Good to hear from you and apologies for the delay in getting back to you.
I find the detail on the Garmin 451S fine and rugged enough too - it's mounted in a quite exposed position no problems, with liberal squirts of water repellent etc on the connectors.
I mainly use it for hazard avoidance, rather than navigation as I know the area. If people were wanting to see their whole panned route and the different islands etc, then a larger screen may be helpful, but then it depends how much you want to pay for that.
Also the 451s interfaces fine with my Standard Horizon fixed VHF radio and the Advansea AIS TR-200 transponder.
That said, I still carry hard copy chartlets (printed on waterproof paper) that I download from Free charts and pilotage
Here's some more information in case you are new to the area (apologies if not) and for anyone else who maybe thinking of visiting:
The Admiralty Tidal Stream Atlas NP209. Orkney and Shetland is also really helpful, due to the strength of the tides we routinely have here, and because in some places slack tide does not necessarily equate to low or high tide, even when only a couple of miles from the tide data station. For example, the other day I was on one island where at low tide in the harbour, the tide was racing past at 7 knots... Slack water was 2 hours later as the tide was coming in. Maybe not a big deal with you have a RIB in terms of speed, but can be critical if wanting to avoid the eddies, standing waves and overfalls you can get here, with tides that at times can be up to 12kts.
Don't let the latter scare you off though - with a bit of planning and preparation you'll be fine.
Another really useful document for up here is the Clyde Cruising Club's North & North East Scotland and Orkney Islands guide Clyde Cruising Club
which is really helpful for both safety and facilities. Well worth the approx £25.
Finally, get a copy of Orkney Harbours Port Handbook - free online here http://www.orkneyharbours.com/pdfs/P...ok-2010-V4.pdf
and in hard copy from the Harbour Offices at Scapa: Orkney Islands Council, Marine Services | OIC Marine Services
. This is also a great website for the weather forecast, details of visiting cruise ships etc.
While that's a lot more than just about the Garmin Chart plotter, all the above will be a real help if you are coming up here, both for planning and as a backup in case of GPS failure etc.
Do also be aware that there are a fair few areas of no/very poor coverage for mobile phones and VHF marine radio; I thus carry a PLB too so that at least a distress signal can be picked up by a satellite system.
Hope this helps