Help Needed. Possible structural issue

[GZigmont]

We have a 2021 R-31 CB that we purchased used before last season. Had a survey done by a reputable surveyor, which showed no issues with the hull or structure of the boat. I of course went through the boat with the surveyor and on my own and found no issues. I have not hit anything or run aground, nor had the PO to the best of my knowledge. I did not need to paint the bottom before last season as the Pettit Vivid on the bottom was still good.

Yesterday while painting the bottom and while in the area on the port side, underneath and maybe about a foot forward of the location of the “day head” I accidentally bumped my elbow on that part of the hull and heard something fairly large rattle inside the hull. The sound would be akin to a two or three foot piece of 2x4 lying on the bottom of the hull and getting jarred when the hull is hit from beneath. I replicated the noise in that area several times with the side of my fist. I don’t have to rap the hull hard at all. Sounds like something that is in contact with the hull in that area is loose and that when the hull is rapped with my hand, it flexes and whatever is on the other side bounces and slaps back. So I went to the same area on the starboard side an when I rap the hull it is solid with no flex or slapback. Just a thud.

My concern is that a stringer has become un tabbed which of course is a serious structural issue. Because the boat has not hit anything or run aground, and the fact that the other side is solid, really heightens my concern. The only non critical object inside that I could think of that might make the slapback noise, would be the waste line from the day head to the holding tank. But then again, why does the other side of the hull have a very solid feel?

I tried searching this forum and the rest of the Internet for hull construction photos or diagrams that are detailed enough to show the location and orientation of the stringers, but came up empty. After painting yesterday, I tried to see of there is any area where I can get a view into this area, but the cave is full of stuff, and my memory tells me there are no access panels under the mattress in there anyway. I have a 4 foot flexible fiber optic camera for my iPhone, which I’ll bring to the boat tomorrow. Hopefully I can find a way to see in there.

Any thoughts, experience, advice, photos or diagrams would be greatly appreciated.

 

[Rode Trip]

You need to isolate exactly where the sound is coming from and then look at that same spot inside, it's the only way you'll give yourself piece of mind. I know that me telling you it's far more likely to be something loose inside the hull than something structural isn't going to ease your mind. Generally when surveyors are tapping uncored hulls out we are listening for dull sounds which indicate voids or delamination. It's not uncommon at all to hear things moving inside when tapping on the outside. Measure from the keel or the chine, and measure from a known point like a porthole or a stanchion, and then use the same measurements inside and see what's there.

 

[S. Todd]

I cannot imagine that a stringer failure would sound like something loose bouncing inside the hull. I suspect it is either a pipe or cable or even potentially a tool or something that got dropped during build. Let us know what you find out with your camera inspection.

 

[GZigmont]

You need to isolate exactly where the sound is coming from and then look at that same spot inside, it's the only way you'll give yourself piece of mind. I know that me telling you it's far more likely to be something loose inside the hull than something structural isn't going to ease your mind. Generally when surveyors are tapping uncored hulls out we are listening for dull sounds which indicate voids or delamination. It's not uncommon at all to hear things moving inside when tapping on the outside. Measure from the keel or the chine, and measure from a known point like a porthole or a stanchion, and then use the same measurements inside and see what's there.

Thank you. I do know what the surveyors look for when tapping, and that’s a little different type of force when tapping with the hammer than I can do with the side of my fist. Unfortunately, this is one of those areas on the R-31, where visual access is all but impossible. I’m hoping it’s the waste hose and I’ll be able to see it with the camera if I can find place to snake it in.

 

[GZigmont]

I got a nice picture of underneath the cave bed from someone on the R-31 group on FB. I feel much better, because the waste hose is lying directly on the hull in that area. I’ll still look with my fiber optic camera to confirm.

 

[GZigmont]

Here's the report: I reached out to Wagner and Peter at Winter Island Yacht Yard, both of whom are stellar in their knowledge of the tugs and are more helpful than I can ever describe. I sent Wagner a photograph of two targets I created out of painters tape to show the area on the hull where I was getting the slapback feeling and sound. I also spent a while on the phone with Wagner, describing the sound, the location and telling him what I had checked. Wagner went to each of the R-31s that were in the yard, including a brand new unsold 2025. Each of the R-31 tugs had the same phenomenon as mine. Wagner then reached out to the factory and I believe he spoke with Kenny Mars. My loose, second hand translation of the word from the factory and Wagner's thoughts is as follows: "This area of the hull is one of the larger areas that does not have a stringer attached to it, but is strong enough on its own. There is a small amount of flex in this area."

I'm satisfied with what I learned, which has allayed my concern. There may be some cables or a pex line running in this area that would help create what I was hearing and feeling, but in order to see, I'd have to remove the inboard plywood piece in the cave, or cut/drill an hole for inspection. To remove that piece of plywood would require removing at least front and inboard walls inside the cave, if not more. I may or may not drill a hole for a borescope, but I'm not in any hurry after learning what I did. Thanks to all for your help and ideas.

 

[AGLyme]

Glad the mystery is solved.
You probably saved a lot of future angst for many RT31 owners and boat appraisers vis this thread.
Good work G !!

 

[serendipity]

Glad mystery solved to your comfort! I want to second kudos to both Peter & Wagner at WIYY! We bought our new R27 OB from them last year and their support before, during, and after our purchase has been nothing short of stellar!