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Old 11-15-2015, 12:09 PM   #1
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Better water flow reshaping

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Would you cut the steel out and make it a tapered return to the prop. I find I have a lot more prop noise and vibration above 600 rpm prop speed on the stern. I think the prop is starving for water. It would probably takes four days to cut it,shape it, and repaint it. Do you think it is worth it? Prop looks good and so does the cutlass bearing. I have not pulled the shaft.
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Old 11-15-2015, 03:16 PM   #2
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There are guidelines about distance between the prop and struts / non-moving appendages covered in Dave Gerr's book "Boat Mechanical Systems Handbook" that say something like 10% of the width of the blade from any strut or non-moving appendage. This is from memory but from your picture, it seems that your prop is quite close to the back of the skeg and this might be part of the problem. If you do taper it, understand that you have no pull around the hub but tapering the skeg might help if you're hearing prop cavitation.

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Old 11-15-2015, 03:26 PM   #3
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It would be worth it if you intend many cruising miles. Ideally the propeller would be further aft, away from the trailing edge of the keel. Leave the keel intact around the shaft, but scallop it away and taper above and below. Minimum clearance from the trailing edge is 30% of prop. diameter, so if that's a 28" dia prop, the leading edge should be 8.5" aft of the keel. The keel taper should run at least 16-18" forward.
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Old 11-15-2015, 03:37 PM   #4
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Thank you for your reply!
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Old 11-15-2015, 03:39 PM   #5
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It would be worth it if you intend many cruising miles. Ideally the propeller would be further aft, away from the trailing edge of the keel. Leave the keel intact around the shaft, but scallop it away and taper above and below. Minimum clearance from the trailing edge is 30% of prop. diameter, so if that's a 28" dia prop, the leading edge should be 8.5" aft of the keel. The keel taper should run at least 16-18" forward.
I would say that means he'll need a longer shaft? Or install an intermediate shaft to give him more length?

Also, if he burns / grinds off part of the keel, remove the cutless bearing or the heat will burn the rubber inside.
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Old 11-15-2015, 03:44 PM   #6
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Like this....

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Old 11-15-2015, 03:46 PM   #7
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You might first try spacing your shaft back 1" maybe with a "DriveSaver". You could also move that big zinc which is in exactly the wrong location for clean flow to the prop. The DriveSaver is a good thing anyway so nothing lost. You will also be smoother with more blades on your prop at a slight expense in efficiency.
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Old 11-15-2015, 05:59 PM   #8
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I have 24in propeller I will change the measurement to fit. I can't move the prop back with a drive saver related to have an Aquadrive installed. I will pull the shaft and replace the Cutlass bearing to be on the safe side. I appreciate all the advice. The steel is 5/16 It is going take work to to bend those shaped in at least I have all winter!
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Old 11-15-2015, 07:45 PM   #9
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You never know. My Mainship Pilot 34 has a squared off deadwood somewhat similar to what is shown above but the keel is tapered below the prop. Take a look at the before pic below. I do experience cavitation sounds particularly at wot and I hoped for a little performance boost. So I decided to fair the deadwood. See the second pic taken before final painting.

It did no good for performance and the cavitation sounds are still there. But it didn't do any harm, except to my wallet.

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Old 11-15-2015, 10:33 PM   #10
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I'm not sure how much good cleaning up the TE of the keel will do but there is a Willard 30 owner that did his keel like in the illustration that TAD posted. He's on the Willard Boat Owners yahoo group. He's in Maine.

Rather than scalloped thke TAD's illustration one could simplify the shape very much and not noticeably suffer in the way it works. Could make the whole thing out of flat plate.

Re your question I would say it's worth doing. Mostly because w steel it should be much easier to construct than w FG. One should go about this w the boat weight mostly off the aft end of the keel and keep in mind that most or all the welding may need to be done from outside the boat.

If you want to get fancy w it the shape of the surface on one side of the fairing could/would turn the water toward the propeller blade and increase performance. Above the propeller shaft the fairing would turn the water flow toward the advancing blade and turn it in the opposite direction below the shaft also into the blade. This would mimic the shape of the keel on my Sumnercraft that I had about 12 years ago. One side of the fairing was curved like the wing of an aircraft pulling the water over into the prop. The other side of the fairing/keel was straight .. flat. I think the stator blades in jet engines work in the same way .. they redirect the air into the advancing blades. I think this increases the efficiency of the propeller. Could be a 1" pitch change would/may be necessary to bring the rpm of the engine back to where it should be. The Sumnercraft is the only boat I've seen w this configuration.

Did I make myself clear in the description above?

Funangler,
I would think you could put the drivesaver aft of the CV joint of the AD at the head/front of the prop shaft. Inbetween the flange on the aft end of the AD and the flange on the fwd end of the prop shaft. That part of the shaft should be very staitionary except for rotating.
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Old 11-16-2015, 02:27 AM   #11
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The Aquadrive clamps it self on to the shaft. I think I will keep it simple with both sides having the same cut. It makes a little unhappy that I will not be able to paint some of this steel in the keel.
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Old 11-16-2015, 06:45 AM   #12
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It makes a little unhappy that I will not be able to paint some of this steel in the keel.
Then what about a longer shaft, longer external cutlass housing and external fairing blocks? Picture available...
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Old 11-16-2015, 07:44 AM   #13
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Then what about a longer shaft, longer external cutlass housing and external fairing blocks? Picture available...
That would be my thought. Instead of cutting up the boat, just get a longer shaft.
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Old 11-16-2015, 08:04 AM   #14
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I think the boundary layer in the flow created by the significant keel surface ahead will create more disturbed water to the keel than will the flat "deadwood" ahead of the prop.

I have done a few Harker's Island wood skiffs with planing hulls with keels and modest tunnels. On each we scalloped the keels just like Mr. Roberts showed on his sketch. On some we were able to run before and after the scalloping and were able to compare directly. Results were a modest improvement, but only apparent at planing speed above 25kts. Below that, could not really sense an improvement. Lots of prop noise in either case.

That's the basis for my comment about the boundary layer formed on keel surface. Although no fluid dynamicist, I did have to study fluid flow way back and remember a tiny bit!!

My experience is backed up by lots of sea trials with keel boats, and at higher speeds, planing and some semi planing, you get LOTS of prop noise. No keel, little noise. Slow speed, little noise. Scalloping helps, but not that much.
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Old 11-16-2015, 02:50 PM   #15
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Telling the wife I'm gonna spend $1500 to make my shaft 2.5 inches longer might be a tough sell. I'm hoping to gain some performance in reverse. I can't imagine having that square block is helping the prop thruster in reverse.
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Old 11-16-2015, 03:23 PM   #16
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Angler,
Cut the shaft and install a coupler. Don't remember the right name but it's a tube split longitudinally whereas the two haves clamp together on the shaft w socket head screws. I think it's called a "split coupling" and one long enough to extend the shaft 2" should'nt be hard to find. I don't think alignment would be good enough w a pair of flanged couplings. Now that I think about it I think "split coupling" refers to flanged couplings that are split. I've used a tube type coupling and it ran smooth and true.

Ski wrote;
"scalloping helps but not that much"
I thought about doing it to Willy when we put in a new stern tube. But considering the number of boats out there w squared off keels of all materials I came to the same conclusion. I'm an idealist though and the faired off keel is/would definitely be better. But it's not a days project
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Old 11-17-2015, 12:51 PM   #17
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If noise/vibration is the major problem, and you can access the hull plating above the propeller, some vibration damping tiles would be worth a try.

Something like this.....

http://www.soundown.com/images/2.3/d...10%201%20A.pdf
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Old 11-17-2015, 02:32 PM   #18
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Telling the wife I'm gonna spend $1500 to make my shaft 2.5 inches longer might be a tough sell.
LOL...Seems many wives would go for this!!!....
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Old 11-17-2015, 04:01 PM   #19
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I like the dampening material idea. The bed is over the prop and this could really help! I think I will still trim the steel back that is a cheap fix, I can do the work myself.
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Old 11-17-2015, 07:55 PM   #20
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I remember a video you posted of your turning radius in the Marina at Erie, Pa. It seems to me you have all the maneuverability you need, whats with the big need for reverse maneuverability? Whats the mission profile that justifies cutting and welding that will add to your situation? I have to say, that I know the marina and was impressed with your boat.
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