Timber beam supporting boat during winter

[ajk1]

A friend stores his 3500± pound boat in his boat house at his cottage (summer home), by lifting the boat up after bringing it into the boathouse and inserting two wood beams, one near the front of the boat and the other near the back of the boat, and then lowering the boat onto the two beams. The beams are currently wood and he says the wood is rotting so he wants to replace the wood beams with steel beams, and he wants the beams to be as light as possible, because they have to be manhandled into place. I have not seen the boathouse. He says that the existing wood beams are "about" 9" square.

I would calculate the resisting moment of the new beams as though they are laterally unsupported for the full span. I think his concerns with wood rotting could be addressed by using pressure treated incised timber, then soaking the sawcut end of each timber in preservative, and as an added measure coat the entire surface with preservative even though the beam is pressure treated. Re-coat every few years.

My question is: would the timber beams eventually twist?

If so, then steel would be the better choice I think, although it would have to be protected from corrosion.

 

[JAE]

Steel is a stronger material than wood pound for pound. A square steel galvanized tube comes to mind here.

Or maybe a larger aluminum tube.

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[msquared48]

You also need to chalk the boat off the beams to avoid excessive deformation to the hull.

ie., you need to spread out the load the BOAT sees to its frame.

Mike McCann, PE, SE (WA)

 

[BridgeSmith]

msquared makes a very important point. The reason timber is typically used for that purpose is that it is soft enough to conform to the hull a little. If a steel section is used, it will require cushioning of some kind.

 

[ajk1]

Thanks very much to all of you. Your comments are very much along the lines that I was thinking. I realized before I posted this question, that an HSS was more efficient and generally a lighter than an "I" shaped section, but I did not quite realize that pound-for-pound, steel is a stronger material than wood (As an aside: I had thought that the reverse was true for axial compression, although the boat support case of course is not axial compression).

I had also thought of an aluminum tube, like JAE, but am not sure that it would be so readily available up in cottage country. It could be checked out though.

Galvanizing I had also thought of, but that increases the cost significantly, and this man has it in his head that the two beams should not cost more than $500. But the cost can be checked out.

I had also thought that there should be some cushioning effect so that the boat would not be damaged. I suppose that this could perhaps be done by placing a wood member such as a 2x6 laid flat on top of the steel member. But like you, I feel that the wood beams are the more natural solution.

I don't see an answer to my question about whether the wood beams may twist/warp with time. Perhaps that is in the link, which I will check out in a moment.

A follow-up question: Is there any deflection limit that the beams should satisfy? I don't yet know the span of the beams (will find out this weekend) but for an assumed span of 16 feet, deflection governs the steel beam design (not strength) if span/180 deflection limit is imposed.

 

[Canuck65]

Wood beams are typically used for this application in Ontario's cottage country. Seen it many times. Provided they are stored properly in the offseason (not lying in the dirt), they should last quite a few years. Whether or not it will twist and warp depends on how green the wood is when purchased, and where in the log the section was cut from. The dryer the timber, the less it will twist/warp as it approaches equilibrium moisture content. I am sure if your friend speaks to someone at Muskoka Lumber or TransCanada Lumber (or other lumber supplier, depending on where the cottage is located), they would provide some good advice. D.Fir and SPF and White Pine (Considered a Northern Species for calculation purposes) are readily available, however, I think D.Fir is cheaper in the larger dimensions (easier to find large D.Fir trees these days).

Bolting (and countersinking) a wood top plate onto a steel beam is commonly done in residential construction. This could provide some cushioning if you decide to proceed with a steel beam.

 

[Canuck65]

Re your followup question. In my opinion, there is no code limit for deflection in this case. I personally would not care if the beam sagged 1" or 2" or 3" while supporting a boat in a boathouse over the winter. I would apply common sense to what you are comfortable with re deflection and make sure it is plenty strong enough. Not much sense in refining the calcs to save $100 in beam cost to support a $50,000?? boat.

 

[racookpe1978]

I would, in fact, go the other way. The axial (side-to-side) sag is beneficial to supporting the boat BECAUSE it allows for sag. See, a boat is intended to remain in the water (obviously) .. but that means its hull is NOT intended to be supported at only two limited points under the keel. The entire hull is intended to withstand a uniform, very-low-pounds-per-sq-foot pressure along every foot of its length, every sq foot of its surface underwater. Drydock and launching cradles are carefully designed to "cup" the hull and keel very rigorously to prevent just such sagging and pin-point pressures your friend is deliberately doing for most of the ship's yearly life!

Thus, I would advocate exactly the opposite: A very soft (literally squichable in compression) softwood the length of the keel to protect the hull's finish and allow for minor hull shape deformations, this liner going at least as far as the last propeller shaft bearing support or shaft seal and rudder post. That on top of a more rigid (perhaps steel I-beam or tube steel) that is itself supported to each side several times by axial beams to spread the load out laterally and prevent tipping over under side loads. Prevents sagging into the dirt/mud/concrete (?) floor.

 

[JAE]

Something like this:

http://namio.co.vu/

Maybe not buy this one exactly but it offers a concept using steel/aluminum beams with a saddle arrangement to properly support the boat - could be custom designed to your boat's bottom profile.



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[rb1957]

I'd put a foam rubber or similar pad between the support and the boat.

Shipping cradles I've used have adjustable pads, to support the faces. Don't like the idea of supporting the boat on the point of the hull (what it sounds like). I'd make a rough cut former from ply and 1" - 2" thick squishable pad ... maybe stick a wider pad on the hull for more distribution of the load ?

another day in paradise, or is paradise one day closer ?

 

[SAIL3]

one concern that I would have on a closed HSS section , even if galvanized, is that any corrosion on the inside is difficult to detect if it is in a corrosive atmosphere like close to the sea....

 

[dhengr]

Ajk1:
Why not a few rubberized and inflatable liquid storage bags? They could be made to length, to fit the approx. width of the slip/channel in the boat house. The boat is lifted as usual, the bags are dragged under/across the slip and inflated with compressed air, to float the boat out of the water, and better support its bottom shape. They would also probably tolerate the frozen water below them during the winter. They would be hooked to a compressed air manifold which was hooked to a small compressor and pressure gage which was left on during the winter. With a little more work, the bags could have temporarily side forms applied, with the boat still on the lifting beams, or whatever. Then, they would be filled with expanding foam and allowed to cure to the bottom shape if the boat. The idea of the side forms is so that you cast an 18”-24” wide cradle, not a 5’ wide sausage, which conforms to the boat bottom.

 

[ajk1]

Thank you all for the erudite comments. I had no idea that it was so involved. I am going to send all the replies to this friend (actually a friend of my brother's; my brother will be up there this weekend so he can show him all these comments). Since the fellow has been storing his boat this way, on wood beams, for some years, long enough for the wood to rot out, I expect that he would know if the wood beams have done any damage to his boat.

Thanks again.

 

[BridgeSmith]

I envision a straight sling system attached to an overhead frame (gantry?) with heavy-duty ratchet straps (like the load tie-downs for flatbed trucks - 2 front and 2 back). This would provide a mechanism to lift the boat out of the water (which is needed anyway, right?) and storage with the load somewhat distributed.

 

[CARunderscore]

If the boat came with a trailer, that might offer some clues as to how it should be supported.

Supporting the boat without damaging the hull is probably the most important thing here, but to toss a structural concern back into the mix: what is the deck of the boathouse made of? Wood? Stone? Home Depot Special concrete? That's what it sounds like is going to be supporting any beams. It's also the one thing that could cause deflection to actually matter: if the middle sags and the ends lift up, the bearing area would be greatly reduced. Even if it's not a functional issue, there might be some cosmetic damage, which may or may not matter depending on how fancy of a boathouse it is.

 

[rb1957]

if something has worked for several years ... don't change it ! If the lumber is NFG after several years ... is that acceptable ? if so get new lumber. if the lumber looks to be on the way out, get new lumber (it'll be the cheaper solution).

I assume boat is something like a boston whaler (or a "tinnie"), and not a sailboat (with a keel).

another day in paradise, or is paradise one day closer ?

 

[msquared48]

Rb...

Being in a bosthouse, yes.

Mike McCann, PE, SE (WA)

 

[Settingsun]

See, a boat is intended to remain in the water (obviously) .. but that means its hull is NOT intended to be supported at only two limited points under the keel. The entire hull is intended to withstand a uniform, very-low-pounds-per-sq-foot pressure along every foot of its length, every sq foot of its surface underwater. Drydock and launching cradles are carefully designed to "cup" the hull and keel very rigorously to prevent just such sagging and pin-point pressures your friend is deliberately doing for most of the ship's yearly life!

It depends on the boat but I wouldn't say this is typically the case. Some 'alarming' images below. Dry stacking boats is common so they are designed to handle being pulled out of the water by various means then supported. They also have to be repainted but half the hull doesn't get painted if half the hull has to be supported to spread the load.

The current scheme is two beams running across the boat. Often they run along the length of the boat (one of the images below). Not profiled to match the hull shape, just standard rectangular sections. But, if it isn't broken...

 

[ajk1]

To Canuck65 - Thank you for the very detailed and useful info. Would the wood beam be the dressed lumber size as listed in the Canadian Wood Design Handbook, or would it be the rough lumber size that should be used? I assume all these species can be properly pressure treated.

 

[Canuck65]

ajk1,

In order to easily get lumber that is 'graded', it would be dressed lumber size, as in the handbook. They typically do not grade rough sawn lumber without jumping through a few hoops. They should be able to pressure treat whichever you choose, however as I have learned, pressure treatment is much more complex than many people understand. The different woods take the pressure treatment differently. There are also different grades of pressure treatment available upon special order. The standard 'deck board' type pressure treatment is ok, but there are much better options available. I spoke with a fellow at the CWC in Ottawa a year or two ago and he was quite helpful. Can't remember the name, but he was knowledgeable in wood preservatives, and subsequently we did specify some upgraded pt for some direct buried wood posts for a tall wood fence.

To make things easier, if it were the beams to support my boat (if I had one , I would likely get the normal preservative treatment and then apply my own stain/preservative, or perhaps stain untreated timbers. There will be a cost/benefit your friend will have to consider. All winter it will be inside a boathouse and generally a dry environment, unless their are bubblers in use.

Some other random thoughts:
- my guess is the most cost effective will be D.Fir No. 1, good combination of strength and $$$
- don't forget that if you are supporting the boat on two beams, the rear one typically has more load due to the location of the engine. The load is not split 50/50.
- to allay some of the comments above, it is quite easy to add some chocks and carpeting to the top of the beams as a method of padding and distributing load away from only the keel.