How to assess bulkhead strength of completed boat

I'm looking at a catamaran built by the current owner in 2000. The builder/owner was a professional monohull builder during his working life. He designed and built this catamaran as a retirement present to himself.

She is 50 feet long, 26 feet wide, and made out of stripped plank cedar. The builder intentionally built her light, and I'm not sure if it was too light.

For instance, when he talked to the local composites company, they recommended 1200 gsm glass on the outside of the hull and 600 on the inside. The builder made some test pieces and halved those values, putting 600 gsm on the outside and 300 on the inside.

Another area he saved weight was the structural bulkheads. For the section of bulkheads running through the bridgedeck, he glued and glassed 2x10 mm sheets of marine ply together. But once he reached the bridgedeck/hull joint, he switched to a just a single 10 mm sheet. All the structural bulkheads are tabbed in with epoxy resin.

The result is a very light boat (estimated at 6,500 kg dry).

The potential issue is in 20 years, the boat has never left the bay that she was built in. The owner has motored up and down the bay numerous times, and crossed across the heads on occasion, but has never taken her to sea.

I had a survey done on the boat, and the surveyor said, "I can't form an opinion on the strength of the boat." He went on the clarify that he doesn't think the boat is dangerous or in immediate risk of falling apart. But he specifically pointed out the single thickness bulkheads that attach the outside of the hulls. He said he'd be much more comfortable if the double thickness had been carried throughout the bulkhead. All that said, the surveyor said that if, and that was a big if, cracks started to appear, beefing up the affected area would be rather simple, as all the bulkheads are accessible from the boat.

There are are no plans for the boat, as the designer just built the boat from an idea in is mind, after building a small model.

I fully trust the competency of the builder for monohulls. He built vessels that won long distance ocean races and delivered some of the boats that he built long distances. My concern is this - are there considerations for multihull design that a skilled monohull designer/builder might miss out on? I talked to the owner, and he said he built the bridgedeck/hull joint extra strong, but didn't want to waste the weight on the outer hull bulkhead joint.

I could always beef up the bulkheads as project number one on the boat, but if the bulkheads are undersized for long distance ocean cruising, what else might give way?

 

Tyler. Please consult with a multihull designer. This bulkhead arrangement as you describe it, is not good standard practice. The cross beam structure should have a top and bottom flange that can handle at least twice the righting moment of the cat. I will make some BIG assumptions here and say the righting moment is 150,000 ft lbs. That mean the top flange needs to be able to handle 300,000 foot lbs of righting moment. That is some solid timber or a lot of unidirectional glass. Cats this size have 20 mm foam and glass or 20 mm plywood thick vertical webs separating the flanges across the full beam of the cat. Please read Multihull Structure Thoughts and you will get the idea. Spindrift 37 plans or Crowther 226 (40 foot cat) main cross beam bulkhead will give a guide but I suggest a few dollars spent with EG Shaun Arbor (if in QLD) or a good designer will save you a lot of future pain..

 

oldmulti - Thanks for confirming what I thought. I'm down in Sydney. Do you have a lead on somebody to talk to here?

 

I disagree with the surveyor. If the bulkheads crack, the structural repair should be more complex than simply "beefing them up". I would recommend increasing the laminate to the original design.

 

Tyler. Sorry, I know more about QLD than NSW contacts. An email to Ray Kendrick of Teamscarab may produce some results. Ray is "retired" now and may be able to help or suggest a trusted person.

 

Unfortunately, there are no designs as the builder just went from a model in his head.

 

Thanks - I'll reach out to them.

 

In that case, the safe method is to measure all the structural members, determine the construction materials, pull the lines of the boat and then backwards engineer the design and verify if the construction meets the structural requirements.