Another prop pitch question..

Bam_bam · 08 July 2016, 19:02

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!

Last Tango · 08 July 2016, 19:33

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.

Pikey Dave · 08 July 2016, 19:35

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.

Don't panic! don't panic!

Bam_bam · 08 July 2016, 19:56

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

.

jwalker · 08 July 2016, 21:21

Quote:

Originally Posted by Bam_bam

...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.

Bam_bam · 08 July 2016, 21:37

Quote:

Originally Posted by jwalker

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 [emoji106].

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

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Pikey Dave · 08 July 2016, 22:42

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.

Lions led by donkeys

Last Tango · 08 July 2016, 22:52

Quote:

Originally Posted by Bam_bam

Might also remove the anti-ventilation plate some evening.
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??? What exactly are you going to remove ?

Bam_bam · 08 July 2016, 22:53

Quote:

Originally Posted by Pikey Dave

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.

Lions led by donkeys

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

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Bam_bam · 08 July 2016, 22:59

Quote:

Originally Posted by Last Tango

??? 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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Pikey Dave · 08 July 2016, 23:00

Quote:

Originally Posted by Bam_bam

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

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Probably not, also if it's that vintage, all the other figures might be a load of billhooks too

Lions led by donkeys

willk · 08 July 2016, 23:09

Quote:

Originally Posted by Bam_bam

When I bought the boat it had one fitted some people call them "anti-cavitation" plates

Those are neither anti-ventilation plates nor anti-cavitation plates - those are doel fins and unless there is something very odd about your 6.5m Stormforce, you shouldn't need them. They may be your problem. Wig 'em off.

Bam_bam · 08 July 2016, 23:15

Quote:

Originally Posted by willk

Those are neither anti-ventilation plates or anti-cavitation plates - those are doel fins and unless there is something very odd about your 6.5m Stormforce, you shouldn't need them. They may be your problem. Wig 'em off.

Right I didn't know that, I will most definitely take them off see how she goes, they must cause a considerable amount of drag...

Cheers!!

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willk · 08 July 2016, 23:23

Quote:

Originally Posted by Bam_bam

they must cause a considerable amount of drag...

Mostly they cause Lift (at the stern). Maybe fitted for slow planing work - towing toys perhaps?

My suspicion is that they have been keeping you "bow-down" when at speed. When you remove them, go out some nice flat day and open her up. Then trim the engine out until it ventilates (blows out/overrevs) then trim back in until it bites. That is your best speed with that prop.

My previous RIB was a year 2000 6.5m Stormforce - nice boat.

Bam_bam · 08 July 2016, 23:32

Quote:

Originally Posted by willk

Mostly they cause Lift (at the stern). Maybe fitted for slow planing work - towing toys perhaps?

My suspicion is that they have been keeping you "bow-down" when at speed. When you remove them, go out some nice flat day and open her up. Then trim the engine out until it ventilates (blows out/overrevs) then trim back in until it bites. That is your best speed with that prop.

My previous RIB was a year 2000 6.5m Stormforce - nice boat.

Yes my bow is down, well at least compared to my last rib, a 7 meter ribtec that had an inboard diesel, wow that thing was heavy at the back.. Yes I'm growing very fond of my Stormforce, softer ride than the bigger ribtec too which was a surprise.

Fingers crossed this "nice flat day" isn't too far away!!

I will let you know how I get on.

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Last Tango · 15 July 2016, 11:37

Quote:

Originally Posted by Bam_bam

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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Hi.

Yep, as others have said, those Doel fins are a big help if your power is "border-line" but I'd guess you're about 170hp over "border-line."

.....in the bin

Just re-checked mine and I'm actually only about 27mph at 3700 (44mph WOT) at 5400.

jeffstevens763@g · 15 July 2016, 11:55

Quote:

Originally Posted by Bam_bam

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

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first lean burn 90 hp in 2008

Searider · 15 July 2016, 12:39

Current 250hp Suzuki has a full throttle operating range of 5300-6300, so at 6200 you have pretty much the best propeller for ALL ROPUND PERFORMANCE. You should be getting a good top speed and good accelleration.

I'd be looking to borrow a 23" and see what happens.

If you can't find one to borrow get a search up on ebay - you never know what comes up.

I'd also not be shy of trying an aluminium prop if your preferred size in stainless doesn't come up.

Lose the fins unless you need to be able to plane at low speeds.