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Sun, December 22 2024, 01:07 [ 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: - Friday - 10/ 1/10 - 12:22am
Author: Randall
Subject: unclear
In reply to: Mike Keller 's message, "Re; prop sizing" on - Friday - 10/ 1/10 - 12:02am

Don't understand the question. what size what am I using? my transmission is a 1:1. Not sure if it's borg warner.

i'm currently on the hard at the san rafael yacht harbor fitting out for a cruise. i'f you'd like to stop by, feel free.

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[> [> [> [> [> Freewheeling is noticeable but not an issue -- Paul - 1971 mariner 31 - #106, - Friday - 10/ 1/10 - 1:02pm

on my Mariner 31. I have the Perkins 4-107 and a 3-bladed prop... not sure what pitch, but I am pretty sure it is higher than Randall's. My Guess is that it is a 13x8 three-blade.

I get hull speed at 1500rpm with a 1:1 Borg Warner Velvet Drive transmission. At max "throttle" speed my engine reaches "only" 2100 rpms but I believe the high pressure fuel pump/governor has been restricted somehow, because the engine "levels off" rather than starts lugging while running (engeged) at that rpm - no smoke at all (no white, no black) - the engine just hums along. Disengaged, the engine tops out around 3000 rpm.

While sailing, the prop freewheels - regardless of whether the tranny is in gear or not. Note that the Velvet Drive is a hydraulic transmission, meaning that, unless the engine is running, there is no hydraulic pressure to move, engage or lock the clutch plates and the transmission basically freespools at speeds of no concern or even remotely harmful to its internals.

I do notice that a fouled prop will not spin as easily but after a fresh bottom scrub it spins quite quickly ans easily and makes a gentle "rushing" sound while moving at 4 or more kts under sail.

A lower pitched, three bladed prop - I believe Randall sayd his is 13x6 - would also freewheel, but probably not as quick (boat speed under sail) and not as fast (rpm).

The jury is still out (in my opinion) on what is "correct": A prop that will "allow" the engine to reach (and run at) design output rpm (note I did not say design output HP) or to have a prop installed that places the engine speed at "proper" rpm for the hp required to push at that rpm while at hull (or cruising) speed...

After some calculating, I have established that my Perkins puts out approximately 21 HP at 1500 to 1700 rpm, which I also established as being the HP required to push the boat at hull (+). So running at 1500, the air/fuel ratio is balanced and the engine runs exactly on it's power curve.

For it to put out 21hp at 3500 rpm, it would be sucking as much air as it would for max hp rating (a diesel is not throttled and is basically an air pump with flow direct proportionate to rpm) but with the fuel intake cut way back - meaning there would be too much air flowing through - the unit runs cool and from idle to full speed never really "see" the resistance that would put it anywhere near on its power curve.

I guess this is a very long way of expressing my opinion, which is that I believe my prop is properly sized for my boat/power plant combination and I would not want to exchange it with a lower pitch one. I am over engined, rather than over propped.

I am perfectly fine with the freewheeling and the effect on my sailing experience is minimal.

Hope this helps...


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