Alternative to timber engine stringers? Layup schedule to reinforce an old engine bed.

Having often worked on old boats with lagbolts screwed into wet soggy glassed in lumber stringers as engine beds, I found this fiberglass engine bed (see photos) to be quite an improvement on my new project boat, a 1984 Nor'Sea27.



Sadly, much of this structure had to be cut out to remove the fuel tank (see photo with removed tank).




Gerr’s book advises on size and layup of old style glassed-over timber stringers (although scantling number is debatable in this heavy but short boat). Modern texts (See photo) and all the install manuals of manufactures (Yanmar and Beta) suggest widening the stringers and mounting the engine mounts to angle aluminum so the engine is through bolted, not lag bolted.



As you see from the photos this (moving outwards laterally) is not possible, as the stringers are too low to the hull. Retaining the faux stringers is appealing because the engine can be through-bolted without the need of angle aluminum, but one's hand needs access to the hollow cavity under the faux stringers.



-- I am proposing adding transverse webbing as shown in green. Maybe made of 0.5” Coosa Bluewater 26. In the photo I draw only 3 on each side, it could be as many as 10 on each side. Bedded to the underside of the faux stringer and hull with 18 or 20oz wovenroven. Glassed over multiple times, tabbed to the hull after course sanding and styrene monomer reactivation. Is this a decent approach?


-- If I do this, what is the minimum layup of vinyl ester resin and glass?


-- I am open to other ideas and bracing designs. Please pitch ideas!! Adding longitudinal web may also be needed? (EDITr maybe just increase the number of green buttresses to 8 or 10 on each side?)

[“webbing” is likely not the term. Maybe buttress? Or Fillet? Or ____?… help me out here folks.]

Thanks in advance for the help. Pete

 

If I understand your proposal correctly, you are basically going to create new beds built from fibreglass, shaped like deep angle bars, with three (or more) 'tripping brackets' on each side of the 'angle bar' bed as shown in the green above?

This proposed arrangement should be a substantial improvement on the old beds for sure.
But what about access under the 'angle bars' for laminating in the tripping brackets, especially further aft - is there enough space to get in there ok?

 

Thanks Bajansailor,
I did not look at it that way, but yes. a "deep angle bar" profile of fiberglass with "tripping brackets"is a good description. THANKS

[EDIT: although... "create new beds" is not true. more shore up the ones that I had. noticed i cut away the part under the engine oil pan but left the faux stringers]

Your question inspired me to quick drive home and see how hard this will be. It is tight, but not as bad as most boat projects. The "faux stringers" are hollow. I admit I have gorilla hands. But with the tank removed I can get in that space and work inside it. Full visibility but not comfortable. It is a center cockpit design so the engine bed if forward a bit and I have access from both sides.
I am worried about glass being sticky enough to defy gravity. If I first bed in the Coosa tripping brackets (Black on 1) by putting a strip of heavy saturated 20 oz cloth (Green on 1), I can then clamp those in place. Next tab it to the existing structure and hull with lighter weight cloth. I ran through these motions just now and have good visibility and access as long as I do not gain weight or throw out my back. In image 2 I consider adding more mass where the engine mount bolt is. But, I do not know if this is overkill.

 

Thanks for this update Cadmus - there are some very good fibreglass experts on here like @fallguy and @Blueknarr and @rxcomposite so hopefully one of them will be along soon to offer their opinion(s) on your proposals above.

 

1. Buy some 1" G10 plate, one plate full under the entire stringer, then a stack of them under each bolt. Glue everything with thickened epoxy, shoot some PU foam in the remaining space, glass everything over. Drill and tap for machine screws. If you want to go overboard, or if you plan a reengine sometimes, make the entire log out of G10. Lifetime unlimited.

2. Take a stainless, bronze, Al, Ti, plate, drill and tap, glue under the stringer, shoot PU foam, glass over. Lifetime depends on the used material, around 20 years with stainless. Plate thickness min. 2x bolt diameter for stainless.

In both cases reinstall or rebuild the oil drip pan between the stringers, glassed in.

 

I'd say @Rumars beat me to the plan. 316ss or g10 plate is far better and easier..

You are a little hard to follow. A drawing of all three views lined up would be needed for a more detailed answer to a detailed problem.

The 316 plate needs a place to be bolted or lagged, so the plate assumes that.

The coosa bw is really not great here because it doesn't really have the same compression as g10 or metals.. for example g10 is rated at 65,000 psi and coosa between 800-1000 psi, 316 ss is like 45,000 psi I believe. Corrections welcome, but not needed, point is made. The coosa is like 2% of the other two and WILL crush under bolting stresses..

Yes. People use coosa in transoms and put a lot of pressure on it, but that is also over/under many layers of fiberglass.

 

Thanks for that idea. I think if i was going to fix-n-flip the boat I would do just that or just glass in wood timbers with a layer of stainless or bronze on top. But in the case of both spray in PU foam and tropical hardwood timber I would be repeating the fate of soggy insides after 10 or 20 years. She will be on the hard in locations with freeze thaw cycles so I would rather not have the stringer full of anything that can trap moisture. Is it possible to glass it such that the dead space under the stringers is open below the engine mounts and drains, airs out, fits my fist with a wrench and is inspectable?

> "The coosa is like 2% of the other two and WILL crush under bolting stresses.."
I intended the strength to not come from the Coosa per se, but from the glass layers between and over them [EDIT: and around them]. I just like Coo$a for small projects because it is more rigid and easier to slowly shape than divinicell, with falls apart in my gorilla hands. It holds it's shape better during layup especially if clamped. It also soaks up less resin. I agree, one can not bolt through Coosa without giant washers. I did not intend to bolt through it. I will take your advice and consider use of g10 if I find a good USA source. I am all ears. But for the sake of this discussion pretend it is divinicell in the tripping brackets and I will glass as many layers of cloth as I am advised to. I just didn't bother drawing layers of glass and tabbing as my original question was "what is the minimum layup [schedule] of vinyl ester resin and glass?"

For the light blue in sketch 2 I intended g10 like fiberglass sheet or a few layers of hand layed glass but note it is labled "overkill?". It is likely not needed as that faux stringer is plenty thick as it was designed to support the 20hp engine. I did not plan to use metal but could. I prefer old fashioned bolts and washers over blind tapped metal. BUT, i will consider this if stainless or bronze. Do I not risk corrosion issues encapsulating metal in FGRP or foam?

> "reinstall or rebuild the oil drip pan between the stringers, glassed in"
I can not do this. If I could, all the reinforcement would be unnecessary. I need to install the fuel tank and be able to rip it out if it ever leaks again. That was a ton of work and if i was stuck paying for time on the hard in Juneau or $anFran I would have gone bankrupt. I can not rebuild the bridge structure for that reason. Who knows where i will be when i do this job again. It also was never a true "oil pan" in that oil would simply shed off but that is besides the point. I have seen ones that have a oil pan in the structure.

> "You are a little hard to follow. A drawing of all three views lined up would be needed for a more detailed answer to a detailed problem."
OK. Sorry. I will take more photos and sketch all 3 dimensions sometime in the next few weeks.

Thanks for your responses and ideas.

 

Yes it is possible to build and open structure with gussets, but it will be very thick, and you would have to glass on the underside of the existing stringer to preserve height (engine alignment). It's noth worth the labour, McMaster Carr sells G10 McMaster-Carr https://www.mcmaster.com/grade-g-10-laminate/multipurpose-flame-retardant-garolite-g-10-fr4-sheets-and-bars/ cheaply enough. Make a full fiberglass log out if it and glass it in. G10 can be tapped and will retain machine screws, will never take in water, and you never have to worry about corrosion. Or make little pockets in the logs and use washers and nuts if that makes you happy. Just put them as low as you can and also tap the G10 above.
Your existing bedlogs probably have a steel plate glassed on the underside to retain the bolts, you can see how badly corroded it is.

Take a sheet of 3/16 Al, cut to shape and bend up all the edges to form a 2" high lip. Seal the corner joints with thickened epoxy or 5200, then place between the bedlogs and screw it to them. Instant structural and removable oil drip pan. Can also be made with two angles arranged transversly and thin sheet for the rest if that is more budget friendly.

 

So the engine is a Yanmar 2GM20 or equivalent? Weighs about 275 lbs? 16 horsepower?

Are you sure you're not overbuilding this? Considering the weight it needs to support the following thought came to me.......

1. On the inside vertical side of the faux stringer, run a thin piece of plywood saturated with three coats of epoxy resin and tab it into that cavity using some strips of fiberglass tape. Once it's tabbed in add a couple or three layers of roving to stiffen it up. You could drill out a couple of ventilation holes using a hole saw.

2. Those studs sticking up are secured from under the faux stringer, right? I'd get yourself a couple of sections of aluminum angle, probable 3/8"x3"x3" . Place the angle on top of the faux stringer and drill it so that the studs will pass through. The vertical part of the angle will run down the inside of the faux stringer. Drill it to accept a couple or three 3/8" stainless steel machine screws. You should be able to get inside that faux stringer with a long box wrench or ratchet if you open up the end with a hole saw, maybe 1-1/2"? The whole faux stringer is only about 24 inches long, right? Secure the aluminum angle with 3/8 SS NyLocs and structural washers.

3. You could drill and tap the piece of aluminum angle installed in step #2 that is installed on the faux stringer to accept a light weight piece of angle. How about 1/8"x1'"x1" or maybe 1/8"x1-1/2"x1-1/2"? Drill and tap the little piece of angle to join the bottom of the heavy aluminum angle that's bolted to the faux stringer. This will provide a flange that you can use to support you oil drip pan under the engine. I'd use 4 machine screws to secure to drip pan and you could probably even remove it (provided you have enough clearance) should you ever want to clean that deep cavity under the engine.

I just don't think you need all these exotic high strength materials but I would use epoxy resin due to it's greater strength and waterproofing properties versus the polyesters.

I did engine stringers on my boat years ago and they support well over 1000 lbs of machinery, north of 200 HP. The former material I used was Formular 250, right out of Home Depot. I'll post a couple of photos below. These stringers have been in the boat since 2013 and I've never had any issues with them in any way. I DID follows Dave Gerr's scantling rules. Maybe they're a little heavy but so what? This was a one off build.

Gerr's right, the former material adds no strength. The wood inserts are for handling compressive loads (clamping) at the beds.

Your angle would be installed over the top of the stringer, different than this more conventional build.
Those foam based stringers hold up a lot of weight.

Good luck with your project!

MIA

 

Thanks MIA

Yes. GREAT POINT!!! Little engine. I can lift it myself. Most Nor'Sea27 owners have, and I will, replaced it with a Beta 20 or Beta 25 or Yanmar 3ym20. Also small engines.

YES!!! I indeed may be overbuilding this. I am asking for what the rule or schedule or code would be for glassing such a structure. If I am overbuilding I am happy to scale back. But guessing will not help me sleep at night. [EDITED TO REMOVE HUMOR}
I weigh 200lbs. I can sit on either one of the cut apart faux stringers as it currently exists and try to bounce with all my weight. When i do so my friend sees no deflection in the structure. So as is, it likely is sound. I doubt much more is needed.But i would still like to follow a proper layup schedule so I rest easy at night.

I planned to propose something like this if people felt more was needed beyond my tripping brackets. Except, in addition to drain holes, i was going to ask if i drilled ovals (5" long and 3" tall) or holes (maybe 4" or 5" diameter) under each engine mount to give wrench access to the bolts heads, how much would i increase the layupschedule suggested by Gerr?

Yes. I agree. I am looking for a glass layup minimum. So strength from G10 and Coosa and Marine Ply are just going to be icing on the cake.

Am I safe in saying that spreading the mass out on the hull is better done with 5" of stacked tripping brackets made of 1/2" Coosa or Divinicell than a single 1/4" bracket of g10?

 

OK. just looked.
No metal [EDIT: in the faux stringer where the engine mount bolts pass through]. Only fiberglass. The bolts didn't even have washers. I planned to back them with big washers with Boatlife of sitkaflex caulk. Or rectangular 1/4" stainless plate simply to spread the load better than washers. I may end up with the WIDE yanmar mounts again, or the smaller Beta mounts unsure at this time and will not know until after this project is done.

Fiberglass faux stringer was not cored where the bolts go through. Solid fiberglass 3/4" to 7/8" thick where the bolts are. Just inspected with light, handlens and sharp awls. I will send photos in the next post.

The "pan" part i cut through to remove the "pan" part of the bridge was foam cored. 3-3.5 mm glass top, 6mm foam, 2.5 to 3mm glass bottom. Heavy weight woven cloth like... well bigger than 20 oz which as heavy as i have used. The "pan" area is 20mm thick so I assume it is cored with foam or ply.

Having a PhD and long career in aquatic ecotoxicology, including hydrocarbon pollution work, I 100% will have a secondary containment oil drip pan below my engine. For sure. And... I suffer from entropy more than most people. My washers, nuts and screws tend to suffer from some supernatural form of gravity making them fall into the bilge more than normal people. So I will indeed have a big oil pan to catch small engine parts. I will consider your idea in a few months, thanks. This will be designed after the engine and the fuel tank and all fittings are in place. I hope to not depend on the structural integrity of the removable pan in shoring up engine bed stringers.

 

interior of the faux stringer

Solid fiberglass 3/4" to 7/8" where the bolts are. Bolts are not tapped into or backed by any metal, or even washers.

Really sturdy. As I said, I can sit on either stringer and try to bounce on it and there is no deflection. So I doubt i need much extra support.

 

I find preserving the integrity of the engine alignment and height well worth extra effort of glassing inside the structure. I have gallons of vinyl ester resin and rolls of heavy, medium and light weight glass cloth for this project. You are right, glassing below the structure will be hard, but with the fuel tank out it is doable. I have done way harder. I am happy to put in extra effort as this is a boat I plan to keep in the family forever.

How thick?
What information can I provide to help with your estimate of minimum layup schedule?

 

If you actually want to do this job, fine. Fillet the existing hull stringer joint on the inside, clean up the cut edge so there are no corners only curves, take a 1/2 PVC pipe, slit it lengthwise and slip it over the raw edge (use a heatgun to bend it to shape) and fillet the corners. Now glass over this pipe with a few layers of glass, then on the underside, tape the hull stringer joint. Then put gussets at the ends, and in between the engine feet, like in your drawing, fillet and glass them in.
How much glass to use? On the slit pipe, 1 layer 1708, 2 layers UD 17oz, 1 layer 1708. Tape for all joints, 2 layers 1708. If you don't have UD at hand, just use plain fabric or 0/90 and double the weight