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Old 28 February 2013, 08:40   #11
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Quote:
Originally Posted by redthunder View Post
They are counter rotating. Getting 45Knots at 6200 RPM Not holding the throttles open for long as I like my warranty. I am looking for good economy not top speed. They are very good so far with 1.5 litres to the mile at 28 Knots. So I need to drop 400 RPM. I believe you lose 200rpm per prop size up. A guy on another forum said that I was getting 11% slippage which is good.
Slip should be as follows

19" is what your prop should push you forward for every revolution, or
19 x 25.4 = 482.6mm
X RPM 6200/ = 2,992,120 mm per minute, or 2.992120 km per minute divide by the gear ratio, from what i find is about 1.87 for mercury (approx)= 1.6000Km per minute x 60 is 96Km/hr
divided by 1.852 gives you knots = 51.8Knots theoretical speed.

45 divide by 51.8 is 87%, so you are getting 13% slip

Unless somone can point out a glaring error in my calculations........
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Old 01 March 2013, 04:01   #12
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Originally Posted by Pikey Dave View Post
The vipers were horrid, blowing out & slipping at every chance.
Ah so it isn't just mine then...

After the 'lazy' Humber which I could leave at a single trim setting from chucking it around in tight turns to a 30kt cruise, one of my few dislikes with the Vipermax is having to fiddle with the trim all the time if you're having fun. I did wonder if a prop change would fix it, I guess that answers the question. The next question is whether it irritates me enough to spend 500 odd
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Old 01 March 2013, 05:26   #13
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Nope. it ain't just you! Same issues here..............
Soon going to try an Apollo 3 blader, but cannot do much as I am in and out of the country at the moment. Should get to try it before easter
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Old 01 March 2013, 11:22   #14
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Ah so it isn't just mine then...

After the 'lazy' Humber which I could leave at a single trim setting from chucking it around in tight turns to a 30kt cruise, one of my few dislikes with the Vipermax is having to fiddle with the trim all the time if you're having fun. I did wonder if a prop change would fix it, I guess that answers the question. The next question is whether it irritates me enough to spend 500 odd
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Nope. it ain't just you! Same issues here..............
Soon going to try an Apollo 3 blader, but cannot do much as I am in and out of the country at the moment. Should get to try it before easter

Can't praise the Rev4 enough. If you are prepared to sacrifice a little on the fuel economy, it will transform your boat, at least it did mine.
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Old 01 March 2013, 12:39   #15
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Quote:
Originally Posted by redthunder View Post
. A guy on another forum said that I was getting 11% slippage which is good.
That guy (on THT, Ken from Propgods) is probably one of the best Prop guys on the planet, I would take his advice as he really does get great reviews and seem to know his stuff, although not necessarily for Ribs.

I have REV4 19", it really does grip in the rough and when cornering hard, it will happily bog down the motor on the back of my boat rather than slip. The Rev 4 is a stern lifting prop, and should allow you to plane at lower speeds, I can hold 10 knots on the plane which is pretty impressive for a big lump but you will not get the top speed or economy that you would from a well sorted three blade. Cruise and slow speed handling is often improved with a 4 blade as well.

Usually sold in 2" increments, Merc have just bought out (or are just about to) some new REV 4 models and I think one might be a 20, as well as some smaller pitch counter rotating ones. Might be worth checking with and a dealer or doing a search.

I have tried a three blade on mine and it was hilarious, just spun up to 5000 rpm with no increase in speed, had to coax it onto the plane, but was slightly faster when it got there.

If you can get a new Rev 4 for 500 I'll have one too!

Best of luck with your research and finding the best prop for your boat.
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Old 01 March 2013, 14:43   #16
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I was getting pretty close to 45 knots with a pair of 19" rev4's and almost getting my max rpm. When I tried 17" I could get the over speed alarm going quite easily but at my normal cruising speed I was getting a significant improvement in fuel economy even though my revs were higher, less load on the engines. I just don't nail the throttles.

If you go the rev4 route I think you'll find the same thing, you can spin a 19 but a 17 could be better on fuel.

I've also tried mirage plus on mine, 21" but at 2.5t they just couldn't grip, straight line or cornering, and the torque steer in corners was horrendous.

Rev4's are definitely the way forward on bigger, heavier boats but they're not cheap, currently not far off 650 inc vat
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Old 01 March 2013, 17:41   #17
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Interesting as we set up the MoD Arctic 28's with Mirage 21", they seem to work better than the REV 4. But the Arctic 24's loved having the REV 4 props.
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Old 01 March 2013, 18:44   #18
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Originally Posted by Turbodiesel View Post
Interesting as we set up the MoD Arctic 28's with Mirage 21", they seem to work better than the REV 4. But the Arctic 24's loved having the REV 4 props.
Interesting as I bought the rev4's on your advice! Transformed the boat
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