Dinghy Design

[Hermit Scott]

Let me revive this thread. I have a bunch or resin, epoxy, roving fiberglass and now some luan plywood and a sheet of pine 1/4". Everytime I go out into my shop and see this stuff it makes me want to build a dinghy.
I don't anticipate my first dinghy to satisfy my standards, so I might as well get to building the first one, so I can get to building my second one.
Roger, what thickness plywood do I need? I have this home depot, really thin luan stuff I am building crates with, will that work? It's a little less than a 1/4" I believe. Do I need thicker for the floor? How many layers of glass to cover it?
I like the idea of building the dinghy in your design and then using it as a mold for an all glass dinghy. How thick would I need to make the glass?

 

[jibes138]

1/4 inch plywood is pretty standard for dinghy

My sand dollar used 1/4 for the floor and side. Here's a trick for you, if you want thicker, bond two sheets together with the epoxy. My transom was made by bonding three sheets of 1/4 inch. Plywood is nothing but thin sheets glued together anyway. Is your plywood marine grade? Marine plywood has all the defects and voids removed so it is solid wood. They punch out the knots and glue in plugs. If you cut your plywood and find big holes it is not marine plywood. I paid about 35 bucks a sheet I think in 1999. It was hard to find at local lumber yards.

 

[Roger Long]

Roger, what thickness plywood do I need? I have this home depot, really thin luan stuff I am building crates with, will that work? It's a little less than a 1/4" I believe.

That's exactly what I planned to use. If you can find fir without voids (hard) or 1/4" marine ply, that would be good for the bottom. Alternatively, add glass to the bottom. I would also glass the inside of just the bottom panel completely.

One layer of cloth on the outside of the bottom should be sufficient. If your beaches are rocky or you plan to use the boat hard, just add another layer. There's no real engineering to do here, just trust your gut about the trade off's between weight and durability. Remember, it will be easier to add a layer of glass later if the boat isn't holding up than it will be to remove one.

 

[DannyS]

When I built mine I modified the plans and used 3/4 " ply on the flat bow section of the pram style much like Roger's. I did this thinking it would give more integrity to the towing eyes that I through bolted with additional backing plates of another 3/4" ply. This backing plate creates the cleat for the forward seat to sit on. In total, the eye bolts go through 1 1/2" of plywood with large fender washers to help distrubute the load. Maybe this was overkill but I had visions of the bow of the dinghy being ripped off if hit by a large wave while being towed. I also used 3/4 for the transom thinking that if I wanted to throw a motor on it, that would be a much better and stiffer mounting surface for the extra weight and thrust. Otherwise the rest of the boat is 1/4 inch plywood, non-marine grade with one layer of glass on the outside of the hull.

 

[William12213]

Mine was built out of the 1/4" luan. It does not have the voids that your typical plywood will have and is totally acceptable for building a dinghy. I did completely seal mine with epoxy to keep the moisture out. The benefit of using the luan is cheaper cost and a very light boat. Mine was less than 70 pounds when finished. Your strength comes more from the design that how thick of materials you use.

 

[Hermit Scott]

I just measured this luan. It is 3/16". I did plan on glass and epoxy inside and out. This stuff is extremly light. A 4x8 sheet weighs 17.5 pounds.
Danny-how much does your dinghy weigh. It sounds like you built that thing as strong as a tank.

One other idea, I have a plasma cutter and 1/8" aluminium sheets. I can make the floor out of that and the sides luan. I think if I rough it up the epoxy will stick to it like the wood.
Then I won't have to worry too much about beaching it on rocks and rotting the luan.
What do you think?

 

[DannyS]

I've never weighed my boat but I gotta think it's less than 100 pounds. I can move it around pretty easily. It's pretty awkward to try and pick up myself but my wife and I can throw it into the back of my truck at the beginning and end of the season with no problem.
I don't know much about the bonding qualities of epoxy and aluminum but I like the way you're thinking. I'd love to see a hybrid plywood/aluminum dinghy! Go fo it. What's the worst that could happen?
When I built mine I was working at a commercial sign shop and I had access to the CNC router. I drew up all my parts on the computer and had all the components computer cut on the router. It would be really cool to cut it all out of aluminum and just weld the thing together. You could make the seats out of wood but have an aluminum hull. That would be cool! I might have to try that.

 

[Hermit Scott]

Danny-I bought a 4'X4' CNC plasma cutter, called the plasma cam. That was a complete waste of $9,000. The software had so many bugs that I didn't even get to use it to make parts. I would painstakingly enter in all the settings and calibrate the machine, and then the settings would disapear and the plasma head would crash into the steel.
Actually it was only a waste of $3000. I sold it to a cop for $6000. I was completely honest and told him that it was a turd, but he said he had a friend that knew how to get them to work right. I saw him a year later and he said it still wasn't working.
I still have the plasma cutter but not the table.
I would definitely consider making it all out of al. but I don't have enough. I haven't weighed a sheet of aluminum yet but I think it would add signifcantly to the weight to build the whole thing out of it.
How do you transport it while you are out cruising? davits, on deck, towing?

 

[Hermit Scott]

From what I understand from a solid mechanics class I took, if you put a layer of fiberglass on top and one on bottom, it is much stronger than putting two layers on the bottom.

 

[sailingdog]

Yes, that's the reason for cored construction laminates....

From what I understand from a solid mechanics class I took, if you put a layer of fiberglass on top and one on bottom, it is much stronger than putting two layers on the bottom.

 

[DannyS]

I would always tow the dinghy. Our typical cruising grounds are fairly protected (Apostle Islands, Lake Superior) but we do venture out beyond the islands occasionally and for that reason, I've gone over to the dark side and last year, we bought an inflatable. The dinghy I built had a lot of rocker to it which didn't allow for it to sit on deck very well without creating some purpose built cradle or brackets to get it to sit and ride on deck without rolling around. Something to think about BEFORE you choose a design. The other reason for going to the inflatable is that we have two boys that we'd like to keep interested in sailing as long as possible and having something they can eventually rip around in was on our minds (I remember being young and wanting to go FAST)
Bummer about the CNC, when they work they sure can be useful. You're probably right about the weight of an all aluminum dinghy.

 

[Ross]

I have given thought to building a dink with 1/2 inch corecell and fiberglass but the cost keeps putting me off. Probably twice as much as with premium marine plywood. Maybe when the economy get healthy again. At this point I am jobless and getting just enough to pay the bills.

 

[Hermit Scott]

Do you happen to know what the cored material you are talking about weighs in a 4'x8' sheet?

 

[Ross]

Do you happen to know what the cored material you are talking about weighs in a 4'x8' sheet?

The A-500 comes in at about 5 pounds per cubic foot that would mean about 6.25 pounds per sheet. Plus glass and resin on each side.

 

[Hermit Scott]

I have been watching craigs list for dinghy motors. There have been quite a few lately. On an 8' dinghy can you put a 10 hp? Does it depend on the weight of the motor, 4 stroke or 2? Can I increase the horse power capacity by making the transom stronger and adding side ties?
Are these unstable at higher speeds? Danny says he got an inflatable to go fast, why can't you go fast in one of these stitch and glue dinghies?
I know this is something everyone knows but I have only been in one dinghy once and I was rowing, so I don't know about the stability with motors on them.

 

[Ross]

There is a limit to the amount of stress that you can safely apply to a light dink. and one to two hp is close to that limit. Rather like putting a 427 V-8 in a Mini.

 

[druid]

Hi

I just came across this forum looking for pictures of Bodger's Nymph. I'm going to buid a rowing dinghy that will fit in the back of my truck so I can get some exercise at a local lake after work. I was going to build a D4, but I liked the flat-bottom, double chine design of the Nymph. Then I came across THIS design... looks GREAT!

One problem I have with most dinghy designs is the aft seat - I need to stretch out my legs to row effectively. I was going to eliminate it, maybe put in some knee-braces... but then I thought I could just make a "cave" under it for my feet! Would that damage the strength of the hull a lot?

I'll be building it with 1/4" ply - probably "SPF" (mystery wood, either spruce, pine, or fir), but probably 1/2" for the transom because I'll probably want to add an electric motor for fishing, and probably cedar planking for the seats and gunwale.

Thanks for the excellent design!
druid

 

[Roger Long]

! Would that damage the strength of the hull a lot?

No, but it will reduce the flotation. Add some foam under the seat.

Be sure to send me some pictures.

 

[Hermit Scott]

I downloaded a free dinghy design. Cut out the wood stitched it together, but I didn't like the design so I am not going to finish it. I would like an inflatable like that speed demon niceNeasy. I heard he went through a marina at 60 knots in a 35' speed boat blasting the rolling stones on his sound system.