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Old 05-15-2014, 09:18 PM   #1
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Prop change like having a new RIB

I have a Brig 450 falcon with a 40HP 4 stroke Yamaha. Last fall on the way back from UTís last home game we spent the night at anchor in a cove. The Admiral took the dink and dog to shore and hit hard oh some large rocks just below the surface damaging the prop. The boat has always been hard to get on plane with 4 to 6 adults and slow wit a max speed of 27 mph at WOT. I took the damaged prop to our local prop shop and he said that it was a large prop for a RIB.(13 inch pitch). After answering a few questions he keyed in on that I was 1,200 rpms low at WTO. He recommended a Michigan 4 blade 12 inch prop. I put it on today and tested it It jumped on plane faster, gained 1,000 RPMís and ran 32.4 mph at WTO.
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Old 05-15-2014, 10:00 PM   #2
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I am a believer that every outboard boat should have a tach. Prop the boat so that with the lightest of loads it can turn at high end of the recommended RPM or just a bit more. If the boat is extremely lightly loaded the operator can back off the throttle a bit. but if the load is heavy it will get on plane faster. If the boat is underpowered, I might put a slightly smaller yet prop on it. With a tach it is easy to back off if the rpm is excessive, but you can't do anything about overloaded. Good rules of thumb for ouboard motors, 2" of pitch equals 400 rpm. Blade cupping equals a 200 rpm fro, but possibly more speed. 4 blades will generally run slower than 3 blades but will plane faster. 4 blade vs. 3 blade will generally reduce WOT by 200-400 rpm. Stainless props of the same size will turn 300-400 rpm faster than aluminum due to the strength of stainless allowing thinner blades. Props of the same size but different mfg. are DIFFERENT.
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Old 05-15-2014, 10:21 PM   #3
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Let's see... that was after the Commodore's beat U.T. correct? Is that the reason you were hiding in the cove?
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Old 05-16-2014, 11:45 AM   #4
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Great info Capn Crag. The dink was hard to back as it would want to go in a circle. Now it seems to back strait. Do 4 blades have less tork steer in reverse? Ulysses we don't hide we lick our wounds hire a new Coach and hope for the best. Go Vols!
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Old 05-16-2014, 12:44 PM   #5
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I can't think of a reason that a 4 blade would back straighter. The greater blade area of the 4 blade does offer more bight, but don't see why it it back straighter.
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Old 05-16-2014, 01:45 PM   #6
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I've kicked around going to a stainless prop on the 20hp suzuki that we have on our rib. It sure makes a difference on a bigger outboard. Plus, they are a lot stronger if (when) you do hit bottom.

Anybody have a source for stainless props for my small motor?
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Old 05-16-2014, 06:02 PM   #7
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Good job on getting a "right" prop on the dink. Now I'll bet it's a lot more fun to zip around in.
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Old 05-16-2014, 08:53 PM   #8
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Stainless props are stronger, but when you hit something you may also take out the lower unit. Here in the PNW where there is much debris in the water I prefer a material other than stainless. To me it is better to lose a prop than a lower unit
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Old 05-16-2014, 09:28 PM   #9
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Scout and all, believe me you can lose a prop AND a lower unit at the same time. I wish it was one or the other, kind of miss shear pins.
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