1985 O'day 28 Depth Transducer Relocation - Something Interesting

[Clutch Cargo]

Haven't posted in a while, but that doesn't mean I haven't been busy!! At the beginning of last season, I noticed that the depth transducer (located outboard of the STBD engine bed was leaking. Actually, it was the plywood backer that was rotted. The PO had tried to tighten it, but to no avail. Therefore, given that all of the instruments were original ca. 1985 Datamarine, I decided to put an N2K backbone in the boat and change the anemometer, STW log and depth transducer. I'll do another posting when it is completed, but for now, I'll concentrate on the relocation of the depth transducer.

I decided to locate the new one in the stem, where is should've been all along, IMHO. It is now on the starboard side, opposite the blackwater overboard discharge valve and just aft of the holding tank. As for the original 2" hole, I employed the classic WEST System method of filling it in. The hull laminate is .5" thick, which translates to a 6" diameter patch area. This is because there is no access next to the beds to do any grinding. I covered over the hole from the inside with a piece of green class and proceeded to remove the outside hull material to create the concave shape to be patched. About halfway through I found a void (see attached)!! Turns out that an athwartship stiffener was added when the hull was laid up and the underside is hollow. I have seen these before when visiting builders but did not realize that they could be hollow. I always thought they would be solid or cored. In any case, it was perfectly dry which means that the watertight integrity has never been compromised. At this point, I'm not sure what to fill it with. I'll make a few calls tomorrow and will also call WEST as their tech support is excellent.

 

[Project_Mayhem]

I've found quite a few voids in my O'Day's hull as well. Layup isn't the best but that's far worse than I've seen. I wonder if water got in there?

 

[Clutch Cargo]

I've found quite a few voids in my O'Day's hull as well. Layup isn't the best but that's far worse than I've seen. I wonder if water got in there?

Unlike the voids you saw, this one is intentional, and it is perfectly dry. It is a pre-made fiberglass channel running athwartships just forward of the engine beds. There appears to be two similar ones running outboard of each bed. They were set in place sometime during the layup, or when the liner was tabbed in. I have attached a sketch showing what it looks like. I spoke to one of the Tech support wizards at WEST and I'm going to fill what I can with Six10 to give the patch something to adhere to. I have about 1/3 of a tube and that should do nicely.

 

[dlochner]

Unlike the voids you saw, this one is intentional, and it is perfectly dry. It is a pre-made fiberglass channel running athwartships just forward of the engine beds. There appears to be two similar ones running outboard of each bed. They were set in place sometime during the layup, or when the liner was tabbed in. I have attached a sketch showing what it looks like. I spoke to one of the Tech support wizards at WEST and I'm going to fill what I can with Six10 to give the patch something to adhere to. I have about 1/3 of a tube and that should do nicely.

Pans and headliners are often not bonded continuously to the deck or or hull. There may be globs of reinforced resin into which the pan (or headliner) is squished onto. This will leave voids between the pan and hull. Sailing Uma recently removed their headliner and they were surprised how much strength this method provided. (The evidence is towards the end of the video.)

 

[Project_Mayhem]

What's the purpose of leaving a void like this on purpose?

 

[dlochner]

What's the purpose of leaving a void like this on purpose?

The short answer, strength, cost, and weight.

First is strength, a hollow rectangular channel is just as strong as one formed around wood or foam. It is the skin that carries the load. The floor pan is built in a mold, thus there is no need to fill the channel with anything. The pan would be just as stiff with as without a filler.

Likewise the pan and the hull need to be connected to increase the strength of both, however it is the separation and subsequent increased thickness that increases the strength. It is analogous to honeycombed structures which can be very stiff and strong, albeit the honeycomb is more highly engineered than globs of fiberglass/resin putty.

Weight and cost are also factors. In boat construction most of the load is carry on the outer skins, the distance between the skins determines the strength and rigidity. That's why decks are cored, to increase the strength of the deck. The core is lighter and cheaper than the amount of glass needed for the same strength. Unlike the deck, the pan is not a flat span, it has multiple bends which increase strength and rigidity.

 

[Project_Mayhem]

The short answer, strength, cost, and weight.

First is strength, a hollow rectangular channel is just as strong as one formed around wood or foam. It is the skin that carries the load. The floor pan is built in a mold, thus there is no need to fill the channel with anything. The pan would be just as stiff with as without a filler.

Likewise the pan and the hull need to be connected to increase the strength of both, however it is the separation and subsequent increased thickness that increases the strength. It is analogous to honeycombed structures which can be very stiff and strong, albeit the honeycomb is more highly engineered than globs of fiberglass/resin putty.

Weight and cost are also factors. In boat construction most of the load is carry on the outer skins, the distance between the skins determines the strength and rigidity. That's why decks are cored, to increase the strength of the deck. The core is lighter and cheaper than the amount of glass needed for the same strength. Unlike the deck, the pan is not a flat span, it has multiple bends which increase strength and rigidity.

I completely agree that forming a channel with hollow space or foam inside provides increased strength. My own boat has a stringer on each side with foam inside. However, the void in the OP's photo doesn't look intentional, at least not from this angle. Feel free to tell me I'm wrong. You are far more knowledgeable than myself

 

[dlochner]

I completely agree that forming a channel with hollow space or foam inside provides increased strength. My own boat has a stringer on each side with foam inside. However, the void in the OP's photo doesn't look intentional, at least not from this angle. Feel free to tell me I'm wrong. You are far more knowledgeable than myself

The foam in your stringers is primarily there as a form for the fiberglass, less for strength.

The area circled below is probably the result of a poor layup. The gap to the left looks more intentional. The photo is pretty limited, seeing the whole area that was ground down and seeing the top of the area where the thru hull used to be would also help.

 

[Clutch Cargo]

Sorry, the image doesn't show enough detail. Looking up close, you can see that the channel was a separately manufactured piece. dlochner is correct, the small void is the result of a poor layup.

 

[jibes138]

I used G10 sheet stock for my backing plates when I replaced my through hulls and valves with seacocks . I highly recommend this, will never rot again and glued in place with epoxy thickened with fibers is solid as a rock. You could also use pieces of an old boat hull that's a half inch thick if cost is an issue and chunks of old boat are available. The G10 is very hard to shape to the shape of the hull, it is really hard material but it cuts OK with a saber saw and drills OK. A sanding disk on an electric drill works pretty good for shaping it. A 12 x 12 sheet of G10 at McMaster Carr gives you four 6 inch diameter backing plates costs about a hundred bucks for 1/2" but worth it for peace of mind.

 

[Clutch Cargo]

I used G10 sheet stock for my backing plates when I replaced my through hulls and valves with seacocks . I highly recommend this, will never rot again and glued in place with epoxy thickened with fibers is solid as a rock. You could also use pieces of an old boat hull that's a half inch thick if cost is an issue and chunks of old boat are available. The G10 is very hard to shape to the shape of the hull, it is really hard material but it cuts OK with a saber saw and drills OK. A sanding disk on an electric drill works pretty good for shaping it. A 12 x 12 sheet of G10 at McMaster Carr gives you four 6 inch diameter backing plates costs about a hundred bucks for 1/2" but worth it for peace of mind.

I used .375" pieces of a solid laminate J-24 lazzarette hatch for the new depth and STW tranducers. Got it at a really great boneyard here in Portland.