Outboard mounting height on Catamaran
Twin step cat hull
Merc 300XS (new)
20 in shaft
1.62 ratio gears
Sportsmaster lower unit
8 inch standoff box
4 inches of standoff on the lifter
Therefore total setback of 12 inches from transom
5 inches of travel on Gorilla lifter
3 blade 26in TXP Turbo prop
Weight 1500kg dry on the crane (so she ain't light)
So having initially setup the engine propshaft 1 1/2 inches below the imaginary line between the bottom of the twin hulls we took her out for some testing. (Theoretical max height could be 3 1/2 inches above the bottom)
Baring in mind that she is in break-in and as such I didn't want to try extended periods of high revs, initial runs showed that she wasn't very happy with the engine at bottom setting on the lifter. Hole shot was bad, lots of wondering around and spray all over the transom.
Having put 6 hrs on her in a mixture of conditions, we established that she accelerates fastest onto plane between 4-5 inches up the lifter (ie 2 1/2 to 3 1/2 in above the hull bottoms)
Water pressure at max engine height was 25psi.
Brief top speed runs resulted in 5000-5200 revs and 65mph
Happy to maintain 40mph into a 6ft head sea with use of bow tank to keep the nose down.
So craned back out of the water last week and we raised the engine one bolt hole on the saddle and 2 holes on the lifter to stand-off bracket - total height increase of 2 1/4 inches. This resulted in a new propshaft height of 3/4 of an inch above the hull bottom with lifter down and theoretical max height of 5 3/4 above at top level. (That sounds very high)
Went out to test at the weekend, Sot'on water was a bit snotty but she still seemed very stable. Again most comfortable with lifter at full height so with everything trimmed up we are now seeing 5600-5800rpm, still 25psi water pressure and speed increase up to 71mph. Engine hours now around 10. Still using double oil so must still be in break-in.
Note there is no rooster tail so prop must still be running sub surface. (It is a surfacing prop)
Setup ideally wants to be with 2 people in the boat, last night, out of interest we went out with 6 adults and it was hard to plane without blowing out the prop. (Needed weight over the front) However once going, we could achieve 60mph trimmed out. Not often that I would want to do this so ignore this for the purposes of setup. Max we would ever want in the boat would be 4.
So question is where to go next. I have 2 further holes potentially to come up on the engine before we have to start drilling holes in the transom. (So another 1 1/2 inches which would put theoretical max height at 7 1/4 in above the bottom)
Is it true that provided the water pressure will allow, just keep going up or does there come a point that we are too high to be efficient.
All this height sounds wrong to me. (my exp is with monos so don't have any cat experience prior to this one) The lighter and similar sized Skater 21s without a lifter are generally setup around 3/4 in above the hulls) they generally seem to be able to turn a 30-34in prop.
So my thoughts are to go up again and see how the boat feels (she is riding very level even with trim fairly well out) it feels like it's running on the Skeg, little bit of wondering around etc but I don't feel like it's going to blow over at any second.
Max rpm is 6400 although the power band tails off around 6200. I could try coming down to a 24in prop to bring the revs up but I assume by bringing the engine up more I will get the prop onto the surface and in turn this will bring the revs up. (I thought I'd end up running a 28 in prop but perhaps not)
Any input gratefully received. There is such little info around on these Cats that I have little in the way of hard facts to go on.
NB: my hope is that when we have found the optimum height, I will re drill the transom and raise the stand-off box so that the engine is only running half way up the lifter for max speed. I don't like the stress everything is under with it at full height. (That should then allow us to drop it on the lifter just a couple of inches when needed to assist with planing with a heavier load or when running in big seas)