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Old 11 February 2024, 03:38   #1
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Selecting a Prop

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
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Old 11 February 2024, 11:36   #2
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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.
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Old 22 February 2024, 06:25   #3
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Finally did my test run and I am very pleased with the results. I've posted my video here.
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Old 22 February 2024, 09:55   #4
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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?
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Old 22 February 2024, 10:43   #5
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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.
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Old 22 February 2024, 11:14   #6
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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.

Thanks again.
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Old 22 February 2024, 15:51   #7
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>>the engine overheat light came on so I assumed it was because I was draining current from the spark plug.

Think I've spotted the made up bit.
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Old 22 February 2024, 22:47   #8
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Wrong Fenlander the only made up bit is that my name is not really Salty Pete.

However to be serious (because you probably don't find me funny) the engine overheat light came on while I was doing my speed check.

I took it off after I had finished because it would have been the only thing that I had changed to make the engine overheat.

The last check I did I had taken it off and I ran for a lot longer time, there was no problem.

And I would not lie to you Buddy !!
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Old 22 February 2024, 23:17   #9
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Hi Pete, if your tiny tach is something similar to this:-

https://www.ebay.co.uk/itm/385639064...mis&media=COPY

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.
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Old 22 February 2024, 23:55   #10
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Or the engine working harder against that larger pitch... long shot but possible.
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Old 24 February 2024, 08:21   #11
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Fenlander you insult me by insinuating that I would not do standard checks to determine why the engine was overheating.

So I thought where is this man I will find him and give him a piece of my mind.

Oh, Cambridge, excuse me, according to Google where they manufacture scientific and electronic equipment.

I'll have you know I live in Western Australia the home of the Bug Zapper where a highly sophisticated electronic piece of equipment intices bugs into its electro field and electrocutes them.

I've suddenly realised my mistake.

I wired up the tacho using the Bug Zapper cuircuit diagram and killed the engine.

Oh, your last post you have things back to front.
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Old 24 February 2024, 08:45   #12
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Old 24 February 2024, 10:23   #13
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Oh dear.
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Old 24 February 2024, 22:09   #14
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Great to see the positive result!
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Old 24 February 2024, 23:26   #15
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Quote:
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Or the engine working harder against that larger pitch... long shot but possible.
I see a potential burned valve or melted piston in the future
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Old 24 February 2024, 23:36   #16
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Old 24 February 2024, 23:49   #17
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I see a potential burned valve or melted piston in the future
Knowing the RPM would help determine if that's a possibility.
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Old 26 February 2024, 01:02   #18
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I see a potential burned valve or melted piston in the future
No Beamishken, Fenlander got that around the wrong way. The overheating light came on while I was using the smaller pitch.

The engine has not overheated since I took the tacho off. By proccess of elimination it must have been the cause, although Fenlander doesn't believe me.

Ther boat is running beautifully and I have a virus. I think it was caused by the tacho.
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Old 26 February 2024, 09:01   #19
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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!
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Old 26 February 2024, 10:02   #20
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I'm sure it was your goodself willk who once said, "our village is big enough for more than one"
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