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Date Posted: - Saturday - 04/ 5/03 - 10:08pm
Author: schifsky
Author Host/IP: es19.interisland.net / 12.45.36.19
Subject: prop size

Any recommendations on prop size for an old wooden M31?
Volvo MD2B, 25 hp, 2500 rpm, ~1.9:1 ratio.
2 or 3 blade(?) for a cruise up the Inside Passage.

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Replies:

[> Re: prop size -- Gene MS, - Sunday - 04/ 6/03 - 11:22am (msp2-401.datasync.com/209.205.137.21)

I run 13/7 two blade with the perkins 4-107

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[> [> Re: prop size -- Speedy, - Sunday - 04/ 6/03 - 8:35pm (NoHost/143.45.252.15)

I run a 13X8 three blade with my Perkins 4107. These are rated at 40 hp, and this prop configuration is too small. {I can easily cavitate it} Might work good with your engine though. In a heavy tide, I'd want a three blade.

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[> Re: prop size -- Jake, - Sunday - 04/ 6/03 - 11:49pm (NoHost/204.60.248.85)

I have a 13/8 three bladed prop on our 32'mariner"maranatha" We also have a volvo 2d2b 25hp diesel.At about 2500rpm's we are pushed at about 5.5kts.I hope this will help you. Jake.

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[> Re: prop size -- Randall Reeves, - Monday - 04/ 7/03 - 12:55pm (64-161-221-132.opentable.com/64.161.221.132)

Murre has a Perkins 4107, rated at 40hp, and a 13/8 three blade prop, which is the prop that engine is RATED for. Any prop shop should be aple to tell you the engine's prop rating.

I get hull speed (6.5+) at half throttle (1100 rpms), but can't get the engine above 1400rpms or so. With the old 12/6 three blade, I maxed at 18-1900rpms. Both got me to hull speed. The bigger one seems to hold its own better in a head wind...emphasis on SEEMS. One mechanic has said that the 13/8 is too big for the engine (i.e. running it hard at slower rpm is running it hotter). Not sure about this.

I get that this is about as opposite from Speedy's above as possible, but would suggest this has to do w/the extreme magnetic variation in Speedy's local :). I only 'cavitate' when I'm docking in a cross wind (always) and backing down hard.

I'd go w/a three blade collapsable if there were such a thing, but I'd not go w/a two blade for anything coastal or where you'll be doing a lot of motoring.

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[> [> Re: prop size -- Chuck Baier, - Monday - 04/ 7/03 - 7:07pm (1Cust152.tnt41.dca5.da.uu.net/67.249.214.152)

Randall, If you can't get the engine over 1400 RPMs then this is not the correct prop and pitch for the boat. I believe the 4-107 is rated to 4000 RPMs to generate 40 HP. You may move the boat but will experience a short life on the engine. Manufacturers will no longer warranty new engines unless the prop is such that you can run at min rated RPMs. You are correct that most good prop shops should be able to determine what size and pitch a given boat will need.

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[> [> [> Re: prop size -- Randall Reeves, - Tuesday - 04/ 8/03 - 9:59pm (64-161-221-132.opentable.com/64.161.221.132)

ya, i don't get it either. i can't reconcile these two facts: one, a recent mechanic attested the engine is in great shape, great compression etc., but that the prop is too big for the engine; two, according to the prop shop next door 13/8 is the rated prop size. either the prop shop is smoking crack or my rpm guage is on the fritz. Murre comes out of the water in a few weeks. will have a closer look then.

several of us on the board are running 13/8's in a 4107...i keep wondering if the props not actually 15 or some such.

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[> [> [> [> Re: prop size -- Speedy, - Wednesday - 04/ 9/03 - 12:55am (NoHost/143.45.252.15)

13 is about max that will fit. My 4107 get's to 3000 RPM in gear no problem. It cavitates badly at this setting, but at six knots. I'm thinking something like 13X13.

The original post though deals with a smaller engine. The 13X8 might be okay for him. I accelerate quickly to 5 knots!

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[> Re: prop size -- Jake, - Monday - 04/ 7/03 - 8:07pm (NoHost/204.60.248.185)

Hi Schifshy, I have a book for the volvo Md2b if you ever need to take a look at it. It doesn't give you any information on prop size. At 2500 rpm's it push our mariner 32' at 6.5kts and at 1800 it's about 5.5kts. My last message I entered the wrong information. Jake.

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