I'm looking to buy a SIB, probably Zodiac Futura, around 4m in size...
The Mercury/Mariner 25 HP will be fine for a futura 3.80; 30 HP for a 4.20, that I know 4.20 uses long tail engines, will you be using a trailer or placing and removing the engine every time your boat is launched and retrieved from water. A big difference that your back and skeleton will surely appreciate.
If your boat is 4.20 or larger there is this nice 2 strokes C-40 HP option, apparently sounds very heavy and powerful to use, but no, is the Tohatsu Compact light version of the 40 HP standard 2 strokes engine. 4 strokes weights 10 Kg more compared to the same 2 strokes same version engine in the portable range.
This Compact 2 cylinder version is only 493 cc with 58 Kg compared to the 25/30 HP version, which has 430 CC and weights 52 Kg. Just 7 Kg more weight and 63 CC of extra power. 2 strokes engines consumes more fuel than 4 strokes, but my motto is, Family diversion is priceless…
We are using this engine configuration for 4.20 & 4.60 Sib boats with nice boating results. On the other hand Zodiac Futura models have 2 small inflatable keels like tubes under the main tubes for better stability, better flotation and more engine weight & speed increase, so will tolerate this engine configuration well. If out of budget a 30 HP will be a optimum election, don’t go for less HP, specially if your boat will be family leisure loaded.
Other good priced option alternative will be a Quicksilver 4.30 Hypalon model with a Tohatsu C-40 engine configuration, much cheaper than pricy Zodiac Strongan Futura model.
Tohatsu manufactures Mercury/Mariner engines in Japan with their own logo and colors in the range of 2.5 to 50 HP 2 strokes engines, The only big difference is that Tohatsu is better priced, If you already are going after a Mercury/Mariner brand, see if you can find Tohatsu in UK, used or new and have a look at the C-40 HP alternative.
2 stroke engines consumes 10% of it’s HP at full throttle, example a 25 HP engine will consume 2.5 gallons per hour at full throttle. You can adjust slightly the steering friction bracket nut clockwise in increments of 5º to have a heavier steering load on your tiller handle if this issue bothers you, the idea is to have a better comfortable/controlled tiller steering specially with bigger HP engines. The ideal adjustment is not tight, not loose
A good cheap alternative will be a flat pontoon trailer for launching and retrieving your boat and an engine dolly for transporting, flushing and storing your engine in a vertical position. See attached practical photo examples.