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Old 09 August 2020, 03:03   #1
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Ribcraft 4.8 prop choice

Our Ribcraft is fitted out with two inline jockey seats and a rear bench. We love the boat, mostly touring, with no water sports currently.

We are two adults, two kids, and 2 dogs, 60l fuel so roughly -340kg payload.

Powered by a Suzuki DF60a with a 11 1/2 x 13 prop I get a cruise of:

15kts @ 4600 and 10lph
&
21kts @ 5150 and 13.7lph.

*I have located a max recorded GPS speed of 31.4 in the GPS Trip Log but assume this was one up with a following tide, wind, downhill etc.

She is obviously under-pitched as we hit the rev limiter easily (I know I'm missing vital top end data but recently it never seems calm enough to record properly whilst keeping an eye on kids dogs etc)

I think (but may very well be wrong) that the limiter kicks in at circa 25kts.

I was thinking of trying a 11 1/4 x 15 in - what do people think.

Reasons for change are - want a spare prop, be fun to be able to go faster in the right conditions, should mean more speed at less rpm (So more economy?)

Any advice gratefully received.
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Old 09 August 2020, 03:51   #2
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The latest Suzuki DF60 appears to have a gear ratio of 2.27:1. That being the case, I'd hazard a guess that you may want to try at least a 17 or 18" pitch prop. No-doubt someone who knows the setup will be along to help shortly.

Here is the test data from Ribcraft with a Honda 60 (2.07 ratio) and a 16" pitch:

https://www.ribcraft.co.uk/documents...erformance.pdf
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Old 09 August 2020, 04:21   #3
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Cheers SixtyNorth
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Old 09 August 2020, 04:58   #4
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Try to confirm what gear ratio your engine has then run some numbers through this...

Marine Propeller Calculator

Of course, you could check it roughly by entering the data you have already gathered from your tests e.g. RPM, pitch, speed. Note that the speed is in mph.
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Old 09 August 2020, 05:09   #5
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My 4.8 and suzuki df60a has a 11x17 suzuki prop which is pretty close thats 75lts of fuel two adults sometimes there is four adults and I think a 11 1/4 x 16 would be better I am getting a 16 for the heavier loads. Hope this helps
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Old 09 August 2020, 07:00   #6
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Country: UK - Scotland
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Ribcraft with 2 in-line jockeys, 75 litres of fuel, Tohatsu 70hp 2-stroke and Mariner 4hp auxiliary. Usually have an Avon 8 deflated on-board aka Thunderbird 4 for exploring coves and beaches.

A 15 pitch is a good all rounder. I opted for a Solas 17 pitch stainless propeller, but could have probably dropped back a pitch.
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Old 09 August 2020, 15:56   #7
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*I have located a max recorded GPS speed of 31.4 in the GPS Trip Log but assume this was one up with a following tide, wind, downhill etc.

Ha ha this might make you smile ,i have been looking at props and max speed,but more for comfort in rough etc on my r/c 5.3 ,where i have gps 750 ,
been ok at 38 but hadnt reset for a couple of trips and your posts today made me inquisitive ,as to the logged max ,was gobsmacked when i saw 45.8 knots , well i racked my brain when i might have reached such speed ,only to remember i had left it on the other day and that was my speed along the duel carriagway on the way home
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Old 09 August 2020, 16:02   #8
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Ah, mine might have been on the M3 then!

Thanks for your input gents.

Looks like I might risk a 16?

Will give RC a buzz tomorrow morning.
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Old 28 August 2020, 03:27   #9
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Ribcraft 4.8m prop choice update

Ribcraft suggested trying a 14 & 15.

I went ahead and ahead and bought a 15 and now get 32kts (gps average) and still hit the rev limiter one up.

Fully loaded (2 adults, 2 kids, 2 dogs, and a bit of kit) I couldn't quite hit 30 (29.5 gps average) and didn't hit the limiter.

So looks like a reasonable choice although I might have got away with a 16....
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