1987 Hunter 31 Mast Step Repair

Discussion in 'Boat Design' started by SEAHunter31, Nov 6, 2016.

  1. SEAHunter31
    Joined: Nov 2016
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    SEAHunter31 New Member

    I have a 1987 Hunter 31 that requires removal and replacement of the wood between the compression post and mast step due to water infiltration. I'd like to know your thoughts on the standard repair, and what I think to be a more durable repair.

    I have attached photos from above and below to give you an idea of the area. There is a hollow fiberglass cabin top beam supported by a 4"x4" compression post under the mast step. There is a 1/4" aluminum plate glassed in below the mast step. Below the aluminum plate is a layer of fiberglass and composite material. Below the composite material (and within the hollow beam) is an 18" long 2"x4" wood member loaded perpendicular to grain.

    The standard repair method is as follows:

    1. Cut out the bottom of the hollow fiberglass beam. Remove the decayed wood, and install an aluminum block in its place. Reinstall the compression post directly against the new aluminum block. The fiberglass beam is typically not repaired.

    I see two possible problems: crushing of the deck materials due to an undersized aluminum shim, and a weakened fiberglass beam due to cutting out of the bottom and not repairing it.

    The proposed solution is as follows:

    2. Cut out the gelcoat from the topside, and remove all layers down to the underside of the fiberglass beam. Fill up the area solid with fiberglass/G10.

    The reason people choose option 1 seems to be cost. It's substantially more to go with option 2. However, I feel option 2 to be the most durable.

    Thoughts?

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  2. SukiSolo
    Joined: Dec 2012
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    Location: Hampshire UK

    SukiSolo Senior Member

    I'd hazard a guess that the real culprit is the PVC tube not being fully sealed along it's length. Rain water and spray has eventually got in with the inevitable result. The structure itself would appear sound in principle, the aluminium acting as a comression plate and the end grain spreading it across the beam. Not sure about the compression strength of the 'non porous' layer - if solid glass then not a problem.

    Personally I'd pretty much replace the rotten timber with similar scarfed and glassed in from the underside. One big diference would be to seal the timber thoroughly with at least 3 coats of epoxy maybe more because end grain can really soak it up. Some species which are suitable like Doug Fir do need quite a bit of saturation. Then ensure the PVC pipeway is dealt with, maybe create the tube as an epoxy lined passageway - a polythene tube of smaller diameter can be pulled through after epoxy has set around it. It could be a shrinkage issue with the PVC and the other materials. I'm also assuming there is a silicone mastic at the upper through ho;e to mast step. Only other thing is to watch out for is I do not see a limber hole in the mast step? This would be worth having so water from inside the mast could drain freely rather than sit around the wiring through hole. Maybe file a half round groove or two so water can escape on the underside of the step? Raising the hole (for the wiring) locally above would also help a little.
     
  3. SEAHunter31
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    SEAHunter31 New Member

    The mast step drainage path is shown below. What happened is that a previous owner had filled the drainage path completely with sealant, and also installed a spinnaker chock against the drainage path exit. For years, all water that went into the mast was trapped. The pipe in there before was fairly tall, so I'm guessing there was a water column of 6"-12" at all times. Recently a rigger drilled a small hole in the back of the mast, and he said water poured out. When the mast was pulled, water was now up to the top lip of the mast step.

    "the aluminium acting as a comression plate and the end grain spreading it across the beam"

    What do you mean by end grain spreading it across the beam?

    "replace the rotten timber with similar scarfed and glassed in from the underside"

    I believe the original design is an 18"x2"x4" notched piece of wood epoxied to the underside of the deck. To get access requires a large hole cut in the beam, which would need to be repaired. My concern was cutting into this beam, I'm assuming it would need to be repaired.

    Why are you recommending wood vs other materials? Aluminum seems to be the material of choice for repairing this issue.

    I have not seen anyone do a topside repair, and everyone has been recommending against it.

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  4. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Thanks for showing the limber hole on the step - it was not obvious on the top shot. I always build one in if replacing a wooden step, and your right blocking it is the worst thing to do.

    In terms of the beam, don't forget that the glass wall are also part of the stiffness and there is a slight return which helps too. The aluminium section the main compression post fixes to is benefitting from the greater compression strength of the end grain timber (part rotted) compared to side grain loading. I can't actually see the problem with digging out the existing rotten timber from the underside right up to the non porous layer. A multitool and a couple of decent chisels should let you do this. Importantly it will let you really find the limit of the rot, it is important NOT to leave entrapped rotten timber in the structure taking the load.

    Is the whole of the timber rotten? or just a part? If you are comfortable working aluminium you could use it but it's a big block and maybe harder to bond in as being a fairly reactive metal it is not easy to get a good bond. Timber is mostly more easily available, easier to work and fettle and actually plenty strong enough. You could even use side grain IF you check the compression loading for that species. For example I would be quite happy to use laminated Doug Fir with very tight grain, oak, rock maple etc. It could be done with the laminate offering the grain in a vertical direction only. This way may allow the designed curve to be easily achieved. It is also possible assuming the ends of the existing timber (beam) are sound to scarf in from underneath new timber in exactly the same grain orientation as you already have.

    If the entire beam is rotten and the internal glass beam needs opening up, I'd still do that and rebuild from the inside. It's not too hard to scarf glass onto glass, just the gelcoat finish to get sweet. However even this is not so hard. The only really difficult thing would be if you HAD to cut the glass sides back flush which would leave an awkward turn to a joint. Best to cut before the turn and dig out the rot above. The new timber can be glued to the old glass just fine and much easier to clamp.

    The only real difficulty is that resin etc will want to drip down so you need some good poly sheet, scrap cardboard etc to prevent messing the inside.

    Yes, you could build up the entire thing bottom up in glass but it will take a hell of a lot of solid filler as well. This also means a considerable resin quantity.

    It is also possible to do it from the top, same logic but you will have the added dificulty of scarfing in a new non porous layer! as you'll have to go through this to take out the wooden part of the beam. It also presents more area of 'loose' glass at the sides (and bottom) of the beam which still have to be bonded - just pouring resin in will result in it exiting through the rectangular slot - but you could seal that with a transparent sheet ie PVC with release wax.

    The roof beam is also acting to spread the load from the step to the shrouds/stays, it looks like there is a stainless tube on the stb side taking this load downwards?
    For this reason you still need reasonable transverse strength in the beam. So the side walls of the beam are important in that regard.

    The tube you have/had for the wiring would still have been leaking at the side where it goes through the roof beam so you still need to sort a slightly better water path. I suggest bonding a small length of tube to the glass inside the step. Even if 20-30mm high and with another poly/pvc tube inside it would give an extra 'step' for water to have to jump.

    BTW the compression post appears to be wooden? Just measure that cross section. Even Sitka Spruce will take 1.5 tons + per sq inch compression and denser timbers very considerably more.
     
  5. kilocahrlie
    Joined: Nov 2016
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    kilocahrlie Junior Member

    I thought perhaps the replacement timber should be submerged in 240 degree wax until it stopped outgassing, then removed from the hot wax bath, allow it to cool wet with wax until it hardens, towel off excess wax, and allowed to dry. If left out for a while so that it looks like old, weathered wood, now apply either the epoxy or boiled linseed oil and install it.

    Your main gain lies in a superior member and superior attachment to other structural members. Wax, epoxy and /or linseed oil will give it excellent wet service. Sealing it up will forever be a maintenance chore. I'd prefer it was vented to allow drying instead of sealing it. Just thinking out loud - consider or disregard as you wish.

    Best of luck.
     

  6. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    I prefer to encapsulate the new dry timber in epoxy. However the through hole needed for the masthead (and any other position) wiring should be run through a fully sealed passageway. This can be achieved by either completely filling with epoxy and redrilling with a smaller diameter or a lot of coats applied in the hole, say 5+. Once encapsulated there should be no real problem. Best to apply the epoxy in a steady or cooling temp so it is 'sucked' into the grain on the first coat.

    Waxes tend to prevent epoxy binding. Did you mean Polyethylene glycol? Linseed oil will prevent a bond to the fibreglass and any epoxy coating and God forbid any silicone in the mix....

    Sealing it forever is not a maintenence chore in this instance as there is no route for water to ingress. In other devices such as electronic enclosures ie navigation instrument clusters (which I have worked on) sealing/venting is a different issue. Same with external woodwork such as grab rails etc
     
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