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Old 02 January 2016, 12:58   #31
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I set my cav plate 1inch above and was breaking out badly and top speed wasnt great. i then lowered the engine to assist in the handling and grip and went to a stainless vengeance and it converted it. Loads of grip and faster tops speed by 3 knots. I know its a different boat but lowering to 1 inch below and correct prop really worked..

What boat and engine is that Matt?
On 1 of my boats, the cav plate is about 4" above.
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Old 02 January 2016, 13:54   #32
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Sounds very high.. Its a ribcraft 4.8.. The cav plate was well above the V initially which caused cavitation/ventilation which the wrong height and prop. After landing from a jump .. The prop found no grip at all and turning it would just let go.. So as said,I dropped it down two holes..
Tbh. I thought it was too low now but im getting the handling and perfomance that i wanted!
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Old 02 January 2016, 15:07   #33
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Originally Posted by matt h View Post
Sounds very high.. Its a ribcraft 4.8.. The cav plate was well above the V initially which caused cavitation/ventilation which the wrong height and prop. After landing from a jump .. The prop found no grip at all and turning it would just let go.. So as said,I dropped it down two holes..

Tbh. I thought it was too low now but im getting the handling and perfomance that i wanted!

It's a Ring 18 with a Yam Pro V 200 on the back and a laser 2 prop 26".
I've had it lower but it performs better at this height.
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Old 03 January 2016, 04:37   #34
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The wood plank is hard up against and level with the planing pad. As you can see, my outboard is about an inch above the wood. I more accurately measured the distance from the underside of the outboard saddle to the bottom of the transom. This came in at 690mm.

I hope this helps you to compare your measurements with mine Stephen.
Thanks for that. Mine is definitely lower than the norm, so making it lower isn't going to help. I think a different prop is worth a go, though I might try and find some sandbags to lob in the stern just for testing purposes, to see if it helps.
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Old 03 January 2016, 12:57   #35
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Thanks for that. Mine is definitely lower than the norm, so making it lower isn't going to help.....
I think you might find that it does. The only reason I can think that trimming in works even though the bow is heavy is the fact that you are substantially lowering the prop in the water as you trim in. After seeing your photos, if it were mine I'd be lowering the engine 25-35mm and fitting a prop 2" less pitch with slightly spooned and cupped blades.
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Old 03 January 2016, 17:23   #36
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I have to say that I am not a fan of the Viper prop. Little grip and would cavitate with very little provocation. Tried mine at various engine heights and still was not too impressed. Someone, i think it was Kieron at Ribshop said that they could be substantially improved by re-working the leading edge, but I forget the exact details. Might be worth dropping Ribshop a PM. Changed mine out and it is much improved. I've still got the old (unmodified!) 19" Viper.
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Old 03 January 2016, 18:15   #37
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I have to say that I am not a fan of the Viper prop. Little grip and would cavitate with very little provocation. Tried mine at various engine heights and still was not too impressed. Someone, i think it was Kieron at Ribshop said that they could be substantially improved by re-working the leading edge, but I forget the exact details. Might be worth dropping Ribshop a PM. Changed mine out and it is much improved. I've still got the old (unmodified!) 19" Viper.
Propping a boat is a bit of a black art, isn't it Ian. Did you have the Viper prop on when we went to France?

Perhaps you could do a deal with Stephen if he wants it?
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Old 04 January 2016, 03:20   #38
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My concern with lowering the engine further is that if the plate is below the transom it'll then be spraying water everywhere, and also will create a lot more drag in the water which will impact fuel consumption.

Sending props back and forth to the UK for mods is not really feasible due to the cost - by the time it's done a couple of trips by air, it will make a fair hole in the cost of a new prop and mean I only have the spare (20" Raker) to use while it's away.

I'd rather try a different prop, and still have the Viper which when all is said and done is perfectly usable. If I get a new one that works well, I'll probably take the Viper back to a repair shop to have the damage on it touched up at some point when I am heading north. How long does this normally take?
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Old 04 January 2016, 03:45   #39
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Originally Posted by BogMonster View Post
My concern with lowering the engine further is that if the plate is below the transom it'll then be spraying water everywhere, and also will create a lot more drag in the water which will impact fuel consumption.

Sending props back and forth to the UK for mods is not really feasible due to the cost - by the time it's done a couple of trips by air, it will make a fair hole in the cost of a new prop and mean I only have the spare (20" Raker) to use while it's away.

I'd rather try a different prop, and still have the Viper which when all is said and done is perfectly usable. If I get a new one that works well, I'll probably take the Viper back to a repair shop to have the damage on it touched up at some point when I am heading north. How long does this normally take?

Steel developments will modify/repair props in 3-4days depending on how busy they are.
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Old 04 January 2016, 06:05   #40
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I would be cautious about quoting trim % figures because these are only based on how you have calibrated your guage, and where you have set the 0% and 100% marks.

We used to have very poor grip, and instant cavitation in turns unless you were almost fully trimmed in when using a Viper 22". When Channel Ribs drove the boat he kept it trimmed in all the way. Since moving to a 4 bladed prop there has been a massive improvement in grip, a huge increase in economy, and very little decrease in top speed (1 knot, perhaps). If you research the Viper prop in general I don't think you will find very much positive said about it, especially for a heavier style boat like a RIB.

We have settled on the BRP Cyclone prop, however I would personally take Pikey Dave's recommendation of looking towards the Mercury Rev4, I have only heard good things about them. The 4 blades have so much more grip on the water it should let you trim out more to bring the bow up, as well as reducing cavitation in turns.
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