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Old 04-06-2021, 01:42 AM   #1
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Stabilizer fins - how exposed are they, really?

Recent thread on gyro vs fins generated several predictable comments about how fins are exposed and "who wants a 2-inch hole in their boat."

Got me thinking- how big is the risk? Seriously, people don't think twice about having a pair of spade hung rudders on a boat. No concerns over exposed props with struts. Yet somehow a well engineered and installed set of fins is vulnerable. Is this a case of true vulnerability, or just sour grapes from naysayers who don't have stabilization? Are there large numbers of broken fins, or just a couple stories that have been repeated a hundred times over the last 30 years?

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Old 04-06-2021, 04:15 AM   #2
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The likelihood of a catastrophic failure is very low but the severity of the potential failure would be high so.... the overall risk would be seen as low to moderate.
Acceptable? Some boaters are ok with this risk, but I choose to avoid it.

As other options are available, I'd steer clear of fins, exposed props and spade rudders. Those sticky-out things give me the heebie jeebies.
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Old 04-06-2021, 05:02 AM   #3
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I have wondered the same thing about "pod drives". They are designed to break off in a catastrophic failure incident with minimum water intrusion. i.e.: you probably won't sink immediately.

Still makes me wonder though. Also I wonder what the cost of repair would be if one was really severely damaged, say by a reef, semi submerged log or whatever.

I know you can spend $10,000 to $20,000 to repair a prop, shaft, rudder and supports, etc. What about a pod drive?

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Old 04-06-2021, 05:15 AM   #4
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I saw a 75-foot (ish) Lazzara in a St Pete area yard last year. It had four pod drives. Given the skinny water in these parts......

The only first hand account of a catastrophic failure due to damaged fin is from OldDan. If I remember, his N46 was improperly blocked in the yard and toppled over. The force of impact pushed the fin into the boat and was a total loss.

I do remember seeing a picture of a PH boat around 50-ft perched on some rocks in the PNW where the fin held the boat upright. Next high tide she apparently floated off with minimal damage. But that's from memory

When something bad happens in boats, seems the same story gets retold for years with slight variations. Net effect is one bad event is multiplied into a trend vs a data point. Wondering if fin failure is the former or the latter.

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Old 04-06-2021, 06:36 AM   #5
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I've seen two designs that deal with the potential of stabilizer fins breaking off due to a hard strike. One on a DeFever and the other on a Dashew FPB. They involved enclosing the inside of the hull fin mechanicals in a bulkhead compartment. Both vessels were aluminum builds.

The two stabilizer breaches I've read about with believable pictures and analysis were due to the vessel powering onto salty sandy bottoms and composite hulls not able to take the load at the stabilizer area. I've also heard from builders that solid GRP hulls do best when fin arrangements are part of the initial build as opposed to adding fins to long cured older hulls.

Really though Peter, avoid a hard grounding seems the best approach to negate hull breaches due to rudders and running gear ripped off and hulls breached. And replace bad hoses. My goodness we're doomed.

Oh well, time to enjoy your redone vessel.
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Old 04-06-2021, 07:35 AM   #6
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This is an ABT-TRAC stabilizer. The fin is hollow fiberglass, the shaft is about a third of the length of the fin. The shaft is designed to shear off upon a hard impact and the fin can be sacrificed by other impacts if need be. The metal plate is thru-bolted and sandwiched to the actuator inside the solid fiberglass hull. The shaft has two seals with oil between them to keep water out.

I sure don’t want to have to replace any of these bits and try my hardest not to but, I don’t lose any sleep worrying about it. To me, the convenience and reliability of these outweigh what I think is a fairly minimal risk.
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Old 04-06-2021, 07:56 AM   #7
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New builds for blue water sail are virtually all balanced spades. Properly designed and constructed they are as strong or stronger than skeg hung. In earlier designs all to often the rudder was holding the skeg on not the other way around. It’s very hard to construct a strong skeg as you need a large surface area where that structure meets the canoe body when building in grp. That limitation is one of the reasons high aspect keels break or compromise the canoe body in a grounding. Failure point is usually at the aft portion between keel and canoe body.
From what I understand quality builders like KK, Nordhavn etc. expect the installation of stabilization so design and execution is in accordance.
We have looked at nordhavn to purchase. One became entangled in the pendent from a mooring ball. That fin was ruined and mechanicals needed to be replaced or rebuilt. Structural integrity of the hull wasn’t compromised. My understanding is some stabilizer designs have break away tips. Most have a failure point designed in to prevent hull failure. But the components inside the hull can be overstressed and fail even in the absence of compromise to the hull. This could cause the fin to be unable to return to neutral. That could make steering difficult as the entire boat would tend to heel underway.
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Old 04-06-2021, 08:06 AM   #8
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My question with fins is how they age. All to often rudders fail due to water ingress. Not only does the interior metal skeleton corrode to failure but the core and grp skin fail due to delamination. To my uneducated eye fins look like a set up for this. Metal rod piercing a grp or plastic skin, multiple different materials inside and no apparent drainage.
Is this a problem? Are particular brands or times of construction more prone to this? Or am I wrong about my concern.
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Old 04-06-2021, 08:15 AM   #9
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The fin is hollow, has drainage holes on the bottom and is normally filled with water when immersed. The shaft has its own anode which barely ever erodes.
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Old 04-06-2021, 08:34 AM   #10
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I always wondered how fins would do in waters laden with lobster traps(?) If the vessel had twins, where the shaft, stanchion and prop weren't sheltered behind the keel, it could really be a tangled mess waiting too happen, but that's just a worrisome mind with too much coffee.....
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Old 04-06-2021, 08:57 AM   #11
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I got a little close to a crab pot last week. As I passed it, it appeared to snag on the starboard stabilizer fin for a moment, then slid right off. Most installs include a “kelp cutter” deflector to help keep objects from getting wedged in between the hull and fin.

Of course I worry that the fin and shaft could tear open the hull, but in millions of miles it hasn’t, to my knowledge, happened to a Nordhavn.

But I witnessed this nightmare scenario play out on an Ocean Alexander that hit a rock in SE Alaska. The fin made contact with the rock and cracked the surrounding hull. The coast guard dropped pumps and the boat didn’t sink. In talking with the owner later, it sounded like the flooding may have been handled by installed pumps, but initially it didn’t appear that way because limber holes were plugged with debris. Interestingly this incident only caused a few day delay to their travel plans. The boat yard in Petersburg simply cut out the entire area where the fin was attached and glassed in a patch. The owner continued cruising Alaska and had the whole thing fixed when back in Seattle over the winter.

Properly engineered and installed fins seem to present little risk, but I think it’s hard for most of us non-engineers to assess the quality of that engineering and installation.
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Old 04-06-2021, 09:29 AM   #12
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Some of the very recent fins aren’t simple nasa foils but rather complex curves. Do you think that presents additional risks of entanglement or damage?
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Old 04-06-2021, 09:35 AM   #13
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Quote:
Originally Posted by Pete Meisinger View Post

I know you can spend $10,000 to $20,000 to repair a prop, shaft, rudder and supports, etc. What about a pod drive?

pete
Shockingly high -- if the old pod can be retrieved and is not totaled, rebuild costs run $30K - $50K.

My fin stabilizers do not protrude beyond the hull so they are largely protected from hitting a dock, etc. Similarly the angled design of the fin, and the kelp cutter protecting it, have been effective at preventing kelp and line tangles (though my props and shafts have been rapped).
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Old 04-06-2021, 09:58 AM   #14
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Run your boat and enjoy it Peter. Don't sweat it, you should be fine.
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Old 04-06-2021, 09:59 AM   #15
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Anecdotal evidence. I have no idea of the percentage of failures of fin installations.

I've seen the after effects of fin damage on two boats.

#1 was loaded on a transport ship, I didn't get to talk with the owner. One fin shaft bent with the fin embedded in the hull. The other fin missing and a large hole in the hull. Twins with unprotected shafts, props and rudders. No apparent damage to the running gear.

#2 was on the hard in Anacortes with a missing fin and hole in the bottom of the boat. The owner said he hit a rock. No damage to the running gear, a single with well protected by the keel.

I've experienced two failures of fin systems

#1 A retro fit, poor installation. A squeak in the system lead us to examine it and find a joint failing. While watching the stabilizers work to diagnose the problem we could see the hull flexing and distorting around the mount to the point we were concerned about hull integrity. We shut the system down and locked it out for the remainder of the delivery.

#2 A failure of the hydraulic pump for the system. It failed with the fins not in the straight fore and aft alignment. We had to shut down and drift rolling beam to in 6 footers while getting the fins aligned fore and aft and locked down.

I watched a friend, an outstanding boat handler, have trouble bringing his boat into the dock. The system had failed with the fins "hard over" giving him extra rudders he didn't need.


I'm not a fan of fins.
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Old 04-06-2021, 09:21 PM   #16
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Stabilizers are great -not really an issue if you follow the OEM PM schedule for seal replacement. If you get a hard strike they are designed to snap off below the seal. This KK had a cored hull. Apparently aftermarket installer didn’t reinforce adequately & just installed the mahogany block over the inner skin.
Rock strike in the Rideau Canal tore a large hole & sunk in 10 minutes. Fortunately everyone got off safely & boat was refloated.
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Old 04-06-2021, 10:50 PM   #17
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I rode over a submerged tree on the Erie Canal a couple of years ago (don’t ask) and felt the bow of our 50,000+-lb boat rise before squashing it flat. Did some minor prop damage but also the only time I worried about what might have happened if I’d hooked a stabilizer. The fins don’t protrude past the width of the hull or below the keel and I know they’re designed to shear off with impact, but I’d rather not put that to the test. Still, if fins were a significant risk, I think we’d be reading more about sinkings (outside of gyro sales literature).
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Old 04-07-2021, 04:20 AM   #18
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Quote:
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Stabilizers are great -not really an issue if you follow the OEM PM schedule for seal replacement. If you get a hard strike they are designed to snap off below the seal. This KK had a cored hull. Apparently aftermarket installer didn’t reinforce adequately & just installed the mahogany block over the inner skin.
Rock strike in the Rideau Canal tore a large hole & sunk in 10 minutes. Fortunately everyone got off safely & boat was refloated.
Yikes! Now that's a catastrophic failure!

I personally am not worried a bit about my fins. There is no way I'd cruise an unstabilized boat, though the W36 is an extremely stable FD vessel due to its very high percent of ballast (6000 lbs ballast, over 20% of her displacement), and very low A/B which drives a very low CG.

Someone uptrend mentioned the appendage-vulnerability comments seem to show up in marketing brochures for gyro manufacturers. Outside of muckraking, seems lie its a lot of hand wringing over extremely unlikely event.

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Old 04-07-2021, 07:15 AM   #19
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To an extent, it's like any other design feature. If you've got exposed bits sticking out, you'll tend to plan and operate a little differently than a boat with no appendages, protected running gear, etc. There's always the unforseen events, but the risks you choose to take (speed in debris prone water, minimum depth you're willing to operate in, etc.) will generally be adapted based on vulnerability concerns of the boat you're operating.
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Old 04-12-2021, 12:48 PM   #20
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Stabilizer Fins

I had a fin knocked off my 58 Hatteras MY when I was forced out the channel by oncoming boats. I did not know it at the time, but the shaft was designed to break off outside of the hull. The thru hull remained water tight. I don't know for a fact but I suspect that is a common design feature simply because fins are so vulnerable.
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