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Old 08 July 2016, 14:02   #1
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Country: Ireland
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Make: Redbay 6.5
Length: 6m +
Engine: Suzuki 250
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Another prop pitch question..

Hi all,

Let me start by saying this is my first post on here and only recently purchased a rib after a few years spent landlocked!

Anyway my 6.5m Redbay is fitted with a Suzuki DF250 and the 3 blade stainless prop (21 pitch) gives it a top speed just over 50mph (6,200rpm). I have only ever done this on a couple of occasions and to be honest am not too interested in raising this top speed. I do however spend a huge amount of time cruising at about 30mph (3,700rpm).

To cut to the chase i presume going up an inch or two in pitch will give me better fuel economy and a higher cruising speed, but is my current set up harmful to my engine? I have heard it being likened to running a car in too low a gear, as long as i don't over rev it should be ok right?

Any feedback will be appreciated!
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Old 08 July 2016, 14:33   #2
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Look up the spec for your engine (max rev range) and if your within that you should be fine. I'm not familiar with your engine but 6,200 I'd imagine is towards or over the top end of the range.
With that power to weight ratio I'd think it would be worth going up to 23" to bring the cruising revs down a bit.
My 6.5 Coastline is doing about 30 at 3700 with a 130hp Honda so I'd imagine you can do better with your setup as you've got a bit more weight but a lot more power.
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Old 08 July 2016, 14:35   #3
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The in built rev limiter will protect the engine from over revving so no worries there. You should be propped to achieve max rpm at WOT ( Wide Open Throttle) I'm guessing that max rpm for your engine is 6000rpm, so you could be slightly under propped. Not enough to warrant a prop change imho. Slightly under propping gives better hole shot & throttle response, better when it turns lumpy. Slightly over propping will give a slightly higher top speed on the rare occasions when conditions allow you to run flat out. Going up a couple of inches will probably drop your max rpm to around 5800rpm, still within the ideal band.


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Old 08 July 2016, 14:56   #4
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Thanks for the quick reply lads!

What both of you are saying makes sense to me. To be honest i'm a bit disapointed i have the same cruising speed/rpm as a similar sized rib with a considerably smaller engine. I will probablly leave the prop as is for now and maybe pitch up next year or if I whack my prop of something between now and then .
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Old 08 July 2016, 16:21   #5
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Quote:
Originally Posted by Bam_bam View Post
...To be honest i'm a bit disapointed i have the same cruising speed/rpm as a similar sized rib with a considerably smaller engine....
As Pikey Dave has said the rev limiter will protect your engine but are you sure you're not on the rev limiter? Try to beg or borrow a 23" prop and give it a go.
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Old 08 July 2016, 16:37   #6
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As Pikey Dave has said the rev limiter will protect your engine but are you sure you're not on the rev limiter? Try to beg or borrow a 23" prop and give it a go.


Yes I'm pretty sure at WOT I'm hitting the limiter. On one particular run with a tank of fuel and 4 people on board!! Yes I will have to try with 23" before the end of the summer .

Might also remove the anti-ventilation plate some evening. The mind wanders on these rainy days !!


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Old 08 July 2016, 17:42   #7
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I think the rev limiter kicks in at 6300 on Suzukis. If it's a lean burn engine, you want your cruising speed to be around 4500 rpm, that's when the lean burn is at its optimum.


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Old 08 July 2016, 17:52   #8
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Might also remove the anti-ventilation plate some evening.
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??? What exactly are you going to remove ?
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Old 08 July 2016, 17:53   #9
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Originally Posted by Pikey Dave View Post
I think the rev limiter kicks in at 6300 on Suzukis. If it's a lean burn engine, you want your cruising speed to be around 4500 rpm, that's when the lean burn is at its optimum.


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Unfortunately my Suzuki is a 2004 so no lean burn on that I presume?


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Old 08 July 2016, 17:59   #10
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??? What exactly are you going to remove ?


When I bought the boat it had one fitted some people call them "anti-cavitation" plates, I'm just curious to know would the prop vent without it. See my pic below, looks pretty easy to take off...




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