Too high?

[Dan Gurney]

Hi all,

I get alot of splashing either side of the leg at speed. It gets worse as you trim it in. I suspect this means the engines in mounted a bit high. You can see from the attached pic that it looks like water is being pushed up towards the lip above the cav plate and then outwards creating the splashing:



...and just to show what I mean about the lip, you can see it here about 2" above the cav plate:


Here's a video taken at the same time as the first pic. The engine is trimmed about neutral here:

Splashing up around he transom - YouTube

Any opinions/advice?

[peterwhite]

Looking at that first picture, your engine is too low in fact hence the reason for all the water spray. I'd try going up a hole at a time till you get the best result.

[A1an]

We had exactly the same thing with our Searider, we lifted the engine 1 hole and although it reduced the spray the prop didn't have the same grip, it would cavitate in tight turns.

We lowered it again and lived with the spray.

[Dan Gurney]

Up you say? But that would bring the cooling water intake further out of the water? It already looks worryingly high to me compared to the waterline at speed. It's obviously pulling in water or the telltail would stop but I'm not sure I'd want it any higher.

[paddlers]

The cavitation plate (the one just above the prop) wants to be almost level with the bottom of your hull when vertical. Water intakes are below it so unless it's extreme they should always be in the wet stuff.

[Dan Gurney]

Originally Posted by paddlers

The cavitation plate (the one just above the prop) wants to be almost level with the bottom of your hull when vertical. Water intakes are below it so unless it's extreme they should always be in the wet stuff.

Yes, I are muppet! Mixing up water intake grills with the the one just above the cav plate towards the rear. Dunno what they are but they damn sure arent where I put the muffs on!

[Locozodiac]

That engine is way too high, lowered it untill water flow passes slightly under small upper deflector plate, that's with well trimmed engine to achieve boat and anticav plate riding near paralell to water level. It's the perfect engine transom height for good hole shot, inmaculate close turns at speed and overall good boating on all water conditions.

Happy Boating

[peterwhite]

The grates you are talking about are just exhaust relief ports. The plate above the cavitation plate is just a deflector the water should run just on the underneath of the cavitation plate when set right. I would definitely be lifting that engine without a doubt. It is just trial and error to get the best balance so you don't loose too much grip in tight turns but then how often are you doing high speed hard turns.

[Dan Gurney]

So it seems that the one thing we can all agree on is that it's too high.... or too low.

I'd need someone with a crane if i was going to raise (or lower) it so it'll probably need to stay as it is for now. If the opportunity presents itself I guess I'll need to experiment. Thanks for the input though guys.

[Pikey Dave]

Originally Posted by Clamchowder

I'd need someone with a crane if i was going to raise (or lower) it

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

[peterwhite]

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.

[Dan Gurney]

Originally Posted by Pikey Dave

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...

[Locozodiac]

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.

Happy Boating

[Projectile]

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.

[bcervelo]

Guys its not a anti cavitation plate, the correct term is anti ventilation plate.


Cavitation is completly different.

[Dan Gurney]

Originally Posted by Locozodiac

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.

[Bigmuz7]

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 ?

[9D280]

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.

[peterwhite]

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.

[jyasaki]

Originally Posted by Clamchowder

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.

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