Hey about RPM of my motor

[y2jericho]

I seem to be turning 4900 at tops depends where the person sits in the boat with 3 people and i was wondering what rpm i should be running at for optimum performance so i know what prop to get..<br /><br />Also can you fit a propellar that lets say gives you 5900 rpm but lets say you must go max only 5500 but the prop lets you do more, Can you fit it but only put the throttle to 5500 or so and not over rev it? is that fine?<br /><br />I have heard some outboards run at 6500 or so rpm's but it can only be run at this for short runs or only for racing engines (which get trashed after 2-3 races)<br /><br />My motor is a 1970 Mercury Thunderbolt 65hp 650cc with 95-100psi across all 4 cylinders<br /><br />Thanks

 

[y2jericho]

Re: Hey about RPM of my motor

anyone

 

[KIWI MERCMAN]

Re: Hey about RPM of my motor

y2jericho, Mate<br />Your engine should run from 4800 to 5300rpm, if your running a pronp that give you 4900 then a change in pitch should help. By going down 2" in pitch you should see about a 400rpm increase. 4900 to 5300.<br />If your using the boat for some sking then you could even go smaller and you are right, this could allow the engine to pull more than 5300rpm at WOT. The book says "when pushing a heavey load proping up to 300rpm above the recommended is advise, eg: water sking However prolonged use at this RPM is not recommended". So you can put on a smaller pitch but don't ring it's neck with it on.<br /><br />Kiwi<br />Still home to the America's Cup

 

[ledgefinder]

Re: Hey about RPM of my motor

No problem to run up to 6500 for a few seconds, then throttle back for cruise. Sustained operation > 6000 rpm, worry about metal reed fatigue, check with Merc experts. Lower prop pitch will help acceleration and skiing pull (a lot). However, if you're at 4900 with your typical load, I'd be tempted to leave it. You may be able to pick up a couple hundred rpm by fine-tuning the engine height & trim angle.

 

[Jack Shellac]

Re: Hey about RPM of my motor

A friend who runs one of the best OB shops in the area insists that the best prop for your motor is one that lets it run a few hundred RPM's over the max. You don't run it there, of course. but he says this is best for your motor over the long haul.