Hi Paul,

The foam seals go on the bow thruster shaft above the propeller gearbox and are designed to compress between it and the top of the recess when the thruster is raised preventing water surge forcing water into the hull when under way. There is considerable pressure created as the boat punches into waves. There is a lip seal inside the boat on top of the housing but without the foam seals stopping that surge it would fail early. The foam seals should be changed at the biannual haulout as should the oil in the thruster and the lip seal on the bow thruster  propeller shaft. A lot of SM have water leaks here but with proper adjustment of the lift mechanism to get the right pressure on the foam seals and regular replacement of the upper lip seal there will be no problem.

Kind Regards

Danny

SM 299 Ocean Pearl