Davit Musings

[Stemar]

Life would be much simpler if Jazzcat had davits on the stern, but they're a bit expensive, and would be awkward to fit, so I'm wondering if a DIY job in GRP would be possible. I've seen Ally ones that are basically a tapered I beam with a bend, and that seems to be a simple shape to model in glass cloth and polyester resin (not as strong, but better UV resistance)

They would rest on a ledge forward of the stern and be bolted through the transom at around 10cm and 25cms from the bottom. I reckon on an expected load of about 60kg for the dinghy, outboard and, possibly some junk to be lifted, so let's say 120kg each before they break, and they'll need to be about 1m - 1.20 high and stick out about 1m 50 to clear the stern lockers aft of the transom. Something like this (Sorry for the quality - I have no skill in drawing on paper or in CAD)



I'm thinking of a maximum depth of the I at about 11cm, tapering to 6cm at the end, and the I being about 15mm thick.

Do it? Forget it? Do it but beef it up? - All thoughts welcome, especially from those who actually know what they're talking about cos, like Manual, I know nothing.

 

[Bouba]

Whenever you have a bend which means the material is under compression one side and tension the other..you either have to be confident in your material sciences or beef it up considerably...I would make them out or stainless...easyier (for me) to weld than aluminum. The bending would either be done by machine shop or just cut and weld or make it out of box section...if it’s home made then it only ever comes down to which material you feel more confident fabricating in

 

[ylop]

I'm wondering why you think DIY in GRP is going to be better than DIY in stainless?

 

[Bouba]

Sorry...just seen your rider about only hearing from those who know what they are talking about (please disregard my post ??)???

 

[Clancy Moped]

What you going to be hanging off the back? 60kg is quite a lot? My old Avon RIB plus an 8hp 4 stroke Yam came to about that.

​

 

[dunedin]

Davits are definitely worthwhile. Like ylop I wonder if stainless steel tube might be simpler - or even just get the inexpensive a Plastimo ones.
What you are likely to need to do is reinforce strongly the cockpit coaming you are planning to attach to. The design you have will put substantial twisting loads on this - do some calculations of the load weight, and then multiply by the leverage of the long arms to the short load bearing area.
Remember to allow for weight of dinghy and outboard PLUS weight of water in dinghy after rain squall or wave PLUS shock load as bounce on wave. Then double again and reinforce coaming accordingly.

 

[Bobc]

Buy a pair of the Plasimo ones. I have them and they're strong as you need

 

[jdc]

May I beg you to reconsider.

1. They look simply horrid
2. They add weight just where you don't want it, hereby increasing the moment of inertia of the boat's pitching leading to inferior sailing performance ('hobby horsing').
3. They add to your LOA so cost more (unless you lie).
4. They are a trap for any passing boat to get snagged by (please don't ask, the memory is still too raw...)
5. They put strain on your boat which it very likely was not designed to withstand.
6. They are unseamanlike, adding windage and are vulnerable to big seas.
7. They scream "here comes a gimmer who's now past it and can no longer get it up" (the dinghy that is).

Look at some examples of those who have to use the dinghy every day, often in difficult circumstances, like Skip Novak's "Pelagic Australis". No davits. Does anyone seriously imagine he couldn't afford them? Instead, contrive a better means to hoist the dinghy onto the deck forward of the mast.

Just my opinion you understand (although points 1 & 7 are probably universal).

 

[Bobc]

If you put a compression beam across the corner, they would probably work ok.

 

[dunedin]

May I beg you to reconsider.

1. They look simply horrid
2. They add weight just where you don't want it, hereby increasing the moment of inertia of the boat's pitching leading to inferior sailing performance ('hobby horsing').
3. They add to your LOA so cost more (unless you lie).
4. They are a trap for any passing boat to get snagged by (please don't ask, the memory is still too raw...)
5. They put strain on your boat which it very likely was not designed to withstand.
6. They are unseamanlike, adding windage and are vulnerable to big seas.
7. They scream "here comes a gimmer who's now past it and can no longer get it up" (the dinghy that is).

Look at some examples of those who have to use the dinghy every day, often in difficult circumstances, like Skip Novak's "Pelagic Australis". No davits. Does anyone seriously imagine he couldn't afford them? Instead, contrive a better means to hoist the dinghy onto the deck forward of the mast.

Just my opinion you understand (although points 1 & 7 are probably universal).

Skip Novak’s boats probably don’t use davits as (a) they need 2-3 dinghies as spares in case of polar bear attacks, (b) they typically have 10 or more crew on board so easy to winch on and off deck, and (c) their usual sailing waters are around Cape Horn and the Roaring Forties.
So perhaps not typical usage for most coastal cruisers on here.

The rest is pretty much of equal debatability - davits might not suit people who sail fully crewed from marina pontoon to pontoon, But are very helpful for people who stay on anchor a lot. And never noticed any measurable impact on sailing performance, as still overtake most cruising boats of similar size.
Each to their own

 

[jdc]

No polar bears in the Antarctic, but fur seals do the same job as far as dinghies are concerned.

FWIW, we sail 2-up and are almost never in marina, I can launch and recover my dinghy alone. Here in Mylor I notice that davits are uncommon on boats on the moorings, but relatively common in the marina...

 

[thinwater]

Your strength is low. The WLL of 60 KG should probably be at least body weight (because people do grab them and there are dynamic loads) and the BS is typically 5x WLL, and even more if the engineering is not very well understood and backed up by some testing.

Unless you are trained in materials and mechanical engineering, you are better off buying something.

Ignor JDC and do what you want. IMO, winching a boat on deck every day is just not worth the labor unless you are being paid. I sail for fun.

 

[Bouba]

I don’t have davits...just a mast and boom...but I electrified it with remote control winches for very little money

 

[Boathook]

I'm not to sure whether the coaming is strong enough for davits. The loads aren't just the dinghy weight but need to include the dinghy 'falling' and being stopped suddenly. There is a catalac Facebook group and a few 8m have fitted davits I seem to remember.

 

[Graham376]

No polar bears in the Antarctic, but fur seals do the same job as far as dinghies are concerned.

FWIW, we sail 2-up and are almost never in marina, I can launch and recover my dinghy alone. Here in Mylor I notice that davits are uncommon on boats on the moorings, but relatively common in the marina...

We also sail 2-up and find having the solar gantry to haul the 70kg of dinghy and outboard up every day is easier and quicker than hauling it onto deck.

I agree with @thinwater, davits and mountings need to be very strong, accelerated loads as boat rolls and pitches are high.

 

[Refueler]

Must admit - I like the Plastimo davits ...... and that they slot into the mounts allowing them to rotate to reduce boats length / overhang etc.
An important factor in many marinas - who measure total distance occupied.

For my boat - rotating so they cross in middle would also provide mast support ...... mmmmmm

 

[Supertramp]

Not an expert on materials technology but buying a simple pair eg Plastimo makes sense. The reinforcement is key and depends on access to the chosen area. I agree with the comments about strength. Mine are massive (Whitlock) and still flex a little when rolling.

I can store the dinghy on deck but it obstructs the foredeck unless deflated. And I tend to store it there upside down making simple lift and launch harder. Sat in the davits it can move and chafe unless padded in the right places. Davits are great for the ease of deployment although I agree with deck storage if your layout and rigging permit.

Do you have the option of some modern boats where you can sit it sideways on a ledge or bracket or bathing platform between the hulls above the waterline and secure to the pushpit? The brackets still need reinforcement but are simpler than davits.

 

[Stemar]

Thanks for the comments, Lots to think about.

1. They look simply horrid
Done right, I think they're OK, but I do see where you're coming from
2. They add weight just where you don't want it, hereby increasing the moment of inertia of the boat's pitching leading to inferior sailing performance ('hobby horsing').
Yes, the dinghy's weight would be better on the foredeck, but either I have to deflate it or it will hide the windlass and controls. As for reduced performance, I can't see it reducing performance as much as dragging the beast behind me
3. They add to your LOA so cost more (unless you lie).
Fair point, but we don't do expensive marinas
4. They are a trap for any passing boat to get snagged by (please don't ask, the memory is still too raw...)
Again, a fair point, but we're rarely in a tight marina where that's likely to be an issue
5. They put strain on your boat which it very likely was not designed to withstand.
This is my biggest concern, and why I'm asking. From what Thinwater says, they will need to be stronger than I guessed, but I don't know how strong my I beam would be. It seems that GRP beams laid up with the main fibres in the length of the beam are stronger than ally of the same weight, but I've no idea how to do the calculations, and I wouldn't expect my home lay-up to be as strong as a professional one. I do know that bolted to the (suitably reinforced) cockpit coaming is potentially stronger than the Plastimo mounting which is simple in a hole in the deck, where all the load is on one edge of the hole.
6. They are unseamanlike, adding windage and are vulnerable to big seas.
Not sure in what way they are unseamanlike, but I'm not going blue water, so if the seas are that big, I'm going to the pub!
7. They scream "here comes a gimmer who's now past it and can no longer get it up" (the dinghy that is).
Guilty as charged

I'm thinking GRP because I have some experience of working with it. I don't weld, and couldn't bend stainless tube of a suitable diameter.

 

[Neeves]

Skip Novak’s boats probably don’t use davits as (a) they need 2-3 dinghies as spares in case of polar bear attacks, (b) they typically have 10 or more crew on board so easy to winch on and off deck, and (c) their usual sailing waters are around Cape Horn and the Roaring Forties.
So perhaps not typical usage for most coastal cruisers on here.

The rest is pretty much of equal debatability - davits might not suit people who sail fully crewed from marina pontoon to pontoon, But are very helpful for people who stay on anchor a lot. And never noticed any measurable impact on sailing performance, as still overtake most cruising boats of similar size.
Each to their own

If their usual cruising waters are Cape Horn or the Roaring Forties - they would be VERY surprised if they have any issues with polar bears.



I don't think Pelagic is necessarily the yacht with which to compare or we will all need to carry drums of cordage on our forecedck for the shore lines.



Jonathan

 

[Neeves]

Lifting 60kgs split between the 2 davits is not going to be a big deal - its only 30kgs each (obviously). The issue is the increased loads when you are sailing in seas. Many cats have an extension to the deck aft and between the 2 transoms on which the dinghy can sit. So you lift the dinghy on the davits, swing it aft and simultaneous lift and then plonk it on the extended deck. It need not be an addition to the deck - it could simply be a piece of a mast extrusion between the two transoms - simply something to take the load at sea.

We have 2 fibre glass davits, no extra bit of deck, no mast extrusion (but I have wondered). We hoist using the mainsheet winch, which is in the middle of the transom. Our davits are bolted to the transom bulkhead between the two hulls. The transom is one of the main strong points on our cat.


The sailcloth bag attached to the transom holds the wash boards when we are on board and the two pieces of conduit on top of the davits allow us to carry one kayak on the davits when sailing locally. When 'at sea' we remove the outboard and it is stored in a vented locker, immediately off photo to the left (partially to reduce weight on the davits but also to get the weight as bit forward.

Jonathan