identification of hull fouling

[C08]

I thought I had seen most types of molluscs and weed on my boat hulls in a variety of areas but the latest has me puzzled. I do not know whether what I am seeing is a mollusc or an excrement left behind. The hard brittle deposits are pure white, about 30mm long in a curly shape and we'll well adhered to the hull, about 3mm in diameter with a round section.
Any ideas, what this is, the area is the top of Portsmouth Harbour?

 

[Hydrozoan]

Tubeworms (keel worms)? https://www.marlin.ac.uk/species/detail/1794

 

[C08]

Thanks that looks like it.

 

[Hydrozoan]

Thanks that looks like it.

My pleasure. Common and widely distributed as the map at that link shows. There may well be other, similar species - I didn't check.

 

[BrianH]

My Italian lagoon marina is infested with tubeworm and requires lifting out every year to clear the critters. Below is first a typical cluster that builds up on anywhere the antifouling wears off and particularly anything metal like my rudder. Next is a closer view of a oyster-shell where they have attached.

The clumps develop because they attach to the shells of others until the build-up is significant. the only minor advantage over barnacles is that they are relatively soft and are easily scraped off. I need to dive at least twice every season to clear my propeller or I cannot proceed out of my berth. In the photograph below I have already done that to reach the crane dock.