Cav plate height for Zodiac Mk4 - Page 5

Projectile · 08 October 2013, 20:42

Quote:

Originally Posted by captnjack

Oh jeesh. I thought the engine was trimmed out to the point of ventilation and it was still bow riding?

After 8 pages I might have misunderstood...

That is correct.

PS. Not all 8 pages are about my boat!

Projectile · 08 October 2013, 20:45

Trimming the engine out made cavitation (ventilation ,,, whatever you want to call it) worse ;unbearable even in a straight line in second hole up (out) from transom. I can post a video of that if you like, or you can take my word for it. Even trimmed out, it still pushed the bow down. Trimmed in all the way (as in the pic) cavitation/ventilation was least.

Projectile · 08 October 2013, 20:47

I'm thinking that this is a problem with the hull ...

Projectile · 08 October 2013, 20:48

I'm inclined to start a separate thread for my boat problems ...

captnjack · 08 October 2013, 20:49

Quote:

Originally Posted by Projectile

Trimming the engine out made cavitation (ventilation ,,, whatever you want to call it) worse ;unbearable even in a straight line in second hole up (out) from transom. I can post a video of that if you like, or you can take my word for it. Even trimmed out, it still pushed the bow down. Trimmed in all the way (as in the pic) cavitation/ventilation was least.

Ok that's what I thought.

The only thing really capable of causing that across such a wide range of motor trim settings is a cup in the hull. Either the tubes are under inflated, the hull fabric is baggie, or the floor boards are too short.

frankc · 08 October 2013, 22:20

The dealer should have picked up on the problem when he ran it. He may have been preoccupied with cavitation though, so I don't want to come down on him without having been there.

The area under a cupped hull could possibly cause air to be entrained in the water flow due to low pressure. Water boils at low pressure at room temperature, but I would not want to bet money on this situation.

If air is entrained in the water feeding a propeller, the bubbles will collapse on the prop blades as the pressure increases. That would cause cavitation which would eat prop blades in time. As the bubbles collapse, small amounts of metal are removed from the surface of the metal. I have seen impellers removed from pumps which cavitated which looked like swiss cheese.

I lean towards what captnjack posted. Cupping of the hull is a probable cause of the problem.

When the hull stretched on my Avon Rover, running the boat was a real experience. I was prepared to get rid of it and purchase another boat when a lady, probably an engineer, at Achilles diagnosed the problem and recommended I add length to my floorboards and see if that did not cure the problem. The boat performed like new with the tighter bottom.

I too was thrown a little by this thread dealing with multiple boats.

SIBRider · 08 October 2013, 22:31

Quote:

Originally Posted by Projectile

Trimming the engine out made cavitation (ventilation ,,, whatever you want to call it) worse ;unbearable even in a straight line in second hole up (out) from transom. I can post a video of that if you like, or you can take my word for it. Even trimmed out, it still pushed the bow down. Trimmed in all the way (as in the pic) cavitation/ventilation was least.

I would love to see one.
My bet is there is nothing wrong with your boat. You need 12 pitch 4 blade prop or 14 maybe even 15 pitch 3 blade. If you are not planning to cary heavy loads get 13" 4 blade prop. Run it at the 3rd hole.

Projectile · 09 October 2013, 01:45

Ok. I will post a video of the non-stop cavitation after supper.

I really appreciate all of the input from you guys. This is a big problem for me, and I feel like you guys are my only resource.

Projectile · 09 October 2013, 05:20

You now, the other thing about all of this is that it just sucks.

I bought a brand new boat from the biggest Zodiac dealer in NA. I had it shipped to Canada and paid duty.

I didn't try to swindle anybody or cut any corners.

I bought a brand new Evinrude outboard motor. Zodiac is a "preferred partner" of Evinrude, and Evinrude advertises their ETEC 30 on a very similar MILPRO inflatable.

Despite all of that, I have to deal with this crap. I just wish that my boat and motor worked well together.

frankc · 09 October 2013, 12:28

Stuff happens, hang in there.

captnjack · 09 October 2013, 16:22

Quote:

Originally Posted by Projectile

Ok. I will post a video of the non-stop cavitation after supper.

I really appreciate all of the input from you guys. This is a big problem for me, and I feel like you guys are my only resource.

The F470 has an odd shaft length. 18" as I recall. You need to use a long shaft (20") engine on it. You haven't ended up with a short (15") by accident have you?

SIBRider · 09 October 2013, 17:08

If you follow thread. Projectile has Zodiac Work boat 465. Similar to f470 but no speed tubes. Same 18 transom. Originally his motor was way too low.
Hang on there Projectile. You got great boat and great motor. Unfortunately they don't match up ideally.

SIBRider · 10 October 2013, 08:38

Took my boat out this afternoon. Trimmed-in to #2 starts bow steering. Faster I go worse it gets. Didn't bother to try #1. Can't imagine using that setting. Trimmed-out to #4. Bit of cavitation rarely, lost a bit of top end. Didn't try #5 since again obviously pointless. #3 was best performance overall. Very stable. Topped out at wee bit over 25 mph in very light chop. Curiously riding glass smooth water I was loosing 1-2 mph. Seems like that light chop gave least resistance.

Circumnavigating Sauvie Island

Nice 40 mile trip. Heading back hit some standing waves/chop in main channel heading back. I was catching air half the time for good 20-30 minutes. Boat shrugged it off. No video. I was too busy holding on. Made me wish I had a jokey seat with steering for sure.

Projectile · 11 October 2013, 04:11

SIBRider: that's with a 25 hp motor on an FC-470?

Amazing! I'm jealous.

SIBRider · 11 October 2013, 09:14

Thanks. Yes that's my Zuk 4 stroke. Yours should do better if you get right prop.

Projectile · 12 October 2013, 03:50

Quote:

Originally Posted by frankc

A hook in the hull will cause bow steer at speed. Are you sure the bottom of the hull is true?

I've been reading about this hull issue ("hook" in hull). It seems that if the hull is hooked, the more one accelerates, the more the bow gets pushed down. That is *exactly* what I experience.

It seems to be nicely explained here:
http://www.marathonmarine.com/boats_guide.pdf

I think that this is my problem. I'll go check out the bottom of the boat this weekend (it's in storage). My recollection is that when I was swimming around the boat this summer and feeling the bottom of the boat, there was a hook just fore of the junction of the transom and the hull: there was a fold in the material there, like a little cup that I could almost grab.

frankc, I think you've diagnosed the problem. I'll confirm it, and hopefully there will be an easy solution.

frankc · 12 October 2013, 13:48

The transom and hull area immediately ahead of the transom are critical. Even the back edge of the transom plays a part and should have a sharp angle, not rounded.

When under power, the force of the motor would tend to produce bending at this point if the transom rotates forward under power, so you may not see a problem with the boat at rest. If you notice a problem with the motor not running, it will only increase under power.

When I replaced the transom on my fiberglass boat, the shrinkage of the glass when drying formed a very shallow cup forward of the transom. It is only noticeable above 23 - 26 MPH.

SIBRider · 12 October 2013, 18:52

Can't imagine hook being an issue on inflatable with fabric hull. Rigid hull yes. Low pressure area will create suction effect pull bow down. Still we talk about speeds far in excess observed. 25-30+.

I firmly believe higher situated motor/prop underloads engine, creates excessive cavitation at correct (neutral) trim. That's all. With excessively trimmed in engine there might be all kinds of problems present but they are not worthy dealing with until engine is positioned correctly.

frankc · 12 October 2013, 23:32

Cavitation and a hooked bottom are two different subjects. Until the bottom is clean, you are fighting an uphill battle. You cannot pick a propeller with a cavitating engine, as a tach would go wild during cavitation.

A hook is very much a factor with an inflatable bottom. Underinflate one and see how well it performs. It is easy to repair in a hard bottom, but not such an easy job with an inflatable.

How does running an engine without a load on it create cavitation? You can over rev it, but the prop is still pumping water.

Projectile · 13 October 2013, 00:15

This photo is taken from directly below the bottom of the boat where it meets the transom, as if you were lying in your back under the boat looking up. You can clearly see the indentations that I mentioned being able to feel while swimming around the bottom of the boat. You can grab those. More to come.