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Old 14 June 2016, 02:11   #11
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I'm also noticing that my trim elevation pin is in the last spot. If I move it the other direction, that would point the propeller up and outwards from the boat, causing my bow to lift even more correct?
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Old 14 June 2016, 02:15   #12
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Quote:
Originally Posted by Fenlander View Post
I wonder if you were in the boat when the image was taken? The important position of the AV plate (with the two visible extra bolts) is when on the plane and they will sit deeper at rest... but that looks very deep if that's just the weight of the outboard on a 4.2m.

Everything else has been covered really... inflatables are a bit like you describe if compared to a hard boat... knowing the actually (not guessed) pressure is crucial before deciding if you have a problem... your outboard is a little small for a 4.2... one up with fingertips on the tiller trying to get weight forward is not great for control.
I was in my boat when the photo was taken, you are correct.

The problem is, a larger motor really isn't an option. I have enough trouble lugging that thing around, I couldn't imagine a 25 hp 4 stroke. If i did have one, I'd never take it out knowing what lies ahead.

A little discouraged.
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Old 14 June 2016, 02:36   #13
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>>>A little discouraged.

Don't be.... you can work towards getting this better.

Next time out make sure you can check the pressures properly... recheck after launch. I'm assuming you don't have a tiller extension... do get one if you have to stay one up. Put more weight in the bow. Try sitting on the seat or down on the floor to get your weight more central. Put your outboard trim in the central position and start from there. See where the AV plate is on the plane and consider blocking the motor up if low in the water.
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Old 14 June 2016, 03:10   #14
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As they all said - important things - weight distribution, tubes and keel etc up to MAX pressure (or more to allow for cold water), tiller extension to get further forward and motor at correct trim and height - use a Tinytach to check performance at WOT as well as practical handling checks.

For engine height Locozodiac was the man but sadly moved on from RIB net it seems, read his guidelines here:

ABC Sib-Rib Installation Guidelines.-

For best piloting the safest position is that adopted by the RNLI in their SIBS and it works great in big waves - kneeling and using upper legs as suspension:
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Old 14 June 2016, 03:18   #15
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the mercury 415 is the same boat as yours with shorter cones weighing 120 kg their site suggests a minimum of 15 hp so it should perform which you have proved at 20 mph but will have its limits,
you say your engine is too deep its not a long shaft is it 20 inch as apposed to a short 15 inch or the transom been cut down.
when i bought my boat it had two shims of 25 mm each so if you lifted your engine 2 inches would the cavitation plate be level with the keel.i didn't lift mine at all [short shaft] and the cav plate is just below the keel line on the transom but in line with the keel when its blown up so its got a step in it of approx one inch.
not teaching you to suck eggs but something not right cheers
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Old 14 June 2016, 07:42   #16
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I'll probably be shot down in flames for suggesting this but in your case where power is a little marginal a pair of doel fins might help to keep you on the plane at a slower speed and maybe get you there easier
People on here don't seem to like them but they do have there uses
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Old 14 June 2016, 13:24   #17
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The engine height does seem to be fairly critical on a SIB.
On my Zodiac l was getting splashback and chine walking at over 15mph, also the steering was vague and it felt unstable.
Although actually getting on the plane was no problem.
Following advice on this forum l put a piece of wood under the engine mounting which only raised it about 2cm.
All the issues were cured.
It looks like you might have a long shaft motor which is not going to work properly with that boat.

I am sure that somebody on here will be able to verify which type of engine you have.
Or you could look online for your engine specifications, which should give you the long/short shaft leg length.
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Old 14 June 2016, 14:24   #18
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Get the tape measure out and see exactly what is what


Outboard Motor Shaft Length
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Old 14 June 2016, 21:29   #19
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Quote:
Originally Posted by HDAV View Post
Get the tape measure out and see exactly what is what


Outboard Motor Shaft Length

I didn't measure the transom, but it appears to be just a normal height. However, my motor, from the bottom of the motor clamps and the AV shield, is 18 inches. I'm guessing there wasn't a standard back in the 80's for these sorts of things?
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Old 15 June 2016, 00:10   #20
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The pic in the manual shows where it should be when running. At rest, the boat will sink down, and the motor will end up much deeper. Take another look in the garage, and if the anti-ventilation plate is within an inch or two of the bottom of the hull, you're close to correct.

I'd agree with those who said to check pressure. Your statement of (though the boat was going to snap in half" implies the boat was flexing a lot. Proper pressures will minimize that (though you will get a lot more slapping and banging and pounding - you'll then think *you're* going to snap in half.)

I had a heavy 40hp on my 14' SIB. At high speed, it was pretty squirrelly. Not really unsafe, just takes some getting used to.

jky
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