beam to support weight of boat 1

[ajk1]

Given:
Need to store boat over the winter, in boathouse. The boat will be supported on 2 beams, that span 14 feet on centre. Owner tells me that boat weighs 3500 pounds (seems a lot to me) and that I should assume that two-thirds of the weight is on one beam only i.e. 2333 pounds point load at midspan.

Note: Currently the beams are timber, 5.5" wide x 6" deep, based on my on-site measurements. I would assume the load is applied as a point load at mid-span of the beam. When I check the flexural capacity based on #2 spruce, the beams are nowhere near to figuring. Even if I assume #1 Douglas Fir, the Mr is only about half of the required moment of resistance.

Required:
The owner wants to find beams that weigh less so that they are more easily manhandled into place. The beam would of course be laterally unsupported.


How to determine a lighter weight beam than the existing 9 pounds per foot wood beams. i.e. shape, material, etc. The only thing that I can think of would be aluminum tubular section.

Questions:
What do you suggest for beam material and beam shape? If you agree aluminum section, would it be 6061-T6 grade and what size? I can size it in steel, but not experienced in aluminum design.

 

[Settingsun]

Long thread, don't mind me if these points have already been covered.

Do the beams have to run athwart? I gather that's how is planned hence the use of a saddle. Do a google image search for 'dry stack boat storage' - the beams are usually run along the length of the boat, each side of centre. A better sharing of the load could be assumed in most cases (50/50) and better than midspan point load.

If necessary, you might be able to stack beams parallel/perpendicular for better distribution as above and better placement of load relative to the supports, still keeping the athwart beams.

Alternatively, you might be able to support the boat on small chocks that result in the load applied to the (athwart) beams closer to the supports, reducing bending moment and deflection. Google image search for 'boat stands', you often need only a few small contact points with the hull.

Boat owners usually know what's ok as far as loading the hull. You should also find out the location of the centre of gravity rather than work from a rubbery assumption of 2/3 load distribution. That sounds like it's from the rule of thumb that COG is 1/3 from stern but disregards the placement of the supporting beams.

I too remember a similar topic.

 

[ajk1]

Long thread, don't mind me if these points have already been covered.

Do the beams have to run athwart? I gather that's how is planned hence the use of a saddle. Do a google image search for 'dry stack boat storage' - the beams are usually run along the length of the boat, each side of centre. A better sharing of the load could be assumed in most cases (50/50) and better than midspan point load.

If necessary, you might be able to stack beams parallel/perpendicular for better distribution as above and better placement of load relative to the supports, still keeping the athwart beams.

Alternatively, you might be able to support the boat on small chocks that result in the load applied to the (athwart) beams closer to the supports, reducing bending moment and deflection. Google image search for 'boat stands', you often need only a few small contact points with the hull.

Boat owners usually know what's ok as far as loading the hull. You should also find out the location of the centre of gravity rather than work from a rubbery assumption of 2/3 load distribution. That sounds like it's from the rule of thumb that COG is 1/3 from stern but disregards the placement of the supporting beams.

I too remember a similar topic.

The beams run transverse to the long dimension of the boat. There is no other way to run them, given the layout of the boathouse which is for just a single boat at the man's cottage.

How can I find the location of the COG? Would the boat manufacturer have that?

I will see if I can find a similar topic...perhaps I asked this question here last year but did not pursue it much at the time, or maybe someone else asked a similar question...

 

[ajk1]

Do you know where FRP beams can be purchased in Ontario?

 

[3DDave]

An interesting factor to deflection is this. If one divides the modulus of elasticity by the density of the material there is a negligible difference between spruce, aluminum, and steel. If there is a fixed maximum depth to the beam then steel is the best bet as the reduced wall to resist the bending moment will produce the greatest section modulus. However, it will also produce a potentially more delicate beam as the ability to resist forces normal to those surfaces, probably not a consideration in this case, but one reason that aluminum is used for airplane skins - wrinkle resistance.

I'm curious as to what the wood that seems to work OK is. Dry spruce should be 6 pounds per foot on a 5.5 x 6 section, not 9, for a total weight of just 90 pounds.

An aluminum beam of 90 pounds and the same outside dimensions will have half the deflection but cutting the weight in half will mean the deflection is almost the same as spruce should be. The stress in the wood part ought to be 6 ksi tensile; the aluminum with half the weight will be 40 ksi.

 

[DTGT2002]

https://www.aluma.com/us/products/formwork_shoring/shoring/aluminum_joists_and_stringers

You would want the 7.5" Aluma Stringer if you are pursuing the formwork beam route, nominally twice the capacity of the 6.5" Aluma Beam.

The FOS related to their noted nominal capacity is typically 2.2:1 for Aluma, so the big question would be what would be appropriate here. It should also be noted that in use in formwork, one typically has purlins at a regular spacing on the stringers, and the purlins are typically secured to sheathing/plyform. In your case, you would need to determine if you consider the boat adequate to secure the system or if some additional bracing would be appropriate, amongst all the other concerns.

Stock sizes are 10' to 20' in increments of 2' for the US market.

 

[TLHS]

There are several FRP manufacturers/distributers in Canada.

Just do a google search and start calling. Since shapes aren't standardized and each company will only have one or two weights of a given depth you might have to check a few companies if you're trying to optimize.

These guys are the ones I have brochures from, but there are definitely others.

https://www.fibergrate.ca/products/dynaform-structural-shapes/

 

[ajk1]

Thanks. I will follow up.

 

[HouseBoy]

This is just a crazy thought..... would it be possible to utilize cables across the boathouse?
The boat would rest on them like a sling. You might need a simple strut or two to keep them spaced properly.
Seems like it would drastically reduce the weight. Of course the sidewalls of the boathouse might need some improvement but that might be simple enough.


One other thing about the plywood box beam... my understanding is that "marine grade" plywood is not the same as pressure treated plywood. I think marine grade more of an appearance and not an end use characterization. I know it is used on boats but I thought it requires substantial finish/sealing.

 

[ajk1]

ok. Thanks.