Building a hard dodger

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Re: Building a hard dodger

Postby Mike Holibar » Fri Sep 18, 2009 6:29 am

Hi Michel,
Nice to see your considered and thoughtful approach to the hard dodger. They make such a difference to your comfort and can really enhance the apperance of the ship if done well. On the other hand, there are many examples of poorly thouught out dodgers that just ruin the appearance of the ship. On my old Rasmus we did the dodger thing a couple of years ago. As I was short of time and bearing inmind the foregoing, I decided to get the professionals to do the job. The first and most important thing was to get the shape right. Two points were that the camber was the same as the cabin top and the angle of the forward window was paralell to the vang. The pros then constructed a mock up using 3 mm mdf and a glue gun and experimented with window sizes and shapes until they got the right look. This structure was then lifted off and used to fabricate a frame work around which the dodger could be built. it was constructed of 18mm ply with a 4 layer laminated roof. A couple of hanging knees to strengthen the aft section. Glass windows fitted with simpson construction adhesive with a uv primer on the glass. Safety glass by the way. 6mm if I remember correctly. This site should give you the pictures. http://www.trademe.co.nz/Trade-Me-Motor ... 139474.htm Cost was NZD11,000 and I did the glazing.
Mike Holibar
S/V Fyne Spirit of Plymouth (Freedom 39PHS-1989)
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Re: Building a hard dodger

Postby Michel » Fri Sep 18, 2009 9:49 am

Mike, nice piece of work on your Rasmus. Very special boat, a wooden Halberg Rassy. Looks like your dodger pros did the same thing as I did, only I start with cardboard. Your dodger was built from 18 mm ply, so it must have been quite heavy. I'm thinking of 10 mm PU or PVC foam and an outside laminate of 4 mm and an inside of 2 mm. But I must check if the roof will me strong enough. The sides with all the window rebates will be almost solid laminate with a lot of strength because of the rebate ridges.

Because I want to work with a female mold, it's hard to test window configurations on the full scale model. The female mold will be put together on board and connected to the deck flange in situ. I'm now working on a more roundish dodger design bent from just two sheets, front and sides in one piece plus a roof. This one will also have rounded windows, so they will not be glass.
Michel Capel, Freedom 44 #4 1981 'Alabama Queen', NED8188, cat ketch with wishbones, home port Enkhuizen, the Netherlands, 52*42.238'N 005*18.154'E.
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Re: Building a hard dodger

Postby whimsy » Sat Sep 19, 2009 3:29 pm

Michel wrote:I think I can bend these window acrylics myself with a mold (a curved sheet of thin sheet metal) and a heat gun. The largest window is maybe 2 'x 2' so that must be possible with a heat gun or even in my kitchen oven. Having safety or tempered glass made is possible, but having it made in a curve is very expensive because they need to make a metal mold for the bending.

I would think that the oven approach is better than the heat gun approach. A heat gun doesn't get a large area at a uniform temperature unless you are really incredibly good with it, and still wouldn't be as even as an oven.

I recall a discussion of replacing the curved sliding companionway hatch which indicated that the method used was to heat the new plastic in some sort of oven, then take it out and lay it on top of the old hatch, using that as a mold.

The other option is to try and design for flat sections where the windows go. Should be very possible to do on the front, but would be hard to do on the top. This gives you a lot more options of glass / plastic, and makes replacement much easier and more flexible in case of accidents.
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Re: Building a hard dodger

Postby whimsy » Sat Sep 19, 2009 3:44 pm

Michel wrote:I'm thinking of 10 mm PU or PVC foam and an outside laminate of 4 mm and an inside of 2 mm. But I must check if the roof will me strong enough.

Sounds pretty reasonable, but I'm giving you a seat-of-the-pants opinion rather than a measured engineering opinion.

I've been working on my deck and I've got a core sample I took out. The factory laminate was ~5mm outside laminate and ~3mm inside laminate. The core was balsa, and it was ~13mm in the side decks, but ~20mm on the main cabin top.

The deck was still felt solid even in a section (almost 1m^2) that had the core pretty completely rotted away, so it is obviously very strong and stiff.

My only suggestion is that stiffness increases greatly with core thickness, so you might wish to increase that to 15~20mm from your original 10mm.

Let me also observe that I am gaining a new respect for balsa core, as I'm repairing and replacing bad spots in my deck. You gotta be careful to keep water out, but it is stiffer and stronger than foam, and does pretty well at keeping water migrations strictly vertical. These days I'm more amazed at how much is still intact than I am discouraged at the damage done by years of leaks. Just make sure that any holes in it go through both cores if you do make holes--Most of my worst problems came from self-tapping screws that didn't go through the bottom skin.
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Re: Building a hard dodger

Postby Michel » Tue Sep 22, 2009 3:38 am

Barry (I think it is?),

Thanks for your observations. The thing about the heatgun and oven is true, I'm reading a lot on this subject now. A heatgun is only suitable to create a fold in lexan or plexi. So if I want to do the bending myself, I need to design the windows to fit in my oven.

About the laminate, our decks also have a structural role in keeping the boat in one piece and keeping the masts on without too much torque in the hull. The dodger laminate therefore can be a bit lighter than the hull or deck. It has no structural function. At the aft edge of the dodger I plan to use a foam core of 20 mm. in a 10 or 15 cm wide strip.

One important property of balsa core I'd like to add is it's resistance to tearing. If you have a foam laminate, you can rip both glass/resin parts off each other when you push a crow bar into the foam. Try this with balsa and you don't succeed because of the strength of the balsa fibers. Balsa also resists denting much better than foam; most foams are not very elastic.
Michel Capel, Freedom 44 #4 1981 'Alabama Queen', NED8188, cat ketch with wishbones, home port Enkhuizen, the Netherlands, 52*42.238'N 005*18.154'E.
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Next generation hard dodger design

Postby Michel » Thu Oct 01, 2009 11:10 am

I am into the second generation of design for my F44 hard dodger. This one has a one piece rounded front and sides with 35* slant at the front. The flat paneled design had 50* slant because the front sheet was a rectangle bent backwards over the 1:10 cambered deck. The rounded design was much harder to make, lots of clipping of cardboard. If you like, have a look at the progress at http://picasaweb.google.com/michcap5/HardDodgerProject# .

Smart comments are always welcome, technical advice also.
Last edited by Michel on Thu Oct 01, 2009 2:46 pm, edited 1 time in total.
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Re: Building a hard dodger

Postby katorpus » Thu Oct 01, 2009 11:51 am

I like the "less upright" look of the greater slant! The absence of a "standard boom" with boom vang gives you a lot more flexibility than what I'd have in the same installation.

I also like the integral "dam" to drain the water that gets past the breakwater. That's obviously something that they "left out" on the F40 deck mold, and it would be a wooly-bear to try to incorporate now. I'm thinking my best bet for a retro-fit would be to glass in a lateral "handhold" at the outer ends of the bulkhead and cut a scupper hole in the "rain channel" forward of that, draining it through an internally-plumbed hole and out into the drainage channel at the hinge-side of the lazarette tops. The aft end of that channel (at the bulkhead) is higher than the channel just forward of it, resulting in a "bowl" that holds water until it evaporates (at the dock) and does not self-rinse the accumulated dirt (ever). This drives me crazy....that...along with the mis-placed side deck scuppers (not at the "low point" of the deck in normal dockside trim), which results in the same kind of dirt & puddle directly in front of the port side lifeline gate...nevermind that the starboard scupper drains down the hull directly above the opening port at the nav desk. Really great planning there!
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Re: Building a hard dodger

Postby Michel » Thu Oct 01, 2009 2:56 pm

I know what you mean about drainage. The first set of F44s (I have hull #4) have a 3' gutter along the outer edge of the deck with a drain hole at the front of the gutter. Now I have a Northern Lights 6Kw generator mounted inside the transom, so imagine where my deckwater drains? Right, it doesn't. Only if I fill the 175 gallon watertank under the V-berth will the water run out of the gutter. Otherwise, it sits there, with the dirt in it, right at the lifeline gates. On later F44s they left out the gutter and put a drain in the deck edge a bit more aft. Still not ideal, but better. I never understood Freedom's ideas about getting water that is outside on the deck into the boat just to get it outside again via a series of Murphy-prone hoses, elbows and clamps. The UK F35 catketch also has an internal deck drain. Really bizarre.
Last edited by Michel on Tue Oct 06, 2009 3:06 am, edited 1 time in total.
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Re: Building a hard dodger

Postby AlanK » Fri Oct 02, 2009 8:11 am

Must be the UK built F-33/35 that has the internally run drains. On my US built F33 There are only the cutouts in the teack toerail and the transom drain for the cockpit. Unless F33s with aluminum toerails have a different drainage system.
I believe the previous owner cut some additional openings in the teak toerail to assist drainage so drainage design by Freedom must have been lacking in general.
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Re: Building a hard dodger

Postby katorpus » Mon Oct 05, 2009 5:19 pm

Michel

I just saw an ad in the back of Soundings. It seems that Webasto is now making a "marine sunroof system". Automotive style (opening) sunroof, available for OEM or retrofit in 7 sizes. Sounds expensive, but maybe worth a look
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