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Old 03 March 2013, 23:01   #1
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Country: Australia
Boat name: SEAL
Make: Zodiac Hurricane
Length: 7m +
Engine: Steyr 260Hp
Join Date: Feb 2012
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Cav plate height for Zodiac Mk4

Hi all, I'm having a little trouble getting trim without cavitation on my 5.3 meter Zodiac.
I have a fibreglass floor which works great.
The boat gets on the plane well with a 70 Hp Yamaha but starts to cavitate and loses speed (obviously).
I know these boats are supposed to get along on the plane with 12 guys on board at speed to keep the outboard in the water. I'm only ever going to have a couple of people on board at any time so don't have the weight to assist keeping the prop in the water and not cavitating.
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Old 04 March 2013, 02:14   #2
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Country: Canada
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Is there currently any space between the outboard saddle mount and the transom, or is the saddle currently resting on the transom?
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Old 04 March 2013, 02:30   #3
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Country: Australia
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I have had 40mm taken out to try and lower the motor. The engine mounting bracket is just clear of the two lower bungs and has helped but is still not right.
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Old 04 March 2013, 04:07   #4
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Country: UK - England
Town: macclessfield
Boat name: Reach Out
Make: Quicksilver
Length: 4m +
Engine: 30hp Tohatsu EFI
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What is the height position of the anti ventilation plate in regard to the keel line? What prop (make and cupping....) are you running?
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Old 04 March 2013, 04:41   #5
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Country: UK - England
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Boat name: Half Cut
Make: Cobra 8.6
Length: 8m +
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Is there any obstruction in front of the prop/leg, e.g transducer, elephant trunk, loose fabric, all can cause cavitation at higher speeds
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Old 04 March 2013, 05:53   #6
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Country: Australia
Boat name: SEAL
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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..
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Old 04 March 2013, 06:15   #7
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Country: Germany
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Boat name: November
Make: Avon Searider 4M
Length: 4m +
Engine: Yam 55
MMSI: 211748930
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Posts: 312
RIBase
Does your boat need a long or a short shaft motor, and what shaft is yours?
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Old 04 March 2013, 06:43   #8
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Not sure what speed tubes are but if they raise the boat out of the water at speed that could be you problem right there!

Where does the cav plate sit in relation to the bottom of the hull?

Have you got a picture from the stern, that would help to see what might be going on?

Did the boat come with that engine or have you added it later? Has this combination worked before or has it never been right?
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Old 04 March 2013, 06:51   #9
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Just looked up speed tubes, looks they are designed to create turbulence at the back of the boat right where the prop is and then lift the prop 3 inches out of the water!

If yours are removable, try it without the speed tubes, if that is OK then at least you know what the is causing the problem, then decide if you keep the motor and ditch the tubes, or keep the tubes and get a longer shaft motor.

Let us know how you get on.
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Old 04 March 2013, 10:18   #10
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Assuming that the speed tubes, center keel and main sponsons are all inflated the right pressure, the speed tubes aren't the issue. Any boat is going to sit higher in the water when on plane than when still. Does your outboard have a 20" shaft length? How deep is the anti-cavitation plate when at speed?

The symptoms you are describing are typical of a spun prop hub. Try making (waterpoof) markings on both the hub bushing and prop body that line up with each other then take the boat out until you get the issue and recheck the alignment of the markings.

Can you borrow another prop from someone and test it?

A 70 hp tiller outboard really is the ideal set up for a Mark IV HD
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