Re: (FlyBoyMark)Transom height v Cavitation plate position
1" above the bottom is fine. The only problem you might come across is SLIGHT cavitation in sharp turns. Most all outboards are setup with their water intakes two or more inches below the cavitation plate. This 1" above setting is the most optimum for speed in smaller boats and HP. Go more than this you will have to change to a heavily cupped prop which is designed to run about the top 1" to 3" out of the water literaly. In addition you would have to modify the lower unit with an auxilliry water pickup. The only reason for DEEP settings on the cavitation plate are boats that use motors for "push" or tug like operations like a pontoon boats, sail boats, ect. They need this deep setting because of the lack of water flow over the lower unit to keep it from sucking air from above. If your boat does not do much more than 25 MPH, raising the motor will do very little for you. In my little 13'10" aluminum bass boat I went 1 " above and gained about 1.5 MPH and 100 RPM with my Merc 40 HP 3 cylinder. Top speed is now 38 MPH on my stock 40 HP. Keep in mind my boats all up weight is 600 lbs without me in it. You probably should have stayed with a short shaft or 15" motor leg and save your self a ton of work unless you're planing to use this motor on another boat that has a 20" transom. Either run the cavitation plate 1" below or 1" above. If you try to run the plate even you will get erratic performance out of the boat as far as speed with diffferent loads and possible porpoising problems. STANDARD rigging procedure is to set the boat up initially at 1" below, then move it around from there. Most of the time you can jump rite to 1" above with no problems...jus' try it, your water ports will be covered...... Top speed = rated RPM @ given HP with proper prop pitch. Example: My 40 HP is rated 4500-5500 RPM but the HP rating is 40 HP@ 5000 RPM. You want to target for example: 4800 RPM for a very heavy load and 5200 RPM for a very light load. This way you are within your envelope for torque and HP. Try not to prop for to low an RPM, always better for a little higher RPM long as you do not exceed the red line limit if you do not have an rpm limiter like the Mercs.
