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snelly

RF18 Repower

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Looking at repowering my 2002 RF18 and I'm seriously looking at the 115PRO XS -  the big delmar - standard gearbox or command thrust ?

To my way of thinking the standard is the go but the local dealer feels

very strongly otherwise.

Anyone got any real world data of this ?

 

Thanks in advance

Snelly

 

 

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Hey Snelly !

Bummer losing the motor !!  The Mercs run fast and sip gas. You do have to have an excellent service center for maintenance, like all outboards.

There may be some repowers here with those new models..  Let's see !

Sorry for the troubles.  Hope you're back on the water soon..  We miss your pics & tournament journals !!

Cheers !

Mod-1

Edited by Moderator

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Snelly,

This is a good question and I am waiting to see some answers. Standard gearbox is 2.07 and command thrust is 2.38.  Why are they saying to use the command thrust where you are? Is it on a flats boat like you are running? it would need to be apples to apples to compare. If they are running it on a work boat (crab boat) or a big cat then they need the extra gearing. The 2.07 gear should be a touch faster on top end and if you go long distances then you may be able to cruise at a faster speed but using less RPM"S therefore saving some fuel.

I would definitely like to know how gearing plays a role in the same boat/engine combo and setup. From my limited knowledge and helping Geeviam with his 2016 RF16/VF115/2.15 gear ratio. I have a 2008 RF16/115 2stroke/2.0 gear ratio. We both ran the same identical prop (Merc spitfire x7 19 pitch 4 blade) He ran 6200 RPM's @ 50 mph and I ran 5600 RPM's @ 50 mph.

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On the Mercury site they run a Mako with both gear cases.  The 2.38 is only 1.2 mph slower on top but holeshot looks much better.  I did not look at the fuel burn.

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Here you go, I found a reasonable comparison using a bass boat and I think this will clinch it for me. I do feel they may have got a bit more with the standard gear box but I doubt it would close the gap.

Also with the command thrust gear box you have a much larger range of props to select.

https://www.mercurymarine.com/en/us/performance-tests/bhb/1172/?units=imperial

 

https://www.mercurymarine.com/en/us/performance-tests/bhb/1176/?units=imperial

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Flatfoot , I'm in Australia so you don't really want to know how much we pay for a new outboard over here other then to say it's roughly double what you will pay in the US.

Cheers Mate.

 

 

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Snelly,

I looked at the data you posted from the Mercury website. I punched in the prop slip numbers for each test. The Merc with the 2.07 gear ratio has a 22% slip (horrible) and the Merc with the 2.38 gear (command thrust) ratio has a 12% slip (acceptable to some). The Merc with the 2.07 gear ratio needs a better prop in order for the test to be accurate. If you adjust the prop slip on the engine with the 22% slip down to 12% like the other test subject it runs faster on top end by 2 mph than the command thrust engine.

I see what they are trying to do in the test is to move you up to the 150 HP engine props with a larger diameter for the Command Thrust (2.38 gear ratio) to have better performance. I am sure you will have a better hole shot and better acceleration with the command thrust gear ratio.

I posted a test above in this thread of me and another member above. Similiar boats, same prop but different gear ratios. We ran the same speeds but at different RPM's. I will say he had more bottom end (4 stroke) and we had to completely close his PVS holes (4 strokes seem to like this as opposed to 2 strokes) and on mine I had most of the holes unplugged (2 stroke needed to spin up the motor)

this is by no means a dig on either engine because I am looking at them when I repower I just want to put this out there for others to consider.

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Snelly, I think Lurem's point is spot-on.  To get an accurate comparison, both gearcase ratios should be tested on identical boats (they got that part right), with props of equal grip/slip characteristics or percentages, and each running as close as possible to the highest recommended WOT RPMs for that model or gearcase.  Like Lurem, I want to see if there is a noticeable advantage in the lower gearing of the Mercury Command Thrust and/or Suzuki outboards.

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Like props generally it can be a "black art" . I'm still undecided, in a perfect world I would like to run both gearboxes on my boat and see for my self. I fell that there would not be a vast difference with the RF 18 I can see how some hulls would benefit though.

 

 

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Snelly,

Good morning! What did you decide on your repower? I would love to hear what you chose for gear ratio, props selection and what you final numbers are with fuel mileage and top speed. Can you tell us the pros and cons as well? Thanks!

Dale

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