Prop size rpms cooling

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Bear
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Prop size rpms cooling

Post by Bear » Tue Sep 26, 2017 6:40 am

Ok 1986 f32 crusader 350s , I finally got the cooling correct previous owner had it salt water cooled then converted it over to anti freeze closed system had cooling problems went thru the usual solutions but lost compression in one cylinder started to get white smoke took the heads off they were destroyed by salt water very thin and to my surprise the block was so clogged with rust nothing would circulate every port was clogged, packed around the pistons walls what a mess cleaned out about a pound or two of rust . Cleaned the block new heads there running great now got good temps good circulation I know this has nothing to do with prop size I thought this information may help someone. I have 16 by 16 props nibrle cupped at OT I get 3600 rpm at 24 to 25 knots, at 3000 I'm at 19 to 20 kt , a t 3200 I'm at 22 kt , I don't push the boat I usually cruise at 2900 18 knots best gas flow older engines. I want to know is this in range Rpm seems low at OT . Different props? 16/15 maybe would that make the engine work harder or easier? I'm happy with speeds I get now I don't want work the engine harder then they have to.
Jim Bear Shut up & reel II ( life is short go fish)

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prowlersfish
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Re: Prop size rpms cooling

Post by prowlersfish » Tue Sep 26, 2017 6:50 am

Going to a 16x15 , while it will increase your rpms it will lessen the load on the engine .
TROJAN F36 Conv.
6BTA Cummins diesels
Life is to short for a ugly boat :D
2009 TROJAN RENDEZVOUS
Port Kinsale 2010 & 2011. Solomons get together 2011.

cappy
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Re: Prop size rpms cooling

Post by cappy » Fri Oct 27, 2017 10:41 pm

Agreed!

Your RPM at WOT with a normal load should be 4400 - 4600 with those engines. One inch of pitch change will change engine RPM by about 200 RPM.. Less pitch = more Engine RPM . The torque curve peaks about 3100- 3200 and that is the most efficient RPM properly loaded. The internal engine wear starts to increase dramatically above piston speeds of about 3200 feet per minute so keep your cruising speeds below that RPM. (You can calculate the piston speed (FPM) from the stroke X RPM X 2= inches per minute, divide by 12 = FPM)

GM blocks are usually happiest at 3000 - 3200 RPM and will go all day at that speed.

Dave

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prowlersfish
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Re: Prop size rpms cooling

Post by prowlersfish » Tue Oct 31, 2017 6:59 am

Over loading will also increases piston wear and cause other issues . But honestly I would not worry about wear do to higher rpm , better to run a few more rpm with less load . 99% of pleasure boats will never wear out a engine . What you will find is failure from lack of use , the environment we operate in or a combination of all . Of course lack maintenance and abuse can also factor in .
TROJAN F36 Conv.
6BTA Cummins diesels
Life is to short for a ugly boat :D
2009 TROJAN RENDEZVOUS
Port Kinsale 2010 & 2011. Solomons get together 2011.

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