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TOPIC: Cavitation

Cavitation 11 Oct 2017 14:45 #1

  • pgiconch
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Wondering if the loss of the plastic ring at the rear of the prop would cause cavitation issues?
TIA...
Dave

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1998 BL Ciera 2655 Sunbridge LX (Special Edition)
"Sea Flights"
5.7L 2bbl w/Thunderbolt Ignition
Alpha I Gen II Sterndrive with raw water cooling
MFG Extra's - A/C, Fresh Water Engine Flush port, HWH and Windlass
Docks @ Punta Gorda, Florida

Cavitation 11 Oct 2017 15:10 #2

  • Mr. Darcy
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Maybe you could call the manufacturer on that one. They likely have a technical help department. Please let the rest of us know what they say about it.

Greg
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Newport, Oregon
South Beach Marina
1986 3270 with twin 110 HP Hino diesels. Name of boat "Mr. Darcy"
Past work history: Prototyping, tooling, and repair for Reinell,. General fiberglass boat repair starting in 1976.
Also worked as heavy equipment mechanic, and machinery mechanic for over 30 years....
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Cavitation 11 Oct 2017 15:25 #3

  • HueyCrew
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Interesting.... My outboard didn't have anything on the aft edge and I don't remember any cavitation issues except when I'd trim up for top speed. I'd take it up till cavitation started then thumb back down till it went away. I'll have a look at the 2655 in a bit. It's been converted to a stainless prop and came with two additional props but don't think there was any ring on them. Hasn't made it to water yet but I'll looking and listening..... Does it sound different? Can you "feel" a difference?
I always associated cavitation with "over speeding", reversing rotation still under fwd motion, bent blades, not enough fluid surrounding the prop (trimmed too high) or disruption of fluids either at or just before crossing the plane of the blades, or rough choppy water that is aeriated. I've not given much thought to fluid past the blades since forward motion in most cases I'd think would prevent "downstream" fluid from circulating back fwd. like with some submerged pumps where the fluid is mainly static around the pump/ impeller.
I'd imagine though that the missing ring would/ could cause turbulence as the water rushes past. That has me pondering now if the ring's purpose is to "streamline" that water, reducing churning and leaving a cleaner center area in the wake "cone" and could that water now be causing a hydrodynamic condition between the stern area and that larger mass of water passing under it......I guess a form of cavitation. I tend to put way too much thought into things I don't have a lot of experience with but hopefully someone will pick up on something above and give you a better (and much shorter) reply......
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Dave
N.C. Boater, fresh and salt water. New to boating in 2011
1990 Sunbridge 2655 "One Particular Harbour"
5.7 Mercruiser Alpha 1 Gen 1
Past:
1995 SeaPro 210 C/C "Hydro-Therapy"
Mariner 150
Towing with:
2002 Ford F 350 7.3L Super Duty
West of Hickory NC

Cavitation 11 Oct 2017 16:08 #4

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The prop had the ring which can be seen in pic below from just prior to purchase. Never hit anything, but when we raised the boat (on lift) after second trip noticed it was missing. Haven't really heard it cavitating (except for once when we went WOT to get on plane and it was, but the outdrive was still raised up a bit and tabs weren't down. Put the OD and tabs all the way down and it went up on plane without further cavitational sounds).

Just wondering if it would cause cavitation or "bite" issues when running. Will try calling MERC as it is original prop. Just thought someone on the forum may know what the "flair" does or if it has an effect on the water or exhaust. Am thinking it may have been done as a weight reduction thing by MFGs.

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1998 BL Ciera 2655 Sunbridge LX (Special Edition)
"Sea Flights"
5.7L 2bbl w/Thunderbolt Ignition
Alpha I Gen II Sterndrive with raw water cooling
MFG Extra's - A/C, Fresh Water Engine Flush port, HWH and Windlass
Docks @ Punta Gorda, Florida

Cavitation 11 Oct 2017 16:20 #5

  • Nauti Mike
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I had a 1993 20' Sea Ray bow rider 4.3 with an Alpha drive that I bought a new prop for and within a week the diffuser ring fell off in the water and the boat started experiencing prop blow out in turns. The place I bought the prop from replaced it and the issue went away.
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1997 Maxum 2400 SCR 5.7LX Bravo II

Mike

Cavitation 11 Oct 2017 16:58 #6

  • Solandri
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HueyCrew wrote: I always associated cavitation with "over speeding", reversing rotation still under fwd motion, bent blades, not enough fluid surrounding the prop (trimmed too high) or disruption of fluids either at or just before crossing the plane of the blades, or rough choppy water that is aeriated. I've not given much thought to fluid past the blades since forward motion in most cases I'd think would prevent "downstream" fluid from circulating back fwd. like with some submerged pumps where the fluid is mainly static around the pump/ impeller.

It also depends on water depth (water pressure), and proximity to air bubbles (air is a compressible fluid - see below).

Nauti Mike wrote: I had a 1993 20' Sea Ray bow rider 4.3 with an Alpha drive that I bought a new prop for and within a week the diffuser ring fell off in the water and the boat started experiencing prop blow out in turns. The place I bought the prop from replaced it and the issue went away.

Yeah, that'd be my guess what it's for. The engine's exhaust gases are mostly vented out through the center of the prop, so you've got a stream of air bubbles trailing back from there. In turns, if an air bubble gets too close to the high pressure region in the back of a prop blade, its compressibility can cause a localized reduction in water pressure (conservation of volume). Basically, the presence of air bubbles allows the back of the prop blade to push water behind it away more easily. The surrounding water tries to fill in that missing water volume, which drops the water pressure to the sides and front of the blade. The low pressure region in front of the blade then drops to an even lower pressure, to the point where it cavitates.

Edit: The flare probably pushes some water aside to reduce the pressure the exhaust gases experience. Basically carving out enough volume in the water behind it to account for the volume of water + exhaust. Without it, the engine probably encounters more backpressure on the exhaust ports due to water pressure increasing rapidly with depth.
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Cavitation 11 Oct 2017 18:15 #7

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I got really curious about this, so I called Mercury Marine. First, they told me this is not likely to be an actual Mercury prop as the lip is cast into the hub on their props. The purpose of the lip is to increase “lift” of the propeller blades. If it is missing it will cause some cavitation at higher prop speeds. So if you do a lot of cruising it would be best to replace the prop. They also said, if you should find a Mercury part number on it please contact them as they would like to know about it.

Greg
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Newport, Oregon
South Beach Marina
1986 3270 with twin 110 HP Hino diesels. Name of boat "Mr. Darcy"
Past work history: Prototyping, tooling, and repair for Reinell,. General fiberglass boat repair starting in 1976.
Also worked as heavy equipment mechanic, and machinery mechanic for over 30 years....
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