Possible Design Flaw Causing Cracks?

Yeh Par, the crack is definitely in the laminate, Will definitely be implementing your suggestions. I'm also leaning towards adding something along the chines like Petros suggest.

In the below picture I have placed arrows where I can see running model design changes in a later version of the boat Including what looks like some sort of extra reinforcement along the chines. Bottom pic is my boat for comparison. I can also see in some some cabin shots of newer boats the reinforcement along the chines follows right up to the front of the boat. They must have found issues during the production run and made some changes. My boat is around 1979 and the other would be around 1985 at a guess.




Just to clarify the make and model of the boat, as to the best of my knowledge this model (carribian crusader) and a very similar hull the Bertram 23 where only made in Australia. I think both companies were under International marine. The hull shape looks very similar to a Bertram 25 but this one has a 21 degree dead rise and 23 foot long with 2.5 metre beam. The Bertram 23 has slightly wider chines and a fuller bow and looks to have a slightly different stringer system, also has a fly bridge whereas this model is a sedan.

 

Chine are notorious for suffering badly, whatever the material. Any reinforcement along these members is always welcomed.

Also, with a bulkhead (half) in the location it is, under a slamming load, where does the load from the bow go? It stops and then tries to continue past the half bulkhead. Ensuring suitable longitudinal structural continuity would be advisable. Similar to the bkts shown by Tanls...it adds in load path transmission and as also noted by Daiquiri..reduces SCFs.

 

Abruptly interrupting the continuity a reinforcing member like a stringer where it meets another reinforcement perpendicular to it (like a bulkhead) is a bad structural practice indeed. And if I remember well, Class societies forbid such practice (Ad Hoc and TANSL correct me if I'm wrong).

So - yes, I reckon that the discontinuity of longitudinal reinforcements could be the main problem here, and hard spots are worsening the situation.

I am attaching two pdf files here, which show a simplified FEM model of this situation. The first one:
View attachment Cut-off stringer.pdf
shows a model of the present situation, from what I can gather from the photos. As can be seen, there is an area of concentrated high stress around the point where the stringer pushes against the bulkhead and against the hull skin. When this stress exceeds the maximum allowed by the laminate or by the gelcoat layer, cracks start to form.

The second one shows the same structure under the same pressure acting on the skin, but with an added piece of stringer on the other side of the bulkhead, which restores the continuity of the reinforcement:
View attachment Continuous stringer.pdf
It shows that the areas of concentrated stress are all gone and the danger of crack formation is now significantly reduced.

The color code is the same between the two cases, to allow a visual comparison of two situations. The actual value of the stress is not shown because it is irrelevant for the purpose of the qualitative comparison.

So, the conclusion is - go for the suggestions given by TANSL regarding the structural reinforcements and, while doing it, avoid creating other hard spots by correctly joining the reinforcements to the hull skin.

Cheers

 

The structure of a ship allows many different solutions, if properly applied. In short, everything depends on the continuity that can be given to the elements and how they are interconnected.
It is not so important to decide where a longitudinal bottom ends as how it ends and how, in this case, is connected to the bulkhead. If that connection gets a good stress distribution, need not be problems in the shell. The bulkhead can transmit big efforts which, absorbed by the hull would reach to break it but, if we transmit to the longitudinal reinforcement, shall be fully supported by the structure.
Regardless of all this, we should always find out if we are dealing with a problem of local loads or global resistance.
In small boats is not normal to have problems of longitudinal strength, but may exist, especially on ships with large deck openings or very trimmed decks.

 

Would I be able to reinforce the chines from bow to stern with something like polyethylene foam and 3 layers of 17ounce bi axial with epoxy?
I'm thinking that the foam would be flexible and mold to the shape around the edge of the hull/chine area.


I also found a pic I took when the floor came off that shows where the outer stringers stopped a few inches short and did not butt up against the bulkhead.

 

I haven't recently worked on one of these Bertrams, but they are a popular design in this area, with many examples, none seem to be breaking in half like this one.

So, I respectfully disagree with the assessment the stringers and chines need additional longitudinal stiffening. Again, in the era these where built the bulkheads, stringers, etc. where installed "floating" in the tabbing. They where spaced off the hull shell and the tabbing installed. This one appears to have had a screw up when built and the bulkhead fell off it's spacers and the guys just tabbed in in anyway, creating a hard point along it's contact line. Cutting the bulkhead out and spacing it up off the hull shell, with new tabbing, will fix this issue. Placing a foam, polyurethane or other spacer is a good idea, but not wholly necessary, considering the way the rest of the boat is built and the general success of the type, in the numbers produced.

The only reason to add any additional reinforcements would be, if the propulsion and top speed where to be substantially upgraded. If she's going to carry similar power and run at similar speeds, simply fixing the issues seems the reasonable route.

 

I totally agree with PAR when he says that "fixing the issue" is solved everything. The problem is to determine, with a good analysis, which has been the cause of the problem. PAR seems to have everything very clear. I, with only a photo, I can not guarantee anything, so you better do what PAR says, "fix the issue".
Cheers.

 

Yep, I think I concur with par, after I think about this boat lasting 35 years without splitting in half and to think it might have even been it out on the water with rotten stringers before the floor started to rot tells me the design can't be all that bad, But the workmanship could have been better. I also found alot of the fiberglass on the stringers were not wetted out properly and resin pooling in areas as well. I'm sure by having better workmanship and using superior materials I should have a stronger boat anyway and hopefully I'll get another 35 years out of it.

I doubt I'll ever want to cruise any faster the 25 knots so I doubt I'll ever make much of a change to the engine size. I mainly want this thing to last and not have to see this part of the boat again in the future.

 

Your assessment of the current structure is pretty much what I figured with this boat. The manufacture just cut too many corners, or it was a Monday morning boat, maybe the "new guy" learned how to tab on this one . . . who knows, but it's unlikely there's a significant "design flaw" considering how many of these puppies still exist.

Fix the bad tabbing and insure you have a tad more then you think you need, on the "questionable" areas and you'll probably be fine.

Of course this is a guess, but an educated one based on several factors. If you want, you could back engineer the structure and find flaws everywhere, but I'm betting she's tough enough to not need this level of scrutiny, if you just fix the broken laminate and repair the shoddy tabbing.

I say go for it and let us know (pictures) how it works out.

 

Prob too late to reply but I only found this forum today, whilst looking for info for my 23CC, which Ive owned for 20yrs or so.

I know of two other 23CCs with a similar problem. Mine & another which was heavily modified with twin OBs on a pod.

In mine, looking into the two small storage compartments, I noticed the hull had become unstuck from the bulkhead so I carried out a small repair. That was 15 years ago & its still holding. Didn't raise the floor, don't know if I've got that crack.

The other 23CC with the 2xOB Pod, this guy actually split the hull twice that I know of, along the same red line you marked on the photo.

Friends in common told me this guy only had two speeds, stopped & WOT. regardless of sea conditions. didn't go on the boat myself.

I think it probably is bad design, but only rears its ugly head when boat is pushed to extreme punishment, such as putting two 200HP OBs on it and goin full noise into rough seas.

Would be interested to know how the project is coming along, mine will probably have to be done as well.

I was contemplating lifting the floor out in one piece, fix the stringers and drop it back in.