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Old 02 June 2014, 07:35   #1
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Country: UK - England
Town: Exmouth - Devon
Make: Avon
Length: 5m +
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Mercury 60 EFI prop change?

Hi All,

To me this sounds like my problem maybe overall power output but would be nice to think I could improve things with a prop change for now!

I don't want to increase the top speed of my boat, with just myself on it it will just about make it to 30 knts but it's screaming at WOT as soon as you add another adult it immediatly you are looking at about 25 knts.

It would be nice to be cruising at around 25 knts without the engine screaming. Like my friend said you wouldnt drive your car around at 6000rpm constantly.

Am I asking too much? My boat is a ~5.5m Avon adventure (I think!)

The engine is only a few years old and I recall the previous owner changing the prop before when he put the new engine on. The boat did have a 75 suzuki previoulsy but that was stolen.

Would love to know your thoughts on this

Is there any measurements I can take of the current prop to get an idea of what I have?
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Old 02 June 2014, 08:45   #2
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Should be stamped on the prop, sometimes just inside, or between the blades
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Old 02 June 2014, 09:01   #3
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Ok Great I'll try and pop out and have a look later.
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Old 02 June 2014, 10:37   #4
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Be aware it's often stamped on the part covered by the prop nut (or various spacers and washers that may be in there.)

Generally, you select a prop that gives you close to the max rpm (manufacturer spec) for the load you'll be running normally. This results in lower revs when loaded heavier, higher when loaded lighter.

You can, of course, prop up or down a bit if that suits you, but you may induce other problems doing so (over-revving or lugging depending on load and how much you decide to change your "normal" operating style.)

jky
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Old 02 June 2014, 13:23   #5
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Is your RIB an Avon Searider 4.7?
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Old 02 June 2014, 14:18   #6
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When engine runs max or near max wot rpm factory stated for that engine screamming is a natural thing, the difference if counting with a tach is that will know it's running safely at wot compared to ear tach in which you could be over revving engine without knowing it.

Check current prop size/diam, install a good quality induction tach, go for a wot spin you alone and lighly loaded rib and check achieved max wot numbers. Compare them to its factory stated min-max wot parameters. Will know if under or over propped with current prop and go from there selecting best prop for combo.

Could install a prop that gives full wot numbers with you alone and lightly loaded boat, bear in mind that when addding extra mates, rpm will drop accordingly. Nobody says you must run wot all the ttime, once correct prop is dialed for thet boat/engine combo can throttle to any position you like cruising at.

Happy Boating
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Old 02 June 2014, 15:59   #7
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Wow thanks for the replies all. I might get onto the yard that looks after my boat in the winter as they seem quite up to speed with it all!

I don't know if anyone can tell me the model of my rib from this photo? That's my other half's son on board!

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Old 02 June 2014, 16:17   #8
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It looks like an Adventure 560. Your motor is quite under-powered for the boat, so 30kts is about as good as you'll get from it. I'd suggest you run a 12" or 13" pitch prop, depending on how heavily the boat is loaded.
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Old 02 June 2014, 16:19   #9
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Quote:
Originally Posted by Locozodiac View Post
When engine runs max or near max wot rpm factory stated for that engine screamming is a natural thing, the difference if counting with a tach is that will know it's running safely at wot compared to ear tach in which you could be over revving engine without knowing it......
You won't over-rev it, as the motor has a rev-limiter that cuts in at 5900ish rpm.
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Old 02 June 2014, 17:47   #10
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I wonder what I'd get on a trade in on something slightly larger. Any ideas? I'd imagine the engine has probably done around 40 hours if that. ~2 years old.
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