Originally Posted by MattD
I had a Honwave T40 with a tohatsu 4 stroke and the same problem. 20hp 4 strokes by mariner mercury and tohatsu are all made by tohatsu. The only real difference is the cowling. There is a transom length mismatch with the Honwave because its designed for Honda engines which have a slightly shorter leg than everyone else makes.
I spent ages on this and ended up buying a 10.5mm (if I remember right, look up the transom height in your engine manual vs Honda) deep strip of wood from B&Q and used it as a shim. It effectively made the leg height the same as a Honda engine and nearly completely stopped the splashing and didn't cause any cavitation if trimmed out with the pin in the second position from the transom.
The Honda 20 hp 4 stroke measures 431mm (honda handbook)
The Mariner 20 hp 4 stroke measures 415mm (my engine engine lowest setting)
So the Honda is 16mm deeper in the water than the mariner to start with and the Honwave was designed around this engine.
Moving the Mariner up 10 mm Means the mariner prop would be 26mm higher than the design height
The transom is 15 inches ,exactly what mariner recommend ,But the Keel is 25mm lower than the transom and in all instructions the cavitation plate should be level or below "the bottom of the boat" in this case the inflatable keel
technically i have 9mm to play with this would bring the cavitation plate level with the keel line but 24mm above the Honda/Honwave match
This aside my set up is on the third hole up which leaves the cav plate level with the hull level on the plane also 2mph more top speed no low nose drag.
raising the engine 10mm using the third hole gives prop slip galore ,dropping back to the second hole gives cavitation out of the hole and sharp turns and lower top speed and slight low nose also if passengers venture forward on the plane cavitation creeps in
ive come to the conclusion that the standard height gives good overall performance as the boat was designed to do and the splashing is caused because the transom angle leaves a large space between the leg and the transom allowing the poorly designed leg to catch the spray
has anyone with a total Honda setup experienced any problembs
Thanks for posting