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Old 15-06-2015, 05:50   #1
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Davits

About to add aluminum davits to my Gulfstar 40 CC. Big debate going on about whether to have then high enough to look under the dinghy or low to look over the dingy. i have 3'6" from the rail to the water and thinking that the 10'5 rib dinghy can hang bottom at the rail they top about the height of the safety lines. From my cockpit I would then easily see over it when looking aft. I will still be able to add solar panels on the davits.

Dock friends are all adding these much taller ones arguing in ocean crossings you need to be up and out of the way in heavy seas. My thought is that I would put the dingy on the foredeck if off shore. I do sail costal along Cape Canaveral inlet as well as the ICW.

Also interested if any reason not to use welded Aluminum vs Stainless.

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Old 15-06-2015, 06:17   #2
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Re: Davits

I'm a bit undercaffeinated to offer a detailed opinion on the height of the davits at the moment (although I'd think that higher's better).
But as to materials. Stainless is a lot strong, especially when you factor in that the majority of types of aluminum that you'd use in this application, lose half of their strength when welded. Plus, how do you plan to account for corrosion if using aluminum? It has some built in resistance, but without adding to such, via coatings or anodizing, it's service life will likely be much shorter (than it either being coated, or vs. stainless).

You can get an idea of the base strengths of various materials here www.Matweb.com Also it's not too hard to find the strength of a perticularly alloy, post welding.
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Old 15-06-2015, 06:47   #3
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Re: Davits

BS on welding and corrosion "issues". Alum has been used for decades for this purpose. Check out atlantictowers.com for DIY versions
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Old 15-06-2015, 06:49   #4
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Re: Davits

I had a transom davit on a previous boat. It occasionally was a problem when a heavy following sea would lift and drop it repeatedly. It never actually caused damage, but it certainly got my attention. I worried that the pounding would cause the cables to break, but they never did. During such pounding there was not much that could be done about it besides worry a bit.

My current boat has the dinghy secured to chocks on a boat deck.
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Old 15-06-2015, 07:16   #5
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Re: Davits

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Originally Posted by 1affiah View Post
BS on welding and corrosion "issues". Alum has been used for decades for this purpose. Check out atlantictowers.com for DIY versions
Aluminum does corrode more easily than stainless and requires more maintenance to keep it in good aesthetic condition. Basically, to keep the anodizing from becoming corrupted and pitting, the aluminum needs to be rinsed with fresh water often. Some wax now and again helps.

I guess all of those sportfishers with the big tuna towers don't know about the weakness of aluminum (but you do see them rinsing it down daily).

Aluminum is sized accordingly, though. One does not use the same schedule or size as stainless when building, but that does not make it "weaker".

BS on the welding thing - I suppose all those aluminum boats are just a wave slap away from breaking apart?.

Disclosure - we have an aluminum arch/bimini structure. It is only 17yrs old and probably only has another 50-60 years left in it, so it may have a shorter service life than a stainless one.

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Old 15-06-2015, 07:25   #6
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Re: Davits

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Originally Posted by 1affiah View Post
BS on welding and corrosion "issues". Alum has been used for decades for this purpose. Check out atlantictowers.com for DIY versions
The man asked about materials, I gave him an informed opinion & some undisputable facts about the two materials.
I never said that aluminum wouldn't work. Rather, I just compared a few plusses & minuses of the materials in question. Along with some information which isn't necessarily commonly known (about alumninum).
Hell, if you want to get technical about it, properly engineered & maintained zinc set of davits could work... in theory.

Back to aluminum, you ever notice that aside from boat hulls, 90%+ of aluminum above decks is either anodized, or coated. And if uncoated, is typically somewhat oversized/thicker than technically called for, to allow for corrosion over time.
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Old 15-06-2015, 08:23   #7
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Re: Davits

Quote:
Originally Posted by Chris Whitney View Post
About to add aluminum davits to my Gulfstar 40 CC. Big debate going on about whether to have then high enough to look under the dinghy or low to look over the dingy. i have 3'6" from the rail to the water and thinking that the 10'5 rib dinghy can hang bottom at the rail they top about the height of the safety lines. From my cockpit I would then easily see over it when looking aft. I will still be able to add solar panels on the davits.

Dock friends are all adding these much taller ones arguing in ocean crossings you need to be up and out of the way in heavy seas. My thought is that I would put the dingy on the foredeck if off shore. I do sail costal along Cape Canaveral inlet as well as the ICW.

Also interested if any reason not to use welded Aluminum vs Stainless.

Chris
Either has issues, but stainless is clearly the stronger material and has fewer issues with regard to corrosion, particularly in the presence of other materials/metals. I have seen large and robust marine aluminium fittings turn to swiss cheese in half a decade on a good number of occasions. If you do go with Aluminium do be sure they are fully galvanically isolated. I would also agree with your dock friends regarding the height. Sometimes you cannot predict the seas you will encounter, and unless you are only going to be in inland waters you are better with higher IMHO.
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Old 15-06-2015, 09:13   #8
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Re: Davits

Loads of boats have bare aluminum spars
These are a bit ugly but actually superior to the painted aluminum spars on my boat in regards to corrosion
Bare aluminum quickly forms a surface film of aluminum oxide which renders it very corrosion resistant. The other thing ( besides painting) that promotes corrosion is galvanic corrosion from stainless fittings mounted on aluminum ( or the other way around)


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Old 15-06-2015, 09:28   #9
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Re: Davits

Either aluminum or stainless will work. Galvanic corrosion can be engineered out. Both form a passive oxide coating and both prefer to not be in stagnant sea water.

IMHO stainless will allow for a smaller neater result. Assuming same static and dynamic loads.

We have all stainless transom arch and solid lifelines so adding aluminum davits would look odd. We are in the process of adding an articulated davit so we can clear our wind vane. This places the dinghy high. We will stow the dinghy on the foredeck for most crossings.

We have a sceptre 41 on our dock with cast aluminum davits. That looks neat on that boat.

Attachment is crucial. That may also drive your material decision. Remember triangles are the lightest and highest specific stiffness shape for a spaceframe. Ensure your design avoids 4 or more sided elements whuch can lozenge and therefore experience fatigue. Low cycle strain fatigue is to be avoided at all costs. If you see your davits flexing then they will fatigue quickly.

As for materials thats a simple choice. Ideally 316 low carbon stainless or 6xxx series aluminum for a welded structure. If you choose T6 temper in 6xxx for example you'll end up with T4 temper after welding.

These kinds of materials selection will impact your sizing. I'd also spec full penetration TIG welding and drain holes for all close sections.

For a static design load, to make life simple, i'd recommend a 4g load. Ideally assuming your dinghy is full of water. This might result in an overly large davit structure and attachment. If so then you'll need to store on deck for rough weather.

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Old 15-06-2015, 12:19   #10
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Re: Davits

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Originally Posted by nshawdvm View Post
Loads of boats have bare aluminum spars
These are a bit ugly but actually superior to the painted aluminum spars on my boat in regards to corrosion


Indeed. I have an AB Lammina rib which is infuriatingly painted aluminium. I tried to get bare but it just wasn't available in the time frame I had at the time I needed it. Bloody stupid idea painted aluminium for a dinghy. Great idea bare.
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Old 15-06-2015, 13:03   #11
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Re: Davits

My new SS davits we made to get the dink high. I have had it on the foredeck and its inherently unsafe on my boat as one cant get to the furler or anchor locker.

There a clip or 2 or the dink in the davits on passage in this vid. The outboard was on its own bracket but short hauls the ob would be left in the dinghy

http://youtu.be/g060dpmr320



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Old 15-06-2015, 13:20   #12
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Re: Davits

My Kato's are SS and I like them very much, but as far as aluminum being inferior, please don't tell my mast that as I've never seen a SS mast?
Aluminum is fine, as long as it's properly engineered
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Old 15-06-2015, 14:05   #13
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Re: Davits

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Originally Posted by a64pilot View Post
My Kato's are SS and I like them very much, but as far as aluminum being inferior, please don't tell my mast that as I've never seen a SS mast?
Aluminum is fine, as long as it's properly engineered
I certainly take the points re aluminium masts and aluminium has its place for various uses. I far prefer aluminium masts to any other type, for example, but then, as you note, steel is just inappropriate in most contexts for that purpose. However there is no direct analogy between aluminium for a mast and the same for davits. Masts are supported by an architecture of stays. Not so davits. Loads can be very high on davits, especially, as one member noted, if the dink fills with water. This load is transmitted in an unmitigated fashion to their base fastenings with high vertical and lateral forces. I know I would not be comfortable long term with aluminium davits, though some may well be and may well be justified in so feeling. Neither is perfect, but for me both aesthetically and structurally steel would be the way to go.
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Old 15-06-2015, 14:13   #14
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Re: Davits

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Originally Posted by MarkJ View Post
My new SS davits we made to get the dink high. I have had it on the foredeck and its inherently unsafe on my boat as one cant get to the furler or anchor locker.

There a clip or 2 or the dink in the davits on passage in this vid. The outboard was on its own bracket but short hauls the ob would be left in the dinghy

http://youtu.be/g060dpmr320

Mark
Thanks for the link. Enjoyed the vid. I like your nav setup and that app for the iridium satellites is cool. I am not especially up to date on that kind of thing myself so, where/how do you go about getting that?

As to Hatteras, yep, worst survival storm of my life was off there, back in early Spring 1989. What a mess.
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Old 15-06-2015, 15:50   #15
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Re: Davits

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Originally Posted by Muckle Flugga View Post
However there is no direct analogy between aluminium for a mast and the same for davits. Masts are supported by an architecture of stays. Not so davits. Loads can be very high on davits, especially, as one member noted, if the dink fills with water. This load is transmitted in an unmitigated fashion to their base fastenings with high vertical and lateral forces. I know I would not be comfortable long term with aluminium davits, though some may well be and may well be justified in so feeling.
Don't tell that to all the sportfishing boats. They would probably invert if they had to make their structures out of steel.

If aluminum is used, it will be sized correctly for the application, so the whole load argument simply goes away. If the davit design is one that is in compression (arch), there won't be much worry about strength. If the design is one where they just stick straight out the back, then steel is the better material, unless they are cast aluminum.

Even when sized correctly, aluminum will be usually be lighter than steel. For simple davits, this may not amount to much difference, but if the structure is going to be larger, like an arch and bimini, it can make quite a difference.

Aesthetically, it is user-choice as to which looks better.

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