Cav plate height for Zodiac Mk4 - Page 2

Projectile · 06 March 2013, 05:13

I'm keen to see how this turns out.

We used to have a 1980 Zodiac MkIIIGR, which had a short shaft transom. It would cavitate (ventilate?) all the time with our short shaft 35 hp Evinrude. When we upgraded to a 55 hp short shaft Evinrude Heavy Duty (a much heavier engine - 215 lbs vs 135 lbs if I'm not mistaken), the cavitation (ventilation?) problem went away - completely.

That's a beautiful boat, btw. I love it.

prairie tuber · 06 March 2013, 05:29

Quote:

Originally Posted by Projectile

I'm keen to see how this turns out.

We used to have a 1980 Zodiac MkIIIGR, which had a short shaft transom. It would cavitate (ventilate?) all the time with our short shaft 35 hp Evinrude. When we upgraded to a 55 hp short shaft Evinrude Heavy Duty (a much heavier engine - 215 lbs vs 135 lbs if I'm not mistaken), the cavitation (ventilation?) problem went away - completely.

That's a beautiful boat, btw. I love it.

Very Interesting! Sounds like I wouldn't need to worry about the extra weight of a 65 hp ETEC or a Yamaha F70 over my current Yamaha 40 hp 2 stroke.

Projectile · 31 July 2013, 22:44

Quote:

Originally Posted by Troy

No obstructions in front of the leg or prop, the boat has "high jackers / speed tubes / high risers' below the main tube and the cav plate sits inline with the bottom of the speed tubes.
It's a difficult situation to be in, as the boat starts to plane it also starts to bow steer, so I trim a bit and then it cavitates.
The prop is cupped (I don't have the size at the moment) and is a stainless steel one.
I can get the boat up on the plane quickly but it goes to crap there after!
I will check the pitch tomorrow (it's at a mates place).
I really appreciate your input everyone..

Troy,

How did this turn out? I'm having the same issue with my new Zodiac WB 465 with ETEC 30. The boat and motor are both long shaft. It planes easily, but as I accelerate beyond a slow plane, the bow digs in, I get a lot of spray coming off the sides of the boat, and its impossible to keep the boat straight. The floorboards creak a lot and the bow boards flex. As soon as I let off the throttle and go back to a slow plane, the bow comes up and everything relaxes and feels normal again. Trimming the engine up from the first to the third trim holes helps a bit, and when I reach trim holes 4 & 5, it cavitates (ventilates?). The boat and keel are all at max pressure, and it does this with the boat empty or my family of 4 in it in a variety of positions for weight distribution, so those are definitely not the issue.

My cavitation plate is 3 inches below the bottom of the transom, but may be level with the bottom of the keel, since the hull has a bit of a Vee to it by virtue of the inflatable keel. I have been advised to try raising the motor ...

Northern Pike · 02 August 2013, 03:33

Quote:

Originally Posted by Projectile

Troy,

The floorboards creak a lot and the bow boards flex.

That should not be happening, and would suggest that you are under inflated. Perhaps you need to test the accuracy of your gauge.

Projectile · 02 August 2013, 04:46

Quote:

Originally Posted by Northern Pike

That should not be happening, and would suggest that you are under inflated. Perhaps you need to test the accuracy of your gauge.

Thanks, but I have over 30 yrs of experience with heavy duty Zodiac inflatables. If there is one thing I am sure of, it is that the boat is well inflated. While I have checked with a gauge, there is little need to do so. I know that when I can't pump anymore in, it's over inflated, and I've been there with this boat when gauges confirmed over inflation. Problem persisted.

prairie tuber · 02 August 2013, 09:25

I'm suspecting that your out outboard is mounted about 2"-3" lower than ideal. I'd try incrementally raising the height by an inch at a time and see how it goes.

Northern Pike · 02 August 2013, 12:36

Its been my experience that when you can't pump any more with the foot pump you are at the correct inflation pressure.

Locozodiac · 02 August 2013, 15:29

Quote:

Originally Posted by Northern Pike

Its been my experience that when you can't pump any more with the foot pump you are at the correct inflation pressure.

That's relative. Not all boaters weights or inflates same, heavy weights much better, skinny less and newbies worst. On some foot pumps valve connector pops out when correct pressure has been achieved, not on the ones having bayonet type end connectors though. Find hand pumps are much better and friendlier to inflate than foot pumps.

That's why is mandatory to monitor your final "assumed" perfect inflation with a pressure gauge, as most of the time will be under inflated...

Happy Boating

captnjack · 02 August 2013, 16:25

Quote:

Originally Posted by Projectile

My cavitation plate is 3 inches below the bottom of the transom, but may be level with the bottom of the keel, since the hull has a bit of a Vee to it by virtue of the inflatable keel. I have been advised to try raising the motor ...

A picture from the rear would be good.

Projectile · 03 August 2013, 17:54

As requested.

camelgas · 03 August 2013, 20:38

I could be way off here but..., is that a long shaft motor on a short transom ?, as the picture with the level and the tape suggests 5" difference.

Projectile · 03 August 2013, 20:58

The cav plate is 3 inches below bottom of transom, not 5. Top of level , not bottom, is even with cav plate.

Zodiac WB465 calls for long shaft engine (confirmed by manufacturer), which this engine is. It measures 20 inches from cav plate to transom rest stop and model number matches for long shaft engine.

Peter_C · 04 August 2013, 01:23

Not sure what the motor height should be as I have never reset mine from the way whomever originally bought and set up the boat. It seems to do okay. Any comments would be welcome as I am open to ideas, but thought I would share another perspective.

The cavitation plate is the lower one with the Stingray plastic wing shown in the lower right of the photo.

Trim is set to all the way down (I usually tilt it up just a little if not loaded heavily with dive gear). There is about 1cm of clearance to the bottom of the boat, as the yellow level is resting on the "bottom" of the cavitation plate.

Projectile, could you please share some photos of your elephant trunk setup both inside and outside? I would really appreciate it.

Projectile · 04 August 2013, 03:25

Happy to but what's an elephant trunk?

Squid Boat · 04 August 2013, 04:35

Quote:

Originally Posted by Projectile

Happy to but what's an elephant trunk?

Like this one, but mines got girth....

Cheers, Squid

Projectile · 04 August 2013, 09:10

Ahhhh ... The high volume self bailer.

Sure, but I'll have to take some photos first. Give me a few days.

Projectile · 06 August 2013, 06:26

Here ya go.

Projectile · 06 August 2013, 06:28

Thanks, Peter_C, for posting the pics of your cav plate. Mine is way lower, and so yours help lend support for the advice to try to raise my outboard. I hope that sorts my issues out.

Projectile · 11 August 2013, 01:19

I just looked at Troy's pics carefully. Not only does the problem he described with his boat match the problem I am having with mine, his cav plate looks to be at least 3 inches below the transom, just like mine.

I have a feeling that raising my engine is going to be the solution.

My transom is 19 inches high, and my ETEC 30 has a 22 inch shaft (not 20 as I wrote above ... that was a poor estimate ... when I measured it properly today, it was 22 inches long). Not a very good match...

The trouble is that I can only raise my engine about an inch before the twisty clamps that secure the motor to the mounting plate on the transom will sit above that mounting plate, so I hope that 1 inch is enough to solve the problem. I may have to bolt this engine to the transom after all. Unfortunately, I was hoping to avoid that.

I wish I'd hear back from Troy with some reassurance that raising his engine also solved his problem...

lineofsight · 11 August 2013, 06:33

Great post locozodiac. Couple comments to add, the relevant inflation pressure is not what comes out of the pump at the launch site but what is in the tubes on the water (earlier this week the ambient temp was 37 degrees, the glacier fed lake was just a little colder).

Here is a helpful article with some depth (ha ha) on cav plate / motor height, personally just cut some 1/2 strips of ply and experimented much in the way locozodiac described (cavitation during 'normal' running and too high).

http://media.channelblade.com/boat_g...tipstricks.pdf (sorry, no idea how to that in fancy manner in phone).

Good luck.