So you want more horsepower, who doesn’t? But this is why an increase in top end power is actually a very bad idea in a boat.
You’re off shopping for more horsepower. You buy a set of higher compression pistons, a ‘lumpier’ camshaft, a set of roller lifters and higher ratio roller rockers, better valve springs, and a bigger carb to feed the beast.
You spend many dollars on getting the block machined and maybe even splash for a new set of vortec heads. If not the vortec, you have the old heads machined, ported and polished, and a single plane manifold to squeeze out the last horsepower that engine can produce.
You’ve assembled the engine with all the meticulous care and cleanliness you could muster, and now it’s in the boat. The drive is on and you have to select a propeller. And here’s where reality takes a great big bite.
Engine power is a trade-off. For a given engine capacity, without adding forced induction, increasing the maximum output usually entails 2 things. Firstly, that extra power comes on at a higher rpm. Secondly, that extra top end horsepower is at the cost of low- and mid-range horsepower. Ultimately you have produced an engine with a SMALLER usable rev range. Not only that, because there is SO much power at the top end, you need to increase the propeller pitch to keep the maximum revs in check, and that adds more load to the engine throughout the entire rev range, not just at the top. So now you have an engine producing LESS power low- and mid-range, but being asked to push a bigger (more pitch) propeller.
So, ignoring all that, you change the prop from a 17” to a 21”, because that’s what the calculations say the engine will need to inhibit over-revving. Let’s go back and ask why you wanted more power… Because the boat wasn’t getting out of the water quick enough, or you wanted to reduce the mid-range cruising revs.
So there you are with your shiny new high power mill ticking away. Everything sounds great, and you drop it into gear. The boat starts moving and you, with a massive smile on your face, drop the hammer. With all that extra horsepower you’re expecting it to launch like a Polaris missile. As the engine struggles up to 3,000rpm far slower than it ever did before, you’re wondering what you did wrong. Where did you mess up? Is the timing out, is there not enough advance, maybe the fuel mixture’s too weak? Well, the answer is ‘none of the above’, you just messed up the power and torque curves, and didn’t understand how engines in boats actually work.
Why your boat is now even more of a dog than before is actually all contained in the above, but to break it down to the simplest, UNLIKE A CAR, in order to get to the top end horsepower, you need to move through the low- and mid-ranges first, and you produced an engine with LESS power in those ranges, and then to kneecap it completely, you added a steeper prop! Just like trying to take off from the lights in 3rd gear.
Let’s face it, 90% of boating is done in the mid-range, and those skiing really want quick acceleration. Building up a fire-breathing dragon is not the way to achieve those targets. For better acceleration, and better mid-range cruising, you actually want an engine with less power at the top, and more power low down. Marine engine suppliers worked for a long time with the goal of achieving the best possible power package with the most usable rev range, and that’s why a marine 350 doesn’t make as much top end as most people think it should. Yes of course a 350 can be built up to make in excess of 450hp, but it’s all at 6,000rpm+, and the low and mid-range are rubbish! Completely useless in a boat.
The only acceptable way to achieve the goal of ‘more power’ for better acceleration and cruising is by increasing the capacity of the engine, and it’s far better to increase stroke than bore.
The alternative way to get better acceleration and mid-range cruising from the existing engine is to drop a prop size. Simple as that. Yes the engine will turn faster for any given boat speed, but it’ll be loaded less (so the throttle won‘t be opened as far for the same rpm), the load on the drive gears will also be less, and you’ll experience less rpm variation between running up and down the ocean swells.
So, the next time someone says “I need more power”, ask them why, then smile quietly and understand that they likely know less about how engines in boats work than you do.
Chris….
. Exactly my issue on the campion. The way she has been set up, all the power is above 4000 rpm. So despite being a rocket ship with lots of pitch...in her quickest form, she is almost unusable down low and terrible everywhere but top end. So 99% of the time, she runs a pitch comprise that will always hit the limiter...but much happier cruising and getting by at acceleration. Not worth having her propped to full on top end, only to suffer everything else. I only rarely do it to prove a point 
). I plan on doing a read on the camshafts when I reinstall them, and I'll publish the specs right here first. 
