Mercury Mariner 200, 225 Optimax Outboards Service Manual, 90-855348, Page 23Get this manual
High Altitude Operation LOWER UNIT GEAR RATIO (ALL MODELS)
(2 Generally improves rough water ride(3 If excessive, can make the boat veer back-and-forth (bow steer)BOTTOM
Operation at high altitudeabove 5000 ft(1524m)may require changes to the lower unit gear ratioRefer to SECTION "Mid-Section" for removal, disassembly, reassembly and installation of lower unitModel 200 DFI 225 DFI High Altitude Gear Set 1.87:1 1.87:1 Standard Gear Set 1.75:1 1.75:1
1Boat Bottom: For maximum speed, boat bottom should be nearly flat plane where it contacts the water and particularly straight and smooth in fore-and-aft directionaHook: Exists when bottom is concave in foreand -aft direction when viewed from the sideWhen boat is planing, "hook" causes more lift on bottom near transom and allows bow to drop, thus greatly increasing wetted surface and reducing boat speed"Hook" frequently is caused by supporting boat too far ahead of transom while hauling on trailer or during storagebRocker: The reverse of hook and much less common"Rocker" exists if bottom is convex in fore-and-aft direction when viewed from the side, and boat has strong tendency to porpoisecSurface Roughness: Moss, barnacles, etc on boat or corrosion of motor's gear housing increase skin friction and cause speed lossClean surfaces when necessary