Mounting double backstays to the davits instead of hull transom?

[randoo]

I agree with those that say "do not do it". The idea may have merit but the issues of stress, strength of materials, and loading need addressed by a very competent naval architect. Your sketch shows some disturbing angles and force multipliers.

my thoughts, be careful.

[AlwaysFORSAIL]

I have a rack mounted much as your picture shows but with 4 braces/struts from the aft end of the rack down to the stern rail. It is strong enough to lift a 100# dinghy in very calm conditions (anchor or dock). Had I beefed it up like a well designed davit system I would be comfortable hanging a dinghy. Your system looks like a light weight rack for relatively light solar panels. I prefer not to hang my dinghy off the aft of my boat but to place it on the fore deck for 2 or more days passage.


To redesign the stays seems dangerous at a minimum.

[hellosailor]

There's no reason that it can't be done by a competent engineer, taking into account all the motions and stresses of, say, a boat hobbyhorsing and pumping the mast in large seas. I'd expect that to mean significant costs and material to beef up the davit structures AND the hull all around the attachment points.


But if you wanted to do it...First find a competent engineer who knows boats, then figure maybe a grand for the initial consultation and estimate? Maybe easily a $10k job before you are done? $15k?

[chris4911]

I have a 12' apex dingy with a 25hp honda and have been told it is too heavy to hang from davits. If they can't support that load they sure as hell are not going to hold up your mast.

[hellosailor]

Chris-
Since "davits" hold up the 150 passenger motor life boats on cruise liners, "davits" can certainly hold up your dingy, and the OP's as well.

Just like "boats", all "davits" are not the same.


Most likely the clerk who told you your dink is too heavy to hang from davits, was punching the pictures on a McDonald's cash register last month.

[chris4911]

I suppose if u build some hideous monstrosity on the back of your boat? I was talking to the guys at Kato

[valhalla360]

Originally Posted by hellosailor

Chris-
Since "davits" hold up the 150 passenger motor life boats on cruise liners, "davits" can certainly hold up your dingy, and the OP's as well.

Just like "boats", all "davits" are not the same.


Most likely the clerk who told you your dink is too heavy to hang from davits, was punching the pictures on a McDonald's cash register last month.

Of course the davits that hold a 150 passenger motor life boat will likely be enough to sink a cruising boat if you just bolt them on the back. So sure if we take it to silly extremes, you could make it work.

For any normal davits I've ever seen on a 30-50' cruising boat, they would need substantail redesign and beefing up.

[roland stockham]

As describe forget it. You can move back-stays aft and it use to be quite common to see some form of brace from the transom (called a boomkin or bobkin) with the chain plate at the base of the transom. Generally this mounted a single not a twin back-stay although I have occasionally seen it done with twins, particularly on chined steel boats. To make it work however you need to have nearly equal angles above and below the brace so that it is loaded in compression. Doing this on your designe with the divats as bobkins you have massive differences in the angles and will get very high bending loads. Not only will you break either the divit or its mounting you will also get so much flex you will never be abole to maintain backstay (and therfor forestay tension) so even if the mast stays up you will destry windward performance.

[Mattyb]

Works fine on mine for the last 14 years but quite a bit sturdier being steel


Sent from my iPhone using Cruisers Sailing Forum

[ErikFinn]

Thank you for all the replies ladies and gentlemen!

I admit the first sketch looked like a support for flower pots, but it was merely to demonstrate where I wanted the backstays to attach to. So let's continue the thread a little bit, not to argue but for the sake of education and interest in boats and engineering, shall we?

So, as far as I see, the biggest challenge is to make sure that the backstay to davits mounting connection is:

1) at least as strong a mounting place as the original spot to the hull
2) will not flex or move to any direction under stress such as in swells, high winds, or dinghy movement, and furthermore, the davits should be mounted to the hull in such a manner that the said stresses would not cause flexing to the deck/hull and compromise the integrity of the deck/hull.

So let's start from scratch. Let's say we replace the say 2 meters /7 feet of the bottom of each backstay with say a 2 inch diameter 316 SS, 4mm thick tube (that should be pretty strong, right? . No davits here, just replace the bottom 7 feet of the backstays. I think we can agree that this would be at least as strong as the original, just more heavy? Because all the forces stay the same (direction) as with normal backstays. And let's presume the tubes are supported so they don't pull the stays down See pic 1.

To make davits, we have to add the out extending arms. See Pic 2.

So here the challenges start. When we hang a dinghy from the davits (say 45 kg / 100 pounds for my 9 feet dink, no engine, just keep the dink empty to minimize weight) we will now have various flexing forces, depending on boat movement, in swells for example. So we need to build reinforcements to the structure to keep it rigid, and we need to to mount the structure to the hull by adding more mounting places, in order to distribute the loads to wider areas, and to minimize the loads at any single point in the hull/deck.

If we add triangular reinforcements (good for pressure reinforcement) to the structure and also add more "mounting poles" (say 1,5 inch diameter SS316 tubes) I believe we could accomplish the two requirements I listed earlier. I ended up with a sketch something like this, see pic 3. for a sketch from rear, see pic 4.

What do you guys think?

I reckon I could have something like this made for around 2000 USD locally here. Of course it would take a lot of measuring, adjusting, trial and error, and I'd have to make sure all the mounting spots on the hull/deck are actually strong, but I don't think this is rocket science. Ok, I used to fly airplanes for a living, but still, I don't think it takes a marine engineer to design this. If in doubt, just make it strong enough, right? I can't make stress calculations, but a lot of this is just common sense. I'm not saying its easy, and maybe there is a reason a google image search didn't yield any results I wished to find, but as Mattyb kindly posted above it's been done (albeit I wouldn't be too keen to compare steel and GRP boats in this design challenge).

There are other challenges also, such as how to make it look at least moderately nice, not a hideous monster, but I believe polished SS316 and professional welding would go a long way.

few links:

Google "davits" image search here

Klacko Marine custom radar arched and davits, nice pics here

[Olde Chief]

The latest sketches look better, but they still do not show how the boat it self will react with the change in weight distribution. You say you flew airplanes, so you should know about weight and balance... I flew and worked on aircraft for many years and would approach this project VERY CAUTIOUSLY.... Always remember, Boats and Aircraft are a lot a like, you can't always find a parking place to work on them, when you get into trouble...

[Jon Eisberg]

Originally Posted by ErikFinn

What do you guys think?

I reckon I could have something like this made for around 2000 USD locally here. Of course it would take a lot of measuring, adjusting, trial and error, and I'd have to make sure all the mounting spots on the hull/deck are actually strong, but I don't think this is rocket science. Ok, I used to fly airplanes for a living, but still, I don't think it takes a marine engineer to design this. If in doubt, just make it strong enough, right? I can't make stress calculations, but a lot of this is just common sense.

"Common sense" would dictate that you talk to a competent rigger about this :-) I'm pretty sure any professional would set you straight about the risks of such a modification...

I must say, I'm completely baffled as to the purpose of all this... As best I can tell, it's mainly to enlarge your bimini, and perhaps gain more mounting area for additional solar panels? As Stumble said earlier, just do what everyone else does, cut slits in a bimini built around your existing backstays, and add davits/stern arch/whatever independent of your rigging...

Can't tell if your mast is keel or deck stepped, but from the drawings I see, it looks like your shrouds are pretty close to being inline... In which case, your backstay(s) are hugely vital to the integrity of your rig...

It's your boat, do what you want :-) But I would be very surprised if any rigging professional would endorse this sort of modification, or suggest whatever gain you think you'll get from it will be worth the cost to do it in a fashion even remotely approaching anything 'proper'...

On the other hand, if you desire the back of your boat to resemble the sort of Jungle Gym you see guys swinging from in those al Qaeda Training Videos, than have at it...

:-)

[Stumble]

Erik,

I think your redesign is again grossly unsuitable. Knowing how strong it needs to be is exactly why you need to hire an engineer, I don't have the background or the experience to even guess at what is appropriate.

If the davits are going to be inline with the current backstays then that part probably won't be too difficult. The tensile pull will be within the design spec of what is already there, and so no bid deal.

Except that you now have to extend the davits back, what 15' to get enough clearance for the dinghy off the back of the boat? Put a 150lbs weight 15' from a cantilever point and the math is pretty easy, you need to be able to support 2250ft-lbs of torque, but this is static load. If we are talking about getting bounced in waves lets assume a 7g momentary force could be expected, then the load skyrockets to 15,750ft-lbs.


Since these davits are now holding up the rig instead of just the dinghy they become life critical gear, and thus need a 10:1 safety factor. So my rough numbers indicate they need to be strong enough to support at least 157.500-lbs.

Now transmit this to the deck plates that have to be mounted to support this thing. How thick is the fiberglass in the area? I have no idea, but its going to be a critical number in determaning how big the plates need to be to support the load. My guess is you are going to need an awfully big backing plate to handle this. And that the deck and transom of your boat are grossly under engineered to handle this, which means rebuilding the entire transom and aft deck.

But keep in mind i am an attorney and boat nut, I could very well be wrong. But I can tell you who would know... a Naval Architect.

As for 1.5" tubing... Thats what my cheap davits just for an inflatable are made of. I have no clue what you would need to do this, but I am guessing that it will have to be custom carbon fiber to keep weight close to reasonable.

[roland stockham]

Can I suggest a different tack. Davits are always problematic and almost always vulnerable to a wave filling the dingy. An alternative is a fold up platform that drops the dinging in the water when lowered and lifts it against the transom for stowage. This is basically an 'L' shaped cradle, the short side is the height of the dingy and attached to pivot points on the transom at water line. It is lifted with simple rope tackles and clamps the dingy firmly in place where it can't fill. Simpler to make and you don't need to mess with the back stays. Then add a conventional stern arch and/or Bimini (which will also provide the anchor points for the tackle that lifts the platform)

[leftbrainstuff]

Originally Posted by ErikFinn

I have had this idea for some time now of having custom built davits fabricated for my boat. I presume davits would be the easiest way to store the dinghy while on the way as I don't care about the windage and I'm not a racer. Also suffering from a slipped disc it just seems like the most convenient way to launch the dink (and to haul out), as opposed to from the deck, or just towing it behind all the time. Furthermore, this would enable me to enlarge my old hard solar bimini, giving me more shelter in the cockpit from the tropical sun and the tropical rains too. And of course more solar watts. I tried to draw the bimini to scale, and I feel it really isn't big enough at the moment.
I would like to use the space on top of the davits for solar panels also.

But for now I wanted to ask if anyone has built a system like the one I sketched below? Hang on there... Specifically, on the pic you can see where the old double back stays attach to the hull's transom (black line), and my idea is to attach those backstays to the bends of the davits instead; the pic I hope illustrates this better than my attempt in writing. I'm a bit concerned about how the new mounting point for the backstays would affect the support for the mast, in a situation where you have say 70-80 kg dinghy hanging from the davits, and get some swells. I guess one should build the Davits almost rock solid so the weight of the dink would not cause oscillating forces to the backstays.. Or not? Anyone ever done or seen a setup like this and what are your thoughts? Is it a bad idea to mount the backstays to the davits? Any fotos or links?

You are taking a tensile load that is fed directly into the hull and creating a huge bending moment in your davit concept. The dinghy and backstays will both create significant bending loads.

Could you do this? Sure. Would you do this? If you understand the loads and can design and build a suitable structure go right ahead.

Tensile loads are relatively straight forward to deal with. Simple tensile elements and inline double shear attachments are the mainstay of standing rigging.

For your structure to work you need to transfer the bending loads back into compressive and tensile loads. You do this through triangles and sizing compressive and tensile tubes. Your attachment points will either be loaded in tension, compression, shear or a combination.

You'll likely find an adequate design will become a jumble of tubes and some seriously complicated interior load spreading structure.

Sent from my SM-N900T using Cruisers Sailing Forum mobile app