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Old 08 December 2011, 21:00   #1
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Checked your RPMs lately?

Do you just assume your motor on your inflatable boat is properly propped?

I have a 1989 tohatsu 40hp 2-stroke on a 13' bombard C3 with the factory 11" pitch prop.

I'm by no means a "Guru", but was a helicopter mech for 8 years in the great US Coast Guard and just 'felt' that my engine was over revving at wot.

I bought an inductive tach/hour meter of ebay for $21 and installed it. I'm running 6400 rpms at wot! The engine specs are 5200-5800 at wot. I'm glad I checked it.

I'm wondering how many engines out there on inflatable boats are over revving as they are usually propped for the general boat population and inflatables are so much lighter.

Just food for thought...
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Old 09 December 2011, 02:03   #2
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As far as I can tell, most SIB owners run what the boat came with. I know I did (Achilles 14'/Honda 40.) Never had to replace the prop, and it ran well enough, so that was that. When I upgraded the boat, the initial prop was far enough off that I had to learn how to select a more appropriate pitch/diameter wheel.

jky
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Old 09 December 2011, 04:48   #3
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Yes I been having isues with my setup for a while and finally checked the tacho against a photo tach and realised that my on board tach was over reading by around 15 percent. I now have the opposite problem that my merc 150 carb model WOT's out at 5200!

What about outboards with no console or tacho? How do they manage this?
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Old 09 December 2011, 09:50   #4
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I tach my engines with an old dwell tach previously used for tuning auto engines. You clip in between the plug and power feed to the plug.

Outboards are furnished with props for average use, whatever that is, so changing prop pitch can add to engine life and get optimum service out of the motor. Small boat owners generally do not tach their motors, but they would find improvements in service life if they did.

The cost of a prop compared to engine repairs is a no brainer. We have two different props for both our 9.8 HP and our 15 HP and use the proper one for whatever use we plan for the outing. If we want to go out lightly loaded or heavilly loaded, we use the proper prop. One would serve for rough water and the other for light loads and more efficiency at speed. Guess I am anal, but the engines operate on the power curve very well.

If you over rev, you are not developing maximum power as well as causing extra wear on the engine. If you under rev, you are wasting the power the engine is capable of.
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Old 09 December 2011, 11:07   #5
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Quote:
Originally Posted by frankc View Post
I tach my engines with an old dwell tach previously used for tuning auto engines. You clip in between the plug and power feed to the plug.
That was what I was going to suggest, as well. I think my late father had 3 of them in the garage. You have to run with the engine cover off, and balance the unit somewhere, but it will work OK. Might have to figure out if you're firing every rev or every other (2 stroke or 4) and make appropriate calculations, but it's fairly straightforward. And, some have an inductive pickup you simply clip over a spark plug wire.

Quote:
If you under rev, you are wasting the power the engine is capable of.
And not getting the fuel economy (or lack thereof) that the engine is capable of.

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