OD cavitation plate relationship to the bottom of the hull?

[Boomyal]

Is there a definitive relationship of the outdrive cavitation plate to the bottom line of the hull? In other words, is there a range of distance above or below the hull line that is considered ideal?

 

[HT32BSX115]

Re: OD cavitation plate relationship to the bottom of the hull?

Is there a definitive relationship of the outdrive cavitation plate to the bottom line of the hull? In other words, is there a range of distance above or below the hull line that is considered ideal?

Evidently not.

If you look through the Mercruiser installation manual,

http://www.boatfix.com/merc/Install/gas/86017211.pdf

You'll notice they talk mainly about the "X" dimension and only allowing small adjustments to it. If one deviates too far from recommended dimensions, you can aerate the (cooling) water that would be picked up by the drive and cause possible overheats.

It doesn't appear to be related to how the cav-plate is positioned in relation to the hull at all...

 

[Bondo]

Re: OD cavitation plate relationship to the bottom of the hull?

Is there a definitive relationship of the outdrive cavitation plate to the bottom line of the hull? In other words, is there a range of distance above or below the hull line that is considered ideal?

Ayuh,.... Generally speakin', followin' the X-dimension listed, ya end up just like an outboard, with the cav-plate even with or up to 1" above the keel-line,...

 

[HT32BSX115]

Re: OD cavitation plate relationship to the bottom of the hull?

Ayuh,.... Generally speakin', followin' the X-dimension listed, ya end up just like an outboard, with the cav-plate even with or up to 1" above the keel-line,...

Could it also be more related to where the water pickup ports are in relation to the waterline? Which might also be the reason low water pickup drives can be somewhat higher than "regular" drives. Cavitation could become more a problem then....especially in a turn...............

 

[Boomyal]

Re: OD cavitation plate relationship to the bottom of the hull?

Well, if I take a straight edge and lay it along my cav plate, sliding forward til it touches the transom, the contact point is about 1/2" above the bottom of the hull. I had never thought of this before but when I redid my stringers, a few years back, I would have had the option to cut them down, if necessary. It's really too late now but I still thought it would be good to know.

 

[Don S]

Re: OD cavitation plate relationship to the bottom of the hull?

1/2" above the keel when the cavitation plate is parallel to the bottom of the boat, is where Volvo, Merc, and OMC drives ended up if you used the base X dimension for the particular transom angle.
It's all about the performance of the boat, nothing to do with the water pickups.

 

[HT32BSX115]

Re: OD cavitation plate relationship to the bottom of the hull?

It's all about the performance of the boat, nothing to do with the water pickups.

It must have something to with the water pickup doesn't it? (if only as a limitation)

It's clearly something for boat manufacturer testing though.

During testing,
X-Dimension should be increased 1/2 in. (13 mm) at a time until desired performance
is achieved...............

There's no way a boat owner could make 1/2" adjustments during a one-off installation unless he wanted build an "adjustable" transom mount point in the boat. Or replace and/or recut the transom every 1/2". That would be pretty labor intensive....

From the Mercury Installation Manual:
http://www.boatfix.com/merc/Install/gas/86017211.pdf

IMPORTANT: The following instructions will provide a sterndrive unit mounting
location that is suitable for most boats. Best mounting location for a particular boat,
however, can be determined only by testing.
1. Below 25 mph (40 km/h): Subtract 1/2 in. (13 mm) from X-Dimension.
2. Heavy Duty Applications: Subtract 1 in. (25 mm) from X-Dimension.
3. Above 25 mph (40 km/h): Use X-Dimension shown.
4. Above 50 mph (80 km/h): The X-Dimension can be increased to improve performance
in some applications. However, pulling power for skiing will decrease. During testing,
X-Dimension should be increased 1/2 in. (13 mm) at a time until desired performance
is achieved but in no case should it ever be increased by more than:
Alpha
Bravo One and Two
Bravo Three
Maximum Increase In X-Dimension
2-1/2 in. (64 mm)
3 in. (76 mm)
1 in. (25 mm)
Extreme care should be taken when raising drive unit to ensure that water supply does not
become aerated. Use clear water inlet hose to monitor incoming water and monitor engine
temperature gauge to ensure engine does not overheat.
In applications where cooling water to the engine is supplied by a through the hull
or through the transom fitting, the sterndrive height will not cause cooling water aeration.
IMPORTANT: Damage to Mercury MerCruiser products caused by too high of an
installed height will not be covered by Mercury MerCruiser warranty.

 

[Don S]

Re: OD cavitation plate relationship to the bottom of the hull?

I think you are taking the Extreme care sentence out of context. That was for when using the max allowable increase in drive height.

 

[emilsr]

Re: OD cavitation plate relationship to the bottom of the hull?

It must have something to with the water pickup doesn't it? (if only as a limitation)

It's clearly something for boat manufacturer testing though. There's no way a boat owner could make 1/2" adjustments during a one-off installation unless he wanted build an "adjustable" transom mount point in the boat. Or replace and/or recut the transom every 1/2". That would be pretty labor intensive....

From the Mercury Installation Manual:
http://www.boatfix.com/merc/Install/gas/86017211.pdf

Owners of Bravo drives can use these to adjust drive height: Drive Spacers : The Bravo Shop - [url]www.bravoshop.net, Your Source for Bmax, Bravo & Mercury Marine Outdrive Parts[/url]

If you're doing that kind of "tweaking", you generally set the drive height on the high side, then space down to the optimum level. Bravo lowers come in standard height as well as 1", 2" and 3" shorter than standard (the 3" shorties don't usually have an anti-cav plate).

As for water pickups, they make a dual water pickup lower that has a water inlet in the front of the torpedo. When/if that's in aerated water you then need to use a raw water pickup either thru the hull or on the transom.

To the OP; the "proper" drive height will depend on the boat and it's particular performance characteristics as well as the type of prop you intend to run (which also depends on the boat's performance). This is one of the areas where the outboard crowd has an advantage; they can set the drive low for coming up on plane, then raise the height using a jack plate as the boat's speed increases and there's a need to get more of the drive and/or prop up out of the water. For the rest of us it's usually a compromise of speed versus planing ability.

 

[HT32BSX115]

Re: OD cavitation plate relationship to the bottom of the hull?

I think you are taking the Extreme care sentence out of context. That was for when using the max allowable increase in drive height.

Maybe.

But I read that as meaning don't install the entire engine drive combination so high that you get the water pickup into aerated water. (which you could mitigate by using the above spacers......but wouldn't that negate the improvement by pushing the "lower" back down further into the water?)

I also read it as meaning you *could* possibly get performance improvements at higher "settings" , but you would probably need thru-hull water pickups to prevent the cooling water aeration that could (or would) occur.

I am also guessing that in most cases, no one would deviate from the recommended X dimension in most recreational boats. AND for the OP, the installation manual doesn't mention (or at least I couldn't find it) any relationship of the cav-plate to the bottom of the hull.

In addition, I measured all this STUFF and read through the installation manual very carefully when I removed the Cobra and installed the Bravo in my Four Winns. The crankshaft and prop (shaft) centerline are pretty much identical comparing the Cobra to the Bravo so nothing changed when I R&R'd it all.

I didn't relish the idea that I might have had to change the dimension (which was exactly where Mercury indicated it should be....so I didn't touch it!)