Engine Mounting Height on Transom - Advice Needed

[skip_mff]

Hi,

I know this is something usually done by the dealer/factory when rigging a boat, but I wanted to check the factors used to determine the ideal engine height on the transom.

The boat is an 18 foot center console, dry weight 500kgs, powered by a Tohatsu 90TLDI standard length shaft, with a 15P 3 bladed prop 13 3/4 diameter.

The Tohatsu engine manual states that the engine should be installed so that the main cavitation plate (where at the end you have the fin anode) and the small cavitation plate above sticking out a bit towards the front sit in between 10-30mm below the V of the hull.

The lowest I could get it was having the bottom of the V almost level with the main cavitation plate, the cavitation plate comes perhaps 5mm above the V.

The dealer who supplied the boat says that they usually install the engine higher up that I have it, about another inch up as they said that improves stability and gives better performance?

On the sea trial yesterday when going to full WOT I had to trim the engine up considerably in order to hit the upper WOT range and get max speed.

Does it make sense that the engine should move up an inch, as I have no experience with these things? It does appear to be a bit slow out of the hole shot as is.

Any help would be appreciated, not sure if there is a rule of thumb or it's just a question of trying these things?

Any help would be seriously appreciated as she's back out of the water for a few days so now would be the time to do it.

Thanks

[mboy]

Search is your friend, but here ya go:

http://www.propgods.com/forum/Default.aspx?g=posts&t=98

[eggsuckindog]

Move it up, ususally approx 1" above the hull bottom for the Cav plate. - after you get that set up then if its still a little slow out of the hole you may have to drop 2" in pitch on the prop, get it moved up before that.

I think propgods is tad overkill for this

[skip_mff]

Hi eggsucking,

Thanks for your suggestion. A one inch lift would be around two holes on the engine plate which is the max I can go at this stage and after looking again today reckon should help alot. Was only carrying half fuel and no water so when I factor in that additional weight a one inch lift should be a good improvement. Then see how see sits in the WOT range with full fuel and typical load and will then as you suggest play around with the pitch etc, maybe even consider a 4 bladed prop?