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Old 27 August 2013, 16:53   #11
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Here's a picture of an alpha one stern drive set perfectly with a bit of trim and the water just under the cavitation plate to give you an idea. Once the height is right you do all your adjustments by trim. Hope this helps.
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Old 27 August 2013, 16:59   #12
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Quote:
Originally Posted by Pikey Dave View Post
No you don't, it's easy. Lower the jockey wheel, block up under the skeg with timber, remove the bolts through the holes in the transom bracket, loosen the ones in the slots just enough to allow the engine to move & wind the jockey wheel down until you move up a hole, simples
Niiiice. Hoping to sneak a trip down to the Menai Strait before the end of Sept...
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Old 28 August 2013, 00:12   #13
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What does a alpha one stern drive has to do with a standard outboard engine ? On any OB you can raise and trim engine accordingly. On the alpha height is fixed, can only play with trim. Sorry are two different engines. Clamchowder, is your trim a hidraulical or mechanical one ?

Right now you have this, check pic, is from a boat with engine sitting too high on transom, same as yours.

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Old 28 August 2013, 00:47   #14
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I don't see a problem here at all. I don't see excessive splashing, and I don't hear cavitation.

Usually, excessive splashing is due to motor too low.

Your cav plate is exactly at the level of the water receding from the bottom of the transom. If you're not getting cavitation/ventilation,then I'd say that it is at the perfect height.

Do you get cavitation/ventilation? If you do, I would definitely not raise it.
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Old 28 August 2013, 02:24   #15
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Guys its not a anti cavitation plate, the correct term is anti ventilation plate.


Cavitation is completly different.
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Old 28 August 2013, 02:36   #16
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Quote:
Originally Posted by Locozodiac View Post
What does a alpha one stern drive has to do with a standard outboard engine ? On any OB you can raise and trim engine accordingly. On the alpha height is fixed, can only play with trim. Sorry are two different engines. Clamchowder, is your trim a hidraulical or mechanical one ?
Not sure I get you here: the trim is adjusted by means of hydraulic rams.

My guess is that the ref to the stern drive assumes that the height of the drive is definitely correct because thats where the builder put it. So if the water runs just under the anti-VENTILATION plate then thats where at ought to run on any installation. It certainly looks to me like it ought to be prone to ventilation at that height. Though dropping the engine would, it seems to me, produce more splash rather than less.
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Old 28 August 2013, 02:45   #17
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It doesent look too high to me, whereever you settle with it, it wont be far from where it is IMHO The splashing is minimal, dont see any coming into the boat ? so whats the problem ?
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Old 28 August 2013, 05:11   #18
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OOOOKay, before this turns into a big barney, Loco has pointed out that it's a tad difficult to lift a sterndrive by a hole. The anti cav / vent thing is, however similar.

There are two thoughts ref. those plates -

1 - the view loco is expressing - is that to make absolutely sure your prop gets no air from above, you need to run the plate slightly below the water. This will drag more bits of metal through the brine (bad news) but will also give the prop more grip (good news).

2) The view most of the other posters here are expressing - is that once on the plane the water scooting out from behind the hull is coming fast enough to fill the gap where the prop has removed it. Hence, running with the engine higher meaning less metalwork in the water & therefore less spray.



Looking at your pic, I would say moving it down is going to need a chainsaw, so unless you are ventilating with monotonous regularity, I wouldnlt bother dropping it.

Moving it up may cause more ventilation, but I had much the same thing (can't remember if I posted pics or not, but they wrere remarkably similar to yours if I did) I raised by 1 hole & improved the handling, spray & fuel economy. But then I spend a lot of my time moving. & it started to "let go" more often. I cured that by upping the prop diameter by 0.5" (it was well over revving anyway) and so I now have a boat that the prop only lets go if I trim out too far and has awesome hole shot (well, comopared to wehat it was like) If I were setting up a rescue boat that was start - stopping & manoevering a lot then yes, I'd consider dropping it back down.

Horses for courses!

As Pikey says, its a dawdle to raise it, having done exaclty that myself.
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Old 28 August 2013, 16:47   #19
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I am not trying to say anyone is wrong here so please do not take offence. I am just giving a little input based on personal experience. We also have a phantom 16 in the family which has a mercury 90 optimax on the back and the best height for it is the anti ventilation plate about 2 inches above the bottom of the boat. It's running a 23" trophy prop. Great hole shot excellent top end and zero prop slip even on full lock. Prop can make a huge difference as well. Stainless will slip far less than aluminium too. As a general rule the higher you can get the engine the more stable the boat will be and give better performance and efficiency.
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Old 29 August 2013, 11:39   #20
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Quote:
Originally Posted by Clamchowder View Post
So if the water runs just under the anti-VENTILATION plate then thats where at ought to run on any installation. It certainly looks to me like it ought to be prone to ventilation at that height.
It's not a static setup. The anti-ventilation plate has most effect at the hole shot, when the boat is stopped and the throttle is run up. At that point, the boat is sitting deeper in the water, and the plate keeps air from being sucked into the prop blades.

At speed, the boat (and motor) lifts up, and hydraulic pressure keeps air out of the prop (the air doesn't have time to penetrate the water between the hull and motor.) While running, the anti-vent plate has almost no effect.

You tend to get ventilation in turns due to additional hull lift, and the asymmetric water flow at the back of the boat, which essentially lifts the prop even more.



Quote:
Though dropping the engine would, it seems to me, produce more splash rather than less.
It will. Too low, and you'll get a rooster-tail of spray to either side of the leg. Not to mention the drag from having too much leg in the water.


Luck;

jky
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