Weird! Zodiac arches it back then cavitates

Peter_C · 01 December 2009, 22:51

I have put a few hours on my new to me Zodiac which is a MKII HD.

It was all going good until we hit this problem.

Then it started doing something I have never felt before. Of course it is the first time I have driven it fully loaded on glassy water. When running full throttle at 23.4 mph or so, what it does is arch it's back so to speak, then unloads picking the transom up out of the water a little. This causes the motor to cavitate for a second. Then it starts the process all over again and releases about 20 seconds or so later. If I slow down to around 19 mph or thereabouts it does not do this. We tried moving people around both forwards and backwards with no results. It seems like it is an air pocket forming or something. Not really an issue since the Pacific Ocean is never dead flat, and diving a lake is something we will not be doing anytime soon.

Anyone else experienced this?

John Kennett · 02 December 2009, 23:01

Hi Peter

You've posted links to some pictures on scubaboard.com but they won't show here. You will need to upload them as attachments.

Bigmuz7 · 03 December 2009, 07:43

I have , although not in that particular boat, shifting weight usually sorts it for me, so I can get the bow up a bit, make sure your engine trim stop, isnt set too low if it has one, and also that your tubes are at the right pressure, of even if you think they are, try making them a little firmer to see if it helps

prairie tuber · 03 December 2009, 14:09

How many psi are you inflating the main tubes and the keel to?

matt h · 03 December 2009, 15:21

if the driver and passengers would sit right in the middle, the problem would go away,

the keel is not deep enough, try it

1ThOFA87 · 08 December 2009, 03:17

I have the same problem. Mostly when I have my brother aboard. He's pushin' close to 400 lbs and I've had him sit about mid front center and mid starboard. I'm 200 pounds so he's double my weight and I can't find anyplace to sit hiim without this problem. I'm going to get an electric pump and see if it gets my tubes firmer than I can with my foot pump.

prairie tuber · 08 December 2009, 05:08

Just in case I hadn't asked earlier;

How many psi are you inflating the main tubes and the keel to?

1ThOFA87 · 09 December 2009, 00:35

Quote:

Originally Posted by prairie tuber

Just in case I hadn't asked earlier;

How many psi are you inflating the main tubes and the keel to?

I don't know about Peter but I have no idea about mine. I have a footpump and fill it until it won't take anymore air. What should they be inflated to?

prairie tuber · 09 December 2009, 01:41

Main sponsons should be 3.4 psi, the keel just a bit higher. I would highly recommend getting a guage, proper inflation pressure is critical to the performance of the boat.

Peter_C · 13 January 2010, 07:33

LOL I forgot I posted this thread.

I keep the all of the tubes set at the spec written on the transom. My pump has a gauge on it, although it is cheesy.

Over the new years holiday it actually did cavitate on the Pacific Ocean because it was glassy flat inside Monterey Bay. Again we tried moving people around to no avail. I plan to buy a really nice gauge to make sure I am pumping to the proper pressures.

Also found out with four divers and six scuba tanks onboard, the boat takes awhile to plane.

raymillard · 13 January 2010, 12:06

I had a similar problem with an airdeck 311. It was caused by the airdeck not being pumped up enough. A wave would accumulate under the rear half of the boat, causing the deck to bulge up, the the boat would climb over it and get back onto the plane, whereupon the process would start again.....
Don't know if your boat is an airdeck, but if it's a wooden deck, is it being held firmly in place??

jyasaki · 13 January 2010, 16:31

Quote:

Originally Posted by Peter_C

Also found out with four divers and six scuba tanks onboard, the boat takes awhile to plane.

Yah. Try 6 divers and gear. You'll run out of gas before you get up on plane...

You did OK with 3 divers, and even better with 2. And, you've figured out that the ride is softer in the roiled water of another boats wake, so you're doing pretty well.

jky

Peter_C · 13 January 2010, 20:19

Quote:

Originally Posted by jyasaki

Yah. Try 6 divers and gear. You'll run out of gas before you get up on plane...

I can not imagine getting 6 divers with gear into my little boat. Obviously you have done it.

Quote:

Originally Posted by jyasaki

You did OK with 3 divers, and even better with 2. And, you've figured out that the ride is softer in the roiled water of another boats wake, so you're doing pretty well.

jky

I already knew that following a bigger boats wake was smoother. Now if that bigger boat would have just slowed down a mile per hour...our ride would have been smoother.

3 divers actually works great! Planes quick enough and there is plenty of room for gear. Although once everyone is in doubles I might change my opinion.

jyasaki · 14 January 2010, 16:05

Quote:

Originally Posted by Peter_C

I can not imagine getting 6 divers with gear into my little boat. Obviously you have done it.

7, actually. Not a pretty sight. Top speed with the Honda 40 was about 3 mph (at least I didn't have to worry about the no-wake zone in the harbor.)

We only went around the jetty to the Barge.

Quote:

Now if that bigger boat would have just slowed down a mile per hour...our ride would have been smoother.

I couldn't run one mph slower. It was that or about 5 mph slower, and throwing a displacement wake.

Later;

jky

avius · 09 February 2010, 01:29

Hey guys I have the same problem I am thinking. this is on a MKVI HD with twin merc 90's. I have to find the boats sweet spot, too much throttle and she starts kicking in water over the transom, not enough and we are just under plane plowing water. I had six tanks+ gear a cooler full of drinks and 4 people, total weight estimate of 1100lbs + the engines are a smidgen over the recommended weight . The pontoons were @ 3.1 the keel& speed tubes @3.4 I am taking her out in the morning to trouble shoot it. any ideas ? ill take some footage and post it when we get in...
-AVI

avius · 15 February 2010, 08:43

after scouring the forums, i believed we had an issue with our motors too deep in the water which begins to create drag and torq down on your tubes. this is what i think may be referred to as arching its back. well we raised the motors and the problem of swamping was solved, it also increased our wot speed significantly.

captnjack · 16 February 2010, 06:41

The floor is compressing and quite literally arching. Its a SIBs version of porpoising. I had the same issue on a Bombard soft bottom with AL floor. The now curved hull is getting too much stern lift and pushing air into the prop for an instant. The hull arch dissipates and you return to plane to repeat. There's not much you can do besides shift weight (aft usually) to set the stern down a little more consistently. Check that your engine is trimmed level to slightly bow up. The bow up manual trim setting will pose issues trying to get on a plane tho.

With manual trim on a soft bottom, you are kinda stuck living with the porpoising

Peter_C · 09 March 2010, 08:34

Quote:

Originally Posted by captnjack

The floor is compressing and quite literally arching. Its a SIBs version of porpoising. I had the same issue on a Bombard soft bottom with AL floor. The now curved hull is getting too much stern lift and pushing air into the prop for an instant. The hull arch dissipates and you return to plane to repeat. There's not much you can do besides shift weight (aft usually) to set the stern down a little more consistently. Check that your engine is trimmed level to slightly bow up. The bow up manual trim setting will pose issues trying to get on a plane tho.

With manual trim on a soft bottom, you are kinda stuck living with the porpoising

Thanks for the reply. I have manual trim, but my Evinrude does have the ability to stop the boat and set the trim wherever I want. I will try trimming it up just a little next time it decides to act up. Right now I would just like to get back to boat diving. Conditions have been craptastic lately.

Projectile · 26 July 2013, 22:17

Peter C, whatever happened with this problem?

Peter_C · 27 July 2013, 07:11

Proper pressures help A LOT! I bought a gauge and added far more air than I thought I needed. So last weekend four of us where out and it still arched its back a little, but no cavitation and it wasn't bad enough to worry about.

If you are not using a gauge it is time to buy one.