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MAN - GA 
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Joined: July-09-2009 Location: Cumming, GA Status: Offline Points: 298 |
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 Topic: Propeller SelectionPosted: August-02-2010 at 12:20pm |
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I have searched and read the prior topics on props, but in the interest of being a newbie I want to ask a general question and a specific question to my boat.
Boat is 1975 Mustang 16, Ford 302 w/ Holly 4160 4bbl, Velvet drive 1:1 - all stock & original 1. General question - what is the recommended way of determining proper pitch - the owner's manual I have says 4,400rpm max - the boat will run more than this before the valves begin to float - at 4,400rpm on nice smooth water and just me in the boat I have had it around 48-49 on GPS 2. Specific question - the prop on the boat now is a 12"x12" or 12"x13" standard 3blade - I have difficulty reading the pitch on the prop as the "2" and "3" as stamped are a little worn and it is difficult to de-code which it is - I believe the prop is Federal as I can make out "Fede" and then it start to fade out Any pointers/recommendations will be appreciated Also note that I have just checked as of this weekend point gap,dwell, and timing and everything is running great so I would like to perform another WOT/speed check to see what I get |
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TRBenj
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Joined: June-29-2005 Location: NWCT Status: Offline Points: 21186 |
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Posted: August-02-2010 at 12:32pm |
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Generally speaking, the 4bbl 302 powered 16' boats (210-235hp) from the late 60's through the late 70's usually came with 12x14 Federals from the factory. If you could turn that prop the "recommended" maximum 4400 RPM on that hull, then 48-49mph would be just about what I would expect to see. I would suspect that either your prop is slightly larger than you think, or your tach is off- a 12x13 or 12x12 Federal would probably have you turning closer to 4800 at that speed.
The 12x14 Federal is actually a pretty decent all around prop for that boat. The comparable OJ would be a 12x13, and would have you turning 100-200 more RPM- and the holeshot would be slightly better as well. Personally, I also like the Acme quite a bit (#1208, 12.5x12)... it will turn about 100 more RPM than the OJ and runs as fast (or faster) than anything up top. It also has a great holeshot. Thats the prop Im currently running on my '71 Skier (same hull as your Mustang) with a 220hp 302 Conquerer/Crusader. I either have a slow hull or tired engine though, as the boat is slower than most at 45-47mph. I turn the Acme 4500, the OJ 4400 and the Federal 4200. I would expect a typical boat (like yours) to turn at least 200 more RPM than than those numbers. |
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MAN - GA
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Joined: July-09-2009 Location: Cumming, GA Status: Offline Points: 298 |
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Posted: August-02-2010 at 1:44pm |
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I believe the tach to be correct as I installed a new one on the steering column specific for the purpose of performance testing - it will read dead on with my dwell/voltage/tach diagnostic device I use at home up to 2,000RPM where the cutoff point is on the meter. I am a little nervous running the boat over the 4,400RPM but as you state it will pull more RPMs as I have gone up to 4,800/4,900 range for just a very brief period of time - if I get over 5,000RPM on the tach it gets a little dicey and this is where I can tell the valves begin to float and the points have difficulty keeping up - I will research the 12x14 and appreciate the feedback
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TRBenj
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Joined: June-29-2005 Location: NWCT Status: Offline Points: 21186 |
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Posted: August-02-2010 at 1:59pm |
I have spun a few tired engines in these boats up that high and I cant say Ive ever experienced exactly what youre describing. I suspect what youre seeing may have more to do with an undersized or tired prop than valve float, though. If youre seeing 48-49mph on the GPS at 4400, then I would be genuinely surprised if you continued to pick up speed as you progressed to (and past) 5000 RPM. The 16' hull and 302 combo is a strong runner right from the factory, but Ive never seen or heard of one that ran faster than 50-51mph. It is far more likely that youre underpropped and would benefit from an upgrade... but thats just a hunch! |
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MAN - GA wrote: