150-200 rpm per inch of pitch is the universal rule of thumb concerning rpm change.
Hi,
thanks for the reply. I'll now try and get some props to try out - I may try the test again to be certain, but if the 6600 is repeated, would going up to an 11" (from current 8") be over doing it? it would to somewhere between the 5700 minimum test revs and the 6000 top limit.
The rev counter turned out to be a great asset in setting the tick over to a decent level - got it to 750 revs with only the occasional stall after a minute or two - its not a new motor after all.
thanks again,
Sailpower,
Before you test other pitch, test that sib with you alone and gear empty, max 2 gal of fuel, tubes, keel must be inflated to 3.0 psi minimum, engine well height/transom seated, well trimmed, engine must be perpendicular to water level, on a wot run on flat no windy waters. With you alone probably rpm will go tad + 200 higher to 6800, but that we currently don't know yet...
That engine is over revving on that small light inflatable, if you pass that same engine to a larger and heavier + 380 will lower its max rpm accordingly. Being a used engine, probably previus owner changed pitch to match other boating application which you didn't about ?
Try going for a 11 pitch if currently running a 8 as to lover 6600 rpm to nearest 6K, Once pitch is correctly dialed, adding a extra boater will drop rpm accordingly. Post results for the record to check if dropping 150-200 rpm per less pitch on a inflatable is a rule of thumb or a myth ?
A good idle rpm adjustment is the one that produces a dry clak while shifting forward at load and engine remains on, if dies, adjust idle carb screw a tad CW to remain on.
Hasppy Boating
Hi, thanks for the advice. I'll check the set up for depth and trim angle before doing as you suggest.
The engine was a long shaft (now a short shaft) on a bigger cabin boat so the 8" pitch was probably useful for a displacement hull. I think an 11" pitch is the largest they do for the 9.9 /15hp range. I have a 9.9 carb for the engine which i could swap - it would limit the revs and produce less power at the top of the range, but maybe that would be enough. Some runs with a speedo would be useful.
I'll report back about the rev drop. Thanks again.