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Old 06-07-2014, 11:10 PM   #1
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stabilizer fin tips

hi. Has anyone added wing tips to their stabilizer fin ends. It significantly improves performance due to hydrodynamic flow improvement like on plane wings. New stabilizer fins have tips. Anyway, has anyone done it and how. Thanks
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Old 06-08-2014, 02:51 AM   #2
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hi. Has anyone added wing tips to their stabilizer fin ends. It significantly improves performance due to hydrodynamic flow improvement like on plane wings. New stabilizer fins have tips. Anyway, has anyone done it and how. Thanks
To be seen by all, the late 50s fins looked cool. But winglets on my Wesmars are hardly like winglets on a Boeing. Once my DeFever gets up to 40 knots like the Oracle then I'll bite.
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Old 06-08-2014, 07:05 AM   #3
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The aircraft wing tips make the wings some percentage more efficient, in ONE direction, lifting UP!

Boat fins work in 2 directions and I doubt if 5% better lift in one direction would reduce the fuel burn any measurable amount.
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Old 06-08-2014, 07:33 AM   #4
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I've got winglets on the fins on my Grand Banks. They were on from day one so I can't provide any before/after comparison. ABT says they improve efficiency, and they are a very competent engineering company, so I believe them. But I believe the advantages grow with speed, so a planing hull (like my Grand Banks) will benefit more than a slow hull. Interestingly, Nordhavn specs their ABT stabilizers without winglets. The rational is that the modest performance gain doesn't outweigh having something else to snag crap on, though that has never been a problem on my GB.
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Old 06-08-2014, 07:43 AM   #5
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Those "winglets" on stabilizer fins help at low speed, not high. They are disadvantageous at high speed. At high speed they just add more drag since fin angle at high speed is very much less to achieve the same force.

prevent water from flowing around the tip which decreases lift and produces a drag producing vortex on the unloaded side, that is why, unlike most aircraft winglets, they work both directions.
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Old 06-08-2014, 07:50 AM   #6
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I've got winglets on the fins on my Grand Banks. They were on from day one so I can't provide any before/after comparison. ABT says they improve efficiency, and they are a very competent engineering company, so I believe them. But I believe the advantages grow with speed, so a planing hull (like my Grand Banks) will benefit more than a slow hull. Interestingly, Nordhavn specs their ABT stabilizers without winglets. The rational is that the modest performance gain doesn't outweigh having something else to snag crap on, though that has never been a problem on my GB.
Here's a Nordhavn 62 with winglets on the stab fins.

Off topic: The little skegs on each aft quarter are kind of interesting, look aftermarket. Wonder if they were trying to cure a yawing problem?

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Old 06-08-2014, 08:17 AM   #7
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Interestingly, Nordhavn specs their ABT stabilizers without winglets. The rational is that the modest performance gain doesn't outweigh having something else to snag crap on, though that has never been a problem on my GB.
It was for this guy:
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Old 06-08-2014, 08:21 AM   #8
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Those "winglets" on stabilizer fins help at low speed, not high. They are disadvantageous at high speed. At high speed they just add more drag since fin angle at high speed is very much less to achieve the same force.

prevent water from flowing around the tip which decreases lift and produces a drag producing vortex on the unloaded side, that is why, unlike most aircraft winglets, they work both directions.
I know very little about this other than basic physics, so maybe you can help clarify it.

Isn't the spillage off the end of the fin present all the time, whether the fin is deflected or not? So a winglet helps reduce spillage and associated drag whether the fin is defected or not, right?

Does the spillage and associated drag for an undeflected fin go up as speed increases? I would assume it does.

And I presume the spillage and associated drag increases as deflection increases, right? So the winglet helps more as deflection increases?

Now of course the down side is that the winglet itself creates increased drag.

So at higher speed where there is less deflection required for stabilization, you are saying the spillage drag is less than the drag of the winglet itself? Calculating that out to figure out the cross-over point is way over my head.

What I don't understand is that an airplane wing flying at cruise speed seems to be more like a fin with minimal deflection at high speed. Where a fin at slow speed with lots of deflection seems more like a plane taking off and landing with flaps down for more lift. But planes presumably have winglets for cruising, not for landing and take off?
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Old 06-08-2014, 08:24 AM   #9
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It was for this guy:
Yes, but I'm not sure I agree with this being a good reason to remove the winglets. If you are going to hit things, you are going to break things. What else are you going to remove? The rudder? The prop? The keel?
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Old 06-08-2014, 08:58 AM   #10
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Yes, but I'm not sure I agree with this being a good reason to remove the winglets. If you are going to hit things, you are going to break things. What else are you going to remove? The rudder? The prop? The keel?
Well you can start with that fiberglass and foam hunk and really streamline things

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Old 06-08-2014, 09:05 AM   #11
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The winglets you speak of are dams or fences. The help some of the water from the high pressure side of the foil from leaking around the end to the low pressure side. I did put a dam on my lobsterboat rudder and noticed quite a bit more power but just in one turning direction (which would be fine with stabilizers or twin rudders)

You will see some ruddrers with a fence in its center, in line with the shaft. These are to isolate the most effective parts of the rudder where the propwash strikes from the remainder of the rudder also increasing its effectivness.

The fence I installed on my rudder was of 3/8" FG (from a cutout) and secured to the bottom w/ epoxy paste and screws. It was soon removed because it imbalanced the steering too much.
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Old 06-08-2014, 09:27 AM   #12
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Isn't the spillage off the end of the fin present all the time, whether the fin is deflected or not? So a winglet helps reduce spillage and associated drag whether the fin is defected or not, right?
If the boat is moving in perfectly calm water and not rolling or pitching at all, the pressure on each side of a symmetrical fin will be balanced and the only disturbance at the tip would be that created by the flat plate area that tip presents to the water flow. It would be minimal and unstable, and for our purposes in this discussion, it twists and burbles unpredictably. Call it a wash.

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Does the spillage and associated drag for an undeflected fin go up as speed increases? I would assume it does.
See the above. The drag goes up with speed. Without going into the very complex factors involved, it is safe to just say that the drag increases proportionally to the square of the speed ... high speed makes very high drag.

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And I presume the spillage and associated drag increases as deflection increases, right? So the winglet helps more as deflection increases?
See above. Drag increases as lift increases and lift increases with angle of attack until a certain point.

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Now of course the down side is that the winglet itself creates increased drag.
Yes, it presents a form and a surface to the water, form is the area of the winglet and surface is the skin resistance to water flow.

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So at higher speed where there is less deflection required for stabilization, you are saying the spillage drag is less than the drag of the winglet itself? Calculating that out to figure out the cross-over point is way over my head.
Proportionally, yes. A great deal of force can be created at a much lower angle of attack (angle of fin relative to the direction the water is flowing) at high speeds. This means that the fin is operating to its best lift over drag ratio. But, in real life since the fins are attached to a boat that is moving (or can) move in 6 different directions plus the fact that the fin is moving in opposition to roll, spillage is a rapidly moving target.

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What I don't understand is that an airplane wing flying at cruise speed seems to be more like a fin with minimal deflection at high speed. Where a fin at slow speed with lots of deflection seems more like a plane taking off and landing with flaps down for more lift. But planes presumably have winglets for cruising, not for landing and take off?
At cruise the wingtip vortices represent a significant source of drag, something around 20 percent or so. During takeoff and landing where both lift and drag are very high (at cruise there is only enough lift to keep the plane flying - at takeoff there is enough "extra" lift to put some distance from the ground) and wingtip devices that prevent air moving from beneath the wing to the upper surface help shorten takeoff runs, load more weight, and climb faster. I think Baker is the subject matter expert in this area, maybe he will chime in.
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Old 06-08-2014, 09:51 AM   #13
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Yes, but I'm not sure I agree with this being a good reason to remove the winglets.
I didn't say that it was, just posted a photo of one that broke when it hit something.
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Old 06-08-2014, 09:56 AM   #14
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This is common well known technology both on aircraft wing tips and boat rudders. Fences on stabilizers will no doubt help but you can get basically the same results by just extending them or making them bigger.

Largely it stacks up like this. You want X amount of effectiveness from your stab. That will require X amount of area from the fins. You can have low aspect ratio fins or HA ratio fins. Tapered or straight. Thick or thin. Fences or no. Bigger fences on one side only. One side will probably work better than the other. All the fence on one side. The real question is what is the best way to increase the effectiveness of the stabilizers? And then there's the question of how adequate are the fins to control roll on the boat? And there's the question of are the stabilizers strong enough to handle structurally any additional forces on the fins? The manufacturers made the fins the way they are. Are they over the top for strength v/s surface area or way far into the safe zone? The manufacturers made them the way they are. By modifying them you're saying they could have done better by employing your idea. And your idea is common knowledge.

So if you ask the manufacturers why they didn't put fences on the blade tips you'll probably find out why you should'nt change them. If you actually asked them they probably would say nothing for legal reasons and calling their lawyer probably would cost money.

And another real question is how much can one gain by doing this? Rudder fences are in the prop wash and high velocity flow whereas the fin fences on stabilizers are in slow moving water and probably won't make much difference unless you made the fences huge and then would you be better off by just making them longer?

Lots of questions and little to gain on this one. However before you call me a hypocrite I'll admit I've modified quite a number of products and at times w considerable risk and come out ahead and very happy w what I had done.
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Old 06-08-2014, 10:26 AM   #15
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Stabilizer fins are or can be downsized by the builder to keep wetted area the same as fins are added, thus no change in drag coefficient.
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Old 06-08-2014, 11:19 AM   #16
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Stabilizer fins are or can be downsized by the builder to keep wetted area the same as fins are added, thus no change in drag coefficient.
Don't know if manufacturers reduce fin area or not when end plates are added, but if an end plate is attached to an existing fin the end plate does provide added wetted area and form drag.
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Old 06-08-2014, 02:51 PM   #17
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One of our stabilizers.Click image for larger version

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Old 06-08-2014, 03:03 PM   #18
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Our boat came with them, a friend of ours boat did not. He decided to add them and is happy he did. He says he can feel a difference and he says they actually produce more lift and less drag. He's an MIT engineer and ex chopper pilot so who am I to argue. The only problem we've had is trusting dockside helpers with advice regarding underwater obstructions.Click image for larger version

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Fix was relatively quick and simple. They are designed to be the weak point in the system.


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Old 06-08-2014, 04:26 PM   #19
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According to the engineer at ABT the winglets on their fins add nothing at low speed to the efficiency of the fin. As I recall he said around 10 to 15 knots minimum so we removed them from ours. If Delfin ever reaches 15 knots it will be because she fell off a truck and I doubt they would help even then.
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Old 06-08-2014, 05:10 PM   #20
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cafesport wrote
"actually produce more lift and less drag"
More lift is a given but less drag is only achieved in aircraft by a reduction in wing angle of attack as a result of more lift. So the "less drag" is referring to the whole aircraft.
In a boat stab fin it's more drag to be sure if you add fences to an existing fin. there will be a reduction in drag when the fin is working hard but I'd guess the reduction would be less than the parasite drag of the fence itself.
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