Cav plate height for Zodiac Mk4

[Projectile]

Here's the same view (still from below the boat looking up) from a different angle:

[Projectile]

This is a pic from the side. With my friend's help, we raised the boat with some blocks under the buoyancy tubes. This let us get a pic of the hook from the side as the hull was raised above the trailer bunks. There is a straight edge, the right side of which is contacting the hull along the center line, in place. The hook at the back of the hull is clearly shown and it's wide enough for my to slip my fingers in between.

[Projectile]

This is a similar pic from the side with the straight edge in place but a little more close up.

[Projectile]

This is a pretty good photo of the hook in the hull:

[Projectile]

Because frankc mentioned that the hook can become more pronounced under power, this last pic shows what a little gentle push in that region can do.

I think that this is very likely to be the cause of my problems. When this is the cause, the more one accelerates, the more the bow gets pushed down, which is exactly what I experience.

Now, what is the solution? Whatever it is, it'll have to wait for spring ...

[captnjack]

Originally Posted by Projectile

Because frankc mentioned that the hook can become more pronounced under power, this last pic shows what a little gentle push in that region can do.

I think that this is very likely to be the cause of my problems. When this is the cause, the more one accelerates, the more the bow gets pushed down, which is exactly what I experience.

Now, what is the solution? Whatever it is, it'll have to wait for spring ...

You can't really diagnose a cupped hull on an inflatable out of the water. And you can't see the shape of the hull underway. But your cup seems pretty pronounced, to the point where its obvious just sitting still - although its hard to know if the trailer bunks are doing on the dry what water does underway.

There are 2 solutions:
1) make the floor boards longer to take up the slack in the fabric and uncup the hull. Some sort of spacer might work, but only if the tubes are long enough that the extra floor board length can actually stretch the hull fabric enough to remove the cup at the transom.

2) return this brand new boat to the dealer. Its defective from the get go and you shouldn't need to be modifying it to make it ride right without bow steering.

I would not waste your money on a different prop, that will not cure the bow steering which is due to the defective hull.

[frankc]

Looks like the bunks support the transom itself which is proper. The photos show what we suspected was the case.

When a boat is being driven through the water, the water under the boat develops pressure, and that hook will be more pronounced with the boat in use. No wonder he has a bow steering problem.

[Projectile]

Here are a couple more pics I found on the web related to hooked hulls.

Looks exactly like what I've found with my hull.

Would appreciate it if anybody might be able to take some similar pics of the junction of their hull with transom ...

[SIBRider]

Mine looks very similar.


I'd like to see perfect shaped keel. Especially at very end where it wraps on itself.

Cavitation happens where pressure change is severe when there is sudden changes in flow direction. Pressure differential in front and back of prop is what propels boat forward. Smaller prop diameter necessitates higher pressure differential. As boat accelerates higher speed requires higher pressure because of higher drag. Pressure depens on flow. Higher flow higher pressure. Impeller type pump flow depends on propeller pitch and RPM. Lower pitch has to spin faster. Lower pitch means higher angel of attack. Higher angle of attach means flow direction is changed more rapidly. Hence higher likelihood of cavitation. Cavitation may also create positive feed back loop. As cavitation bubble develops resistance load applied to prop drops. Therefor at constant throttle you engine will speed up higher possibly over-revving. Over-revving at speeds significantly below theoretical means likely cavitation.
And/or ventilation. The later can also be a factor with smaller pitch prop as it creates more turbulence at given speed.
Really important to match prop with load/speed.

[SIBRider]

What you/we see is somewhat different from the hook. In case of hook if you continue keel in strait line it drops down at the stern. Effectively acts as trim tab. Ours turn up a bit with a small cup. It would be darn impossible to make perfect hull shape with so many loosely fit inflatable and fabric pieces. Unfortunate imprecision of type of boat we have I think.

[SIBRider]

Speaking of evil anti-cavitation plate. or anti-ventilation plate. Why then every outboard manufacturer calls them anti-cavitation plate? (see #5) Wouldn't they know better? One thing for sure if motor is trimmed -in it will act as one big mighty trim tab lifting stern up and pushing blow down.

[The Gurnard]

Originally Posted by Projectile

Would appreciate it if anybody might be able to take some similar pics of the junction of their hull with transom ...

Not sure if this will help you but here is a photo of the rear of my 4.3 Quicksilver SIB.
As far as I am concerned, mine is well trimmed and has no bad handling characteristics, either lightly loaded ..with only me, the fuel tank an engine at the rear of the boat.. or laden with more weight up front. No water spray either over the back from the 25hp Mariner 2 stroke even at full throttle.
The nose is higher in the air when lightly loaded with weight at the back..but as soon as it starts planning ..she is level. Heavy loaded with weight up front..the nose doesn’t go quite so high before planning..but once she does...the boat zips along level.
From the photo below .. there is quite a hook on mine too .. so Im not so sure that is your problem ?
I drew the white line to help highlight the hook..the greyish lines are the keel strip on the bottom of the boat

[Projectile]

SIBRider:

Thanks again for all of your input. I really appreciate everybody here trying to help me out.

The photo below shows that a hook includes an upturn in the hull, like an upside down cup, just as you describe. This will act like a trim tab, too.

You have a futura hull on your boat, so when you plane, your boat rides considerably if not mostly on the speed tubes, much higher up and out of the water than my boat which has a conventional SIB hull. I could imagine that that might mitigate the effect of a hook in the center of your hull.

Nobody disagrees that a motor that is trimmed in will bring the bow down. I just can't keep describing over and over how trimming the motor out does not solve the problem that I have with the bow coming down as I accelerate beyond planing speed.

When my A/C plate was 3 inches below the bottom of the transom, I trimmed the motor all the way out. The bow down attitude got slightly better, but was still a huge problem. In the 4th or 5th trim holes up and away from the transom, the A/C plate started to become visible at the water line and cavitation emerged. That's not a big suprise since the prop was brought close to the surface of the water. Now that my A/C plate is about 1/2 inch below the transom, the bow down attitude is better, but still problematic. Furthermore, now I have cavitation all the time that got worse as I trimmed out (to address the bow down attitude) and got better with an additional 200-240 lbs in the boat. Since the bow down attitude is present as I accelerate from planing speed whether the motor is low or at proper height, and whether the engine is trimmed in or out, it is IMPOSSIBLE for trim to be THE cause of my bow-down problem.

Either the boat was designed and is supposed to run with the bow down as I experience, or there is a problem with the hull. I find the former hard to believe.

[Projectile]

The Gunard:

Thanks for posting the pic. I think that I have a much more prominent hook in my hull than that.

[SIBRider]

Well. If think it's such a concern. You can glue/screw piece of 1/2 or whatever is appropriate size plastic (HDPE for example) to bottom of rear panel to push keel down. Shouldn't be that big of deal.

That's one huge cup/hook on that boat.

[Projectile]

That's not what my WB465 looks like at WOT (but I wish it did). Spray is coming up just fore of the aft carrying handle, while it comes up fore of the fore carrying handle on mine.

That's just not normal.

[frankc]

Seems we are comparing a boat supported by speed tubes plus the hull extension sponsons to a straight inflatable with no speed tubes. They are two different designs and susceptible to different hydraulic conditions in use.

What The Gurnard posted is pretty close to ideal for the design.

The picture Projectile posted is what he should see with his boat, not a bow steered hull which could be dangerous at sea.

[Projectile]

Not exactly the same angle, but still a reasonable side by side comparison:

[SIBRider]

Originally Posted by Projectile

Not exactly the same angle, but still a reasonable side by side comparison:

then again even judging by tiller handle angel. Yours point up theirs just about level. Your motor is significantly more trimmed-in then theirs.

[frankc]

The gentleman reported that even when trimmed out to the point that cavitation became extreme, it still bow steered. He then posted pictures showing a cup in the bottom of the hull. I certainly take his word for that.

Why do you feel that his only problem is cavitation and the bow steer does not exist? What radial thrust component results from cavitation which would depress the bow of the boat? Have you ever run a boat with a hook in the bottom?