Aluminum Framing for Wooden Boat Hatch

Steel and aluminum boatbuilding. See: "Boatbuilding Methods", in left-hand column of the Home page, for information about alloys.

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BayouBengal
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Aluminum Framing for Wooden Boat Hatch

Post by BayouBengal » Sun Jun 01, 2014 8:19 am

I'm wanting to use a fairly large (approximately 3' x 5') single engine hatch for my present build. Because of its large size, I'm thinking that I'd like to build the underlying framework out of aluminum angle iron instead of wood. The hatch must be strong enough to stand on. Would the aluminum framing maintain its strength in the hot engine environment? I'm not very familiar with metals, are there any cons to doing this that I've not thought of?

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mrintense
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Re: Aluminum Framing for Wooden Boat Hatch

Post by mrintense » Sun Jun 01, 2014 8:50 am

Well aircraft engine nacelles are made from aluminum and I can testify that they get quite hot. As long as you don't get a cheap grade of aluminum I think you will be fine. Consider too that regardless of how hot the engine compartment gets, if it gets too hot for aluminum, then it will be too hot for the wood in the boat structure as well.
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Bill Edmundson
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Re: Aluminum Framing for Wooden Boat Hatch

Post by Bill Edmundson » Mon Jun 02, 2014 6:58 am

Jeff

Structural grade aluminum is 6061 T6.

This might be a source. http://www.metalsdepot.com/index.php

Bill
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Kevin Morin
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Re: Aluminum Framing for Wooden Boat Hatch

Post by Kevin Morin » Sun Nov 23, 2014 5:09 pm

The topic is kind of old, probably all the work done is past but the question of mixing wood and aluminum is a good one, and there are some points that can be made to enable this combination of materials reliable instead of potentially problematic.

Just as we coat wood to keep out water, and in the worst case wood boring worms, so aluminum's surface is only truly self healing (automatically forming oxides that protect the underlying alloy from further corrosion) if the metal is allowed to have free access to the atmosphere, which is pretty hard to insure if the metal is bolted to wood; or even to itself!

What happens to both materials, wood and aluminum, is that external surfaces are not naturally sealed from exposure to various chemicals, including water, that can and do break down both materials. To use them with one another, they both need protection from the other and if they will be joined they both need protection from water as well.

A brief review of wood's problems. First water can come and go inside the cellular structure so the 'paths' become worn. Cellulose cell walls become weaker than original allowing the fibers to weaken, and also organic factors, yeasts, fungi and bacteria as well as some compound galvanic activities, all water born, can all contribute to 'rot' or reduced strength. It should be noted that bolting any bare metal to wood, allowing the joint to be wetted and dried repeated, or just wetted to say immersed, is not good practice.

The problems begin at the surface and go deeper into the wood fiber structure so coatings like epoxies, glues and sealants all help the wooden surface resist water and the problems that come with water.

Now, since I'm more of metal oriented builder I'll spend some more time on aluminum in the same vein. Aluminum marine alloys are generally labeled in the 50 and 60 series but the numbers are as critical as knowing the tree your boat lumber was cut from. If you use cotton wood to frame a boat, you know it won't last and if you use 6063 alloy extrusions below the waterline, you have realize they will dissolve in the salt water in a short time. So all aluminum alloys are not created equal anymore than Ironbark or Rosewood are equal to Hackmatack or Black Spruce.

Bare aluminum alloys will form a 2-3mil oxide layer from exposure to the oxygen in the atmosphere and that film will remain and stop all future degradation/oxidation of the metal if left outdoors on a non reactive surface (rock) indefinitely. If we scrape or sand that plate the oxide reforms in a few seconds. BUT... if you lay to pieces of aluminum together and leave them to have a thin film of water, they will begin to form a corrosive cell in weeks and in a couple year's time will have corroded the two inner faces with pits.

To use aluminum joined to wood there are a few rules of thumb to follow. Coat the wood with epoxy to seal the fibers, and seal the faces so water cannot get in between the two materials and form a thin film. The interface wooden surface can't trap or hold water which could act as a catalyst to create and acidic state of the joint.

Next isolate the aluminum from the epoxy sealed wood with a non reactive film of plastic. This can be sheet goods like UMHW, Nylon or a liquid sealant like 5200. To apply a liquid type sealant, fit the joint, apply sealant and tape the outer edge so the sealant can be screeded off on the tape before it sets up, and leave the fit without full force of clamps until the 5200 is set up. Then adding the bolts tighten so the film of sealant has good pressure insuring that no water will infiltrate the joint faces.

The idea is to keep a film of water from staying on the surface of the metal and forming a corrosion cell by deareating the water film which causes the water's neutral ph to shift toward the acidic.

Another step that can help is to have the aluminum pieces powder coated prior to assembly, where the heat flowing acrylic paint has a good bond to the metal keeping water from the underlying native alloy. Last and similar to powder coating is to have the surface of the aluminum anodized or electro-plated with another oxide film that is more durable than aluminum oxide.

Bare aluminum can be joined to wood if they are both treated with forethought with an eye to preventing water being in contract with either or BOTH surfaces for slightly different but very similar reasons.

Aluminum shapes, especially extrusions can be purchased in cross sections that are very helpful for coamings, brackets, hinges and other handy boat building uses. By thinking ahead, treating both materials as we know by experience is best for each substances' longevity both can be used to make better boats that last longer and give safe enjoyable service.

If anyone had questions about a direct application of mounting aluminum to wood please don't hesitate to post.

Cheers,
Kevin Morin
Kenai, AK
Kevin Morin

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