115HP intuitively seems light to me.  I often look at boat/engine packages and think the engines look too small.

Personally I like to go toward the upper end of the recommended HP range.

As boats get bigger they proportionally , within reason get heavier...also tend to have bit more gear, under floor tanks and bit in pieces. and extra person.
Also we are talking plaining hulls...not displacement, where length is an important factor ... Plaining hull, once up on the plain tends to it on a small area of the rear 1/2 of the hull...sometimes they have a flat bottom, others a curved V and a few other differences, but basically all modern trailer boats have a very similar performance on the plain...crouches formula constant between 195 and 205...and that constant increasing slightly as speed increases.

Most 5.5m cabin boats.. from 80s sea nyph commander to modern rayglass/ buccaneers, and even modern ally  (now have heavier plate) come in around the 1200 to 1350 kg fully loaded... for good economy thru to good performance, cruise speed etc  a 115 min power thru to a 140, with 150 being a little over powered. Well propped a cruise 3800 to 4K 32 to 37 mph

5m boats one is looking at around 1000 to 1250 kg... full gross weight on the water...so looking at 90 to 115 hp.

If wish to go larger  6m getting up to  the ton and over (jump to things like 4 ppl comfortable, 110L tanks to 120/ 150L, bigger live bait tanks etc so very min 175 hp to 200 hp.
Well propped would have a cruise around 3800 to 4000 @ 28 to 32 mph

There are also significant differences in how the same hull performs and rides in chop , wakes, rough water and bars , and on flat water...between a min powered and well powered. Which also gives impressions that certain hulls 'ride better' beyond how they actually have.

So yes there is a ball park for good reason to relate length to hp range.

Looking for a 5.5m boat but 80℅ of them have around 115hp. Doesn't seem enough for sea conditions let alone skiing?
 yes you are correct, and thats why the previous owner of our boat sold it and got a newer better powered rig.. the wake/ sking for family/ friends.

Suggestion:
 you have a boat that is top/ maybe little over powered, and good engine (???). So that engine would sit well on the back of a 5.5m. why not look around for a good 5.5m boat, thats under or min powered (90 to 115 hp ) ?
 Swap out the engines and flick the old boat and 115 hp engine.
 It may take 6 or 12 months to find a suitable boat thats priced fairly to the market to sell...  They dont last long on the market, usually 2 to 4 weeks at most. The others.. like outboards tend to be on the market for 6 to 36 months
 We have just done the reverse ... got a bigger engine at top end of fair market price and sold off the old one at middle fair market price.. net just over couple K for the upgrade.

My 6m (heavyish) alloy boat goes well with 2 x 70HP.  Offensive amount of hole shot, easy 25 knot cruise and ~35 knot top end.

X2 Steps though if happy with a small motor fine but I have seen and experienced difference of more power on a boat from min to acceptable and it is amazing.

For what its worth, my Buccaneer 565 (550 by a newer name) has a 150 4 stoke. Its overpowered. WOT is verging on dangerous, and it is a heavy motor. But it pulls me up on a single ski (no easy task) and for most fishing trips is not even trying, so relatively economical. If I was buying new, I'd put a 130 on and am confident that would do the job. 115 on 5.5m is okay for skiing if you are young fit and light. I am none of those.

RaggedJoe wrote: For what its worth, my Buccaneer 565 (550 by a newer name) has a 150 4 stoke. Its overpowered. WOT is verging on dangerous, and it is a heavy motor. But it pulls me up on a single ski (no easy task) and for most fishing trips is not even trying, so relatively economical. If I was buying new, I'd put a 130 on and am confident that would do the job. 115 on 5.5m is okay for skiing if you are young fit and light. I am none of those.

My 6m aluminium boat has a 115hp motor, it cruises easily at 4000rpm doing 20-22 knots. This is a safe speed for the usual sea condition I fish in. It does a max of 30 knots if I open it up. At cruise it uses around 18 litres per hour. My boat is heavier than most but it's not a hardtop so not as heavy as some. I cross a bar every time I go out and it has more than enough to keep me out of trouble. It is possible to go up to a 150hp with my boat but it is perfectly balanced as it is and it's well within my speed, comfort and economy expectations. Seriously a 200hp would just be stupid and totally uncontrollable in anything but flat water. I would rate my hull (Its a White Pointer) as one of the best sea boats for its size and it's certainly not underpowered.

My 6m aluminium boat has a 115hp motor, it cruises easily at 4000rpm doing 20-22 knots. This is a safe speed for the usual sea condition I fish in. It does a max of 30 knots if I open it up

Rem my question above? expectations one expects from the boat?
 Other who say travel out to channel on a regular basis .. day trip... around 6m boat will cruise around the  32 35 mph with a well propped boat... different expectations.

On those numbers gross boat weight would be about 1200/ 1300 kg and if a a  V4 type, merc/ johnson/ evenrude  with around a 1.86:1 gear ratio and WoT in the 5000 to 5500 mark around a 15 or 14 pitch prop...
 At cruise you would be using around 60 hp of approx 75/ 80 available at the prop at that rpm... which still gives you a good 20% in reserve....
Basically well set up to meet YOUR EXPECTATIONS of 21 knotts (about 24 mph cruise)

 Assuming the numbers (Weight etc) are correct..drop a 150 hp on a and WoT goes to about 40 mph  cruise @ 4K about 30 mph... about the ball park as a slightly over powered 5.5m fully loaded gross weight cabin boat