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Will_3Offline
Post subject: What is hull-speed for the CD28 ?  PostPosted: Jun 09, 2014 - 02:03 PM



Joined: Sep 04, 2012
Posts: 36
Boat Summary: Miss Bobbit, 1985 Flybridge Cruiser, Hull # 140, East Central Florida
Status: Offline
Just wondering if anybody knows the 'hull-speed' for the Cape Dory 28 Cruiser?

Or the other question is... what is the length at the water line (LWL) ... then you can calculate...

Hull Speed = (square root of LWL) * 1.34

Using this relationship and a LWL of 27 I get 6.96 knots.

Of course this is for a displacement-hull boat and I guess the Cape Dory 28 would be considered a semi-displacement hull... right?

The reason for the question... other than just basic curiosity... is finding the most efficient cursing speed when trying not to push an older motor to force the boat get up and travel on a plane...

and alternately not wanting to have the stern dug in a hole somewhere between hull-speed and ideal planing speed.

And to be able to compare fuel consumption at the ideal speed on a plane and the ideal speed when not on a plane... where ideal speed equals best mpg.

Then there is the question of what is the specified cursing rpm for the TAMD41A?

(I've heard WOT=3800 and cursing = 3600... which is different than the 80% to 90% of WOT I've always heard)

Regarding the rpm sweet spot... I have hard running diesel's at an rpm lower than that they are designed for can reduce engine life... is this anyone else's understanding.
 
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larryfellOffline
Post subject: hull speed  PostPosted: Jun 21, 2014 - 07:04 PM



Joined: Mar 13, 2007
Posts: 102
Boat Summary: "White Sand", Hull #167. Panama City, Fl, 1989FB
Status: Offline
Yes, the hull speed is about 7 knots. I cruise my Chrysler 360 (1.9: trans, 18x17x3) and cruise at 1700-1800 RPM. Depending on load and sea conditions my speed is 6.9-7.3 knots. My fuel flow meter shows a hair more than 3 GPH. Gas consumption rises DRAMATICALLY with each additional knot achieving one NMPG at 13 knots. And yes, it goes higher after that!!! Years ago I spoke with Jed and Connie ???? about their Great Circle Cruise. He kept careful records of diesel fuel purchased and decided that 7.3 knots was the most economical (for their overloaded) boat. I am about 50% sure he was burning about two GPH. Both Jed and I close our cruising day with a few miles of high RPMS to check out the cooling system and exhaust. I also think that trim tabs or lifting strakes would improve low speed fuel consumption/performance, but I don't think anyone has their boats equipped with fuel flow meters & trim tabs to verify.
Larry
 
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Will_3Offline
Post subject: RE: hull speed  PostPosted: Jun 28, 2014 - 02:16 PM



Joined: Sep 04, 2012
Posts: 36
Boat Summary: Miss Bobbit, 1985 Flybridge Cruiser, Hull # 140, East Central Florida
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Larry,

Thanks for the info. Our boat has the original Volvo TAMD 41a diesel and we have been operating it at about 8.5 knots so we had a 10 mile per hour speed... but maybe we need to take it back to 7 or 7.5 for a long days run.

By the way, is NMPG mean No Miles Per Gallon?

_________________
CD 28 FB - Hull # 140 - Miss Bobbit
Volvo TAMD 41A Diesel
East Central Florida
 
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vibound2000Offline
Post subject: RE: hull speed  PostPosted: Jul 13, 2014 - 12:24 AM



Joined: Jul 23, 2012
Posts: 63

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I also have a TAMD41A diesel. I generally cruise around 8 knots and burn about 2.5 gals per hr. give or take. Anything over 8 knots obviously increases fuel usage. According to the GPS I have gotten it up to 15.9 knots on flat seas but don't push it like that. I'd rather cruise slower, save money and enjoy the ride.

_________________
Emotional Rescue
1990 Cape Dory 28 FB
200 hp Volvo
St Thomas, VI
 
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ttasmaOffline
Post subject: RE: hull speed  PostPosted: Jul 15, 2014 - 04:54 PM



Joined: Dec 02, 2008
Posts: 19
Boat Summary: Freelance Houston, Texas 1990 Flybridge Cruiser Hull #211
Status: Offline
Will
Speed range for the 200 Volvo seems to vary from boat to boat I am sure depending of a variety of factors. My average cruise speed is 7.5 knots, a little over 2 gallons per hour, and when everything is perfect, water, boat bottom and load, my boat maxes out at 16 knots, probably 10 gallons per hour. Loaded for cruising it maxes out at 14 knots. Anything over 8.5 knots and I increase rpm to 3200 and cruise at 11.5 knots. Usually only when passing, or in rough choppy water. I wish I knew most efficient cruise speed. Max rpm for me with 18x14 four blade prop is 3800 to 3900.
Tom Tasma
 
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jwandlingOffline
Post subject:   PostPosted: Jul 27, 2014 - 02:35 PM



Joined: Jun 19, 2014
Posts: 5

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I just purchased an 89 model with 200 hp volvo. Did about 550 miles on ICW, stayed mostly at 8.5 knots at 2700 rpm. Used a bit more than 3 gph. I plan to install 30 in Bennetts and will post results.
 
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Will_3Offline
Post subject:   PostPosted: Sep 30, 2014 - 12:30 PM



Joined: Sep 04, 2012
Posts: 36
Boat Summary: Miss Bobbit, 1985 Flybridge Cruiser, Hull # 140, East Central Florida
Status: Offline
Good info folks. Thanks. Based on this info I think we are going to back down from our 8.5 knot cruising speed to 7.5 +/- .

_________________
CD 28 FB - Hull # 140 - Miss Bobbit
Volvo TAMD 41A Diesel
East Central Florida
 
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