Cavitation problem with new engine

[Nightfisher]

I raised the transom height 1.5inch to accodate 3/4" between the cav plate and lower edge of transom after tilting the merc 9.9 one slotback from vertical. It's working well.

[lightning]

Yes I did think about raising the engine as it is a bit lower than the recommended 1 inch. Worth a try.

[richardjawilson]

A soft boat will cause cavitation in my experience. On my Zodiac 325 with Tohatsu 9.8 2s, I under-inflated to tubes yesterday and suffered from similar issues at higher speeds. Correct pressure should sort your problem out.

[lightning]

This outfit gets cavitation as you try to get on the plane, open the throttle at say 5mph and the engine seems to lose grip on the water and it races.
I'll try trim positions plus raising the engine an inch or so with a block of wood.

I contacted Tohatsu and they said I needed the 10" pitch prop on the engine, as opposed to the 8.5" that it comes with.
I might give that a try, but not if I cannot get a prop for less than the £160 + VAT that Tohatsu want for one.

Note on the picture above the keel is not inflated as I did not have the wooden floor in.

[Locozodiac]

Some tech issues >

-Inflate tubes and keel to factory specs, minimum 3.0 psi once on water, re inflate if needed. Tohatsu has more tail issues compared to other brands, it's tail design.

-Float sib with engine next to you (you must be on water) place trim angle hole as to have engine perpendicular to water or Tohatsu side cowl sticker paralell to water level.

-Place a medium passenger up front at bow.

-Go full throttle and while driving sib check visually where is water flow passing with respect to lower tail.

-If passing over small upper defector plate, you will experiment excesive tail drag and water spalshes issues at back tail/transom area.

-The sweet spot is water flow passing slightly under small upper plate. At this point you can experiment full engine power, best top speed and inmaculate performance under choppy seas, close curves.

-If flow is passing between upper plate and cav plate or close to cav plate will have cavitation specially on choppy seas, wakes, close tight turns at speed.

If your sib transom height border to lower hull in straight line is 40 cm you should be ok. If not, try placing shims under engine untill sweet spot is achieved.

Check this video, this is the sweet spot. Must be reached under trial and error and only once, forever and ever amen!!

Tohatsu 20hp w/ 3/4" shim and proper trim - YouTube

Happy Sibbing....