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Old 30 July 2002, 01:21   #1
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Ribcraft 5.85 + 115 Ficht what speed ?

I have an oldish 5.85 Ribcraft fitted with a new 115 Evinrude ficht . Has anyone got any idea how fast she should go with 2 up?
Also any advice om prop sizes .Is stainless steel worth the money ?

Thanks for any help .
John
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Old 30 July 2002, 03:42   #2
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Dunno about speed.......

errm say 35-40kts two up? As for prop size, well what RPM's are you pulling at Wide Open Throttle (WOT)? Your engine manual will give you the WOT range which should be somewhere around 5800rpm. If you are pulling significantly less you need a smaller pitch prop and more you can swing a bigger pitch prop. Rule of thumb is that a bigger pitch will give you more speed for less revs but a smaller will give you better acceleration (say for pulling a skier out.) Of course there is a lot of black magic surrounding props - cup, rake etc etc but this is the basic.

Alu vs Stainless? Stainless in theory gives better performance as it doesn't flex so much as Alu whilst in motion. If you look at most RIBS (he generalised) they seem to have Stainless props. Alu are cheaper (1/3rd price stainless.) On my last RIB I actually got better performance from a Alu prop than the equivalent sized stainless!

HTH,
Alan
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Old 30 July 2002, 03:46   #3
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My Ribcraft 5.85 with Mercury 90HP 4-stroke does about 35 Knots WOT (4 up)

Not sure what Pitch the prop is but at WOT gives 6000 RPM

Lots of discusion on stainless props on the forum - just do a search!

Mine is currently an Alloy - may consider "upgrading" at a later date - I believe it improves fuel economy and top speed.

Out of interest I'm doing about 1l/m and comparing this with various other boats at the Thames cruise most other 4-strokes are achieving about the same. So when you look at litres/mile bigger engine dosn't actualy mean bigger fuel bills!
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Old 30 July 2002, 04:51   #4
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Makes me feel better Jools........

as I got roughly 1 litre per mile from a 150 Optimax and get the same from my Yam 200 HPDI!! (both fuel injected 2 strokes.) Thats unless I push the loud stick forward at which point my wallet explodes!

On your prop sounds like you might get away with a larger pitch but then again if you load the boat up with fuel, kit and friends then you probably want to stay the same. Might want to switch to a stainless of the same pitch as your ali?

There is a highly scientific method of determining what pitch your prop is. What you do is look at it carefully, muttering knowledgably to yourself and amaze your friends by declaring what pitch it is by reading the number stamped on it somewhere!! I would imagine yours is either a 15 or 17inch if its a three blader.

Cheers,
Alan
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Old 30 July 2002, 09:17   #5
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John,

I have F115 Yamaha on my Ribtec 585 which will yield around 40 knots in smooth conditions with 2up. Best speed achieved is 42.1 with some tide assist. I'm currently running a 21" alloy prop.

Fuel economy is better than you indicate, though. I would normally get somewhere between 1.2-1.3 miles per litre, obviously depending on conditions, but for example we did Warsash-Salcombe this year, 3 up, 108 miles and used 87 litres.

All the Best

Brian
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Old 10 August 2002, 16:22   #6
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Jools - Have you found out what pitch your prop is yet?
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Old 11 August 2002, 02:33   #7
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115 ficht

Thanks for all the info , just had a week playing with the boat and we get 46 mph with 2 up and a load of junk on board.
Economy is very good not measured but credit card still in 1 piece !

Hummingbird GPS plotter with Solent chip in is just fantastic , the accuracy is quite amazing .

John
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