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Old 15 June 2016, 03:02   #21
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18" I think is standard shaft not short, I think you will find transom to be 14" ish in which case a 15" short shaft would probably be better. Only way to know is to measure the transom (or get manufacturer data sheet) and try easing motor with some timber spacers.
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Old 15 June 2016, 09:23   #22
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With standard boats, the cav plate should be ~0.5-1" above the keel.

Futuras are a little more different to set up, but that's another boat with another story entirely.
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Old 17 June 2016, 04:26   #23
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Also my boat tended to bang about and the floor would creak and flex on even slightly choppy water, spoiling any enjoyment of going fast.

As already mentioned, it was my tube pressures. I was using a RULE electric pump which did seem to blow the tubes up pretty well.

But after buying the Bravo pump (which also has a compressor) l found that the RULE was only inflating the boat to around 1 psi.

I now inflate it to 2.4psi (max is 3.4) and it's a different boat, still a SIB of course but much better if there are a few waves, and no creaking/flexing of the floor sections.
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Old 17 June 2016, 11:29   #24
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Pump it up to 3.4 and leave the dust caps off in hot weather as a relief valve just keep and eye on them to stop them going too soft
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Old 18 June 2016, 02:44   #25
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Quote:
Originally Posted by jwalker View Post
I think 15hp is not enough. Sibs are not efficient hulls and have a lot of drag nor are they very directional because they are flexible and the keel line is very rounded, also, the hull will turn into a gull wing section because it's simply a piece of fabric being pushed upwards by the water. Because the hull pushes up it gives two other characteristics; Number one, as well as the gull wing shape it forms a bit of a tunnel hull between the hull fabric joint and the tubes and secondly, in a side on view the hull fabric is flexed upward but has to curve down to the transom and this forms a hooked hull shape at the stern. Plus, of course, any wave under the boat will effect the hull shape at that moment in time so, in effect, the hull is constantly changing shape under the various loads produced by the waves. That's why the equivalent sized small rib is so much better a hull than a sib. This doesn't mean you can't improve your boat but at a distance we can only guess at a solution for you.
4.2m sib, 15hp should easily plane it with at least two adults. ours jumps onto the plane with two adults and child and luggage.
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Old 19 June 2016, 02:15   #26
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Thanks guys.

I'm going to try and take it out next weekend to try a couple of the tactics below. I have a feeling the motor is just too deep but I'm going to keep the fingers crossed and see what the results are. I had a blast otherwise.

Now only if I had the vehicle, space and money for a nice 20 foot fishing boat
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Old 19 June 2016, 19:17   #27
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JUst for kicks I took a look at the prop. It's 3 blade stamped with 9 1/4 x 10 1/2. I don't see or can find any other information.

Do you think a lower pitch or a 4 blade prop would be helpful? Top speed is nice, but I'd rather get on/stay on speed if it only cost me a few mph. I was at 21, so 15-18 wot with a 4 blade etc would be fine b me.
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Old 20 June 2016, 00:37   #28
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Shorter prop would help you get out of the hole and up on plane quicker, at the expense of speed (like a lower gear in a car.) Have to watch that you don't over-rev the engine for too long if you run WOT. A tach would help ensure that doesn't happen.

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Old 20 June 2016, 04:56   #29
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>>>Shorter prop would help you get out of the hole and up on plane quicker

As the standard dia for your Mariner's prop is 9 1/4" I think 10 1/2" will be the pitch.

The standard prop on most 15hp 2-strokes is 9" and on all the SIBs I've used with 15hp 2-strokes I've rarely ever needed a larger pitch than 9".

Given your Bombard 420 is a heavy alloy floor version that weighs 181lbs the last thing it needs is a prop with higher than standard pitch.

Not only is there its weight but it will have a very large floor area pushing a huge wall of water just off the plane and a 10 1/2" prop will tend to bog the motor down as it tries to transition onto the plane.

Your current prop pitch will also tend to produce the effect you describe of it easily falling off the plane and then taking ages to get back up.

I've experienced similar recently with a new 20hp that was supplied with an 11" prop... I guessed it was too large a pitch and a drop to 10" has greatly improved things.
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Old 20 June 2016, 22:49   #30
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Quote:
Originally Posted by Fenlander View Post
>>>Shorter prop would help you get out of the hole and up on plane quicker

As the standard dia for your Mariner's prop is 9 1/4" I think 10 1/2" will be the pitch.

The standard prop on most 15hp 2-strokes is 9" and on all the SIBs I've used with 15hp 2-strokes I've rarely ever needed a larger pitch than 9".

Given your Bombard 420 is a heavy alloy floor version that weighs 181lbs the last thing it needs is a prop with higher than standard pitch.

Not only is there its weight but it will have a very large floor area pushing a huge wall of water just off the plane and a 10 1/2" prop will tend to bog the motor down as it tries to transition onto the plane.

Your current prop pitch will also tend to produce the effect you describe of it easily falling off the plane and then taking ages to get back up.

I've experienced similar recently with a new 20hp that was supplied with an 11" prop... I guessed it was too large a pitch and a drop to 10" has greatly improved things.
Hi Fenlander.

Excuse my novice, but are you suggesting a lower pitch as I suggested? Thus reducing my top speed and getting out of the hole faster? Or do you think a 4 blade would suffice?

You mentioned my scenario and that it falls off plane. This is frustrating and caused me to bring the boat in early.
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