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Old 17 November 2007, 14:18   #1
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X-RNLI 40 hp

I have a 4.30m Infatable boat, its rated to a max of 30hp 2/4 stroke short shaft. i want to put a 40hp 2 stroke standard shaft, i beileve the boats where rated to 40hp 2 stroke but changed it to allow for the added weight of the 4stroke. the engine is 75kgs and the max engine weight is 85kgs. i believe it will hold the engine but im just wonder what effects the standard shaft will have as it is ment for a short shaft, thanks guys!!
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Old 17 November 2007, 14:56   #2
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when you say "standard shaft" do you mean "long shaft" (20")?

I believe some ex-RNLI 40 HP engines were special "mid shaft" engines - presumably somewhere between 17-18".

Longer shaft should still work but will be a little slower due to extra drag. Many engines can be converted from short<-->long so if it can be "fixed" at a later date if a problem.

I'm not sure I understand the argument about derating when 4/str came along - normally there is a weight and power number - and NEITHER should be exceeded. So unless it is purely a marketing thing to stop people trying to buy a now impossible (new) combination I can't see why you would change it.
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Old 17 November 2007, 15:31   #3
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the 40 hp is a mid shaft engine, 17''. from 2004 on the max engine was downsized, will 40hp realy put that much extra force on the boat? i was talking to a good few people who sell this boat and the just said that as long as its under the max transom weight i should be ok, its a new boat so it has not had any ware and tare. i just wanna be as sure a i can be before i spend my hard earned cash. the boat is a quicksilver 430hd xs 2007model, im a powerboat instructor and i have been boating since i was 12yrs old driving and hndling the boat is no problem, i just wanna be sure its not gonna rip the back of my boat.
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Old 17 November 2007, 15:42   #4
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I nearly fitted a 40 to my Futura mk3 . After playing with it a bit in some smallish swell with a 25 on the back I came to the conclusion the 40 would do nothing more than distort the floor . It was the HP air floor model and Iam sure the hard floor model would have been better but the way the inflatable hull flexes I dont think it would have gone as fast as it should with a 40.
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Old 19 November 2007, 09:03   #5
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Quote:
Originally Posted by andreworiordan View Post
will 40hp realy put that much extra force on the boat?
Err 33% more than it's rated for? If I'm reading your first post correctly, it sounds like the older version of the hull was rated for 40, but now 30? I would hazard a guess that with the older boats they found transoms and boats weren't staying together!

I had a chat with Humber about my new boat - turns out its the grandaddy of the Ocean Pro, but were I to drop an 80 on the transom (as per current O-Pro), I'd probably end up trawling the bottom looking for it! To put it in perspective, mine's rated for 60Hp.

Also remember if the 40 takes your transom off, your insurers will fall about laughing if you claim against it, as you've exceeded the max.
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Old 08 April 2008, 15:50   #6
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one of the reasons for the larger engine size was that the rnli boats started carrying more wieght in the boat , ie oxygen kit and entenox gas cylinders.
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Old 09 April 2008, 03:38   #7
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Also if it matters to you your warranty will be invalid too!
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Old 12 April 2008, 18:54   #8
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If the shaft is too long, the engine will produce much more torque on the top of the stern, I believe that you never should use a longshaft engine on a shortshaft stern...
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