Hi, for those interested following the problems I had wit the speed of my boat I have uploaded a Youtube Video of the procedure that I went through to determining what prop I should use on my Highfield 380
Your outboard needs to be in the rated max revs range (5300-6300 for yours) at max speed and knowing that you will see if you are about right or under/over pitched overall. I prefer to see revs in the upper half of the allowed range, more like 6000 for you, so if anything, forgetting your issue, you might prop down for overall performance.
My original thinking was if it was an unlucky coincidence the revs at which the problem happened coincided with the hull speed to lift on the plane it wouldn't matter which way you moved the pitch as long as it removed the coincidental revs vs speed situation.
My worry is that if you are already slightly over propped then increasing the pitch even more will bog the engine down and spoil the overall feel of the boat.
Now we know your existing pitch and revs at max speed it would be interesting if anyone else with a similar setup has comparison figures.
So it was helpful advice that Fenlander gave you when he suggested fitting a tiny tacho. With your new prop fitted it appears to have helped alleviate the problem. Do you know what the rpm is at WOT with your new prop?
Yes the WOT speed/revs would have been interesting to know to fully complete the picture (unless it was mentioned in the video and I missed it).
Anyway good to get the result you wanted. I was convinced the key to this was moving the self increasing rev speed away from the hull speed spot with a pitch change and as long as it was achieved it matters not if you went up or down pitch assuming the revs or speed haven't dropped off at the top end. That's likely not a problem if you always run quite light.
Thanks for assistance in this matter Fenlander and to answer both your and Steve's question about the tacho.
Well, had I installed the tacho in accordance with the manufacturers instructions, something I have never tried before, I could have possibly left it on there and got the final revs etc.
But, the cable supplied was not long enough so I added some more so I could mount the tacho ontop off the GPS. It might have been OK except the cable that came with the tacho was shielded and the bit I added wasn't. Had I had any brains I could have wrapped some aluminium foil around the cable.
Anyway, to cut a long story short, which is a bit late at this stage, the engine overheat light came on so I assumed it was because I was draining current from the spark plug.
So I took it off. It must have been the cable since it ran OK afterwards.
That's the truth the whole truth plus a bit made up.
then I think you have misunderstand how a tiny tacho works. It doesn't draw any electrical power from the HT lead it is wrapped around, it has its own internal battery. It uses a single cable which measures electrical impulses to the spark plug through electro magnetism. Even extending the cable with unshielded cable wouldn't make the tacho draw any current/power from the HT lead (if it did, it would fry your tiny tach). I suspect the cause will have been something else, maybe weed blocking your water intake.
Crikey. Remember the fella who dug up the local waste dump looking for his lost lottery ticket? That's what reading this thread reminded me of - there's value in it somewhere but damned if I can spot it. Suddenly, Dennis sounds like the Voice of Reason!
I'm sorry, but there IS no Mars Bar.