Quicksilver 3.8 mtr SIB nose diving - help!

kerravon · 31 January 2011, 22:56

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.

willk · 01 February 2011, 06:41

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

kerravon · 01 February 2011, 07:58

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?

Channel Ribs · 01 February 2011, 08:03

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

m chappelow · 01 February 2011, 08:36

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 .

kerravon · 01 February 2011, 09:25

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.

Channel Ribs · 01 February 2011, 09:40

Quote:

Originally Posted by kerravon

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?).

kerravon · 01 February 2011, 21:52

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.

Nos4r2 · 01 February 2011, 21:57

Are the stringers missing from the floor? (Do QS 380's have them? )

willk · 01 February 2011, 21:59

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!

kerravon · 01 February 2011, 23:28

Nos4r2 and Willk - thanks for the suggestions. The stringers are installed but the boat is flexing at the join between pieces 2 and 3 of the floor - the stringers don't reach that far forward and don't cover that join by design.

Channel Ribs · 02 February 2011, 05:33

Quote:

Originally Posted by kerravon

Nos4r2 and Willk - thanks for the suggestions. The stringers are installed but the boat is flexing at the join between pieces 2 and 3 of the floor - the stringers don't reach that far forward and don't cover that join by design.

They should! Unless we are talking about two different boats.

There should be two pairs of metal extrusion that have a groove into which the top flange of the floor sides fit tightly. These stringers make all the oblong floor pieces sit flat.

Duck Witch · 02 February 2011, 07:04

Quote:

Originally Posted by malthouse

They should! Unless we are talking about two different boats.

There should be two pairs of metal extrusion that have a groove into which the top flange of the floor sides fit tightly. These stringers make all the oblong floor pieces sit flat.

The stringers only hold panels 3, 4 & 5 together. They stop just before the join between 2 & 3 as that's when the taper starts, hence why it can bend there.

azzurro · 02 February 2011, 11:12

Quote:

Originally Posted by Duck Witch

The stringers only hold panels 3, 4 & 5 together. They stop just before the join between 2 & 3 as that's when the taper starts, hence why it can bend there.

You should have 4 stringers, 2 long and 2 short. They are placed alternatively (short-long, long-short).

jyasaki · 02 February 2011, 15:29

Don't know about that one, Azzurro; my Achilles was set up the same way Duck Witch describes (must be a story behind that username, no?)

Rear 3 sections held firm with stringers, front 2 sections held in place by compression between the rear floor section and the tubes at the bow.

Inflation pressure of the tubes and the keel seemed to be what controls the flex towards the nose; there certainly isn't a whole lot of mechanical structure to keep it firm.

I'd suggest getting a gauge so you know where you are inflating to; else, pump it up until it feels like a taut sheet of canvas. Thumping it with a fist should not deform the tubes much at all. A half inch indentation with the thumb sounds a little low to me (but maybe my thumbs are weaker than yours.)

jky

John Kennett · 02 February 2011, 15:57

Assuming the boat is correctly assembled, my guess is that the keel isn't properly inflated.

Channel Ribs · 02 February 2011, 16:04

Quote:

Originally Posted by John Kennett

my guess is that the keel isn't properly inflated.

Wilkee "You should get the pressure sorted first, especially the inflatable keel"

Malteaser "the floor and keel work together to make the boat work"

M Chap "more likely to be under inflated keel"

Wilk II "ruined when the keel wasn't inflated"

Jyaskee "pressure of the tubes and the keel seemed to be what controls the flex"

Is it me, or is there a theme developing here.

kerravon · 03 February 2011, 01:55

Sure does seem to be a theme developing, namely that the most likely explanation is incorrect inflation, particularly of the keel. Pretty typical newbie mistake, right?

It was precisely this type of helpful feedback I had hoped for from my post, so I thank everyone for their considered input. I will be checking the inflation and re-testing the boat this weekend, so will post my results.

Jyasaki's description of the boat is the same as the one I have - 2 stringers per side, both equal length, with floor pieces 1 and 2 not attached to stringers and held in place by the side of the boat only.

Thanks RIB (SIB) net

kerravon · 04 February 2011, 08:59

Just a quick followup to let everone know how the test on the water went today - in a nutshell, problem resolved - it was as most people suggested a lack of proper inflation - not just in the keel but the whole boat in fact. 3.6 psi turns about to be a lot firmer than I thought. I knew it would be a classic newbie error. She's going well now. Thanks for the help all.

Channel Ribs · 04 February 2011, 09:03

Quote:

Originally Posted by kerravon

She's going well now.

Result

(The keel should be higher pressure than the tubes)