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View Full Version : King Cobra (reverse spin) props 23p+?



EstorilM
03-25-2012, 11:10 AM
I'm having a tough time finding large props for reverse rotation OMC drives. My boat is getting a new engine and I need to start figuring out the best prop for the boat. I know there are many variables, but unfortunately with the reverse rotation and the (rather high) hp for an OMC setup, I can't go up to a marina and expect a good selection. The new motor (as some of you saw from my 454 thread) put out just under 500hp. I was thinking a 26" 3 blade, but that would probably give me a crap-tastic hole shot.

From Solas I see I can get either a Titan 3 24p or a Titan 4 25p in left rotation. For cruising the Titan 4 would be great, and with the larger pitch I almost wonder if top speed would suffer at all over the 3 blade.

BUIZILLA
03-25-2012, 11:22 AM
Turbo makes lefty's..

EstorilM
03-25-2012, 03:27 PM
No clue man, that's just hot I got 'er.

"daddy" King Cobras (similar to Volvo SX drives, has cone-clutch and F-N-R gears in the upper unit - these were the apparently bullet-proof ones) - from what I understand, they were all reverse rotation.

-Alex
http://www.athomasphoto.com/cobra1.jpg
http://www.athomasphoto.com/cobra2.jpg

MOP
03-25-2012, 04:35 PM
I was an OMC tech yours is the first single that I have seen setup reverse, which side of the boat is the helm on. If it is on the left like in an auto then it makes sense, but if it is on the right like the classics you will suffer some speed lose and roll way over in right turns.

Edit I just pulled up your old posts, you should be running a right hand wheel. Forget staying with a lefty!

EstorilM
03-30-2012, 11:01 AM
Hm sorry, I thought I had subscribed to this thread and haven't been around a computer much lately..

I'm confused though, with that kind of drive and limited parts, how could someone convert to left rotation in the first place?

Do you think they just got a good deal on a left-rotation king cobra and threw it in there as a replacement?

The two props I'm looking at right now are a 25p 3 blade or a 26p 4 blade for starters, both are left rotation obviously so that's fine - I think I can get them for around $360.

Again, with the price of the engine install and boat (both cash) I'm not really in a position to mess with stuff if it "aint broken" - a guy on another forum just said rip off my perfectly good king cobra and put a bravo on it :confused: lol :mad:

MOP
03-30-2012, 11:38 AM
I am not sure how to change the rotation been away from this stuff for awhile, it is done in the boat get some knowledgeable guidance. The prop on any single engine boat should turn in the direction of the helm, yours being right the prop should turn right. Having it the way it is now does cause adverse handling issues, you will maximize speed swapping back to right hand.

Phil

EstorilM
03-30-2012, 11:52 AM
I'm not doubting you, but I'm curious about the physics/fluid dynamics behind your statement.

Technically if the weight of the boat is the same (say I have a passenger/cooler over there) the displacement of water from the hull would be identical, with zero impacts relating to the position of the helm right? It's just a bunch of cables and connectors.

Additionally, what changes based on the rotation of the prop anyways? One turning direction will cause slip / cavitation and less bite but this would simply be the opposite direction if the prop turned left vs. right - the actual impacts should remain the same correct?

Confused now hah.

Ghost
03-30-2012, 02:27 PM
You're right about weight and Archimedes, except that the hull is (reasonably) symmetric along the centerline, but the weight is not necessaeirly so. So, depending on how it is loaded, the boat may list to port or starboard.

And underway, prop torque, depending on direction of rotation, will also attempt to induce list relative to the static floating position.

I *think* the question of where the helm should be has to do with the driver's weight (as well as a bunch of weight from wiring, cables, steering, etc.) offsetting prop torque. I *think.* Which may sound a bit silly, but I bet most of these boats do spend most of their time with more human weight on the driver's side. (With only one person aboard, or a big dude behind the wheel and a lady in the shotgun seat.)

And if that weight and the prop torque are additive, rather than counter to one another, on the little 7' beam, it's probably not what you want.

I *think* that's what was meant earlier by "adverse-handling issues."

krock66
04-04-2012, 01:06 PM
Your King Cobra pic looks like the back of a Blackwidow. My engine upgrade in my Blackwidow is 514hp and runs at 5200 rpm. The original setup was a BBF 460 and also is left hand twist. I put a 3 blade 24pitch on it, up from a rather old 21 pitch 3 blade. Gets out of the hole fairly well and 73 mph on GPS. If you feel inclined to switch it over to a right hand twist it is a very simple fix. Where the two cables link together at the manifold bracket you simply switch the cables around so the shift cable pushes the cable to the outdrive the opposite way. It takes 2 minutes.

EstorilM
04-04-2012, 11:02 PM
Thanks again for your input Ghost, I think we are on the same page - I actually installed 3 Interstate SRM-27 deep cycle batteries on the port side of the boat to counter-act my weight slightly.. I also have an Optima marine batt on the starboard side (24 I think?) for engine start, but there's still slightly more weight to port.

krock66 - Good call, it's a 24' black widow!! That's funny, I was looking through 610 pages of OMC manual to look at the construction of the drive, and discovered pretty much the same thing. With a cone clutch setup, there is no "forward" or "reverse" gear, it's simply a function of upper and lower gears which switch rotation 90 degrees. They are both splined inside and engage the vertical driveshaft based on on how the cone clutch is engaged. One is on the upper portion of the input shaft, one is on the lower portion (hence how one drives opposite the other) - so unlike a dog clutch unit, this can spin either rotation with a simple cable switch as you stated! I did want to understand the mechanics of it before I did anything though, but I'm becoming more confident with the drive every day - these things are beast!

Can you give me some details on your engine? The one I purchased was listed at 490hp, 10:1 compression, forged internals, roller rockers, etc.. but I don't know much beyond that (he also listed cam profiles though) - but your input is very helpful to my prop selection!

So you saw 73mph at 5200rpm with a 24p left rotation prop? Was it a 14 3/4" prop? Solas?

Sounds like you have her tuned out already - I was hoping for 75 but it doesn't sound possible based on your input and my slight HP disadvantage, unless you think I've got the torque for a 25p which might give me a bit extra. Apparently it dynod ~620lb-ft but I don't have that documentation in hand yet.