Re: 4.3 in a 20 ft boat...enough hp?
It is not so much the amount of HP as it is how you use it. Additionally, HP is rated at near peak RPMs (4800 to 5000 for example). Most boaters are looking for more HP to help them get "out of the Hole", up on plain. This occurs at the 2300 to 2500 RPM range. The additional HP of the larger engine is less than half of the "rated HP" differential. For example you may have a 4.3 V6 with 200 HP and a 5.0 V8 with 240 HP. The differential is 40 HP at peak RPMs but at 2300 RPMs the differential may be only 18 HP. Now add extra weight at the stern for the heavier V8 and the boat is now more out of balance which off sets some of the benefit of the extra usable HP.
Go for balance with reasonable HP!
The hull design dictates the boat performance as much if not more than the power plant. There are many considerations when choosing a boat, it's design, and intended use. If you intend to pull skiers, don't buy a tug.
Once you have chosen your boat and assuming it serves your intended use, you can still alter the hull design to improve performance, and handling. Actually you can do this quite easily with Trim Tabs. At slow speeds they can be deployed down to create extra lift to get "out of the Hole", when cruising they can be retracted to improve speed and etc. Trim Tabs modify the hull design to coincide with the boat and sea conditions.
If getting "out of the Hole" is your prime consideration, think about this. How much HP at 2500 Rpm's is used to climb a 15 to 17 degree grade ( "the Hole"). Also think about this; when driving in the mountains you notice warning signs that indicate "Steep" grades of 6 degrees. Your asking your boat to climb 15 to 17 degrees, and do it quickly. If your boat could talk I don't think you would like what its saying nor the place where it is suggesting you go. Hint, it is very hot there.
Let's say you could reduce the grade (hole) to 6 degrees or less. How much HP would then be available to get "out of the hole" or now we might call it a "bump".
Lest look at it: Hypothetical
Engine option 1 (4.3 V6)
Rated HP at 2500 RPMs is 100 HP (1/2 the peak HP)
16 Degree grade requires 80% of the HP to get "out of the hole" onto plane
Total required to get on plane is = 80 HP
Engine option 2 (5.0 V8)
Rated HP at 2500 RPMs is 120 HP (1/2) the peak HP)
16 Degree grade requires the same 80 HP
(this does not account for the extra weight of the V8)
Net gain in HP from the larger engine is 20HP
Option 3 (4.3 V6) with trim tabs
Grade reduction in hole shot = 6 degrees or 60% less
Available HP for forward thrust = 48 HP (60% of the 80 HP)
However, bigger and more powerful engines are sexy. Trim Tabs are not!
