Larger Props For Efiency And Engine Life When Running Big Engines Slow

[Boss Lady]

Variable pitch props would allow us to have the best of both worlds. you could add more pitch for more thrust at lower rpms and adjust loading of the engines at any rpm you wanted to operate.

We do this in airplanes all the time. We use less pitch to let the engine make maximum rpm for take off, and climb to altitude. Once at altitude we reduce rpm, add pitch to get more speed and then lean the engine to get max cruise without melting the heads or exhaust.

Boats in europe have had variable pitch props for years. We have been lazy because we have historically had cheap fuel. I bet you will begin to see VP on new boats here real soon for the same reasons they have them in europe (maximize fuel efficiency across all operating rpms)

The most cost effective way to do it now is to have two sets of props, one for normal operation and the other for max economy.

[krush]

Airplanes mostly have variable pitch because the density of air changes so much with altitude. Even at the same power setting, a good climb prop sucks up high at cruise (won't get into nerd details).


Water's density remains constant so the advantage of a variable pitch prop isn't as great as in an airplane. A continuously variable transmission may be another alternative. "Our" problem has to do with the linear production of power from an engine vs the exponential (J curve) power absorption of the prop--not the changing density of air and great variance in forward velocity.

[Dan Mapes]

Are the exhaust gas temps in the manuals? This is all quite interesting. We have Covington 6-71TIs and are going to be cruising at reduced RPMs. We can run wide open at 21K on glass at around 2200 but we are thinking that 12-1300 rpm is where we will run, perhaps lower.
I like the idea of increasing prop size/pitch but we already idle at 5.8 knots.

[Brian Degulis]

I haven't ever been able to find anything from DD regarding stack temp. The common wisdom on commercial aplications is for continous running above 500 but not over 700 for long life.

Brian

[Dan Mapes]

We don't have pyrometers installed so I am wondering which ones are available. Presumably since we both have 71's the exhaust manifolds are the same, and they would be mounted prior to the coupling that goes up to the turbo?

[mikfin]

Hi,

Nice engineering! Which prop calculator did you use? Is it available online?

Thanks,

Mike

[akintosyali]

I think the props on my gas boat are original. So, the question is, has the prop technology changed so much in the last 40 years that my props are obsolete? Or can the existing props still be shaped and trimmed into something a lot more efficients??

[Brian Degulis]

We don't have pyrometers installed so I am wondering which ones are available. Presumably since we both have 71's the exhaust manifolds are the same, and they would be mounted prior to the coupling that goes up to the turbo?

I like Etna for pyrometers the same folks that make the digital tachs (I like them also)

The prop calculations where done by General Propeller In Bradenton FL they have some very good programs that not only take into acount the normal HP reduction lengh weight etc But can also factor in the prop your using now and how it loads the engine and what speeds it provides. By factoring some real world data you can get much closer on a calculation.

Brian

[Brian Degulis]

I made a mistake Hewit is the pyrometers not Aetna they are only tachs

Brian

[Boss Lady]

Airplanes mostly have variable pitch because the density of air changes so much with altitude. Even at the same power setting, a good climb prop sucks up high at cruise (won't get into nerd details).


Water's density remains constant so the advantage of a variable pitch prop isn't as great as in an airplane. A continuously variable transmission may be another alternative. "Our" problem has to do with the linear production of power from an engine vs the exponential (J curve) power absorption of the prop--not the changing density of air and great variance in forward velocity.

Hi Krush, I like it when you shoot holes in your own argument. Chris