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Mon, January 06 2025, 01:31 [ Login ] [ Contact Forum Admin ] [ Main index ] [ Post a new message ] [ Search | Check update time | Archives: 1, 2, 3, [4], 5, 6, 7, 8, 9 ]

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Date Posted: - Tuesday - 08/10/10 - 12:24pm
Author: Randall, Murre, Mariner 31
Subject: 12x6 is where I ended up
In reply to: Paul - 1971 mariner 31 - #106 's message, "Engine propping" on - Tuesday - 08/10/10 - 8:35am

Paul,

I had a 12x6 to start. Put on a 13x8 at the reco of a mechanic, but had the same experience you are having. Changed back to 12x6. I have a 1:1 tranny and so the 13x8 was lugging the engine much as you describe. Don't know my current RPMs as the meter is kaput, but as comparison, I couldn't get the RPMs much above 1/2 potential with the bigger prop and now get, I'd guess, 2/3rds potential. Plenty of reserve power with the 12x6 for headwind and chop.

Again, the engine is plenty strong for a 13x8. The bigger Mariners (M36 and M40) were built with the same engine we have but differeing tranny ratios allow more/less prop. (I think I'm right about that...?)

Hope that helps.

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[> [> Big trip this weekend -- Paul - 1971 mariner 31 - #106, - Thursday - 08/12/10 - 11:26am

I am heading out for a raft up (in Port Jefferson, NY) this weekend - the engine will see a lot of action and I will evaluate the performance some more before I decide whether to make this an issue or not.

After a whole bunch of calculations, I confirmed that at 1600 rpm the engine is putting out exactly the amount of "work" it was designed to do at that rpm...

The power curves of the prop and the engine seem to cross at around that number, which explains why the engine starts bogging down when I try to push it beyond there. But the I ask - why would I want to push beyond hull speed?

Again, I will think about this some more.

Funny thing happened in the process - I contacted a prop shop for their advise - gave them a bunch of information about my set-up and their only response was that my engine did not have a 1:1 tranny. I was wrong on that, they said.

Oh well.


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