Standing cabin build on 19’ aluminum runabout

I have a 1974 Starcraft Holiday (19-foot, 90hp outboard, gutted it out, etc etc) that I am building a standing cabin for. Pretty tight fitting on a boat this size!

I replaced ~240 rivets, installed a new transom and sole (nautilex over) with a custom under deck gas tank, shortened the splashwell by over half its original length, and made provisions for fishing specific wiring, etc.

Currently have framed out the cabin in white cedar (low weight, not exceedingly strong) and am about to ‘glass the frame, ‘glass both sides of the 1/4” plywood that will provide essential strength, and then glass over the whole assembly.

While the framing has some structural value, I (intentionally) did not use typical careful wooden boat joinery because the plywood will provide basic torsional rigidity and I planned to rely upon 1708/polyester (two layers) with subsequent final layups of 6oz cloth before fairing and painting for actual strength.

My intention was to ensure multiple layers at the three (really four fore/aft, and two abeam) locations where the highest stress loadings occur: the ‘post’ tops forward of the cantilever will have the ‘weakest’ joint (highest loading) as it has little to transfer the energy elsewhere and will be stressed in four directions, and the assembly housing the three forward hinged windows. The side windows will also be hinged. Why not? Not much difference in the work.

Above is the setting for the question I arrived at today.

➡️ Is two layers of 1708 with 3 layers of 6oz cloth at those high-load points sufficient to provide rigidity without fracturing?

I hadn’t thought about it extensively until this morning when it occurred to me that two layers of 1708 are not particularly “thick,” and even though 3 subsequent layers of 6oz will be fairly strong for its thickness going over that I do not have it in my experience/knowledge base to determine this scientifically-it just feels light when I thought about it. Made me wish I’d done more robust joinery for a moment, but then again there’s a lot of boats successfully out there that use the wood in the boat merely as a structural form (my intention) for the fiberglass.

Thanks in advance.
I’ve lurked here now and again for quite some time in the past but this got me to finally join
I have a picture of the current state of affairs but the 45* angle braces are temporary. Permanent 15* sides above the gunwales are planned at the ~15” window sills height.

I’ve tried to proportion as tastefully as possible, but 73” headroom is asking a lot of a boat this small. I don’t think it will be or is as comical as a similarly-sized C-Dory, and my New Englander esthetic sensibilities are somewhat satisfied with the hint of downeast / sea coast details common in Maine, NH and Nova Scotia that I tried to lean into.

 

This is not the first time we’ve seen such remodels. I’m not a fan. The boat will be unstable in turns.

If you want to reduce weight; avoid csm in the laminate and switch to jist biaxial fabric and switch to epoxy.

Your post is too vague to remark well. If you are putting plywood over the entire outside, a single layer of 6 oz will be plenty on the outside. The inside reinforcements would be good at somewhere between 2-4 layers of db1200 tape. But the vessel is never going to perform well as an offshore wave pounder, so trying to make it into that is another story. You don’t need to glass the inside plywood. Use a precatalyzed epoxy paint.

 

That's a fair whack of cloth on 1/4 inch plywood, certainly more rugged than the windows bolted in.

Personally If your set on a couple layers of combo I'd do whatever resin is cheapest. Standing up in the bow that far forward can be a wild ride.

Puget sound crabbers have retrofit boats like that for years, makes them sail a bit in the wind but has still worked.

 

I'm with fallguy in that adding weight that far forward in a small boat and at such a height won't make it a better boat.It will provide some shelter but that comes with a cost in terms of weight.Do you have any idea of the total you will be adding?A lower cuddy would add less windage and keep the c of g lower,you could add a sprayhood in the central portion for standing headroom.This should give some idea of the principle.

 

Can you?
Should you?
Different answers for more than one reason, most of them already mentioned.

You mentioned comical looks and C-Dory vs. your project in the same sentence. I'll just opine you have a curious sense of proportion.

 

Thank you for your note. Fortunately it’s never going to see any salt ever and if it does it will probably only be one trip for stripers in the Providence River.

Glass on just the exterior will indeed protect the plywood adequately from weather, but adding the interior facing fabric will be significantly stronger as a sheathing than simply covering the exterior because it completes the mechanical system of the plywood base.

Thanks. Ya, if it were a bluewater boat. The helm is only 4” forward of the oem location. But your observation indicates to me that my proportions have created illusions I expected - some beneficial, some negative, but at some point in a small boat function has to win over form.

That is interesting to know! Around here (inland, Vermont) I haven’t seen anything like this. Nothing new under the sun, is there!
I ran a 14’ MFG Niagara on Champlain for years. Been to Oneida a few times. Just my bimini up would frequently let me fish (trolling) with the motor down (steering) but off, and sometimes it was too fast so I know of what you speak. Such a poor sailing vessel it was- usually just resorted to running the motor into the wind for the magic walleye speed.

Thank you.
While I have not weighed the actual new materials, I removed the oem front windows, ski racks, step-up, dash, and vinyl that was at least 140#. The white cedar framework is ~1lb/BF and therefore ~40lbs. 1/4” ply will be ~25lbs/sheet so that brings it to 115#. Window glass and assembly will be (guessing) 70lbs, and add (arbitrary guess) 50lbs of cloth and resin for about 235lbs additional weight “up front.” So it’s only like 100lbs additional weight not counting that my handmade seats shave about 45-55lbs off from the oem folding doubled up seats.

I was initially concerned about the cabin weight until I did the above math and decided it was inconsequential for a boat rated for 7 150lb people aboard.
I further considered the addition of weight aft: a 6hp trolling motor, 140 pounds of additional batteries, and the plano box storage cabinets located up high in the empty airspace of the two former jump seat backs. The belly tank moves some fuel weight forward yet altogether the gross wet weight without passengers hasn’t really changed the center of gravity / balance dramatically.

The ‘cuddy’ is kneeling room only; the hump was created 95% to avoid the floating-phone-booth effect. It’s about 38-40” in height, 26” fore to windshield.

I believe the weight cost is mildly irrelevant. The hardtop goals were to allow a heatable space for frostbite fishing on Champlain, permit sleeping aboard in a covered area alone or with grandkids (soft windows and marine vinyl to fill the rear and aft sides of the ‘cabin’), provide a sunshade, make the boat feel roomier than its size (why I cut the splashwell shorter and installed the fuel tank below the flooring), and allow mounting of a couple solar panels to power essentials like the electric coffeemaker. In that order.
So I was and am willing to accept the consequences of my actions LOL

Thank you for taking the time to post that picture! I’ve worked on a number of boats like the one pictured. Lake boats for fishing Champlain and Ontario. I have a few friends with the ills of Grady Whites, Crestliner Sabres, etc so I’ve experienced them. I just didn’t want canvas for my purposes and I have underutilized skills (…and issues with boredom).

 

I like C-Dory. Great boats.
On Champlain we see smaller ones and they roll a lot compared to the harsher rides of the mod-vees and v-bottoms typical of the inland northeast. So the image a 275# man squeezed into a 16-17’ C-Dory on a blustery day in January rolling and bobbing in the waves of Champlain is seared into my mind
Admittedly, he was probably warmer and dryer than we were in my remodeled ancient 14’ MFG Niagara and thinking how silly we looked as well!!
A Maine small-time contract lobsterman would probably think it stable and secure


Meanwhile I’m still welcoming commentary on the layers of ‘glass on the high-stress T junctures of the cabin which is already framed out and begging for sheathing.

 

You only need to tab the joins on the inside. The added strength to laminate the plywood inside is -neg-.

glass the structure, longest pieces first 4”,3”,2” beyond the joins for any you deem critical and 4”,2” for less critical, over fillets for smooth glass transistions

precoat all wood before glassing and either sand it or glass wet on green

 

The house framing technique used is quite bulky, but not very strong.
I’ve had some success building similar cabins using an inside out technique (framing outside, melamine inside) to build a one off female mold for a light, strong all fiberglass cabin.

 

Thank you

I would agree.
The lap joints are about the only ‘strong’ part of the whole structure.
I did mention I intended to relay upon the fiberglass laminations for the vertical torsional rigidity. Not as bulky as it looks as the verticals are 2-3/4” across, but white cedar is light enough that extra isn’t offensive and weak enough that you wish for mass. So I used some mass.

I’d imagine so!

I used the framing for several reasons, the first being I designed it in my head and didn’t draw anything this time. So flexing battens, a sharpie, and scribing / taking off the baselines straight to boards was my ‘plan view’ while a bevel gauge and speed square worked out the initial three angles pretty quickly.
I’ve been working wood and doing a lot of structural carpentry along with construction management for, well, since the 80’s so I was comfortable “designing” in the wood.

But as mentioned, I hadn’t worked out the laminations at the stressed inverted T and L joints ahead of time until the thinking of that stage arrived upon me. As you stated, it is not a strong frame beyond the lap joint resting on the ‘post’ so I thought I should inquire of experts regarding adequately handling the axial and lateral loads with fiberglass laminations. Of course the plywood cabin roof base plywood is going to far more than adequately absorb any torsional loads at the top of the posts, and in my experience 1/4” plywood when glassed both sides is incredibly stronger than the plywood alone.

So the fore-aft strain at green, the latitudinal load at orange, and the transfer of the load at yellow is my concern to combat with how many layers of 1708? Everything above needs to transfer potential energy to the green line points (below there the plywood will exceed anything, sufficiently transferring loads to the gunwales).

My uncertified ‘calculations’ suggest the two layers of 1708 with three layers of 6oz cloth over will be “adequate” but I’m not actually an engineer- I just play one on the internet.

 

Aluminum boat rebuilding project - The Vonda Lynn
You might go back to this thread from 2020 and see if the builder of this cabin on a small aluminum completed and how his project worked out

 

The problem isn't the structure it's your background preventing you to understand how this particular structure works. This will be a glued construction where all of the loads you are concerned for are taken primarily by the plywood. The frame acts as a local stiffener and to increase bonding area for the corner glue joints. The plywood is acting like a diafragm, in order to have force acting on the joints the plywood has to fail first.
You can add all the laminations to the corners you want, they will do nothing unless the plywood fails first. If the ply fails the corner lamination is also useless, the posts will break below them.
To better understand how the system works, take a square piece of plywood and glue two battens along two of its edges. It doesn't matter if you just butt them or do a mitered cut, it doesn't even really matter if you glue the battens together beforehand, you won't be able to open or break the joint before you break the plywood.

You could calculate the entire structure as a sandwich, but that would disregard the static contribution of the wood and result in an overly heavy construction. It would only make sense if using foam or endgrain balsa as a core.

Your lap joints are neither good or bad all that matters is that you fully glue the ply to the frame. Thickened epoxy is the first choice, but any waterproof wood glue would render the same result if used correctly.
One layer of 6 oz glass with epoxy as weatherproofing and paint substrate on the outside is enough. If using fir plywood, add another 6 oz layer on the inside to prevent checking. Hardwood ply is ok with three coats of epoxy followed by paint, as is the frame. If you want to glass over the frame you will need to fillet the corners to the ply using thickened epoxy and round over every other corner (minimum of 1/4" radius, bigger is better). 6 oz glass is also appropriate here, there is no need for more, but it won't gain you anything, it's just adding work. The cedar is quite rot resistant and paintable by itself, adding glass is just for your piece of mind.

Your biggest concern should be the plywood to hull intersection, keeping it permanently watertight despite the widely dissimilar thermal expansion requires some good planning.

 

Thank you for your reply.
However, you have greatly misjudged my understanding of structural loading. Which is forgiven easily due to my realization (with apologies) that I neglected to mention that the plywood will only be applied to the top and bottom of the roof beams, and the lower 16” of the cabin sides. The verticals were not intended to be sheathed in plywood- just glassed.

At the top there will be no plywood to mechanically transfer the axial and longitudinal loads.

Yes. I’ve been messing with ‘glass boats for 30+ years. But you did not know that.

It will be fastened to the (reinforced) gunwales by eight bolts. The cabin sides’ bases will be laminated with fiberglass and polyester resin prior to final install and fully bedded in polyurethane caulking (which is an incredible adhesive and sealant in the unlikely situation that you did not already know that).

So that perhaps enlightens regarding why I am asking about the segment I do not have an understanding of: the fiberglass laminate thickness to do the work that plywood would do had I built it that way.

 

Sorry, I tought you were building an enclosed cabin. If I understood correctly you plan a plywood roof and a plywood on three sides 16" high. The rest is open. You want to wrap the joints in glass to keep them thight, and you don't want to use any sort of knee or other diagonal bracing.
If I got it right you will need a fair amount of glass, like a few pounds on each joint. Not because of the strength but because glass is much more flexible then wood, and you need stiffness, so it needs to be thick.

 

Yes. You got that right.

Exactly.
I’ve done a lot of fiberglass repairs over the years and some structural constructions, but because ‘new construction’ of boats where the glass laminate itself was going to be the primary structural element is not in my experience set (nor am I an engineer) I have little to go on for this application- which in this case isn’t own personal boat project.

There is a shallow ‘knee’ under the cantilever as shown in the photo, and there will be a 25” height, 11* diagonal aft of the vertical post (“B-pillar” if you will) to the gunwale which lowers the horizontal longitudinal tension at the 16” height window sill. All things else aside that will place the bulk of the transitional load at the B-post tops.

Maybe I’m overthinking…

FWIW, the “B-post” tops will have lap joints of roof beams both fore and aft, and these will be glued with titebond2. So although ‘mentally’ I have been considering the (knowingly) poorly cabin framing as limitedly structural, it does have some significance in the structural integrity.

Nevertheless, I understand your point of ‘several pounds’ of glass at the joints. I appreciate the time people have taken respond; it has helped me think in ways I hadn’t included, yet.
My conclusions at this juncture have me reconsidering a bit. I think that the bending moment at the cantilever is adequate, but letting-in and gluing a horizontal piece of 1/4” x 2-3/4” over the ~60” fore-aft segment will lend enough rigidity by its tension that less than several pounds of glass will ‘tighten’ (as you say) the assembly to an acceptable margin.

As weird as it sounds, running on the water has been the lesser of my concerns versus the violence of trailering over vermont’s roads.

I haven’t built any bad porches or decks myself, but I’ve seen some incredibly poor decks (structurally) that have survived 20 years of overweight vermonters and been fine… as the instructor (an engineer) in a weekend wood I-beam floor joist design class I took years ago said: deflection is not failure. For my boat hardtop, I have taken a step back after considering that once the roof is sheathed and plywood and the glue is set I would be willing to stand my 170# on it - or even hang from the cantilever- and not expect it to fail as is. If 3-4 layups of 1708 is added it isn’t going to get weaker, now is it?

Thanks for that- I skimmed through.
That vid showed it was def bow-heavy! I felt (still do) that what I was doing on a 19’ boat was not over the edge but I never would have attempted on a narrower (and shorter!) hull.

I knew I needed to pay attention closely to what I added for weight and where versus what I took out. I had done some notebook napkin math years ago when I started the project, but now that wood is in place and I’ve calculated window glass weight I’m better off than I thought.
The cuts and sizing on the frame members wound up at half the board footage I expected, and I’m going to need less then three sheets of 1/4” sheathing plywood so that’s over 65lbs less right there.
Time will tell but with the additional aft weight (fishing equip) I’m probably going to be weight distributed near OEM if not a bit heavy aft.

The wind-sailing problem; I knew that was going to be an unavoidable consequence from the get-go. I thought about my goals for it and decided I could suffer.
(I have other boats)

I have some very dry western red cedar and a bit of (local) northeastern white cedar. I’m maybe going to bandsaw out 1/4” x2-1/2” to plank the interior ceiling, varnish it. Mostly white cedar with a few strips of wrc for effect, and it is lighter than 1/4” ply