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Old 15 October 2013, 03:24   #11
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The Ant Cav plate needs to be basically level with the bottom of the hull!!

Tha first picture is set way to low!
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Old 15 October 2013, 05:35   #12
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Quote:
Originally Posted by SIBRider View Post
...Quite simple math. Changing pitch by 2" from 13 to 15 is equivalent to 15% reduction in gear ratio. so if you able to maintain approximately same speed as before you will observe 6000 *0.15 ~ 900 rpm drop...
Right that makes sense! However I did a lot of research on whether to get a 14 or 15" pitch prop and decided to go for the 15 because I was told a 1" change in pitch makes between 100 - 200 rpm difference, so I was expecting a drop of about 400 rpm NOT 1000 rpm!!!

This rule of thumb that everyone uses seems absolutely pointless as in my case it didn't work, but your simple math is virtually spot! I don't understand! Why have this rule of them?!
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Old 15 October 2013, 13:03   #13
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Those with thumbs are generalizing to much and may stem from high power applications where for example change from 23 to 25 pitch would match those thumb rules. On the opposite side change from say 9 to 11 pitch would have even more effect. It also depends how much spare ouph your motor has. If you do then reduction wouldn't be so great you will see gain in speed if you don't then you see what you see.
Math rules!
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Old 15 October 2013, 13:59   #14
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Was the old prop damaged in any way, or was it ever re worked? If yes then this could cause it rev higher than it should. I have never seen a 2" increase lower revs by 1000 rpm, if both props are in good nick, different manufacturers wouldn't vary that much.

Post pics of your set up so we can see how it looks.
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Old 15 October 2013, 14:49   #15
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Quote:
Originally Posted by SIBRider View Post
Those with thumbs are generalizing to much and may stem from high power applications where for example change from 23 to 25 pitch would match those thumb rules. On the opposite side change from say 9 to 11 pitch would have even more effect. It also depends how much spare ouph your motor has. If you do then reduction wouldn't be so great you will see gain in speed if you don't then you see what you see.
Math rules!
I wished I was told about the maths way to it out from day one and not the rule of thumb which obviously applies to engines bigger than my 50hp, more likely 100hp upwards at least!


Quote:
Originally Posted by breghall View Post
Was the old prop damaged in any way, or was it ever re worked? If yes then this could cause it rev higher than it should. I have never seen a 2" increase lower revs by 1000 rpm, if both props are in good nick, different manufacturers wouldn't vary that much.

Post pics of your set up so we can see how it looks.
My original prop is in an excellent condition! Its only just over a year old and has no dings or dents or scratches whatsoever! and no it hasn't been modified either.
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Old 15 October 2013, 15:07   #16
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Also today I checked the engine height on the transom, the water deflector plate is above the bottom of the keel, its about the same height as the top of the water outlet bung in the transom. Also the anti cav plate is virtually level with the bottom of the tansom/hull, so I am 100% sure my motor height is fine. Like I said the motor was mounted to the rib at the Tohatsu UK headquarters, so they should get it right!!

Also I managed to take some pictures whilst we were cruising at about 3000 - 3500 rpm (3 up in the boat)...

http://i24.photobucket.com/albums/c2...0-15163812.jpg

http://i24.photobucket.com/albums/c2...0-15163822.jpg

http://i24.photobucket.com/albums/c2...0-15163835.jpg

http://i24.photobucket.com/albums/c2...0-15163844.jpg

and finally...




How do the pictures look to everyone? (the extra splash in the pic above on the left hand side is because I am sitting right at the rear on that tube!)
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Old 15 October 2013, 15:17   #17
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It all looks good wonder what RIB you got but with 50 ponies you are just maxing out your motor. 14" pitch might give you couple mph but that's about it. What is the maximum HP your rib can take?
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Old 15 October 2013, 15:34   #18
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What you are experiencing is normal ,what is happening is the prop is at the limit of your boat setup weight length and power with this prop.the prop will try to push more water than your old prop drawing more air from the hull wake and the overall resistance of this prop and the boat hull verses power available leaves higher losses and poor performance.raising the engine will provide more cavitation,on your setup 14p is probably the limit and this would probably require power trim to keep the best trim as it will change through the rev range and the larger the pitch the bigger the problem will get
I hope this is of some help
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Old 16 October 2013, 05:52   #19
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Quote:
Originally Posted by SIBRider View Post
It all looks good wonder what RIB you got but with 50 ponies you are just maxing out your motor. 14" pitch might give you couple mph but that's about it. What is the maximum HP your rib can take?
Quote:
Originally Posted by sparky666 View Post
What you are experiencing is normal ,what is happening is the prop is at the limit of your boat setup weight length and power with this prop.the prop will try to push more water than your old prop drawing more air from the hull wake and the overall resistance of this prop and the boat hull verses power available leaves higher losses and poor performance.raising the engine will provide more cavitation,on your setup 14p is probably the limit and this would probably require power trim to keep the best trim as it will change through the rev range and the larger the pitch the bigger the problem will get
I hope this is of some help

Thanks guys. I have the maximum hp on my 4.2m rib. As you say I think everything is normal, I also think I would probably be better off with a 13.5p as opposed to a 14p to be honest. All this messing around is too much hassle for me I think and I will stick with the OEM 13p and keep my eyes open for a cheap 2nd hand 11 .1 x 13.5 prop maybe.
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Old 17 October 2013, 14:03   #20
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Thought I'd post up a couple of pics I took yesterday when I took my rib out of the water and cleaned it...


As you can see the engine height is fine, but thought I would ask re trimming. I normally have my engine fully trimmed in as when I have played with the trim I have never really felt it was any better with it trimmed out tbh.

But When I was taking pictures I noticed that when fully trimmed in the engine is not parallel but it is if I trim it out 25% on the trim gauge it is then parallel!

Fully trimmed in...




and trimmed out about 35% (just past parallel)...




Once on plane should I trim my engine out about 25% then so its parallel with the hull?!

Sorry for the silly question but thought I'd ask!!
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