When it comes to performance boating and efficiency in general, it is known that the less lower unit in the water -- the better. In actuality there is a sweet spot where the prop, lower unit, and water pickups are in harmony. This is where the boat is it's fastest and flows most efficiently through the water.
"THE SWEET SPOT"
This sweet spot is typically just above the nose cone during normal cruise conditions and slightly below during wide open throttle runs. Mercury has developed a lower unit specifically catered to this sweet spot called, "Sportmaster Lower Unit". This lower unit features a sharp, streamline design with water pickup only on the tip of the nose cone. Though this style is the fastest, it has it's disadvantages on heavier boats in rough water or heavy loads.
THE DIFFERENCE BETWEEN SPORTMASTER + HD/STANDARD LOWER UNITS
The more common, "Standard" or "HD", lower unit is more robust and larger when compared to the "Sportmaster" style. This provides much more stability is changing sea conditions, netter ability to plane, and overall durability in the long term. This style lower unit is primarily found on majority of outboard boats and even our Yellowfin 39 with four Mercury Verado 350 engines. This style lower unit features different style wing plate, wider profile, and both side/nose cone water pick ups. Though this is the most common lower unit on outboards, it has it's downfalls when running at optimum height. Sometimes if used on stepped hull boat, padded, or boats with pocket -- the side water pick up holes can ingest air. This air pocket starves the engine of clean water and can cause low water pressure, which leads to the engine overheating.
This issue is very common. Some boat manufactures do this at the factory before boat is delivered to customer. Our USP Marine Tuning Performance Lower Unit Plug Kit provides all hardware + tools required to plug the side holes of the lower unit -- fixing the aeration issue and allowing the lower unit to ingest clean water for proper cooling.