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Old 11-12-2018, 01:51   #1
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Design ideas for davits/solar arch

We're looking at buying a new boat and equipping it for bluewater cruising. On most boats we have considered so far, we would need to add davits and/or a solar arch. It seems to make sense to combined these functions into a single structure.
Just to complicate things, we will also be adding a windvane (probably a Hydrovane) so need to consider airflow around the stern of the boat. I am fully expecting that we only use the davits for overnight storage of the dinghy, and for use on shorter passages where we are happy to hand steer or use the autopilot. The Hydrovane would be used for longer passages where the dinghy would be on deck, not in the davits.

I would like the arch to support a minimum of 400w of solar, and a dinghy+ob weighing up to about 80kg.

What sort of tubing size (thickness and od) should I be considering? And how best to fabricate the attachment to the boat? Boat is likely to be a centre cockpit GRP design in the region of 38-40ft.

If anybody has pictures or description of a similar design that would be fantastic.
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Old 11-12-2018, 06:40   #2
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Re: Design ideas for davits/solar arch

I think that a google search will provide a great number of ideas, and pictures. Any design will be dependent on the design of the boat, and where you plan on putting the solar, whether over the cockpit, or hanging off the stern. just my .02
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Old 11-12-2018, 07:10   #3
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Re: Design ideas for davits/solar arch

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I think that a google search will provide a great number of ideas, and pictures. Any design will be dependent on the design of the boat, and where you plan on putting the solar, whether over the cockpit, or hanging off the stern. just my .02
As a combined structure, it would have to be over the stern if it's to double up as davits- especially on a centre cockpit.
Some info on tubing size would be great, so that I can start getting an idea of what this is going to cost. Fortunately I know some people who are excellent stainless welders, but I need to know raw materials costs as a starting point.
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Old 11-12-2018, 08:43   #4
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Re: Design ideas for davits/solar arch

From my research and what I have been told by a number of people, they went with a 2" schedule 40 SS. Up in Michigan we do not see many arch/davit systems, but my time spent in Florida and other places this seems to be the norm. Mounting entails large flange base plates at the deck, bolted through the deck to the undersides with a backing plate below. Some people like the davits that have a swing out arm for the dinghy, that you can use to raise the dinghy. This would move the arm out of the way for the windvane when it is in use.


Here is a gallery from a well known maker of arches/davits for ideas:
https://www.flickr.com/photos/325036...7640291671424/
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Old 11-12-2018, 09:03   #5
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Re: Design ideas for davits/solar arch

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Originally Posted by SY Kelpie View Post
We're looking at buying a new boat and>>>>>>>
>>>>>>>>>>>>>>>>>>>>


What sort of tubing size (thickness and od) should I be considering? >>>>>>>>>>>>>>>>>>>>>>>

There are a variety of diameters and wall thicknesses. Since you don't have the boat yet which would include distances and dimensions, it is hard to say other than eyeballing other boats that may be around you. It's like asking how long is a piece of string. If, however, all you want to do is budget, them make some assumptions on distances and dimensions, find out what materials are available (1" to 2" and in between; Sch. 40 or Sch. 80 wall thickness) and maybe do two or more budgets and options. Your stainless friends should be able to help you out on what materials are available, wouldn't they? That'll give you some time to do some more research. Good luck.
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Old 11-12-2018, 10:30   #6
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Re: Design ideas for davits/solar arch

My Irwin 38 has a very stout arch with davits. Only two panels but could accommodate more with minor adjustment.
I think it a good idea to look at the mounting with different shear points. Deck and transom share the load. Aluminum was used but I am not certain the wall thickness of the various sized tubes.
https://www.dropbox.com/s/eluli5j042ui622/arch.jpg?dl=0
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Old 11-12-2018, 10:39   #7
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Re: Design ideas for davits/solar arch

Depending on your planned boat size I would reconsider putting all that weight on the stern. Davits, dinghy and outboard can dramatically affect the boat trim.

Depending upon the boat design and boom length solar panels can be mounted above the bimini.
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Old 11-12-2018, 10:40   #8
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Re: Design ideas for davits/solar arch

Two reference points for you - the first being my research, and then my design drawings, for our arch, which has davits (fixed), no swing-arm for engine (dammit, my mistake), wind generator stub (and pre-weather-forecaster satellite antenna stub), several antennae, and a solar panel frame, and the second being my solar upgrade, which will show you the bracket and the modification made to accommodate 2 larger panels in place of the 3 originals:

Pictures: Flying Pig Early Refit + Projects/Research_Before_Building_Our_Arch

Pictures: Flying Pig 2015-2018 Shake-and-Break-Down/Electrical Upgrade/Solar
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Old 11-12-2018, 10:44   #9
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Re: Design ideas for davits/solar arch

As to material, I am forever angry at my fabricator, as he did a lousy job. I compounded it by not stiffening my very tall (but now common-heighted; maybe they saw mine and liked it?) structure on the inside legs, as well as in an "X" reinforcement of the davit arms, as the entire structure can be shaken with ease, let alone when I'm climbing it for whatever purpose.

I expect that tubing, and not pipe, was used, as this guy was otherwise nearly all/only aluminum fabrication, and mostly for power boats, and perhaps didn't know that it had to be pipe.

So, if you're doing SS, for sure use harder/thicker material than my 2" tubing, and don't skimp on bracing...
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Old 11-12-2018, 10:59   #10
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Re: Design ideas for davits/solar arch

On my 34' Beneteau they used 1-7/8 Sch 40 aluminum tubing, it was anodized after welding for good corrosion protection. It has been on a year and has performed perfectly on the gulf coast of FL. I don't recall the weight, but the installers said it was much lighter than the smaller diameter stainless steel normally used (when the arch was laying on the dock, my old muscles could pick it up). The large diameter tubing makes the arch very stiff. On the inboard side of the arch they added a bracket for the dinghy motor. With the crane arm I added, I can get the motor on and off the dinghy easily, (with using one arm in calm conditions).
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Old 11-12-2018, 11:10   #11
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Re: Design ideas for davits/solar arch

Depends on the boat mine on 38 ft boat, carries a 100kg dingy, uses 1 3/4inch tube........but integrates the bimini so comes a ways forward it also holds 2 325 watt panels......I see weirdly braced aches because they want to just have it over the transom......I thought mine through from dodger to transom and it includes a cockpit enclosure.
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Old 11-12-2018, 11:30   #12
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Re: Design ideas for davits/solar arch

Quote:
Originally Posted by Dan_K View Post
On my 34' Beneteau they used 1-7/8 Sch 40 aluminum tubing, it was anodized after welding for good corrosion protection. It has been on a year and has performed perfectly on the gulf coast of FL. I don't recall the weight, but the installers said it was much lighter than the smaller diameter stainless steel normally used (when the arch was laying on the dock, my old muscles could pick it up). The large diameter tubing makes the arch very stiff. On the inboard side of the arch they added a bracket for the dinghy motor. With the crane arm I added, I can get the motor on and off the dinghy easily, (with using one arm in calm conditions).
Its more likely that your fabricator used 1.5" Sched 40 pipe. Tubing isn't measured with "Schedule" that's for pipe and pipe is measured by ID So 1.5" Sched 40 pipe is just over 1 7/8" diameter. Its a good choice at its very strong and relatively light.
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Old 11-12-2018, 11:31   #13
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Re: Design ideas for davits/solar arch

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Originally Posted by SY Kelpie View Post
We're looking at buying a new boat and equipping it for bluewater cruising. On most boats we have considered so far, we would need to add davits and/or a solar arch. It seems to make sense to combined these functions into a single structure.
Just to complicate things, we will also be adding a windvane (probably a Hydrovane) so need to consider airflow around the stern of the boat. I am fully expecting that we only use the davits for overnight storage of the dinghy, and for use on shorter passages where we are happy to hand steer or use the autopilot. The Hydrovane would be used for longer passages where the dinghy would be on deck, not in the davits.

I would like the arch to support a minimum of 400w of solar, and a dinghy+ob weighing up to about 80kg.

What sort of tubing size (thickness and od) should I be considering? And how best to fabricate the attachment to the boat? Boat is likely to be a centre cockpit GRP design in the region of 38-40ft.

If anybody has pictures or description of a similar design that would be fantastic.
Since you are buying a NEW boat I would try to make this addition part of the purchase transaction, hopefully during construction so that the stern arch is built into the stern pulpit assembly...otherwise you most likely will have a stern railing remove and replace situation to deal with as well as all the specifications you are asking about now. Some manufacturers build semi-custom boats which can accommodate this type of add-on during construction.

My second concern is for the windvane which will require a somewhat clear area on the stern...this complicates the design. You will see very few davit boats with windvanes because of this but it is possible. I removed the davits from my boat to accommodate a Monitor windvane which you don't want to go offshore without. Later I added a stern arch (no davits) with 2 x 140W panel that are almost six feet long. I also added 2 x 140W panels on top of the dodger, 560W total.

Adding a stern arch-davit to an already completed boat brings up the matter of finding a reputable stainless steel fabricator experienced in this work at a reasonable price. I had difficulty locating one in my area and had fabricators turn down the job as they did not do "on site" work. Four years ago I got one estimate of $7,000 plus shipping across the U.S. plus the local install...not at all recommended from my experience as not all boats are the same and issues develop during the process. I finally found someone reasonable who built the stern arch "on site" while the boat was hauled. If you go this route I strongly recommend you install the windvane first and then have the arch-davits built around it to insure the windvane will fit/operate as planned...the windvane is more important than the davits to your offshore plans.

Good Luck.

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Old 11-12-2018, 13:28   #14
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Re: Design ideas for davits/solar arch

Thanks again for the replies and info.
Just to clarify, no unfortunately we are not buying a 'new' boat, I should have been clearer- it will likely be at least 30 years old!
Just to reiterate, it will probably be centre cockpit and about 38-40ft LOA, although we are considering some aft cockpit designs as well. All fairly old fashioned designs without open transoms or excessive beam.

I'm aware that having davits and a windvane has the potential for complication, and until quite recently I didn't even think it was an option, but a bit of research shows some people managing to make it work. My solution is to simply not use both at the same time- the windvane is for use on long passages, where the dinghy would be on deck.

Again reiterating, the dinghy will likely be around 35-40kg with a 6-10hp engine weighing up 25-40kg, so total max weight in the region of 80kg, possibly a bit less depending on what we end up buying.

I like the idea of aluminium, but I am going to struggle to find someone to fabricate it, at least at a mates rates sort of price.

My gut feeling was that 38mm SS tubing would be about right, but I take on board the number of people who have said they have 2". What weight of dinghy are you hanging off this?
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Old 11-12-2018, 14:38   #15
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Re: Design ideas for davits/solar arch

Aalco in aberdeen will do you 41 X 2mm 316 @ ( +/- ) 5 per foot. but as you suggest prob 38 mm will do. weight wise the 50' that i used ( of 41 X 1.5 mm ) ( or 2 mm i cant remember off hand )) on a 20' beam catamaran weighs less that my wife does . i used ss for ease of adding on lugs etc. at later stage is easy otherwise i would have used aluminium. if you know some " excellent welders " then there is no reason why they cannot weld the All,, other than that a more industrial type "AC" machine is needed , welding / fabbing Al is just as easy as SS, maybe even easier as it cuts easier and contraction is much less than with SS. re deck fitting, make as wide as poss and use throo deck bolts with big back plates,, all easy stuff.. i could prob lift my whole boat with mine !! ( but remember , without lots and lotsa work or industrial polishing facilities then show room finish is not possible . ) a finish polishing disc to fit on a grinder are 5 a pop,, you will need about 5. and then some pickling paste which ( i think ) comes in 50 jars, about 2-3 pints.., you will need about a tea cup , bst if you know people from fish farms they can get small amounts. so you are prob looking around 200 materials for 10' beam monohull , ( twin bar hoop ) + very best of " mates rates ".
(ps,, Aly would have to be 2" x 3mm )
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