1/4" plywood strong enough?

[96720]

Will 1/4" plywood covered with 6oz fiberglass and epoxy inside and out be strong enough to build a 4.5' wide by 2.5' high and 5 foot long half cab shell similar to the one pictured below? All mounting points and steering console will be backed or made of 1/2" ply.

 

[BWT]

Re: 1/4" plywood strong enough?

structurally wise I think it might be strong enough (borderline). A couple of things to think about. first, trying to make a shell out of 1/4" that will be stiff enough so that the panels don't appear to be bowed or have a wave in them is going to be tricky. The one pictured has nice crisp lines and I don't know that you'll be able to get that same appearance with such lightweight plywood. Once the glass is layed up it will be stiff, but until then the layup is going to be tricky and the fairing will be tedious. I would consider going up to 3/8".

Also, I would personally look at a heavier glass layup. For example on each side of this enclosure I would layup in this order : 1st layer CSM 1.5oz, 2nd layer 1708 biax then 2 layers csm 1.5oz each. If all these weights are added up inside and out you're looking at roughly 45oz of glass (22.5 each side). Also, I would consider using poly resin rather than epoxy. For cost, but also you'll be able to use standard csm rather than epoxy compatible csm; not a big deal, but probably easier to find on the "big island". Just my opinion... A couple of other Q's, what kind of plywood and how would you finish the enclosure (paint, gel, etc)

 

[96720]

Re: 1/4" plywood strong enough?

structurally wise I think it might be strong enough (borderline). A couple of things to think about. first, trying to make a shell out of 1/4" that will be stiff enough so that the panels don't appear to be bowed or have a wave in them is going to be tricky. The one pictured has nice crisp lines and I don't know that you'll be able to get that same appearance with such lightweight plywood. Once the glass is layed up it will be stiff, but until then the layup is going to be tricky and the fairing will be tedious. I would consider going up to 3/8".

Also, I would personally look at a heavier glass layup. For example on each side of this enclosure I would layup in this order : 1st layer CSM 1.5oz, 2nd layer 1708 biax then 2 layers csm 1.5oz each. If all these weights are added up inside and out you're looking at roughly 45oz of glass (22.5 each side). Also, I would consider using poly resin rather than epoxy. For cost, but also you'll be able to use standard csm rather than epoxy compatible csm; not a big deal, but probably easier to find on the "big island". Just my opinion... A couple of other Q's, what kind of plywood and how would you finish the enclosure (paint, gel, etc)

I was considering 3/8" ply initially but weight is a factor in this project that's why I'm stretching myself thin with the 1/4" ply. I didn?t take the flexing factor of the ?? ply into account and you?re right it will be difficult to create the lines as in my model. I?m looking for a minimum amount of weight in my fiberglass layer(s) your layup sounds strong but a little heavy for my project. You?re right poly is easier to find here on the Big Island but, may not be too fresh. And, the ply isn?t marine grade, another difficult item to find here on island. So, AB ply will have to do. As far as the finish I?ll have a friend spray gelcoat.

 

[BWT]

Re: 1/4" plywood strong enough?

how is your supply for foam rather than plywood? That would really cut down on the weight. One other consideration with using epoxy is that you may want to paint rather than gel. As a general rule, epoxy can go over top of polyester, BUT polyester cannot go over top of epoxy; doesn't bond real well. Not saying it can't be done (west system claims that if properly prepped poly will bond with their epoxy but I have not done any testing personally to confirm this.). Another thought, if you strategically placed some 'stiffeners' solid strips of wood on the inside of the 1/4" ply that might give the rigidity needed for layup... Just a thought. Sounds like a fun project!

 

[96720]

Re: 1/4" plywood strong enough?

how is your supply for foam rather than plywood? That would really cut down on the weight. One other consideration with using epoxy is that you may want to paint rather than gel. As a general rule, epoxy can go over top of polyester, BUT polyester cannot go over top of epoxy; doesn't bond real well. Not saying it can't be done (west system claims that if properly prepped poly will bond with their epoxy but I have not done any testing personally to confirm this.). Another thought, if you strategically placed some 'stiffeners' solid strips of wood on the inside of the 1/4" ply that might give the rigidity needed for layup... Just a thought. Sounds like a fun project!

Foam board was another alternative I was considering. I can have it shipped to me from Oahu but rather pricey. And, I wasn't sure of how strong it would be compared to ply of equal thickness. I'd also have to put wood backing to high stress mounting areas.....guess it would be the same as using thinner plywood. What core material and layup would give me the lightest weight and strength enough so you'd be able to hold on to the half cab with this size and dimensions in rough conditions.

 

[BWT]

Re: 1/4" plywood strong enough?

What core material and layup would give me the lightest weight and strength enough so you'd be able to hold on to the half cab with this size and dimensions in rough conditions.

I think your last 3 words say alot.. What size boat are you looking to mount this on? Also, if you generally know the dry weight that may also be helpful.

 

[96720]

Re: 1/4" plywood strong enough?

I think your last 3 words say alot.. What size boat are you looking to mount this on? Also, if you generally know the dry weight that may also be helpful.

The boat is 16' X 6'08" approx. 1400lbs w/ 60hp 4 stroke. A marginal trade wind day is about 4'-8' wave ht and winds 10-20 knts.

 

[BWT]

Re: 1/4" plywood strong enough?

Given the narrow beam and relatively light weight I would really try to find a way to do this with canvas and stainless tubing. I am assuming that this shell is mostly to keep the spray off? Even a light weight cab of this size is going to be over 200lbs and that's really skimping on the glass layup. Properly built would probably be closer to 250-275lbs. In my opinion that's too much weight above the water for this hull. Are you able to take some pics of the helm area of your boat for design ideas?

 

[96720]

Re: 1/4" plywood strong enough?

Given the narrow beam and relatively light weight I would really try to find a way to do this with canvas and stainless tubing. I am assuming that this shell is mostly to keep the spray off? Even a light weight cab of this size is going to be over 200lbs and that's really skimping on the glass layup. Properly built would probably be closer to 250-275lbs. In my opinion that's too much weight above the water for this hull. Are you able to take some pics of the helm area of your boat for design ideas?

This is actually at total rebuild. The half cab will be part of the helm station. The half cab/helm prior to the rebuild was about 2 feet shorter in length and about a foot shorter in height and it was built out of 1/2" ply and a layer of fiberglass.

 

[BWT]

Re: 1/4" plywood strong enough?

a few Q's come to mind. If this is a rebuild, what happened to the original? Do you simply want something bigger or did it fall apart from stress, rot out etc? How did the boat handle with the original cab? now figure that with the re-designed model you will be using 1 additional sheet of ply, probably 6 more yards (minimum) of fiberglass, as well as roughly an additional gallon or so of epoxy; while adding an extra foot of height. With the size of the swells mentioned (4-8 feet) I see this as a bit risky if you happen to get caught on your beam and start to pitch. This is just my opinion. Hopefully others will weigh in on this.. I am still recommending a smaller helm station/pod and the "cab" portion to be built of canvas. Weight wise, this will be the lightest way to go...