Cav plate height for Zodiac Mk4 - Page 4

Northern Pike · 31 August 2013, 12:16

Yes.
And its easier to say and type AV plate.

SIBRider · 31 August 2013, 17:32

Quote:

Originally Posted by simsy

Sounds like the full length of the keel of the boat is concave, making the water suck the boat down when at speed.

Interesting thought. I just raised my motor 3.25". Home grown solution: 2 AL 12"x12" plates with HDPE block sandwiched on top. Currently "you know what plate" is at about 0.5" below keel at middle trip hole. I also changed prop to 13" pitch. Certainly maintains better speed with less splashing. Cruises at 18-20 mph with 600lb on board. Seems adequate by thought of bigger motor hasn't left me. Didn't have opportunity to take it out solo. My boys stick around if I try to head out.
At first though I observed heavy bow steer, and prop ventilating periodically. My keel has a leak. Once inflated to proper pressure problem seemingly was gone.

And some scenic pics from trials.

Projectile · 05 October 2013, 02:28

Went on a test drive today with a very helpful rep from my local Zodiac dealer.

All the problems I've described were present when I went out on my own. I'll try to post a video later.

However, when he got in the boat with me, the boat handled quite well. He admitted to weighing over 200 lbs. It seems that I need some extra weight in the boat for it to run properly!

He suggested that I should try getting a bigger motor. The WB465 is recommended for a 40 hp and max is 60 hp. My ETEC 30 is pretty light at 150 lbs. Maybe a heavier and bigger motor would do the trick?

SIBRider · 05 October 2013, 17:29

What trim position your engine is at? AV plate generally should be nearly parallel to keel if you extend the line. Place a level or something on it to see. For me it lined up nicely at middle trim hole.
Your are saying you trim it up or down. It's confusing. IMHO right term is in or out (close or away from transom). What is the position of AV plate at middle trim hole relative to bottom of transom.
What prop pitch are you using?
It's only about weight then do some experiments. Get few water cans and see how their distribution affects handling.

Projectile · 06 October 2013, 21:01

If I trim out past the second hole, it just cavitates all of the time.

In second trim hole from the transom, it cavitates if I'm in the boat by myself. With the additional passenger (240lbs?) - no problems.

Trimmed all the way in (first hole), it only cavitates rarely with me in the boat, but I feel that the bow is pushed down too much.

The best performance was in second trim hole with extra weight in the boat.

Its bothersome how finicky this boat (and, by the looks of this thread, other inflatables) can be...

The Zodiac dealer rep suggested a heavier engine to add weight, especially to the back of the boat (to try to get the stern down and the bow up a bit more). My ETEC 30 only weighs 150 lbs. An ETEC 40 weighs 240 lbs! The max engine weight for my boat is 243 lbs, so I'm looking at an ETEC or an older 2-stroke or a new Yamaha 40.

A test drive in a heavier and more powerful engine would mean bolting to the transom - I don't know of any that have the twisty clamps except for the ETEC 40 with rope start, but I don't know anybody who would let me take one for a test drive. My Zodiac dealer would let me take an older Yamaha 50hp 2-stroke for a test drive but I'm a bit reluctant to do that, since I don't know how its going to work out, and I don't want unnecessary holes in my transom...

frankc · 07 October 2013, 01:15

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

Projectile · 07 October 2013, 04:03

Here's a video of the Zodiac dealer rep driving my boat.

He gets the boat gently on plane, and everything is fine at that point. The troubles start after accelerating from a plane to full throttle. Watch after he makes a left turn and comes toward the camera.

You can hear the engine cavitating. Also, does the boat not look too "flat" in the water? What I mean is, do you think that the bow is too low ... that the forward portions of the hull are making contact with the water when they shouldn't be? Spray is coming up from where the keel is contacting the water and that is fore of the fore carrying handle! Shouldn't more of the fore of the boat be up and out of the water (as it is when on a slow plane)?

frankc · 07 October 2013, 10:14

The fabric strips on top of the tubes are anti-hogging devices designed to keep the boat from bending and allowing the boat to run with the bottom basically cupped, ie, assuming the shape of a hogs back. If they stretch, the floorboards may need to be lengthened to keep the hull bottom straight.

If the hull is cupped, you have a low pressure area formed under the hull which may be the cause of the cavitation. I had to lengthen the front floorboard in a 10 ft Avon to cure a problem like yours. I was about to trash the boat until a nice lady from Achilles clued me in and recommended the cure. New lengthened floorboard and the boat is still in service after many years with no problems.

I would believe that as forward pressure is applied to your hull, the boat is bending and causing your problem. The motor is bending the hull.

Projectile · 07 October 2013, 19:15

Thanks FrankC, but its a brand new boat and I've examined the bottom of the boat and its pristine, so I think that's unlikely.

SIBRider · 07 October 2013, 19:22

Does look like you are bow riding. There is no point in trying to counteract ventilating by trimming motor in. You have to achieve good performance in neutral trim. Moving forward your motor creates a force produced by low pressure in front of blades and higher pressure behind. When you trim in direction of that force changes somewhat upward.

You can separate it into horizontal and vertical part. Horizontal moves it forward but vertical actually lifts your aft up forcing bow down over center of all forces acting on boat. What's more there would be low pressure area close to transome. Since you basically suck water from under boat. That would make boat stick more.

You are ventilating (are you actually ventilating or/and over-revving?) at neutral IMHO because of insufficient back pressure. I believe you should be able to cure it by using higher pitch and/or 4 blade prop. What is you current pitch? Do you have a tachometer?

Of course dropping engine a bit might help as well.

40 hp recommended rating is not to for purpose of aft load. That would be stupid. Lighter motor should plane lighter boat just fine. Rather because rated load for you boat is north of 2000lb and to plane that (or at least maybe like 60-70%) you need some power.

frankc · 07 October 2013, 22:12

What were the dealers comments other than using a larger engine? Did he have the motor tilted in as shown in the sketch above? If tilted in, that would cause bow steer.

You need less HP for a light load, so the comments to use a larger motor seem out of line.

You would not see the transom twisting except when power was applied. If the hull caused bow steer whether a new boat or not, it could still have the same problem. The video does indicate a bow down attitude.

Projectile · 08 October 2013, 04:44

The driver gets the boat up on plane very easily and all is well. Then, right at 0:20, when he accelerates from that slow plane, you can see the bow get pushed down! That can't be right...

Projectile · 08 October 2013, 04:50

SIBRIDER:

I have no tach, but it feels like last 25-30% of throttle provides no more speed, if that's worth anything. How could it: the bow is being pushed down and there is so much drag in the water?!

The prop says 10.3x12 on it.

SIBRider · 08 October 2013, 05:32

Quote:

Originally Posted by Projectile

SIBRIDER:

I have no tach, but it feels like last 25-30% of throttle provides no more speed, if that's worth anything. How could it: the bow is being pushed down and there is so much drag in the water?!

The prop says 10.3x12 on it.

There is so much drag because bow is pushed down. Was engine trimmed-in heavily?
When you open last 30% of throttle do RPM increase without gain in speed? Up to ventilating possibly?

frankc · 08 October 2013, 12:03

The motor should have the propeller shaft parallel with the surface of the water, but if it is tilted facing up as in the sketch, it will force the bow down.

Imagine a boat fitted with trim tabs which are tilted down, ie, the back of the tabs are lower than the front of the tabs. That would force the bow down. If there are no tabs but the bottom of the hull is hooked so that the back of the boat is lower than the area of the bottom directly in front of the transom, you would effectively have the same problem.

As the line of force of the propeller is below the bottom of the boat, it will tend to force the bow up with a good bottom on the boat. That combined with the boat being forced forward against the surface of the water should allow the boat to be driven forward in a normal planeing attitude.

If the propeller cavitates, the boat would still move forward on a normal plane. It would not attain sufficient speed, but that would not interfere with the boat running at a normal attitude.

Either the motor is improperly tilted or the bottom has a hook in it. If the dealer was really experienced, he would have spotted this when he ran the boat. He would have trimmed the motor out to prevent bowsteer. You can feel the stern of the boat rising and the bow steer when at speed.

Your video and explanation of what you saw indicates that the bow is being forced down on that hull. That is dangerous and could cause an accident.

Putting a larger motor on that hull will just cause worse problems than what you have. He is banking on putting a heavier motor on the boat forcing the transom down, but should recognize that there is a hull problem instead. The hydraulic forces will only increase with speed and offset the weight on the rear of the boat.

Is the motor fitted with hydraulic trim control? Tilting the motor out would raise the bow, but a problem with the hull would still be evident.

It appears to me that it was run with improper motor trim or there is a problem with the hull.

I have a slight hook in the bottom of my fishing boat. With 15 HP, it runs just fine. When I had a heavier 25 HP on it, it would bow steer slightly at high speed. My not so little weight plus the heavier motor did not cure the effects of the slight hook. The extra 4 MPH and weight was offset by hydraulic forces on the hull. No bowsteer with the 15; bow steer with the 25.

captnjack · 08 October 2013, 17:18

Quote:

Originally Posted by Projectile

Thanks FrankC, but its a brand new boat and I've examined the bottom of the boat and its pristine, so I think that's unlikely.

Assuming its pumped to rated pressures...

The hull is defective. The floor fabric is too loose and when under way its creating a cup which is causing your bow steering. On the trailer/ out of the water you won't be able to detect this.

Slightly longer floor boards would fix the issue, you might be able to make a spacer to keep the floor fabric tighter.

A heavier engine won't fix the problem at all.

SIBRider · 08 October 2013, 18:21

Quote:

Originally Posted by captnjack

Assuming its pumped to rated pressures...

The hull is defective. The floor fabric is too loose and when under way its creating a cup which is causing your bow steering. On the trailer/ out of the water you won't be able to detect this.

Slightly longer floor boards would fix the issue, you might be able to make a spacer to keep the floor fabric tighter.

A heavier engine won't fix the problem at all.

From what I gather bow riding only happens when engine is trimmed-in heavily. Which is expected of course. Sound like only ventilation/cavitation is a problem at neutral trim. If I'm not mistaken.

captnjack · 08 October 2013, 19:01

Quote:

Originally Posted by SIBRider

From what I gather bow riding only happens when engine is trimmed-in heavily. Which is expected of course. Sound like only ventilation/cavitation is a problem at neutral trim. If I'm not mistaken.

Bow riding occurs on trimmed in motors OR when there's a cup in the hull. Since SIBs have fabric hulls, they depend on the exact length of the floor boards and tube inflation to stretch the hull fabric tight enough so that it doesn't have cup. Underinflation, short floor boards, or stretched hull fabric can all lead to cup. And a cupped hull experiences bow riding no matter how far out the engine is trimmed.

Ventilation can occur at any trim setting, not usually when trimmed in however.

From the description of the problem and the video, I strongly suspect the hull is cupped. Since its new I can only conclude its either improperly inflated, the floor boards are not installed right, or the hull fabric is too baggy. Any of which can create a cupped hull which will never work right.

SIBRider · 08 October 2013, 19:13

We speak the same thing. From what I see. Look at the trim pin position.

Engine is at furwest trim-in position. So boat beheaves as expected. Heavy bow riding. I don't understand why this "rep" didn't pay attention to such obvious issue.

captnjack · 08 October 2013, 19:37

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...