It could actually be stronger. Grain direction, pith inclusion, knot inclusions and method of "sistering" would all play a role in the end product and strength there of.

The other consideration is, define "stronger".
(a) More load capacity before breaking?
(b) Stiffer, less deflection, under the given load? A "stiffer" bunk board may not be desirable.

I doubt the hull is perfectly flat. A one piece bunk board may flex just the right amount to conform to the hull shape, providing more evenly distributed support. A stiffer bunk board may only contact the hull in two or three localized zones.

There are quite possibly some wood beam construction, online engineering calculators which could give you information from which you could formulate an educated answer to your question. Otherwise a local full service lumber yard may be of help.

My gut tells me a laminated/sistered beam/board is going to have a higher load capacity, as well as less deflection.

 

The timber doubled will be double the strength. More if they are glued together.

 

Reread what Quickstep wrote. The overall size is NOT being "doubled".

 

Making a certain size timber by sistering or laminating smaller pieces together does NOT "double the strength".

If that were true, then Quickstep could use four pieces (each being 3/4" by 7" by 11') to build his 3" by 7" by 11' beam and have quadruple the strength. Common sense tells us that can not possibly correct.

Yes a twice laminated beam/board may be stronger than a single piece the same overall size as the laminated board, but no where near "double the strength".

I'm out the door on other tasks, or I would look up a chart as I mentioned earlier. Show me a chart saying it doubles the strength and I will eat crow.

 

Standard beam theory would say that two unglued sistered beams have the same bending moment, strength, and stiffness as a single beam with the same overall dimensions with regard to a vertical load but much lower bending moment, strength, and stiffness for a horizontal load.
Gluing the two beams together would not change the calculation of the vertical bending moment but would change the calculation for the horizontal moment.

 

Apparently spoken by someone with engineering expertise?
Those terms bring back my days in Architectural Design about 50 years ago, about which I remember very little.
With these exceptions, a beam can have a distributed load across it's length in the horizontal orientation OR a concentrated load.
The distributed load would be like a floor joist, supported at either end.
A concentrated load would be like a support beam with an intersecting cross beam on top.
Or a shop crane with a hoist at the center:

This "beam" is made from 2 - 2 X 6's spaces apart with support posts at either end.
It was for lifting these 3/8" thick steel plates, truck cabs and engines weighing less than 1000 lbs.

What need to be understood in designing a beam are the forces at play within the beam itself.
The bottom flange in an "I" beam is in tension, trying to be pulled apart.
The top flange is in compression, trying to be pushed together.
Wood is not a strong in tension as it is in compression and will fail be breaking along the lower edge first when overloaded.
Steel has very different properties, like yield strength and modulus of elasticity, neither of which I remember much about.
I really enjoyed those engineering classes, but didn't think I'd ever use that technical information.
I always consulted and actual structural engineer when I needed that expertise.

 

Just to be clear - I’m looking to replace a beam that has actual measurements of 3” x 7-1/2” with two 2 x 8’s. The sistered 2 x 8’s would also have actual measurements of 3” x 7-1/2”. The question is would the the sistered beam have similar weight bearing capacity as the single beam.

I’d liken it to comparing a single 3” x 7-1/2” joist to a joist made of sistered 2 x 8’s

 

The sistered 2 x 6’s would also have

That's a typo, dude!

 

Making a certain size timber by sistering or laminating smaller pieces together does NOT "double the strength".

That why I bolded these words:

It's not clear what you mean.
I think you mean a "doubled" member will be "double the strength" of a single piece.

Steve did not include that important part of the reply!

 

That must be a big boat