Quite a lot of people use those white aerials, but I've broken one on a boat similar to your Shetland - there's a lot of weight and leverage on the plastic base.
The raider type seem good - allegedly designed specially for RIBs. Ours is just a plain steel whip type and we get really clear reception. They used to suffer from failure due to fatigue at the base (if you had looked behind when we were at speed around St Catherine's Point last month, you would have seen how much wobble the aerial gets) but the new versions have a plastic coating over the lower half to damp vibration. Our current one is two-and-a-half years old and going strong.
VTronics web site is down at the moment, but I think this is the model: http://www.force4.co.uk/ProductDetai...4-6af131974522
I'm sure I paid less at Shamrock though!
Here's the "Raider": http://www.force4.co.uk/ProductDetai...egoryID/119/v/
If my radio aerial theory is correct, the straight aerial should out-perform the coiled one.
We also carry a spare aerial like this: http://www.force4.co.uk/ProductDetai...4-6af131974522
Or at least we did until I lent it to Andy Stevens!