Hard dinks, nesting dinks, and why we like them

ProaSailor

dreaming my life away...
6,123
809
Oregon
When the lifting sling is not vertical, the load goes up considerably.  Let's assume it has only enough water in it to match its normal displacement with two people - say 500 lbs.  That's 250 lbs. on each davit.  Then assume the vertical distance between the davit and the dinghy attachment points is one foot.  Given the 2.5 feet overhang, that's a triangle like this:

triangle.png

Given a=1 and b=2.5, c = 2.69258

https://www.calculator.net/right-triangle-calculator.html?av=1&alphav=&alphaunit=d&bv=2.5&betav=&betaunit=d&cv=&hv=&areav=&perimeterv=&x=39&y=22

So tension on 'c' is 250 lbs. (the load on 'a') X 2.7 = 675 lbs. at a single point on the bow, split between two points at the stern (though stern bridle geometry is slightly more complicated).  Add shock loads when the boat bounces and/or more water and/or shorter 'a' value and the tension on 'c' goes up rapidly.

If the dinghy were full of water, I would guess a weight of at least 2,000 lbs.?

 
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ProaSailor

dreaming my life away...
6,123
809
Oregon
On second thought, I believe my math is very wrong since tension on 'c' goes down when 'a' is reduced... that can't be right?  Oops!

I'll dig deeper but maybe @Zonker knows how to compute this?

 
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Zonker

Super Anarchist
10,176
6,370
Canada
He does but doesn't care enough to do it. Make sure your lifting slings are more vertical than shallow horizontal angles. 

 

ProaSailor

dreaming my life away...
6,123
809
Oregon
Make sure your lifting slings are more vertical than shallow horizontal angles. 
Naturally, that's the key.  But the 2.5 feet overhangs make that difficult.

I dug up a Grasshopper model I wrote two years ago for two, three or four point slings on a 4,000 lb. boat, adjusted the numbers to match this scenario, and get nearly the same value: 673 lbs. tension on 'c' when 'a' = 1 foot.  This model makes more sense when 'a' is reduced, showing 870 lbs. tension on 'c' when 'a' is 9" (0.75 feet) and 1,275 lbs. tension on 'c' when 'a' is 6" (0.5 feet)

If the flooded dinghy weighs 1,000 lbs., it shows 1,346 lbs. tension on 'c' when 'a' is 1 foot, 1,740 lbs. tension on 'c' when 'a' is 9" (0.75 feet) and 2,550 lbs. tension on 'c' when 'a' is 6" (0.5 feet).

I remember the model was much more complicated than I expected so won't dig any deeper.  Why should I be the only one who cares... ?

 
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rustylaru

Member
400
138
earth
Thank you everyone for advice.

I think i will have to use some kind of spreader bar as the davits are just not high enough off the water to allow for a relaxed angle.

 

ProaSailor

dreaming my life away...
6,123
809
Oregon
Thank you everyone for advice.

I think i will have to use some kind of spreader bar as the davits are just not high enough off the water to allow for a relaxed angle.
Broken wind surfer masts are relatively cheap and super strong.

By the way, my old model shows that if 'a' is 2.5 feet, same as the 'b' overhang giving a 45° angle, the tension on 'c' would be 354 lbs. (each, at both ends) with a 500 lb. dinghy.

 
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rustylaru

Member
400
138
earth
Hang it from the three corners and fill it with a garden hose. I'm kidding, but I think it would take it. 
What do you think of the extra holes through the gunnels as pictured above?

I have some 1/2" G10 tube I could line the holes with.

You were so clear in the building manual how important the gunnel reinforcement was to the shape of a nesting dingy. I really don't want to compromise it.

 

toddster

Super Anarchist
4,346
1,045
The Gorge
After labor day, people have taken to just abandoning worn out gear around the launch sites.  Presumably clearing out the vans before they fuck off to Baja.  

 

Fah Kiew Tu

Curmudgeon, First Rank
10,418
3,502
Tasmania, Australia
Naturally, that's the key.  But the 2.5 feet overhangs make that difficult.
Why?

Are you assuming that the lifting points will be at bow & transom? What makes that assumption reasonable?

As for the load, commercial davits aren't rated to carry 500+kg each so they'll collapse if the dink gets close to full of water. The better answer is to make sure it doesn't.

FKT

 

Russell Brown

Super Anarchist
1,760
1,441
Port Townsend WA
What do you think of the extra holes through the gunnels as pictured above?

I have some 1/2" G10 tube I could line the holes with.

You were so clear in the building manual how important the gunnel reinforcement was to the shape of a nesting dingy. I really don't want to compromise it.
The G-10 tubes are the way to go, but challenging to install, especially after finishing. 3/8" or 1/4" ID would be better. Keeping the holes inboard near the hull wouldn't weaken the gunwales, unless they leak water into the gunwales, of course. There are methods...

Where are you, Rusty?

 

rustylaru

Member
400
138
earth
The G-10 tubes are the way to go, but challenging to install, especially after finishing. 3/8" or 1/4" ID would be better. Keeping the holes inboard near the hull wouldn't weaken the gunwales, unless they leak water into the gunwales, of course. There are methods...

Where are you, Rusty?
I'm in Arizona with the PT11. Heading north in a few weeks to Tacoma to get the big boat ready for the summer.

Maybe easier to drill at 3/8" with a chamfer and fill with fumed silica or high density filler and then re drill it at 1/4". 

1/4" spectra would still be overkill I hope. IMG_3395.jpeg

 

2airishuman

The Loyal Opposition
1,004
477
Minneapolis area
What do you think of the extra holes through the gunnels as pictured above?

I have some 1/2" G10 tube I could line the holes with.

You were so clear in the building manual how important the gunnel reinforcement was to the shape of a nesting dingy. I really don't want to compromise it.
Why lift at the gunwales?  Aft, bond a gusset with a hole in it down lower, at the first chine maybe, and use that.  Or put the gusset between one of the planks and the buoyancy box. Forward, put an eye (or two, if you want four points) with a backing plate and/or doubler under the bow deck.

The gunwale is the weakest part of the boat when it comes to resisting forces that pull the gunwales together.  Lift from somewhere lower and you'll benefit from hull strength, and will not have as large an angle from vertical on the sling.

 

fufkin

Super Anarchist
No way to fit it under the boom or on the foredeck? It does nest pretty compactly for an 11’ dinghy. I keep mine under my boom. 
Davits on a 32 footer when Rusty has a great nesting dinghy does seem counterintuitive...not to mention somewhat unfriendly to waterline trim, especially on a boat of this length.

If it were me, the davits would be gone if the PT 11 was the support boat...seems like there’s gotta be 6/7 feet of space somewhere on deck/cabin house aft of mast.
 

 

Veeger

Super Anarchist
The other problem with davits is that you need to have a low point drain hole for when the dinghy is in the stowed position.  Very often it isn't actually level so just a centerline drain may not be enough.  Very thin plywood is not conducive to off the shelf type drains as they all have a lip that stands proud and leaves a puddle of varying size depending on the design/angle.

 

Elegua

Generalissimo
Davits on a 32 footer when Rusty has a great nesting dinghy does seem counterintuitive...not to mention somewhat unfriendly to waterline trim, especially on a boat of this length.

If it were me, the davits would be gone if the PT 11 was the support boat...seems like there’s gotta be 6/7 feet of space somewhere on deck/cabin house aft of mast.
 
The biggest issue under boom on modern boats is clearance with the rigid vang.  It looks a bit tight on the J32.  I have to squeeze mine between a mid-boom sheet and a vang.  

 
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toddster

Super Anarchist
4,346
1,045
The Gorge
In my marina there is a too-small boat with a too-big hard dinghy hung on davits off the stern.  Apparently not a problem because AFAIK it never leaves the dock.  But yesterday I noticed that the dinghy hangs bow-down.  IDK why it doesn't fill with water???  Bung hole in the front?  Somehow it gets through snow and ice season - I never thought to look.

 




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