Sister Stringer

[PaulJ]

I need to replace the port side, inboard sister stringer on my B28. What type of wood is appropriate? It appears to be a 2x12 sandwiched between 2 sheets of marine plywood. What is the appropriate method for mating the marine plywood to the 2x12?

The Stbd side, inboard sister stringer was replaced in 2001. The Port side, inboard sister stringer bolts are very rusty and the wood near 3 of the bolts is soft and appears that water intrusion is coming from the bolt
holes. Top side of the stringer is solid. Bottom side near 3 bolts is soft.

Thanks for advice.
Paul

[CaptPatrick]

Paul,

The marine ply sandwich isn't necessary and isn't even good technique...

Use un-treated fir or yellow pine to make the sister. Get it cut and shaped to fit, & round over the long corners with 1/8" or 1/4" radius. Using 6 oz 0/90º weave fiberglass cloth & epoxy wrap the 4 long sides.

On a table/bench longer than the sister, lay down a large piece of waxed paper. Cut the cloth wide enough to fully wrap around the sister and lay it on the waxed paper. Wet out the cloth with the epoxy, leaving a 3" dry strip along the top and bottom. Lay the sister, face down on the wetted fiberglass. Pull the top over to be a good fit and staple it to the sister spacing about 1" for the entire run.

Lightly pull the bottom edge of the cloth, keeping pressure on the sister until there is no slack along the top edge of the sister. Pull the bottom edge over to complete the wrap and staple it down & finish wetting out the cloth.

Carefully turn the sister over and work out any existing air from the layup. work from the top edge back to the bottom edge and from the center out to each end. Let the epoxy cure until hard.

Clean & prep the stringer & prime with epoxy, wiping away any excess epoxy. You want the stringer to be fully coated but with minimal epoxy left behind. Scrub the back side of the stringer with denatured alcohol to remove any amine blush. Amine blush is caused by un-linked hardener molecules that have risen to the surface during curing. It's an oily feeling film and will act as a release agent if not removed.

Mix a batch of epoxy & thicken it with cabosil, microbaloons, or even saw dust, to the consistency of creamy peanut butter. Using a 1/4" notched trowel, trowel onto the sister the thickened epoxy. Register the sister into place and clamp it lightly to the stringer. Fill any gaps with the epoxy mix and clean up any excess. If you don't have suitable clamps on hand, you can screw the sister to the stringer. After the epoxy sets up enough to hold the sister in place, the clamps or screws can be removed.

Finish as desired... The epoxy bond area between the sister and stringer is going to be around 600 sq inches. At around 1,200 psi of bond strength for the epoxy, no through bolts or other mechanical fasteners are necessary. Bertram used through bolts because that was virtually the only thing that held the sister to the stringer.

Br,

Patrick

[PaulJ]

Thanks Patrick...
Additional questions...
1) Do I wrap the 2 ends that abut the cabin bulkhead and rear bulkhead?
2) Both stringers currently have notches cut at rear to step-down the sister to receive the rear engine mounts. They would be too tall otherwise. When I wrap the wood with the cloth, how do I deal with the resulting bunching of fabric at the notches?

[CaptPatrick]

Paul,

The ends will be fully bedded within the thickened epoxy. You could wrap the sister without the notches, let that harden & then cut in the notches and epoxy glass the exposed wood individually. The 0/90º is also very drapeable and can be worked in with gussets depending on the configuration & depth of the notches.

Br,

Patrick

[PaulJ]

Thanks again Capt. Patrick.

The stringer work will be done in preparation of repower. I plan to stick with gassers. I currently have mid-1980 Mercruiser 350 GM (260hp) carburated, FWC with 1:52 Borg-Warner Velvet Drives. Any recommendations on new power/trans combos that will fit in 1973 B28?

PaulJ

[cmccool]

Hi Paul

I am going through the same thing right now with my 1972 B28 so I will throw my 2 cents in.

The cheapest way to go is to replace your 1980s with either rebuilt, reman'ed or new 1980s style engines - depending upon budget. There are quite a few places out there where you can get them - best bet from a service point of view would be get them from a dealer near where you live - just have to make sure you have the low profile intake manifold.

That way you could re-use your trannys and you are good to go. When you do your stringers - if you make them to the height of the bottom of the notch, you will have a flat surface throughout - no notches to worry about. They probably got notched out at your previous repower. Just get a schematic showing engine mount location and heights for the new engine before you finalize your stringer location.

Personally, I am going with new Mercruiser 5.7l Horizons and new transmissions. With the fuel injection, I will have to raise the cockpit over the engines, but that is such a small portion of the overall project that it is worth it to me. With the increased fuel economy over carbs, I should pay for the raised cockpit fairly quickly.

I am also going with smartcraft guages and full digital throttle and shift control. For me, I figured if I was going to spend that kind of money, I want all new technology.

I am actually converting my flybridge to an express and moving my steering station from the flybridge to the cockpit, so the DTS controls will make that much easier. The other reason for raising the cockpit over the engine is the increased visibility for my new steering station.

Regards
Cliff