Impeller pitch Utopia 205 with optimax 250 at 7k altitude

[Daniel D]

I'm looking to get my rpms up to the 5600 this boat can do. Just bought it and I live and boat at 7k ft.

Right now I'm only getting about 5,000 rpms

Any impeller recommendations for high altitude boating?

Solas 22/39 seems like the only option or get my current one repaired. But isn't that stock pitch? Wouldn't I want to drop to get better hole shot and increase ability to build rpms?

I'm getting up to about 42mph with almost 1000 lbs so it's running decent but I can feel cavitation when I push it. I'm going to repair or replace the wear ring but would also like impeller recommendations and any other suggestions.

Related question. If I'm getting cavitation is the impeller able to spin faster? Because my rpms never get above 5000. I'm wondering if that's a clue I've got a separate issue to look into



Thank you

 

[Daniel D]

There's a 3 blade 18/27 that I've heard is run on the lower hp motors.

The 4 blade 22/39 is always suggested on whatever sites I've found.

 

[Dr Honda]

the 3 blade was for the 175hp engine. DO NOT run it on your engine.

The solas is good... but Mercury made a "High Scew" impeller. They were known for good hole shot. But looking for all that extra RPM isn't a reality at 7K ft. At that altitude... you've lost a lot of power.

 

[Daniel D]

So what do you think is my best option on that boat? Look for that Mercury "high screw"?

Do they still make them?

 

[Dr Honda]

Either one of those impellers will be good. But short of moving to a lower altitude... I don't think there is much you can do. If this was a 4-stroke engine... I would say to slap a turbo or supercharger on it, to compensate for the lost air density... but that's not really an option on a 2-stroke of this type.

****EDIT****

After pulling out my handy Jepisen calculator.... at 7000'... you are looking at 11.3 psi air pressure. And, with a 250hp engine... you have lost 52hp. Only way to get that back is to feed it more air.

 

[Daniel D]

So how does the optimax air compressor compare to a super charger. Sounds like the same concept to me.

Get more air in there by making it smaller...

I thought the Opti would lose less power because of that...

 

[Dr Honda]

The air compressor on the Opti is part of it's fuel system. Basically... it's an early direct injection... and it has to overcome cyl pressure. So it uses a relatively low fuel pressure (for a DI engine) and it's boosted with the help of the compressed air, to push the fuel into the combustion chamber while the cyl is under pressure.

A supercharger, or turbo charger is used to force feed the cyl's more air. In turn, achieving over 100% volumetric efficiency. (Makes the engine think it's bigger than it really is) These devices don't really work on a 2-stroke like this, because there is a point where the intake (bypass) and exhust ports are exposed. The timing of these ports is what helps the engine run properly. Also, the exhust is first top be exposed, so you can't pressurize the cyl. There are som big 2-stroke diesel engines that run a supercharger (like the Detroit 4-71 and 6-71) but these engine work because they still have a physical exhust valve, and only intake cyl ports.

 

[AirSniper]

Awesome explanations Doc! Much thanks for the added detail, very interesting.

 

[Daniel D]

So I completely misunderstood...

Thanks for clearing that up with great detail.