CF davits?

Zonker

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I think a 10' dinghy is too small to install davits but that's just me.

More seriously it's not trivial to design these. You can use the same dimensions as stainless steel tube davits (slightly more wall thickness for robustness) except for the deck connections where metal ones will have a bolted plate or similar. That might not be the best for a composite tube connection. Instead embedding them against the transom and glassing them to the inside of the transom (foam gussett if needed)

Do not weld the junctions. That will fail. Instead you can mitre one tube and use a thickened epoxy fillet on the inside of the join. Then wrap the joint in narrow strips of pre-wetted out unidirectional. Compact with heat shrink tape or electrical tape wrapped inside out (over peel ply). The joints will be strong but a bit bulging.

Hard to advise you further - I assume you're not an engineer and are just looking for rules of thumb.

 

Zonker

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I'd make sure to join the davits together with a tie bar, suitably bracketed. That way it's a lot better with side loading (boat heeled over)

 

12 metre

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If you want to go the composite route - why CF?

While CF is much stiffer, standard FRP is almost as strong - and much less $$$.  FRP would be more bendy but that is about the only drawback I can think of when compared with CF.

McMaster Carr has square FRP tubes (which are inherently stiffer than similar sized round tubes) of varying sizes and thicknesses: https://www.mcmaster.com/plastics/shape~rectangular-tube/material~fiberglass/

You can also find pultruded FRP tubes from various web vendors.

At joints, you could epoxy in FRP gusset plate(s) to strengthen the joint.

 

Zonker

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Stiffness is really the answer. You don't want a flexy bouncy davit because you don't want the dinghy moving significantly (chafe/impact with big boat).

Not saying you can't do it in FRP, just that isn't as desirable. 

I've bought and used pultruded FRP (McMaster Carr sure is costly) but I don't think it's ideal for this situation. (I've used FRP for fiberglass handrails (tube & solid round bar), traveller track mount (C-channel), and angle (hatch coamings, hatch supports)

 

MiddayGun

Super Anarchist
1,061
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Yorkshire
I think a 10' dinghy is too small to install davits but that's just me.

More seriously it's not trivial to design these. You can use the same dimensions as stainless steel tube davits (slightly more wall thickness for robustness) except for the deck connections where metal ones will have a bolted plate or similar. That might not be the best for a composite tube connection. Instead embedding them against the transom and glassing them to the inside of the transom (foam gussett if needed)

Do not weld the junctions. That will fail. Instead you can mitre one tube and use a thickened epoxy fillet on the inside of the join. Then wrap the joint in narrow strips of pre-wetted out unidirectional. Compact with heat shrink tape or electrical tape wrapped inside out (over peel ply). The joints will be strong but a bit bulging.

Hard to advise you further - I assume you're not an engineer and are just looking for rules of thumb.
I've often thought something similar could be built for the solar arch that so many boats have on the back end. It just has to bear the weight of the solar panels (could use light weight ones) and maybe an AIS antenna. 
Could save a lot of weight, & even make it removable for when not cruising. 

 

12 metre

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English Bay
As far as stiffness goes - shape (cross sectional inertia) is in many structures a bigger factor than the material.

As Zonker mentioned, tie bars and the like add a lot of stiffness.  Design has a lot to do with it.

 

Zonker

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I've often thought something similar could be built for the solar arch that so many boats have on the back end. It just has to bear the weight of the solar panels (could use light weight ones) and maybe an AIS antenna. 
Could save a lot of weight, & even make it removable for when not cruising. 
Yup. See my solar arch on my catamaran. Made from CF/glass windsurfer masts. And a fiberglass ladder which was surprisingly useful for attaching solar panels to and running wiring. The uprights were glassed into the inside of the hulls, extending inward about 6" below deck level. Used windsurfer masts were quite cheap; about $35-50 each (that's 3 masts in the photo I think). The ladder was about $225 a decade or so ago. Then a few bucks more for CF uni to tape the joints...

image.png

Not many people noticed the ladder once it was painted white and hidden by panels. Probably weighed about 40 lbs, maybe 50 with speakers.

image.png

 

Zonker

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As far as stiffness goes - shape (cross sectional inertia) is in many structures a bigger factor than the material.
True enough. Big and boxy GRP versus black, slender and sexy CF. Your choice. Except of course paint the black tubes to protect from UV :)

 

MiddayGun

Super Anarchist
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@Zonker Nicely done. 

I'd envisioned something with a tube end fitting, similar to the sort you get on sprayhoods, the four points would get fastened in that way & providing the weight carried was kept low, it could be fairly sturdy. 
 

The idea would be that I'm maybe cruising / weekending on the boat for a month of each year & its nice to have the solar power to cover the coolbox & autohelm. But the rest of the time it doesn't need to be there, so easily removed. 

I'm not sure how the carbon tubes would get on with those end fittings though. 

 

Zonker

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Well they're too small for windsurfer masts :)  (and the pieces all have varying diameter)

If it's just carrying solar panels you could use smaller 1" diameter tubes and s.s. end fittings. But with smaller tubes and the slop inherent in the end fittings, use lots of triangulation... Make the arch in 1 big piece or 2 big pieces with a few removable cross connectors.

The nylon ones are too flexible for anything that will be loaded. I'd gusset the joints with GRP plate brackets. Easy to tape to tubes.

image.png

You can now buy relatively cost effective CF tubes. And very costly connectors...

image.png image.png

https://www.rockwestcomposites.com/shop/connector-accessories?utm_campaign=c&utm_term=&utm_source=adwords&utm_medium=ppc&hsa_acc=8114822638&hsa_cam=12016006556&hsa_grp=116434378975&hsa_ad=490177721497&hsa_src=g&hsa_tgt=dsa-1178685882157&hsa_kw=&hsa_mt=&hsa_net=adwords&hsa_ver=3&gclid=CjwKCAjw682TBhATEiwA9crl370iwoO0RQl24---gAym4O0Wp-grnfutOQE7aiU6pz_4sCssgVxmsxoCYzgQAvD_BwE

image.png

 

Max Rockatansky

Max Rockatansky
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816
@Zonker

If you would, offer your thoughts:

my extant davits are cantilevered stainless. I would love to lose the weight aft, but it seems to me that to mimic the design, the weight difference would be nominal.

Quick pic for illustration:

98C6BD55-9158-4117-A973-21AC6ECF8D74.jpeg

 

Zonker

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Canada
Honestly you'd be shocked at how much weight you would save if well engineered. Rule of thumb - Carbon is like "superlight steel". If you have a carbon laminate with the same thickness as steel and same outside dimensions, it will be pretty close in strength to steel. Density of 316 stainless steel is ~7870 kg/m3. Carbon is around 1600 kg/m3. So about 1/5 the weight. Depending on details etc. maybe only 1/4 the weight. But still you can achieve very significant weight savings. If those are 50 lbs each maybe the carbon one is 10-12 lbs x 2 davit arms.

My arch was about 11-12' wide. The fiberglass ladder was about 25 lbs. The carbon tubes were maybe 5 lbs total. Add about 2 lbs for joining the tubes, 1 lb for glassing to the fiberglass ladder. MAYBE 35 lbs total. Would have been lots less if I had done more carbon horizontal tubes but the glass ladder was cheap and off the shelf.

Atkins and Hoyle makes a stern arch that is similar in dimensions. 130 lbs in stainless steel, 80 lbs in aluminum.

https://www.atkinshoyle.com/products/hardtops-and-arches/hardtops-and-arches/radar-arch.html

 

Hans Genthe

Member
72
62
Dubai
If you you have some experiences in composite, there´s s simple way. I expect it has to carry appr. 200g weight and one meter out of the boat. I would shape the davids freestyle in foam (80h divinicell) and cover them with carbon/epoxy , one layer 300 biax, 5 layers 400g unidirectional (follow the direction of forces), one layer of biax again and one layer twill 200g. 2 layers addidional UD in the area where you mount them to the boat. You can make a nice aerodynamic design. If you have a pump, better but them in a vacuum bag (can be a big rubbish bag), peel ply, perforated film, breather fabric. These numbers are without any guarantee, but feel free to post a drawing here. I´ve have done a lot similar parts. For example this Catamaran roof is made of carbon.

hg1_21_IMG_2668.jpg

hg1_21_IMG_2848.jpg

hg1_21_IMG_2484.jpg

 
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Zonker

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Canada
Beams are often governed by depth (to obtain adequate stiffness) however in that case it might have been engineered by the "TLAR" method. That looks about right.

Or they were being conservative. I think they could have gotten by with 2/3 the depth by eye, but if the Owner had a 14' RIB with a console and a 40 HP motor...

Anyway KATO only works in metal to my understanding so that might have been just painted stainless.

Correction. They are aluminum. 28 lbs each so not too heavy, just a bit bulky.

https://www.katomarine.com/image.php?type=images&img=Voyager2big

 
If your own freestyling leads to somewhat different results than Hans’ work above too I can happily recommend these guys ijn Fréjus:

http://www.petitjean-composites.com/manutention-annexe/

 
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