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Old 31 January 2011, 18:56   #1
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Quicksilver 3.8 mtr SIB nose diving - help!

Hi Everyone,

SIB newbie here who recently purchased a Quicksilver (Mercury) 380 HD (3.8 mtr Hyphalon SIB with removable aluminium floor) and a Mercury 2-stroke 25 HP engine.

Yesterday, I took it out for the first time in relatively flat water. Wow, great fun.

I had one issue though: each time the boat goes onto the plane, after about 15 seconds, it seems to take a nose dive (i.e. bow sinks back into water, the boat slows down very suddenly, and then the nose of the boat bends up at about a 45 degree angle).

The bow is not actually going underwater, but it sinks back down into the water and creates a big bow wave which makes the boat go from high speed to nearly stationary in a short time.

I thought it might be a trim issue but I tried all of the different trim positions and nothing stopped this happening.

Maybe I did not inflate the boat enough. The instruction manual says to inflate between 2 and 3.6psi (0.14 to 0.25bar). I don't know what pressure I inflated it to because I could not get the pressure gauge that came with my dual action hand pump to work. However, the guy who sold me the boat told me that I will know when it is the right pressure if I can push my thumb about half an inch into the side of the boat without the hypalon creasing. So this is what I did. The boat certainly seemed firm and well inflated to me, so I do not think this is the issue.

Has anyone else experienced this or got any suggestions.

Thanks in advance.
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Old 01 February 2011, 02:41   #2
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Welcome to RIBnet.

You should get the pressure sorted first, especially the inflatable keel (I assume it has one?). Rule of Thumb inflation on SIBs is not a great idea if you want performance and they do take quite a bit of pressure. My QS Airdeck is rock hard when properly (to the gauge) inflated and I would think that the keel is a critical point. The keel in mine is rated higher pressure than the tubes.

hth
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Old 01 February 2011, 03:58   #3
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Thanks WillK, you are right - I certainly need to properly check the pressure. I will have to buy another pressure gauge that works and make sure I am inflating it properly.

Anyone ever experienced this phenomenon with a properly inflated SIB or have any other ideas?
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Old 01 February 2011, 04:03   #4
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Hello and welcome.

Are you sure it is Hypalon? Not that it relates to your question.

As Wilky says, pressure is very important. Also make sure the floor is fitted exactly as per the instructions, the floor and keel work together to make the boat work.

They are great boats, I have owned six and sold more than I can count
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Old 01 February 2011, 04:36   #5
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You say it was flat conditions, sometimes though a long swell can cause that sort of problem even though the sea looks calm .
though it may as the others have said more likely to be under inflated keel or the like .
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Old 01 February 2011, 05:25   #6
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Thanks Malthouse. Yep, it is the 380 HD Hypalon model.

M chappleow - good point. Maybe it was due to conditions and not the boat. I guess I need to get out onto the water again on a calm morning with totally flat seas and see if the problem is still there.

Good news is that both the manfacturer and company that sold the boat are saying they would like to take it for a test drive to see if they can solve the problem, so can't complain about the after sales support from Mercury, at least down under anyway.

Keep these great suggestions coming RIBnet! Great forum.
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Old 01 February 2011, 05:40   #7
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Originally Posted by kerravon View Post
Yep, it is the 380 HD Hypalon model.
Odd, must be for the Pacific market. They are all PVC here.

I doubt very much it was the conditions, I have taken these boats out in all sorts and never had one fold up and fall off the plane (I think thats what your describing?).
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Old 01 February 2011, 17:52   #8
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Hi Malthouse,

Yes, that is exactly what happened. The boat fell off the plane, felt like the bow "dug into" the water (although it was not anywhere near being submerged) then the front of the boat "bent" upwards about 45 degrees for a second while the boat came to a complete stop, then the front went back down into the water. The "bend" in the boat happened at the join between floor pieces 2 and 3.
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Old 01 February 2011, 17:57   #9
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Are the stringers missing from the floor? (Do QS 380's have them? )
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Old 01 February 2011, 17:59   #10
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FWIW, I've seen an ali decked SIB ruined when the keel wasn't inflated - the stringers got ripped out with the constant flexing that ensued.

Keep 'em under pressure!
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