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Tue, November 30 2021, 13:43Login ] [ Contact Forum Admin ] [ Main index ] [ Post a new message ] [ Search | Check update time | Archives: 12[3]45678910 ]

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Date Posted: - Tuesday - 08/10/10 - 8:35am
Author: Paul - 1971 mariner 31 - #106
Subject: Engine propping

I know there has been some discussion here already about some of our boats being "overpropped" and I believe some have taken steps to correct this.

My question is - for those that now have a smaller prop - what size did you settle for and how happy are you with the performance?

My M31 has the 4-107 Perkins and a 13 x 8 three bladed prop (so I was told) - the engine runs at 1600 rpm with the boat at hull speed and she'll push her at 7.4 at 2000rpm but peak out right there... I cannot get the engine revs higher than 2000, maybe 2100, unless I put the gear in neutral and then I only get 3000.

No smoke - not white, not black - never. the engine seems very happy at 1500 - 1700 rpm; and pushes the boat easily through heavy chop and winds (been there, done that); engine temp stays at 180F, no matter what rpms.

My only concern is that I may be stressing out the enginee - for those that changed the prop to something less agressive - what rpm's are you running at with the new prop and, again, are you happy with the results?

Sorry for re-hashing this, but it's been on my mind lately.

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[> 12x6 is where I ended up -- Randall, Murre, Mariner 31, - Tuesday - 08/10/10 - 12:24pm


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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