Not trying to hijack but the -2 sportmaster I bought for mine does not have one on it....wonder if I need to order one??

mostly correct as most all drives come with them cast on, but some like the IMCO SC do not.It has a bolt on one available if so desired.

Look at your lower and a stock bravo lower. yours is 2" shorter correct?? When using a shorty, that add on plate is mounted to the upper, because with a shorter lower the prop (depending on size) would hit one on the lower.

Correct it is 2" shorter....the bravo lower has the splash guard cast into the housing and the shorty does not...I guess my question is do I need to buy one of the add on ones? The lower I have is a sportmaster and not an imco

It bolts to what really is called the ANTI VENTILATION PLATE. You want that above the prop as you do not want open space above the prop on stock sterndrives. The reason and the main purpose of the plate or an extended plate is to eliminate or reduce air being drawn from the surface into the negative pressure side of the prop blades. Thats the main reason for the ANTI VENTILATION PLATE and or the added extender plate that is bolted on from whomever. Usually a IMCO product... Also they can add in planning and reducing those times for jist of it

^^ I agree. I have been running my cat for several years with both sportmaster and Imco lowers without a splash plate. Both Imco and Mercury call it a splash plate.
Boat comes right up on plane and runs to top speed with no issues. This is more of a surface drive application. V-bottom with a submerged drive may be affected differently. I would test without first to see how boat reacts before spending the extra money. It is easy enough to install after testing.

Aye , I'm learning something here . I've never heard of these things before and didn't know there was such a thing that bolted on (extended) the running surface of the ant-ventilation plate. I think that maybe a few extra words could have been added to the original post/question to clarify it .
In retrospect , your reference ... You want that above the prop as you do not want open space above the prop on stock sterndrives. reminds me of the fact that on a B1 the prop is under the 'plate' but on a B3 the plate is extended to cover the rear prop. HOWEVER , exceptions abound ! On the Volvo DPS/a unit the rear prop is not covered by the plate .
More useless info , eh ? haha

Was not talking about extension boxes or surface / near surface props in the mix. .

Early Bravo 3 has an extender plate that bolted up and then later Bravo 3 the extended ANTI VENTILATION PLATE was extended as a one piece unit. Here is the early version of Bravo 3 with the bolt on extender and IT WAS NEEDED. Number 12 in the diagram and number 12 in the scrip tells what it is.

And if I recall every Teague Platuim drive has the bolt on extender that I have seen in person.

MerCruiser Bravo Three Gear Housing (Bravo III) Parts

Has anyone installed anti-ventilation plates on SSMIII? I have a tough time getting out of the hole without blowing out