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Old 10 May 2005, 09:01   #1
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Country: UK - England
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Make: Humber
Length: 5m +
Engine: Twin 50Hp
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Engines & fuel consumption

I run a 5.5m Humber Destroyer RIB with twin Yamaha 50HP two stroke engines for diving.

Thinking of changing to a single outboard of 100HP (two or four stroke).

What fuel consumption should I get?

The very best we have got with the current set up is 2.5 miles per gallon.

This was done with six people on board plus all their dive kit and a straight run up the Sound of Mull over flat water on a windless day for 25 miles, our 2 x 5 gallon tanks were virtually dry on arrival at the dive site.

Should I get better or worse consumption with a single engine?

Any thoughts?
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Old 10 May 2005, 09:36   #2
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Country: UK - England
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Make: Humber
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Engine: Suzuki 70hp 4* outbo
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Posts: 205
Donít know about the 100hp size but we managed to get 4.2mpg out of our Suzuki 70hp 4 stroke on our Humber Assault 5.5. the other week end with 5 divers on board in lumpy seaís.
If that helps.
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Old 10 May 2005, 10:29   #3
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Country: UK - Scotland
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Boat name: Crusader
Make: Redbay 8.4 Exped
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Engine: yamaha sterndrive
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Merc 90 4st

I had a 6m delta with a single merc 90. doing 30kts it used about 4-5 gallons of petrol an hour
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Old 10 May 2005, 11:31   #4
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Country: Netherlands
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f100

The Yamaha f100 uses 31 liters at full speed and by that is Yamaha's most economic engine.

Know few people who haven them and are very, very pleased with the engine's fuel use.

Danny.
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Old 10 May 2005, 11:48   #5
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Country: UK - England
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30% better

A single 100hp should be 30% to 40% more efficient than twin 50s as you only have the drag of one gearcase.

Can't comment on the 100 four stroke, as have not used one, but it should be pretty good. Perhaps you might get nearly 5mpg?

I get about 4-5 mpg with my 150hp Opti on a 6.5m boat at a steady 25-28 knots.
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Old 10 May 2005, 17:01   #6
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I reckoned on averaging just over 2gph with my single f100 on a humber assault,but generally cruised at 3800-4000 rpm with a 21 inch prop(max 5600 lightly loaded),so harder use would increase consumption.This worked out at between 0.55 and 0.85 litres per mile depending on type of use.
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Old 11 May 2005, 05:59   #7
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Country: UK - Scotland
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Make: Northcraft
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Engine: Mariner 135hp ob
MMSI: 235029446
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Ten gallons per hour, WOT at 5400rpm, 44knots, 4 divers, twinsets, coxwain, 5.6m Northcraft widebeam 135 Mariner 2stroke, 21 inch LaserII, dolefins.
Ouch.
51knots just cox fuel and anchors.
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Old 11 May 2005, 16:48   #8
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Country: UK - England
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Look at etec's! compared to Yamaha
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Old 12 May 2005, 09:04   #9
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Country: UK - England
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We have a Yam 100 4 stroke (on carbs) on a Delta 5.5 the fuel use is low and we are pleased with it. In comparison to its sister boat with 75 Mariner 2 stroke it uses half the fuel. I would be interested in how much fuel efficient the newer 100's are that are now fuel injected.
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Old 15 May 2005, 08:14   #10
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Country: Belgium
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Make: Ribtec
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Engine: 90HP Honda
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Honda 90 hp on a 5.85 Ribtec uses 19l/Hr at 4500 rpm/24kn heavy loaden,with 17" alu prop.
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