Cav plate height for Zodiac Mk4

[Troy]

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.

[prairie tuber]

Is there currently any space between the outboard saddle mount and the transom, or is the saddle currently resting on the transom?

[Troy]

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.

[simsy]

What is the height position of the anti ventilation plate in regard to the keel line? What prop (make and cupping....) are you running?

[Oscarguitar]

Is there any obstruction in front of the prop/leg, e.g transducer, elephant trunk, loose fabric, all can cause cavitation at higher speeds

[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..

[Mextli]

Does your boat need a long or a short shaft motor, and what shaft is yours?

[Oscarguitar]

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?

[Oscarguitar]

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.

[prairie tuber]

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

[prairie tuber]

One more thought. If you do have hard floor boards (rather than the roll up slats), the futura hull design should be pretty tolerant of less than ideal weight distribution within the boat. Nonetheless, if you are loading up the bow with a lot of heavy gear, that would make the symptoms worse.

[Oscarguitar]

That (weight up front) and a high stern planing height would also create the bow steering problem that he is having.

[prairie tuber]

Originally Posted by Oscarguitar

That (weight up front) and a high stern planing height would also create the bow steering problem that he is having.

Definitely.

[Locozodiac]

Post a nice rear transom pick at engine side and keel level to have a look. Which engine brand is that horse ? What's the keel to middle transom height in centimeters ?

Happy Boating

[Troy]

Photos as requested, I think the height looks ok....
The prop is fine, I can wind the motor over (with effort) using the prop when the engine is in gear.

[Troy]

I forgot to get prop size, details to follow.

[Troy]

There was a total of about 8-10 Kg forward of midships.

[Locozodiac]

It's not a matter of just looking good, you should perform a visual inspection of the venting problem with the following :

-Inflate sib tubes and lower speed tubes to at least 3.0 psi with gauge.

-Trim engine to be perpendicular to sib when floating.

-Place a mate near same weight as you up front.

-Go full wot, with driver sitting on rear tube or inside deck next to transom visually check water flow passing through lower leg.

-This test must be done preferably on flat calm no wind seas, close bays, etc.

-With respect to transom, probably is intended for a long shaft engine and you have a short shaft horse or viceverse issue.

-Spotting at plane is the only way to know how much you need to lower or raise the engine, depending if having venting or transom back splash issues.

-Engine height as stated on all owners manual is just a starting refference, if wanting boating perfection engine/transom height must be adjusted/done under trial and error...

If with excessive airation, ventilation water flow must be passing slight over or under cav plate (2) worst at close turns or at choppy seas, your ideal water flow should be passing slightly under small upper plate (1) best performance under any sea conditions, superb top speed and inmaculate close turns at speed. All you need to do is distribute sib's passengers well for a perfect sib ballance and know your engine power capabilities.

PD : Your sib is intended for a long shaft engine, probably using a short one, that's why in the need to chop transom down 40 mm, will need to chop down more to be at the correct engine/transom best performance height. Try deflating completely your lower speed tubes and test that way if the problems betters a bit.

Happy Boating

[Peter_C]

I just walked outside and looked at my cavitation plate location and it is within a cm or so of the bottom of the keel, not equal with the speed tubes. In other words it is much higher than what the picture above shows. I have a short shaft on a Zodiac Futura MKII Sport GT.

[Troy]

New photos