My geuss is it was antifouled in the slings on a pre sale haulout .....what material is the rudder made from?

The hull probably has been coated up in a copper based antifoul. The saildrive is always an issue as it must be coated in an aluminium based antifoul, so as not corrode the leg.
I amongst others, have not come across anything durable as yet that performs remotely well.
Volvo did have product in a spray can intended for sterndrives that was brilliant, but was taken off the market 3years ago.

Perhaps they(previous owners) may have used the same product on the rudder as per the leg that maybe a reason for the excessive growth there.

The prop looks to have been done in Propspeed which performs pretty good and hence why it's not bad.

Nice hull profile by the way. What brand of yacht is it?

C.R.

Oh, and I found the hull molds are for sale if you're interested.

http://yachthub.com/list/yachts-for-sale/used/sail-monohulls/northshore-370-369-building-molds/132349

FR,
make sure the boss doesn't go up forward when on the slip like that, I'd like too see a prop under the bow somewhere.

I didn't have much time with it on the slip so it was a fairly quick job.

First used starbrite medium rubbing compound paste - mostly easy rub on rub off by hand and did a good job on stains/marks/oxidation

Then starbrite premium marine polish liquid - very easy rub on rub off by hand.

I did first try using a 3m polish wax and power buffer but it was taking too long and I found it hard to get a good finish.

Ideally would have liked to get a second coat of the starbrite polish but ran out of time.

sunlight would shine on the rudder more than the underside so maybe its encouraged growth?