Sunday, September 20, 2015

Turbulence City

There are a some things right with the picture below and things really wrong, at least from my perspective. I labeled the pic so there weren't any doubts about orientation. I'll speak slowly for the benefit of the more boating challenged. This is an aft bilge compartment. The arrow pointing down is the direction towards the pointy sharp end of the boat. The one pointing upward is towards the square back end of the boat. This is a boat that has a "Liner," meaning there is a layer of foam in between the hull and the top shiny fiberglass surfaces you see can see. The foam layer provides floatation and the laminated nature of the construction makes the boat structurally stiffer.

But our mystery deepens (bring in the Twilight Zone theme music here). What are the two liner cut outs about and why are they there in the first place? Are these the boating equivalent of crop circles? Is Turbulence city a real place? We're going to explore these weighty concepts and see if there are any real answers.

You can tell the two liner cutouts aren't exactly precision pieces of work, but they are fairly symmetrical. I would guess that a template was used to draw the squares.

Some tool sharper than beaver's teeth was then used to cut the top fiberglass layer. This was pried off and a scraper of some sort was used to remove the foam stuff exposing the fiberglass hull.

The exposed surfaces were hand painted with gelcoat. So in a form follows function sort of way we have the two exposed rectangular hull areas. What isn't clear is why bother with this in the first place?

Let me wax poetic on this this subject for a couple of minutes. You remove a vessel's liner to expose the hull for a couple of reasons. If it's a really thick foam layer it may make it difficult to install a through hull transducer and have room for the stem's retaining nut. Okay that's one reason. The second reason is you may want to install a inhull transducer that needs direct contact with the hull. I don't have much after this.

I'm going to dwell on the transducer application first. What they did at the plant was to place the water pick ups all the way forward. Like in the picture below created the Rant's wonky graphics department, what has happened is the fittings on the underside stick out from the bottom of the hull. This creates downstream turbulence which will adversely impact any transducer you wanted to install in these cutouts.

Just so I'm clear, if the water pick ups were placed at the aft end of the cutouts instead of the forward end, transducers could have been installed. Alas this was not the case, and the result is now Turbulence city on both sides downstream of the fittings.

I'm going to go one step further. The full hull thickness including the foam is an inch and a half at the most. This means almost all water pickups, transducers, underwater lights and about anything else could have been installed without bothering with all of the cutout work and effort in the first place.

Can I move the offending fitting back? Sure, but it's a P in the A to do. Buy new longer hose, beat the pickup out of the boat, drill a new hole, reinstall, and clean up the gooey mess. Here is the catch, I now still have the original hole and it's right up against the edge of the cutout. This results in having to remove even more liner to install a new transducer to accommodate the big nut.

I'm sure someone somewhere in the factory meant these cutouts to have a purpose, but what it is a complete mystery to me. Also the boat is devoid of any documentation regarding this subject and who would ever be surprised about that? This will remain an enigma for all time, and a place to trap water and debris.

I suspect that even if the liner hadn't been removed the water pick ups would still be in the same place resulting in exactly the same problem. Most boat builders have little to no understanding about what transducers are and where you can install them on a boat. I know this because they always put water pickups in the worst possible places making my life more difficult. I guess they think most boaters still use a "Lead and Line" and holler Mark Twain. Alas I put a transom mount transducer on the boat instead. It was the most pragmatic option. Oh the inhumanity of it all.

The Turbulence city graphic uses a small portion of a drawing by Wikipedia user BoH.


  1. Is that a thru-hull and seacock? I would think you'd want to install a thru-hull in solid glass. If there's liner and foam between the outer flange and the backing nut, it'll allow some compression and undermine the integrity of the joint, no?

  2. Thanks Anon, I certainly don't disagree and understand your point. These aren't big through hulls and the foam layer is fairly thin. In this case both the hull and liners are fairly substantial. If the both the hull side of the flange and the top side nut are well bedded in 5200 the sealant by itself is largely sufficient to retain the fitting and there would be no need to have a roid raged installer to do more than tighten slightly more than by hand with the 5200 acting as the thread lock. You can see this approach with some Airmar transducers that have a rubber gasket and the retaining nut is just hand tightened. Now if it was a cheesy thin hull and liner... you betcha.


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