You may not have seen the threads but I'd run a 20hp on my 3.8m SIB for the past 2+yrs. Due to the difficulty of handling the OB weight in my own particular circumstances I decided to downgrade to a smaller OB. On holiday we used an 8hp for a while and found close to the exact performance you have... give or take a bit 13kts 2-up.
So comparing with how our outfit was with a 20hp I can say you'll feel a massive improvement in performance. Ours was easily able to plane with 4 folks but for me the most impressive and useful aspect was the ability to jump on the plane with a quick twist of the throttle rather than waiting for it to creep over the hump.
So yes if you can easily manage the 20's weight very worthwhile.
As fenlander says just watch your back correct propping and trim plus weight distribution it will do all you need enjoy post some pics and performance when you sort your rig out.
Plymjack you asked over on the "Aerotec with 8hp" thread how your outfit might go with 15hp. Answered here as it will keep the replies re your situation together.
I looked your SIB up just now and assuming it is the alloy floor seems it weighs about 190lbs. You do very well to get it on the plane at 12kts two up as that's a really heavy SIB for an 8hp. It must have a flattish easy to plane underwater profile or perhaps your crew/kit load is light??
Anyway I'd say a 20hp would be a far better power match to a 4m alloy floor than a 15hp to carry four folks.
But regarding the question of a 15hp and two people I guess if you were getting about the same speed as us with an 8hp then from our experience a 15hp should be around 17-18kts.
As always it's not all about the top speed but the ability to nip on and off the plane at will in a difficult sea. This ability increases going from 8hp to 15hp and is perfect with a 20hp which will allow you to instantly recover speed after slowing down to match large waves.
Thank you Fenlander .. yes it is a flattish underside with inflated nosekeel - I up (100kg) it did 15 knots and went on the plane easily at 6knts... but yes I need a 15 or 20 to make it more enjoyable and practical.
Looking to head out to waves but not till we upgrade the engine.
spy-sea-one
