cavitation height?

[phoenixgold2100]

I have read some people say to have the prop shaft 3.5" below the and I have read some people say to have the anti cavitation plate level with the hull of the boat or two inches higher.

So what is it? Because If I have my prop shaft 3.5 below the pad the cavitation is no where near the bottom of the hull.....

 

[Sea Rider]

Re: cavitation height?

What's been stated on most engine owners manual is just a refference from where to start, really there's no difference if cav plate is sittting one or three inches over or under boat's bottom, because each boat has a different hull type and not all brand engines have same lower leg configuration. Best engine height transom match will only be achieved under trrial and error.

If yo want top boat/engine performance, go for a wot spin on flat glassy, no wind sea with boat as usually loaded, trim engine to have a paraalell boat running with respect to water level, have a back transom spotter check where's water flow passing throught lower tail, If it's passing slightly under small upper plate you have dialed the sweet transom/engine height spot.

If water flow is passing slightly above anticav plate, will need to raise engine accordingly with wooden shims to achieve flow passing under small upper def plate. Once you meet this condition engine will rev at it's max with minimum lower leg drag, achieve good hole shot and superb close turns at all water conditions.

I know raisining larger heavy lump engines sitting on transom is a PITA, but if you want top boating perfection is the way to go, you decide.- If possible post a pic of your transom engine rear view to have a look at your actual height setting.

Happy Boating

 

[jestor68]

Re: cavitation height?

The pad to prop shaft height is for boats with inset pad hulls with the motor usually mounted on a jack plate that sets the motor further back than usual. If the motor is mounted without a jack plate, the height is different.

It has to do with how water rises after leaving the pad or keel. The further back the motor is, the higher it can be mounted.

For boats with a straight transom, the motor/drive is mounted lower because it is closer to the transom/keel. Hence; the AV plate even with or slightly above the keel.

 

[phillnjack2]

Re: cavitation height?

at planning speed the cavitation plate must never have water over the top of it.
when running at max speed get the engine up as high as possible ,but make sure you still have plenty of
water getting over the intakes on the side of gearbox.
if the cav plate is on the water its causing drag and the plate is trying to lift the boat,THIS IS WRONG, the plate should be
just skimming the surface not being used to lift it.

when people put on doel fins they think the fin should ride on the water !!! this again is wrong,it puts pressure on the plate.
the fins are only designed and meant to AID in getting it up.

if your engine bracket has the bia bolt patter )fitted to nearly every engine 40hp and above), then try it on the holes so the
engine is up the highest.
if this gives plenty of water and dont cavitate bad in the turns then this is better for your boat/set up.
But also you can buy different props that will work best at different height !!!!!!!!!

this is extreme but is very true with what ive got for my 60hp rude triple.

12 3/4 x 23 pitch alloy this prop likes to run deep down with engine on its lowest setting (cav plate 1 inch under the boat)
13x 19 stainless prop, this one likes to be with cav plate 2 to 3 inches above bottom of boat.
13 3/4 x 23 stainless semi-cleaver, this prop will only work correctly when the engine is way up and the centre of the bullet
on the gearbox 1 inch below cav plate ( thats cav plate 5 inch above the bottom of the boat)

so props and heights can be a strange thing to get right.
with a straight out the box engine and motor i would go to highest possible setting and then test it out, much easier to
lower the engine a bit at a time with a block of wood under the skeg and someone using the jockey wheel as my crane up
front to raise or lower the transom.

when raising to extremes is when the jack plates come into play, for these its never going to be the average alloy prop, they
just dont work any good full stop.

if that is your boat in the avatar with the jack plate, then 3 inch below pad is fine to start with as long as you have a
nice stainless prop that can run a touch high and water pressure is still good.
if you want to go realy fast then take it so the drive shaft is dead level with the bottom and get a quality surface prop.
BUT just make sure you still have water pressure !!!!!!!!!!!!!!

once the jack plates and good props come into play ,its also where the low water pick ups come into play as well.

jo average just needs no lower than the boat with the cav plave plate or his performance all round will suffer and
expensive fuel is wasted.


what engine and prop is on your boat NOW


phill

 

[phoenixgold2100]

Re: cavitation height?

I have a 17 foot stratos with a pad. 150 johnson I have a jack plate and 14.5 24 raker prop. I have lowered the engine all the way down on the plate and this is the only way I can get any bow lift. it chine walks now at 68-69 mph gps. would rasing it a 1/4 inch stop this or should i keep raising it until i lose speed?

I cannot get any of my buds to climb on the back of the bass boat to look at the cav plate going those speeds and I don't blame them. So I have no idea if the water is over or under the plate right now.

I know that the boat was not getting up on the pad with the prop 4.5 above the pad. and having it lowered gets it on the pad now. just not sure if i should raise it up some more now or not.

 

[Texasmark]

Re: cavitation height?

I have a 17 foot stratos with a pad. 150 johnson I have a jack plate and 14.5 24 raker prop. I have lowered the engine all the way down on the plate and this is the only way I can get any bow lift. it chine walks now at 68-69 mph gps. would rasing it a 1/4 inch stop this or should i keep raising it until i lose speed?

I cannot get any of my buds to climb on the back of the bass boat to look at the cav plate going those speeds and I don't blame them. So I have no idea if the water is over or under the plate right now.

I know that the boat was not getting up on the pad with the prop 4.5 above the pad. and having it lowered gets it on the pad now. just not sure if i should raise it up some more now or not.

Get another bud with his boat to ride along side. At those speeds wake is minimal and no reason you can't get your answer.

On chine walking, probably the more of the keel you have in the water the less it will wallow; course that means drag and you don't want that. I had it happen once on my pad boat but never got to the possible answer as I changed props due to too much pitch (WOT rpms too low) and never got back up to the speed where the boat became unstable.

Might borrow a different prop and see if that helps. A different rake or progressive pitch variance may help. To be honest with you, I was always amazed at how you could take a spinning prop and put it behind a water ski and have any kind of stability without the boat trying to counter the prop torque.....course that is just what chine walking is....moving into and back (reaction) out from the boat countering prop torque.

Mark