seaboy, I am not really poking fun at you, just the boat you seem to want does not exist. Thirty miles offshore is a long way out. There are those who don't go that far out in much more substantial vessels than one that can be rolled in a closet. Don't misunderstand, I love inflatibles and RIB types but as they get into the 16 foot plus range they cost as much or more than a good glass boat or certainly no cheaper. They are also a very good choice for offshore.
I have had my 14 foot Novurania about 10 miles offshore--I think. There is no rule of thumb other than common sense.
OK, I am looking in my West Marine catalog, the Zodiac Pro Open eighteen footer with Merc 90 could carry you that far offshore safely with good conditions, price listed, 25,499 dollars. Looking at the Pro (not Pro-Open) it runs for a bare hull for the seventeen footer at 11,499 dollars. That would need a 90 horse engine also,with Yamaha 75 it runs 21,399 dollars, the twenty footer with 115 horse runs 28,699.
For comparison sake, an unsinkable, new, Boston Whaler Outrage 190 with 150 horse Mercury engine will run mid thirties with trailer depending on options, more or less. Good used glass boats are easier to find than inflatibles---more of them to choose from.
The Zodiac closest to my Novurania is the Grand Raid MK II GR. It is right at 14 feet and costs bare 7,899 dollars and can use up to a fifty horse engine. The Zodia Bombard series is also intersting at just 4999 for a bare hull for a 15.5 footer. None of these just listed in this paragraph are suitable in my opinion for thrity miles offshore, ten maybe. I also feel a wheel and/or console are needed, I have a wheel in mine.
In theory I could paddle thrity miles offshore in my kayak, I don't think I will however. Maybe I am a sissy but unless I knew a place really well (weather, tides, conditions etc), I would not go that far out in my Outrage 190 without another boat as company.
Just a possibility but you might look around for a used Boston Whaler Montauk. These at 17 feet are suitable dive boats for maybe four divers if you like a crowd and while I have seen them that far offshore under super ideal conditions it is pushing it I think. Montauks really hold their value, buy a good used one, relatively new, use it a few years and sell it for as much as you bought it for--could hapen if well cared for.
Just some thoughts. It is that thirty mile thing with four divers and two sets of doubles each that worries me--lol. Where are you going to put the electronics---sonar--radio--GPS and back up GPS. Where will the battery (dual) and fuel tanks for that distance of a trip go (1/3 to get there, 1/3 to return and 1/3 fuel for emergency). Howabout your anchor rode--most anchors use about four to one scope, 80 feet means 320 feet of line plus about five feet of anchor chain. O2 bottle for medical needs, first aid kit, life jackets? Maybe a shotgun too. Don't poke a hole in it with that spear gun!!!!! Consider getting Sea Tow insurance.
One other thing, roll up boats ride really rough and bouncy and wet, by any comparison to roll up boats my Outrage 190 crusing at 30 MPH (sixty gallons of fuel and approx 240 mile range) slices through the water like a dreadnaught battleship. N
