transom composition question

:?:

Most older boats and some new ones use marine plywood for the transom
core. Some newer boats also use one of the foam composites. Why do they use the wood or foam? Why not just build up the transom with glass? Is it that much heavier or just not strong enough by itself?
Just wondering before I redo my transom. I plan on bolting on a flotation bracket as well, so I don't need the transom 1.5" to 2" thick to hang a motor.

 

Pogden, a core makes a panel stiffer. Foam as a core is well know. But plywood between glass laminates also acts as a core. It is of course much denser than your average foam (thus stronger), but also heavier.
On the other hand you don't need a very thick core if your panel (transom) is curved and plywood could be ideal. Your glass will stick much better to plywood than to foam, using epoxy. And 2 layers of 5,5mm plywood glued together, after bending it in the curvature you need for your transom, would not even be too heavy.
A foamcored panel needs more glass laminate than your plywood cored one. On our 30 footers a ply-cored (2 x 5.5mm) transom would take 600 grams of biaxial e-glass as an outerskin and 300 grams + local reinforcements where needed om the inner skin. Foamcored we use at least 1kg glass each skin on a 15mm core (density 65-80).

Building it up from glass will be strong, but you really need a lot to make it stiff as well. (or use stringers/frames)

Good luck

Lucas

 

Lucas is right.

I'll add just for the pleasure of writing that with plywood the job will be easier. An all glass laminate will require far more work and will be heavy as a dead donkey. Rigidity is needed for the transom as it closes the boat and maintains the shape of the aft part of the hull.

 

Thank you both for the replies. I had always wondered why we use the wood or foam, now I know.

I have a 1978 Seacraft with a bad transom and I am almost ready to begin laminating the glass, I just wasn't sure what type of core if any I would use.
My transom is fairly flat so either one of the two would be okay. I just have to decide if the extra cost of the foam is justified. I think Coosa is the best but $$$$.

Thank You for the info!!!

Stay dry,
Pat

 

transom compisition question

Hey sounds like my project! I have a 78 reinell 217 I am restoring (21 foot hartop with alaskan bulkhead, it is cold and rainy out salmon fishing 23 degree deadrise). Will outfit for offshore albacore trips to, and sturgeon. I bought it for a song with the IO missing. The transom was 3 plys of 1/2 plywood lightly glassed in. After 25 years this had turned to mush.

As background, I am a engineer repairing ships, mostly steel work though.

I dug, cut and ground it out the old transom. Laminated a new blank using treated 5 ply CDX .4 lb CCA (three layers). YA YA can hear you think CDX!!! Had a sheet in the shop which had airdried a few years and it doesn't have voids, although the veneer looks to be mostly second growth Doug fir. Another benifit is the plywood is no longer in the shop taking up room. It did warp a bit during storage/drying, treated marine ply would solve that, but it worked ok and should be fine as the main structure supporting the OB bracket is structural aluminun shapes inside the boat, and I am increasing the amount of fireglass. Covered with glass, sandeed to fitGround to fit and glassed it in. (Sealed all the edges) Foamed it all in fair and glassed some more. Way heaveier than new.

Using mostly 24 oz woven roving in polyester. Anybody kow what Reinell used for layup in 1978?????

Fiberglass is coming from fiberglasssupply.com, great outfit by the way.

The bracket is 30 inches long with 3/8 al plate fore and aft with 1/4 plate top bottom and sides. It was water cutt by IRC and welded by L&J I am restoring a 91 johnson 200 hp 25 inch (Only 450 lbs) to hang on it

Interior bracing includes aluminum angles lagged and beddedto the engine bedd 3x4x1/4x28 and welded to 3x5x1/4x36 angles thru bolted thru the transon and bracket All fasteners are 1/2" SS. In addition I have 8 1/2 inch bolts through the bracket and transom only The upper end of the transom angles will be bolted to a five inch aluminum c channel running transversely which will tye into the sides using more channel and bolts where I have built up the sides with more fiberglass. Sealing with 3m 5200 and 101 and resin.

The motor end of the bracket is designed to support a 6,000 pound force in any direction without deflection apperent to a layman. Some flexibilty is required to prevent hardspots and cracking.


Looks cool to, the old reinell has kind of boxy lines to start with. So far so good, should be able to go faster than reasonable. After rigging the engine the interior will get updated,

 

transom compisition question

Hey sounds like my project! I have a 78 reinell 217 I am restoring (21 foot hartop with alaskan bulkhead, it is cold and rainy out salmon fishing 23 degree deadrise). Will outfit for offshore albacore trips to, and sturgeon. I bought it for a song with the IO missing. The transom was 3 plys of 1/2 plywood lightly glassed in. After 25 years this had turned to mush.

As background, I am a engineer repairing ships, mostly steel work though.

I dug, cut and ground it out the old transom. Laminated a new blank using treated 5 ply CDX .4 lb CCA (three layers). YA YA can hear you think CDX!!! Had a sheet in the shop which had airdried a few years and it doesn't have voids, although the veneer looks to be mostly second growth Doug fir. Another benifit is the plywood is no longer in the shop taking up room. It did warp a bit during storage/drying, treated marine ply would solve that, but it worked ok and should be fine as the main structure supporting the OB bracket is structural aluminun shapes inside the boat, and I am increasing the amount of fireglass. Covered with glass, sandeed to fitGround to fit and glassed it in. (Sealed all the edges) Foamed it all in fair and glassed some more. Way heaveier than new.

Using mostly 24 oz woven roving in polyester. Anybody kow what Reinell used for layup in 1978?????

Fiberglass is coming from fiberglasssupply.com, great outfit by the way.

The bracket is 30 inches long with 3/8 al plate fore and aft with 1/4 plate top bottom and sides. It was water cutt by IRC and welded by L&J I am restoring a 91 johnson 200 hp 25 inch (Only 450 lbs) to hang on it

Interior bracing includes aluminum angles lagged and beddedto the engine bedd 3x4x1/4x28 and welded to 3x5x1/4x36 angles thru bolted thru the transon and bracket All fasteners are 1/2" SS. In addition I have 8 1/2 inch bolts through the bracket and transom only The upper end of the transom angles will be bolted to a five inch aluminum c channel running transversely which will tye into the sides using more channel and bolts where I have built up the sides with more fiberglass. Sealing with 3m 5200 and 101 and resin.

The motor end of the bracket is designed to support a 6,000 pound force in any direction without deflection apperent to a layman. Some flexibilty is required to prevent hardspots and cracking.


Looks cool to, the old reinell has kind of boxy lines to start with. So far so good, should be able to go faster than reasonable. After rigging the engine the interior will get updated,

With a bracket I used plywood instead of foam for it's crush strength

 

Sorry for the double post. BTW do not underestimate the forces caused by a bracket. Have seem jobs turn out poorly.

 

Hi TT,

I think I will probably use a couple of layers of mat followed by 18 and 24 oz WR and then 3 layers of 1/2' marine ply separated by M-WR-M-WR-M finished off with the same sequence. I'm using epoxy from Raka.

I would like to raise my deck up 3" as well to finally make it a self-bailing boat
(the way it is now with any weight in the stern the scuppers go beneath the surface).

I was going to epoxy a 3" dia length of PVC pipe along top of the stringers, to build them up instead of using wood and glass and then just tab in the floor to the top of the pipe----I know that is not what is usually done but it seemed reasonable????

I also want to build my own flotation bracket molding it in the shape of the hull, using glass and epoxy.

I have a 2001 Merc 200 25" shaft.

I'm a doc, but like this stuff much better, only I'm so slow at it I could never make enough money to eat or support my family.

 

If you go with foam make certain it has enough compression strength (a 14+ pound density will usually provide this). Plywood is very good material if you use it correctly in conjunction with epoxy and directional glass.

As far as building your own bracket goes, do you have the engineering background to calculate the forces and the scantlings to build a bracket strong enough? If not, then you will probably over build it to make sure it is strong enough, which leaves you with a very heavy weight hanging off your transom that costs a fortune in resin and glass to make. If it were me, I would buy one.

Joel
Boatbuildercentral.com

 

Joel,
Thanks for the input. I will probably go with the marine ply and epoxy. I'm going to enclose my transom completly. I want to limit the number of holes i put in the new transom, there were over thirty holes in the old transom where water go enter. As far as the bracket goes I would like a Armstrong floatation bracket they run around $3500. Over at the Classicseacraft site one of the members made a flotation bracket very similar to the armstrong. I figured I could attempt to follow his work, it is documented in pic's and he is pretty good at answering questions. But you are probably right I will probably make it too heavy. But I have to try, it's my nature, I don't like to think I can't do something just because I never tried.

 

Pogden, building a bracket and expecting a reasonable level of success will be quite low if you don't understand the concepts and issues related to such an undertaking.

You've said you're stern heavy now, adding additional weight back there would not help this issue. A bracket can be designed to provide some additional displacement to help hold up the motor(s) and prevent squatting. This will mean an engineer or designer will need be in the mix or an aftermarket bracket manufacture's product, this isn't a backyard engineering job, though installation is reasonably easy.

Don't separate you're ply laminations with so much goo and mat, in fact, just wet out each panel with epoxy, then screw them together with temporary (I use drywall screws for this) fasteners on 8" centers, removing them 24 hours later. I would only put as much ply in the transom as was in the transom, you have a weight issue remember. You don't need to over build this, just have it done correctly.

You'll find little will stick well to PVC pipe. It can be chemically welded and I think also with heat, but little else sticks very good to it. To shim up your sole, I'd just make up some 3" shims to tab in on top of your stringers. You could use foam, but this will require more 'glass work or you could use wood which will require less 'glass work, but may rot if attention isn't paid. Do your self a favor and bond your fasteners with epoxy when installing the sole. This will provide a much higher yield in fastener strength and help stave off rot. I bond all fasteners that will be out of sight, in the bilge, under the water or where ever hardware or gear must be held down. In areas where machine bolts or screws may need be removed for inspection, access or repair I bond in a blind nut (T nut) on the underside of the stringer, backing plate or whatever and coat the threads with a release agent (wax basically) The fastener can be removed and installed repeatedly without stripping the hole or threads.

Good Luck,

 

Thanks PAR, that's attention and great care of the details.

True boatbuilding (I mean true boats, not polyester shoe boxes made by underpaid employees, decorated with chromed zamac fittings) does not accept the "à peu près" (the "look like" or not cared job). It takes years for a pro to learn good boatbuilding.

A bracket is not just a box hanging at the transom, it may become a dangerous nuisance. For a "heavy transom boat" you have several solutions from a skirt adding dynamic lift and thus getting aft the plane center to brackets designed to give some buoyancy taking the weight of the engine(s) and to act dynamically as the tail of a plane for stabilizing the angle of planing.

 

PAR,
You are right a bracket is not something to just throw together. The Armstrong bracket I mentioned is a quality piece of hardware. I think I can do something similar, actually I think I have a better design... at least in my head, whether or not I can be successful is another story, but I will try.

Thank you for the tips...the transom had two sheets of 3/4" ply originally, I thought three sheets of 1/2" would be better... epoxy would penetrate more leaving less untreated wood to rot. Does that make sense??

I'm sliding the tank and the console forward to help with the stern heaviness
also.

I'll skip the PVC that was a easy way out, I'll use wood or.... Some of the guys have been using molds and making up glass stringers and filling them with foam that may be what I will end up doing.

Thanks,

 

Unless you have a good engineering background, I'd strongly recommend, you reconsider the home made bracket. It would be a shame to go to the trouble of building a bracket, just to find it ripped your freshly repaired transom clean off with the first hole shot.

I'd think about addressing the trim first and moving weight(s) around, like the helm and tanks is a good start. With the boat trimmed better the bracket may not need additional displacement built in, though typically this would be an issue better suited to mathematics then experimentation.

The extra layer of ply will make a stronger transom, but at the expense of more materials, labor and cost. This third layer will rot just as any other layer inside a transom. Rot will only occur if the transom coatings ('glass work) have been breached and not dealt with in a timely fashion. Bonding the holes in the transom will cut this down considerably (drilling an over sized hole and filling with thickened epoxy, then drilling for the fastener after cured)

Glass stringer risers or extensions will not take fasteners very well (one of the reasons wood is used) but you could rely on just the tabbing to hold down the sole. I like hard fasteners holding things, even if just a backup to tabbing. I find they keep things in place as the goo sets up if nothing else.

 

Thank you Ilan and PAR. Your experince is amazing.

"Bonding the holes in the transom will cut this down considerably (drilling an over sized hole and filling with thickened epoxy, then drilling for the fastener after cured)"

That's a great idea, I will do it that way. I can mark the holes I need, drill a small hole on the outside and then drill a larger hole from the inside stopping at the skin so I only have a small hole from the outside for water to penertate. Then fill the hole with epoxy and redrill the correct size I need.

Wood is so much easier to use than everything else. I am leaning more and more to using it for the risers on the stringers. I have only had this boat three seasons, it's a great boat. I take care of it much better than the previous owners, so I think the wood I use will not rot as readily. I'm sure it will still rot eventially but by then my kids can rebuild it (because I will be long gone).

Actually with the deck raised, the tank and the console moved slightly forward, I really won't need a bracket. I can raise the transom to twenty-five inches and I will be done. The boat will be self-bailing. I can run the scuppers out the sides, so they won't go through the transom, eliminating a hole in the transom(and a chance of water intrusion into the wood and rot).

Thanks,
Pat