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Old 08 December 2003, 02:27   #1
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Fuel economy, Twin set up

Does anyone have approx fuel consumption figures and top speeds expected for the following:
7 m Rib

2 x 90 hp 4 stroke v 1 x 200 hp 4 stoke.

I would like the 2 x 90 hp for the safety aspect, but am concerned about the economy, or lack of.

(1 x 200 hp with 8hp aux is cheaper, but what if you have 20 miles to go against the tide ( apart from waiting for the tide to turn) )
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Old 08 December 2003, 07:34   #2
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The normal way to work it out is.......

Fuel consumption is approx a third of the horse power at wide open throttle so a 90hp would use 30ltrs an hour. (2 stroke that is)
4 strokes use about a third less again so.....

a four stroke would use about 20ltrs an hour at wide open throttle.


So twin 90s will use approximatly 40ltrs an hour and a 200 will use about 44 ltrs an hour. But its not that simple as your running twin 90s you have more drag as you have two gearboxes that brings the consumption down a bit so your probably going to get about the same consumption. Althought the single 200 will be quicker than twin 90s by a fair bit. but you do have reliability with two engines.
These figures are for Mariner/ Mercury engines not sure about other engines.
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Old 08 December 2003, 07:49   #3
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that's the economy, anyone have a guess at top speed with Twin 90's on a Ribcraft 7m.........
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Old 08 December 2003, 08:15   #4
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Quote:
anyone have a guess at top speed with Twin 90's
Hmm, are you offering a prize for the winner then ?

38 mph is my guess for twin 90s. However I recon you would be going a lot faster with the single 200 hp, probably 50 mph.

Pete
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Old 08 December 2003, 08:19   #5
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I reckon 32knts ish with twin 90s not sure about a single 200.
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Old 08 December 2003, 12:28   #6
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Quote:
Originally posted by Diesel
not sure about a single 200.
OK I can help you out here: 46kts max (53mph).

Fuel burn: 1.0 to 1.1 litres per mile at cruising speeds.
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Old 08 December 2003, 12:31   #7
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Thanks Louise.
Is this fact or are you after the prize money.
46knts with twin 90's ? Pls confirm.

Thanks
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Old 08 December 2003, 12:34   #8
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Sorry that was me!

Yes - fact. Boat + engine as per profile. Not twin 90's - they'll be going backwards!


Will Pm address for prize. Thanks.
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Old 08 December 2003, 17:22   #9
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Twin 90ies

I would concur with Diesel in approx 32 knots on a 7m Ribcraft bearing in mind that its block co. and prismatic co.( increased due to its greater under water area/displacement) would be relative to the 7.7m H.

Fuel useage will obviously be again relative to weight/loading. On a 7.7m with 1200kg payload we would achieve 25lt on a mixture of 20% WOT, 50% cruise and 30% tickover.

I would advise on a twin set up pushing the underdeck tanks further forward than the manufacturers like. (approx forad of midships by 5%). This has proved very sucessful with us and accounts for overall increase of engine mass and thrust calculations compared to a single ( seen many RIBS down by the back - on a larger RIB power the bow up rather than trim it down - far better in a big sea).

Diesel figures are based on a standard carb 2 st but with a DI they will not be far off a 4st so adain I concur with his calcs.

Peter, I am glad that you are still looking at the twin set up. Whilst many are warry of it due to the increased cost of purchase I am convinced that in your circumstance it will be worthwhile.

Best of luck, at least we know that we got you on this train of thought !

Cheers

John
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Old 08 December 2003, 18:29   #10
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Pete,
think you should definitely go for the twin 90's after reading these posts. It will then save me having to buy a new boat to give you a run for your money next summer !!
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