Ive always bought boats to suit my 4/5 family. One of the smaller ones was just 3.85 and probably the most used by the whole family as it was so easy to tow on holidays. On the lake we towed donuts with the three kids holding on and my wife as the rear view observer. I only took this boat 4-5 miles offshore as its aluminium hull was hard riding in chop, in saying that we regularly dived three adults with scuba gear and six dive tanks. The hull was very open and the flat design made it incredibly stable. This 3.85 boat was used to put the kids 3 person dome tent on while traveling and stopping overnight (came in handy on crocodile locations being able to keep the kids well off the ground in the boat).
A 4.5m centre console I had also did several big trips with the whole family 5 around the country (one clocking 14000km). My position was behind the console with a double cooler box in front which seated two and another cooler box at the back that seated two. The cooler boxes held all our junk and food, once again this boat was used on several Island hopping adventure 80km offshore along the Barrier Reef.
My 5.2m zodiac (recently sold) was legaly able to carry 12 adults but 5 was my max normally heading offshore. The boat I have now is 17ft is legally able to carry 7 adults offshore. The seating is two forward seats and a long bench seat at the stern which can easily sit 3 adults, the hull carries a 150lt underfloor fuel tank and the rest of the underfloor spaces are designed to carry gear, literally nothing needs to sit around and obstruct the deck area.
Something that is being touched on in another thread is hp, which unfortunately many UK boats don't have a lot of for pushing 4/5 people. In saying that there is difference in taking the family of kids to taking 4/5 serious adult anglers. I had 4 adults on my boat all with dive and spearfishing gear on the weekend two of which are over the 100kg (rugby players), the boat was a mess of gear all over the floor!!!!
Often when taking the family you will find the trips are more about exploring than fishing, when you do fish its often just the kids for their limited time of interest.
This was my little 3.85, I also had a 3.85 fibreglass boat which was a weapon of a little boat, they even get used far offshore chasing game fish.
Aluminium 3.85
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Stability was good even lifting little sharks onboard.
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This was the little 4.5 without the cooler box seats in it.
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I swapped the engine to a little F70 which gave me over 300km range from the underfloor fuel tank, this boat was built to a comercial hull standard with 45mm of glass in the hull (brick ****e house).
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Im sure you will be able to find a boat to suit your requirements, I think people are talking more about 2 onboard for hard core fishing.