:19 PM   #1
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Fuel efficiency

Hi everyone... oh, and Happy Holidays!
Just recently purchased a Cape Dory 28 with a 100 hp Westerbeke.
What should I expect for a cruising speed and at what would that most likely be. How much fuel per hour etc.
Newbie here, so all advice appreciated.
Paul
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Old 12-14-2018, 02:28 PM   #2
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I would expect hull speed or maybe 1/2 knot shy.

(HS = 1.34 x √LWL

Not sure but assuming 27 foot waterline maybe 6.5 or 7 knots? Fuel economy should be great. Maybe 2.5 -3 mpg would be my guess. 3-4 gallons per hour?

Enjoy the boat. Are you on the Vinyard?
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Old 12-14-2018, 03:22 PM   #3
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A good cruise speed would be 1.2 times square root of waterline length.
Guessing 1 gallon per hour.
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Old 12-14-2018, 03:22 PM   #4
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My CD 28 gasser had a rudimentary Flowscan device installed. It showed 3 gph at 6.5 kts and 4 gph at 7.2 kts. Your diesel should do a third better so maybe 2 gph at 6.5 kts and 2.7 gph at 7.2 kts.


Fuel will be the least of your ownership expenses.



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Old 12-14-2018, 03:40 PM   #5
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Thanks everyone from The Vineyard!
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Old 12-14-2018, 03:48 PM   #6
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Any of you have a sense of what rpm’s that would be?
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Old 12-14-2018, 05:21 PM   #7
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It will take about 40 hp to hit 7 kts in that boat. Westerbeke doesn't give a prop hp curve in their spec sheet, but I would guess it should be in the low 2,000s to make that hp.


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Old 12-14-2018, 05:26 PM   #8
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Hi Paul my brothers, sister and I own an inherited house in Chilmark. Good general rule of thumb is 20 HP per gallon. So, a 100 HP engine should burn a maximum of 5 gallons per hour. My guess on fuel burn is:

6 knots 1 GPH
7 knots 2 GPH
8 knots 4 GPH

Ted
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Old 12-14-2018, 05:34 PM   #9
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Please get in touch if you’re gonna be here. My wife is artistic director at the MV PLAYHOUSE in VH.
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Old 12-14-2018, 08:03 PM   #10
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Quote:
Originally Posted by Pauliemv View Post
Please get in touch if you’re gonna be here. My wife is artistic director at the MV PLAYHOUSE in VH.
Oh to back the clock up one month. It's been a family tradition to do Thanksgiving on the Vineyard since the early '70s. While I will be up next Thanksgiving, not sure what my spring and summer schedule are this coming year. You won't see me there during the winter.

Ted
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Old 12-15-2018, 02:50 PM   #11
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Happy cruising speed on mine is 7-7.5 knots. 2K RPM's on the Volvo, and about a 2 GPH fuel burn. I'd expect you'd have to spin the Westerbeke a bit harder but I'd expect the fuel burn to be very close..
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Old 12-15-2018, 03:10 PM   #12
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My guess is 7kt at 1.5gph, less at 6.5kt. The Westy 100 I think is a Mitsubishi or Nissan with indirect injection (glow plug diesel) so is a little less efficient than the direct injection. Not much worse, but some.

If you have to preheat to start, it is the indirect injection or pre-chamber type.
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Old 12-15-2018, 05:10 PM   #13
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Forget the math. cruise at the max speed where the bow is down. more than that the extra fuel is being used to push the boat up with minimum increase in speed
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