There alre lots of different style of cowl (Dorade) vents around and I wondered if anyone can tell me the practical difference between the low profile type (Donald) and the normal types (Jerry) on eg this page ?
I would have thought that if there was a likelyhood of sheets or other rigging fouling on the vent then you'd use a low profile style. Another issue could be if a hatch or similar feature was likely to disrupt airflow then you'd use a high profile type. Otherwise it's mainly a matter of style/appearance.
Yes, the higher ones are probably more effective because they are above other obstructions, but are often not practical because they catch sheets, so the low profile ones might suit better. The other difference in the examples you choose is the material. Stainless steel is smart looking, but is rigid whereas the plastic ones may stand up to more abuse - like being kicked or even stood on.
Ive just taken delivery of an 'air-only' ventilator to replace the aged Plastimo one on our pegasus, since Plastimo out of business for the time being and no one has any of their Dorades left in stock I also had to do the rounds to chose a replacement.
My budget dictated plastic - the metal ones are very expensive and a wood dorade box would look out of place, so plastic it was. The choice of high or low profile was straightforward for me, on my boat the vent is on the coachroof so a low profile simply looks better and gets in the way less while still allowing the facility to 'direct' air by turning the funnel. If I had a larger boat where the vent was near bulkhead I would probably have gone for the more traditional high profile shape because of its better airflow.
I'm not connected with these guys in any way but the product does appear to be first class though its slightly dearer than the Plastimo equivalent it is easy to see why. Theres a heck of a lot of bits and bobs to put together so I would be wary of doing it on the water in case I dropped something i the drink. Theres a video on the website that shows how it works: http://www.air-onlyventilators.com