Rolf,

I used the belt sander on the outside of the Alfresco 920 @90 deg to the planks and it worked well. Although the fine cedar dust used to get into the sealed bearings and I went through two sets of bearings in the sander. Using 90 deg probably wouldn't be suitable on thin planks as it rips the wood back pretty quick.
The sander polisher, I used on the inside and it worked great, It had a foam disk of around 7 or 8 inch dia.

Richard,
The belt sander is all very well on cedar planks that are 11mm thick but when you pull out a belt sander to use it on ply hull where the first laminate in the ply is around 1mm thick. I can't help myself.