Update of keel backing plates -2

Seeking advice for a project or do you want to share the outcome of one? Various technical tips and questions.
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Bruce3477
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Joined: Wed May 11, 2011 6:10 am
Location: South Africa

Update of keel backing plates -2

Post by Bruce3477 » Sun May 15, 2011 10:33 am

Dufour 34 keel structure re-build – 2004 34P
Pre-amble
The original hull is thick solid GRP (19mm?)
Keel stud grid is (from aft) A:20mm /B: 2 x 20mm/C:2 x 20mm/D:2 x 20mm/E:2 x 20mm/F: 1 x 14mm
Original backing plates were 60 x 60 x 5mm low spec s/steel
Nuts were A4-70
Problems:
1. The keel floor had not been cast flat, but was very irregular as moulded, and the backing plates had buckled into floor to take torque load of nuts.
2. Areas C & E had the plates at about 20* sideways from nut base level with result that only a small part of nut was in contact with backing plate.
3. Areas B & D had the plates sloping down (fore/aft) between at about 10* from nut base level, and about 5* sideways variation, with result that only a moderate part of nut was in contact with backing plate.
4. It appears an automotive wax coating such as “tectyl” had originally been sprayed over nuts/plate to protect from initial corrosion.
5. GRP base areas of A, C and E were at a low point in bilge and would have been exposed to any water ingress on a regular basis.
As time went on, and keel loads caused minor movements of nut into plates (due to ‘hot’ loadings) and stale water collected between the nut and backing plate, severe corrosion and electrolytic reactions took place.
6. In the case of A, the nut ate right though the backing plate, and underwater keel (aft) moved away due to lack of support and caused water ingress from broken underwater bedding and keel flexible beading.
7. At time of this project, all plates were in a state of bad corrosion and nuts had eaten into backing plates in a serious manner. In some cases the hex shoulders of nuts were the only part in contact with the backing plate. The studs were in good condition.

Part 2 – rebuild of inner hull areas.
8. All keel pad areas were ground to remove gel coat and prepare a solid base.
9. Multiple layers of glass (Chopped strand mat) were glassed over with polyester resin to form level bases and to raise base level above floor level a small amount for drainage away from said support areas.
9.1 In the case of D & E, and with hollow casing in floor between said areas, substantial build-up was made with a “dam wall” due to extreme angle of floor to nuts base levels.
10. The areas were finished off with gelcoat to protect finished areas.

Part 3 – Mechanical and fasteners
11. Special threaded plugs were made to fit around keel studs while glass levels were raised, and when removed provides clean studs and no resin over-flow into stud area/flex sealer. Coating with release agent when used.
12. Special carbide tipped hole saws were thereafter used to clean area around studs and provide a good clean 25mm recess for flexible bedding filler and about 6 -10mm deep depending on area built up.
13. Special temporary backing plates were made 80 x 60mm with 20mm tube to pull down into glass (when setting) and provided a good and level flat bedding area. A nut was tensioned lightly above (on threads) to provide pressure on plate. Plate underside covered with release agent.
14. Special plates were made of 8mm thick 316 s.steel for all new backing plates, with edges beveled underneath to prevent “hot spots”, and with a counter bore underneath centre hole to provide a better “o-ring” bond of filler below plate. 9 pce of 80mm x 60mm (replaced 60 x 60mm) and plate A was 150 x 50mm x 8mm (replaced 60 x 60 x 5mm)
Part 4 – final assembly
15. All 20mm studs were re-cut with Die nut (M20-2.5mm) to remove damage from plates buckled against studs thread, corrosion, and for clean tightening of new (A4-70) nuts. Then cleaned with thinners to remove any residues.
16. All recess holes around studs were filled (proud) with flexible PU and plates fitted firmly onto glass beds.
17. Tef-gel paste was fitted on top of plate between nut to provide galvanic protection.
18. Lockite 270 was used on thread for secure hold.
19. Nuts were tightened equal and off-set from centre and working outwards. A long tube was fitted on T-bar (19mm bar/30mm socket) for approx 1m leverage and torque to about 250NM. The nominal setting with extended T-bar socket was extended by about 1/3 of turn from tight, impact of about 0.80mm compression under full load.
19.1 The keel join (underneath hull) was fitted with sika 291 around hull/keel join and into all recesses, and hand faired.
20. Final finish was to fit secondary locknuts above primary nuts, fitted with tef-gel between nuts to prevent galling and secured with Locktite 270
21. Backing plates then sealed around edges with Sika 291

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amusgrave
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Post by amusgrave » Sun May 22, 2011 2:10 am

Was this covered under warranty?
Alan Musgrave
VICTURA
D34-P, Hull #106
Toronto, Canada
http://sailorman-blog.blogspot.com/

Bruce3477
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Joined: Wed May 11, 2011 6:10 am
Location: South Africa

Post by Bruce3477 » Sun May 22, 2011 7:32 am

Had very little joy in past with warrantee issue(s), so did not bother this time. As yacht was 7 years old (#22x), would be pushing luck!

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amusgrave
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Post by amusgrave » Sun May 22, 2011 6:41 pm

Interesting. Did you purchase the D34P new or used and do you think this was a build quality issue. Haven't heard many complaints about keel joint issues on the D34P on the newsgroups. My warranty on the hull is 10 years here so I wonder if this type of problem would get any assistance from Dufour as it sounds like there were some underlying build issues. I haven't had any warranty claims on my boat since the first year when we had a few voids repaired.
Can I ask how expensive this repair was. It sounds a lot more involved than just dropping the keel and resealing.
Alan Musgrave
VICTURA
D34-P, Hull #106
Toronto, Canada
http://sailorman-blog.blogspot.com/

Bruce3477
Posts: 20
Joined: Wed May 11, 2011 6:10 am
Location: South Africa

Post by Bruce3477 » Sun May 22, 2011 10:11 pm

amusgrave wrote:Interesting. Did you purchase the D34P new or used and do you think this was a build quality issue. Haven't heard many complaints about keel joint issues on the D34P on the newsgroups. My warranty on the hull is 10 years here so I wonder if this type of problem would get any assistance from Dufour as it sounds like there were some underlying build issues. I haven't had any warranty claims on my boat since the first year when we had a few voids repaired.
Can I ask how expensive this repair was. It sounds a lot more involved than just dropping the keel and resealing.
I purchased this yacht new in late 2004. With earlier reports of the Bavaria 42 Match which dropped a keel in Croatia?, I was looking for a solid boat for our rough ocean off coast of S. Africa. At 5.5 ton, the D-34 is built very solid. Subsequent to the above loss, there were reports of a Farr 38 losing a keel (Australia) and in S. Africa a local 42ft race boat lost its keel/sank in previous Mauritius to Dbn race.

With the D-34 heavy build, perhaps the backing plates were enough if are at least 3/4"/19mm solid glass build above the keel. At time of rear backing plate failure, all of rest of backing plates were re-torqued, so keel movement on rest of keel area was nominal to nil. The keel seel between keel and hull was still good (with exception of last 100mm of keel area which was cleaned out and packed with new PU flexible filler.

If the original build/assembly had torqued down the keel nuts at approx 250-270nm, the plates would have bent to fit the (inner) hull profile, at the expense of local 'hot spot' contact points between nut and plates. With the tectyl/wax ? coating over the nuts/plates, there should have been no corrosion until any movement/re-torquing between the nuts and plate broke the waterproof bond and dirty bilge water started the corrosion process.

As mentioned above, I often sail in rough waters, and the forces on keel are huge enough that original (5mm) backing plates may flex when drop into a trough between large 'wind chop' driven waves.

Warrantee issue? I did not consider this issue at all, but when sailing in big seas, the safety of yacht and crew was prime reason to make sure build was to highest specifications...

From an engineering point of view, could not accept a torqued and bent plate as sound locating of keel loads, hence the grp building up of beds to provide a level base for backing plates and a solid spread of nut surface against the backing plate. i.e. the nut/backingplate surface was at very close to 90* of keelbolt angle.

The cost. Most of this project was owner managed, and direct cost of labour, materials and special parts would put at about US$500-750 excluding crane and yacht parking costs

Lars
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Location: Sweden, Soedertaelje

D34 keel

Post by Lars » Sun May 29, 2011 8:53 am

Hi my D34 is no 42, from 12 july 2003.

I have not exeperienced any keel problem.

Even if Dufour has high quality build spec
--a singel workman--
if he takes the wrong item and use it--is always a warranty issue-.-because of;
" The Dufour Yachts are all built to Bureau of Veritas production approval"

If proven that the yacht is not built to these data, the Dufour Yacht is responsible

Peter Richardson
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Joined: Thu Jan 31, 2008 11:42 pm
Location: England, Ipswich

Post by Peter Richardson » Fri Jul 22, 2011 4:54 pm

I've come across too many manufacturing faults on my D34 - Hull No 631 to have any trust in Dufour workmanship.

Last winter the sealant at the back of the keel was coming away. This is fairly normal in any yacht with a short keel base and heavily tapered back end. The sealant is placed with the boat's weight on the keel but when the boat is in the water the boat bends away from the keel a little as the rear bolts are a long way from the back end of the keel. However, the sealant was also coming away at the front end. I scraped this out and found a complete void between the top of the keel and the fibreglass up to about a foot from the front of the keel; about 5mm deep at the front tapering away to the keel being in contact with the fibreglass at the back. Apart from the sealant round the edge there was no trace of any sealant ever being in place between the keel and the hull.

I got as much Sikaflex 291 as I could into the void and then took the cabin table out to get at the keel bolts. The front bolt nuts were very loose. Once they were torqued up a lot of the Sikaflex squeezed out but the keel did not come into direct contact with the hull. None of the rest of the nuts were anything like as loose but I got the best part of a turn on all of them. I am now waiting to see what the peripheral seal looks like when I lift out this winter.

I know you cannot manufacture GRP to metallic engineering precision but I would have expected the base of the hull to at least be sanded flat to take the top of the keel which it clearly has not. I would also expect the keel to be offered up to the hull with a thick bed of sealant in between as is standard practice in most firms. Again this clearly did not happen. It is impossible to say whether the nuts were never torqued up properly in the first place or whether they have worked loose but I know which option I believe. I also find it odd to have just one thin S/S stud at the front of the keel rather than 2 thicker ones as further back. The keel is not particularly narrow at the front.

I strongly recommend those who have not already done so to get hold of the appropriate sockets and a large torque wrench and test the security of their own keel bolts.

Bruce3477
Posts: 20
Joined: Wed May 11, 2011 6:10 am
Location: South Africa

Post by Bruce3477 » Fri Jul 22, 2011 7:41 pm

The keel to hull join on my D-34 was tight ( < 0.5mm) alongside all the M20 studs, and about 1mm at front of keel (M16 stud). When I retorqued the M20 nuts, was very little movement of original sealant, but the M16 when torqued did squeeze a small ridge (white sealant) out of this join.

Can only think on your yacht, that when the hull was lowered onto the keel, that torqued the nuts from aft... Perhaps I was lucky, as the 40th Anniversary yachts had a bit more effort put into build?

But, the aft 20mm nut is the most critical item, and any flex on backing plate would lead to movement and seal failure.

NLL
Posts: 1
Joined: Sun Jun 28, 2020 11:54 am

Re: Update of keel backing plates -2

Post by NLL » Sun Jun 28, 2020 12:34 pm

very interesting account , thankyou ..A couple of questions
Did you not drop the entire keel and check that the keel profile in both directions matched the hull profile
Why did you feel the need to use Loctite on the keel bolts when lock nuts are always used
Could you not have made individual 316 stainless steel plates which were wedge shaped to match the hull slope ( and still tapered of on all four sides underneath to avoid a "hard" contact
Kind regards
Nigel

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