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Old 25 July 2009, 19:40   #1
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How fast?

......should my Zodiac 2.85m SIB with Mercury 6hp engine go?
I was on the canal today and the boat does about 2mph on tickover. Increasing the revs a bit produces 4mph or so with no effort and little wash.
However increasing the RPM beyond this has little effect. A large foaming wave builds up behind the boat, the stern lowers and the bow rises 6 inches or so. Further RPM (say up to 70% full throttle) just sinks the stern lower, raises the bow more, and produces a large following wave and wash. No real speed increase. Maybe another 1 or 2 mph.
I only tried this briefly on a section of the canal with steel banking.
However, after being waved past a couple of slow moving canal barges I was really struggling to make any real speed over them to get past.
What could be wrong? I was sat at the stern and my wife on the seat near the front. I tried altering the angle of the engine but this made no noticeable difference.
The cavitation plate is a bit lower than the recommended range of 1.5-2 inches below the bottom of the transom, but it's only just under 3 inches (less than one inch too low?) Should I try a spacer to raise the engine a bit or is there nothing wrong here?
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Old 26 July 2009, 03:35   #2
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You are bascially reaching the limit of the hulls displacement speed and as such it is trying to get "over the hump" and onto the plane I would suggest you may need to move some weight forward to try and achieve this. However it may simply be that your Sib is not going to plane with 2 adults on board with a 6hp pusing it along. You may need a larger engine.
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Old 26 July 2009, 03:39   #3
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Doel fins might help? Cheaper than a new outboard.
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Old 26 July 2009, 04:26   #4
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Have you "played" around with the trim settings?
Its not stuck on "shallow drive" is it?

That said - I think 6HP is going to struggle to plane.
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Old 26 July 2009, 08:16   #5
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Boat speed

I've seen some videos on YouTube (type in Mercury 6hp or Honda 5hp etc) and there are videos of small SIBs with these engines, granted you are not going to be doing 20mph but they seem to manage 10mph with no problems. Many seem to have 2 people on board.
Clearly my boat is OK, I mean there can't be anything "wrong" with it, but I was hoping it might manage more than 5mph. I may as well have gone for the lighter Mercury 3.5hp if the upper 50% of the engine RPM on my 6hp don't really do anything!
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Old 26 July 2009, 08:20   #6
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You need to get someone to hang right over the bow as far forward as they can - as it starts to plane they need to move aft. That's assuming it will of course!!!

And don't think you may as well have a smaller engine - if you are at sea and there is a fast tide running yopu will need every bit of extra power..........
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Old 26 July 2009, 08:34   #7
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Boat speed

So if I understand it correctly, the boat will achieve its "displacement speed" of say 5mph with little power, maybe even a 2.5hp. To achieve more you have to get it to "plane" which guess means the hull lifts out of the water and it rides on the V-keel? And then it might do 8-10mph, but you may need a lot more engine power for this.
The maximum recommended engine power for this boat is 6hp, with 8hp as a "maximum safe limit" according to the instructions.
I had no idea there was this "transition" where the boat goes "on the plane". It explains a lot. Over the years I've been on a few small boats (engine under 5hp) including my dad's SIB years ago, and on all of them, increasing the engine RPM past about 50% just makes more noise with little extra speed.
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Old 26 July 2009, 08:38   #8
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you need to get the hull to the stage where it is producing 'lift' out of the water , but with lift you have drag - so getting over this 'hump' is the hard bit.

I'd think with a bit of moving weight around and playing with the trim you shoudl get on the plane with 6hp ( unless the floor is sucking itself to the water).
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Old 26 July 2009, 08:49   #9
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Boat speed

What do you mean by "playing around with the trim"? Do you mean the engine angle/height? I can't think of anything else that I could alter.
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Old 26 July 2009, 08:52   #10
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The engine angle.
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