Cav plate adjustment

[Greg Lukin]

Hi all. Our new boat has a cav plate but is angled down a fair bit~ about an inch below the hull. They reckon it makes the wake better for skiing which I understand but we ski in a narrow lake and need to turn sharp. The problem is that when we turn the bow of the boat pushes down and makes turning very difficult due to the amount if water it is pushing... Question is. If we lift the cav plate to half the angle will that make the boat turn easier? Any thoughts?

[BraceMaker]

Which boat?

[Cusefan78]

Is this for a supra? The cav plate only adjust the boat riding height. Only make little adjustments. Should be even to 1/4 below for best ski wake. To loosen steering you'll need to check the helm, steering cable, and rudder packing. Any of it stiff it'll need to be changed. Most common issue is the cable. Also in have read that filing the leading edge of the rudder helps

[Waternut]

Don't know what your boat is but typically when the nose dives down, it digs in and makes turning even tighter. If you're struggling with steering stiffness you probably want to look into steering cable and rudder packing. The steering cable can get rusty and nasty and create extra resistance when turning the rudder. If the rudder packing is worn out, the steering will feel loose when not moving but will feel very heavy at speed. If you suspect rudder packing, you can drop the rudder, add a little grease in there, and if it makes a huge difference, you'll know what the problem is.

[Greg Lukin]

Hi all. The steering is fine. It's just that the nose of the boat digs in when turning sharp. We are taking it back to see what they can do. It's a custom protege. The boat they use in the barefoot worlds but ours is an inboard. PCM 450. Should have gone with the 409 as we are stuffing around with props and no one knows what to expect from it as not many are in boats over here in oz

[Wayne]

Do you know if the hull has any hook in it? Place a long (3 feet or longer, preferably over 4) straight edge on the bottom of the hull on either side if the cav plate. Set the cav plate to the same angle the straight edge follows. It should be pretty close to an "extension" of the running surface. This will get you to a neutral setting and tweek the plate up or down from there. Down an inch is a lot. Malibu has something similar as an option to play with the barefoot wake. I've used one on my old malibu Sunsetter for slalom skiing and it doesn't take much downward angle to make a difference. I think mine was less then 3/16".

 

Are you using the boat for barefoot or skiing? With your plate set that low I would think it knocked a few MPH off your top speed if you barefoot.

[Greg Lukin]

Wayne - that is exactly what my thinking was - thanks for that - I'll get it set to roughly 3/16" and see what that does to the handling of the boat.

 

Top speed isn't a problem - especially once we get the right prop - with the PCM 450 engine there is plenty enough power down low and at top end.

 

We tried a 13.25x14 four blade and it sucked air causing cavitation. Now we are going for a 12x14.25 with a bigger cup size - 3 blade - (sorry don't have the Acme codes with me) hoping that will give us a max rev of 5500 and still plenty of hole shot.

[GregHind]

Greg, stick with it. That will be an awesome boat. That motor should be brilliant in there!!!

 

Not sure why you'd drop down to 12" diameter if you are having cavitation problems. It will probably be much worse. That motor packs so much punch it should just overcome a 12 by 14 and spin to the rpm limiter instantly. Increasing pitch will increase bite/traction. You have a light boat and a very stout motor.

 

I would see what prop people run with that same engine in Nautiques, Mastercraft and Malibu - or even Sanger. Especially Sanger (google ABC barefoot forum and ask there). Your boat is lighter but if you go with the same model prop but higher pitch it should have enough traction.

 

Your engine is particularly light. It is likely that you don't need as much down trim from the cav plate to get your boat to the right running angle. No harm in playing with the angle of the plate. For reference, if you want, just lift the thing up high out of the way and see how the hull handles. You then know what the two extremes are and can weigh up what kind of compromise in the middle works best for you.

 

Your engine is quite light so you may find you just don't need the plate on your boat.

 

Please post pictures of the boat and also of the plate. Do you have zero off?

 

I'm interested as to why you optioned up to the 450hp engine? Do you barefoot a lot and fast? Understanding why you got that engine would help you achieve whatever your goal was.

 

I want my next boat to have that engine!!

[DW]

@Greg Lukin: what is the boat a Sanger of some sort I presume? You mention the normal engine application is an outboard, perhaps the Center of Gravity with the inboard is too far forward in your case coupled with the cav plate effect on the dynamic pressure?

[GregHind]

http://www.customskiboats.com.au/#!__boats/protege

[Greg Lukin]

Greg - thanks for that. Got the prop size wrong. We are going to trial a 13x14, 0.135 cup not a 12 diameter.

 

The cav plate is "not" adjustable. I'm not sure how they have set it but I presume they use a packer of some sort to create the desired angle (I'll get some photo's soon)- to do that we need to undo the bolts and change the angle and then bolt it back up. Also the rudder goes through the cav plate so that bush might need adjusting if we change the angle. It's not a 5 min job and since it's new I'm not keen on adjusting it myself until after I talk to the guys in the factory about how to do it without breaking things - problem is they have closed the factory for another couple of weeks so I need to be patient and wait (although I have been close to pulling it off and seeing if I can adjust it - it should be easy)

 

We have gone the 450 - why? We just thought get the latest and greatest as we tend to keep our boats for a while. We certainly foot a fair bit but my main aim over the next summer is slalom course work - yes it will have zero off as soon as it goes back to the factory - it was an option we ticked but they didn't get "time" to install it before they closed the factory (quite annoying).

 

I'm confident the boat will be great once we iron out the issues. Cav plate adjustment, prop selection and installing zero off should make a lot of difference.

 

Cheers

 

 

[Wayne]

I just did a Google image search for "custom ski boats protege" and found a picture of one. Your prop and rudder is similar to a Moomba boomarang except those don't have a plate that extends the running surface.

 

How is that plate attached to the hull? Looks like it bolts to the bottom of the hull. Interesting set up though.

[Greg Lukin]

The plate is attached with three hex bolts through the running surface of the hull - that small part of the hull has been molded to fit the plate flush to the running surface - I'll get pics which will make more sense.

[Greg Lukin]

Photos as promised

[Greg Lukin]

 

[Intheday]

I am not sure you will get much change. I have heard that these boats have very flat wakes though quite wide but not to good a turning circle

[Wayne]

Does the manufacturer offer any pieces so you can adjust the cav plate?

 

If not, it can be done but will take some custom pieces. Is the cav plate flat or is there a bend at the transom? Also what does the upper support for the rudder shaft look like? I'm also trying to figure out if there is a shim between the cav plate and the hull. It's hard to tell from the pictures.

 

If the upper support for the rudder shaft is adjustable front and back that will help accommodate changing the angle of the cav plate. If not a wedge shim can be made or there are some spherical bearings you could use to replace the existing bushing to allow for different angles on the rudder shaft.

[Greg Lukin]

Hi again,

 

Interesting prop testing yesterday.

 

Tested a ACME 13x14 3 blade - WOT rpm was 5600 - good.

 

However 1 sec of ventilation/cavitation occurred only when we flattened the throttle for hole shot test - which was the same as the previous prop (13.25x14 4 blade) - but not as bad. Remember the gap between the bottom of the hull and the prop was only about 6mm hence why we went down in diameter - which worked better but perfect.

 

HOWEVER - when we had someone sitting at the rear of the boat there was no ventilation/cavitation?? AND the boat turned beautifully like a boat should - obviously lifting the nose of the boat helped - so that means the cav plate must be the cause of the poor turning.

 

So now my thoughts are that I hold onto this prop and adjust the Cav plate and see how it performs then. The only other prop is a 4 blade 12.5x15 - not sure I really want to drop that low in diameter but worth a test I guess.

 

Wayne - the rudder shaft from my limited knowledge looks to go through a nylon bush with three bolts holding it down to the cav plate - I "should" be able to simply loosen the rudder safety collar and loosen the bolts and then move the cav plate. I think your right that the cav plate has a shim creating the angle - I want to remove the shim and take it back to flat but I just don't want to stuff it up - the guys are back next week so I'll be patient for now and get them to do it and then I'll just ask them how to do it in the future so I can play around with different angles etc.

 

There is light at the end of the tunnel!! thanks for your help so far everyone.

[GregHind]

Greg when you say cavitation, what rpm does your engine spin up to in the first second after you nail the throttle? I would have expected as much as 3500rpm with that engine. If it's up at 4500 or more then your prop is slipping excessively. 3000 to 3500 would be perfect.

 

What rpm is it in the first second then what happens in the next 5? How about a holeshot video showing the tacho and gps speed?

[Greg Lukin]

Good idea Greg Hind - I'll video next time I'm out - didn't really watch the rpm for hole shot test.

 

What actually happens is that in the first second after nailing it - it's fine - it grabs and goes - then a second or so later (whilst the boat is still getting on the plane) it cavitates for less than a second and then it stops and takes off - for the very brief moment it does cavitate id say the revs get up to approx 4500. Hope that explains - in the video I will take you will hear the moment it all happens.

 

We are going to adjust the cav plate tomorrow after work and try and get out for a quick test late. Hoping that will stop the cavitation but it could also make it worse...

 

 

[Greg Lukin]

Ended up working on the cav plate last night - it's now sitting only slightly lower than the hull. Testing this afternoon - fingers crossed it helps.

[GregHind]

Very interested to see how it goes. Your engine is very light so in theory you should need less down trim on your cav plate than standard engine boats. Without looking up the power curve of your engine again it is probably making 350hp at 4500rpm. The prop must still be gripping a lot otherwise your engine would simply hit the limiter.

 

My bet is the change you have made will help a lot. The nose will come up a touch more on holeshot and the prop will be in the water more.

 

I would be aiming for 3000 to 3500rpm on takeoff ideally but you do have a very big engine so it may be best to set it up to spin even 4000rpm on holeshot.

 

While getting holeshot vid please also get wake pictures at 55, 58 and 66kph.

 

What town/city are you in?

[Steven_Haines]

I'd like to see what this boat looks like. Pics?

[Greg Lukin]

I can't upload the video from my phone so I'll try and put it on youtube later. But it's very clear that adjusting the cav plate made no improvement - probably worse for cavitation.

 

Hole shot is fine up till 3000 rpm and then it jumps to 5500 rpm before kicking back in.

 

The cav plate did have an effect - it gave us an extra 200 rpm at WOT - now 5800 (too high prop was 13x14)

 

The cav plate help the bot turn at "higher" speeds - over 32mph seemed to turn "ok" but still not great. Under that speed was terrible.

 

When we loaded one person in the rear the boat turned well.

 

So now its back to the factory for them to sort - my hunch is that they wont take the time to test and fix the handling issue which is frustrating for me.

 

What else could make the boat perform better? Moving the engine back a few inches to get the weight off the nose? Massive job and no guarantee it will work. It might make the wash worse as well?

 

So options are

 

1. Leave it and put up with the bad turning - drop the cav plate slightly more to improve wash a little.

2. add weight (lead) to the rear of the boat - not a great fix but might be the easiest answer - but could increase wash.

3. Try and move the engine to the rear of the boat - big job???

 

Then we have the cavitation issue - Any prop guru's around? I don't really want to drop down in size for diameter but I think with the prop shaft angle we have no choice. Next to test is a ACME 2209, 12.50, 15.5, 0.150 cup, 4 blade - not sure a 450 HP engine should have such a small diameter? but we need to gap under the hull to increase in order to stop the cavitation (well at least thats what I think is causing it)

 

This is really frustrating... oh - and yesterday we couldn't engage neutral - it stayed in gear - pretty sure its just the cable throttle needing adjustment but still on top of everything else it got me cranky.

 

So now it stays in the shed till next week when it goes back to the dealer to sort out -

 

Things to repair by them.

 

Handling issue

install Zero off

fix bimini - flapping around

move mirror to windscreen mount (not bimini)

hot water shower install

fresh air exhaust install

throttle cable adjustment

engine cover insulation

 

and a few other minor things

 

rant over....

 

[Dacon62]

In one of the pictures it looks like there is a fresh water intake immediately in front of the strut base??? That is an unusual location for it. Could it be causing enough of a disturbance to contribute to the cavitation? Also I measured my Malibu Response TXi prop to hull clearance and it is approx. 40 mm or 1.56 inches. Your clearance at 6 mm seems extremely tight. I would experiment with a smaller diameter prop and see if the increased clearance reduces the cavitation. Failing that relocating the fresh water intake would be the next thing to consider.

Was in OZ in 1989 and loved it. Would probably still be there if my wife didn't want to be closer to her family. The hull design looks similar to the proven and popular "planked" bottom design that was being used by Flightcraft, Successcraft, etc at that time. Hope you figure it out. Will keep watching this post to see how it turns out. Cheers!

[Greg Lukin]

@Dacon62‌ - come back for a visit I got a nice little course you can have tow through.

 

We will get there - It will take some time though - just getting into summer so its frustrating. I'm still recovering from a broken foot - the ski gods are against me !! lol

 

cheers