# Thread: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

1. ## Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

From the moment I first saw a photo of the Mirror 16 Dinghy I was fascinated by the particular hull shape with the chine rising all the way up to the sheer.

There are several designs with this special feature. Another one is the Sabre dinghy with a hull very similar to the 125.

Sabre:

125:

Based on the the information provided by the Rules of Construction and Measurement August 2017 for the 125 Sailing Dinghy and by studying various photos and videos of this dinghy I tried to create a 3D model of the hull true to these very rules.

Here is the result:

2. ## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

Nice, Flo-Mo.
Are you able to calculate the basic hydrostatic values from your model?
I am particularly curious about the prismatic coefficient and also the position of centre of buoyancy relative to the centroid of the waterplane area. I have recently modelled a variety of hull shapes and would like to compare the values with the likes of the Finn and Laser.

3. Gareth
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## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

I'd never heard of the 125, so I looked it up http://www.125assoc.com/125info.asp

I was very exited when the Mirror 16 came out. Wikipedia says 1963, but I seem to recall it later than that.

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## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

A very good visualization, flo-mo! Thanks a lot.
I am not able to imagine what hydrodynamic advantages such a hull shape might have. I do suspect some added drag due to eddies at the chine, and some loss of secondary stability, and I think that the wave pattern of such a hull might be worth a close look. Even when heeled a lot, this chine seems to be too wide aft to have an effect on the entry angle. I wish somebody of the experienced folks could tell us more about the hydrodynamic behaviour of such hulls.
Last edited by canoe_sailor; 02-05-2018 at 12:05 PM.

5. ## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

Originally Posted by canoe_sailor
A very good visualization, flo-mo! Thanks a lot.
I am not able to imagine what hydrodynamic advantages such a hull shape might have. I do suspect some added drag due to eddies at the chine, and some loss of secondary stability, and I think that the wave pattern of such a hull might be worth a close look. Even when heeled a lot, this chine seems to be too wide aft to have an effect on the entry angle. I wish somebody of the experienced folks could tell us more about the hydrodynamic behaviour of such hulls.
It is a fairly classic planing hull

6. Gareth
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## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

The British Seafly and Mayfly (not to be confused with the Michalak one) conform to a similar chine layout I remember the Seafly as being very fast

http://seaflymemories.uk/

7. ## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

Originally Posted by whiskeyfox
Nice, Flo-Mo.
Are you able to calculate the basic hydrostatic values from your model?
I am particularly curious about the prismatic coefficient and also the position of centre of buoyancy relative to the centroid of the waterplane area. I have recently modelled a variety of hull shapes and would like to compare the values with the likes of the Finn and Laser.
I use Prosurf for hydrostatic calculations. So yes I can provide these numbers.

For 225 kg displacement:

Prismatic coefficient: 0.56
LCB (center of buoyancy from x=0): 214.4 cm
LCF (center of flotation from x=0): 231.4 cm

I created the model of the hull in Rhino. What program do you use for your calculations? Maybe we could share files.

8. ## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

Above numbers are for a slightly immersed transom.

For 225 kg displacement with the transom just above waterline the numbers are:

Prismatic coefficient: 0.53
LCB (center of buoyancy from x=0): 197.6 cm
LCF (center of flotation from x=0): 217.9 cm

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## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

The Mirror 16 was a great racing / cruising boat. You could sleep 2 on the sole. There's not many around now, most sucumbed to rot in the bottom panels.

The stitch and tape method of construction was fine in theory but failed in practice due to the low peel strength of polyester resin securing the web frame/floors to the bottom panel and 3/16 ply. The whole boat really needed to be 1/4 ply not 3/16 used. Sole / frame connection was fine since the sole was fastened to cleats on the webs. Bouncing up and down the webs became detached from the bottom panels and the rot started from there.

Some people overcame this by major surgery. Cut the sole out around the webs, chisel off remaining sole from cleats, remove all traces of polyester then epoxy fillet and tape the webs and then fit a new sole.

As a fast camp cruiser it was great and it's a pity the plans etc were lost when Bell Woodworking went bust.

Nick
Last edited by NickW; 02-05-2018 at 03:16 PM. Reason: sp (again)

10. ## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

This introduces an interesting degree of convexity in the plywood forward. I've been trying to figure out where the limits are on that sort of bending, over in this thread. On page 4, you'll see my current iteration, still much harder to build than your 125 exercise.

11. ## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

Originally Posted by flo-mo
I use Prosurf for hydrostatic calculations. So yes I can provide these numbers.

For 225 kg displacement:

Prismatic coefficient: 0.56
LCB (center of buoyancy from x=0): 214.4 cm
LCF (center of flotation from x=0): 231.4 cm

I created the model of the hull in Rhino. What program do you use for your calculations? Maybe we could share files.
Thanks for the numbers.
I use Solidworks, so to get the Cp I have to model the mid-ship section prism, get the volume and manually calculate Cp.

12. ## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

I am busy building a 10ft Spindrift tender with a chine design similar to its larger Core Sound sisters. I have created a cad model from the plans and the waterplane shape looks very similar to that of the Mirror 16, but the bow angle of entry is a lot finer on the Mirror, while the Cp of the Spindrift is much higher at 0.63.

Have you done any panel development and nesting to see if the Mirror panel shapes have any inherent advantage of material usage on standard sheet sizes?

13. ## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

The Spindrift is a very nice and versatile design - looking forward to your build.

I had a look at the developed panels of the Mirror 16 but did not bother with any attempt to optimize a panel layout.

The panel layout for the 125 according to the building instructions is pretty decent.
Only 5 sheets of plywood (4 to 5 mm, 244 x 122 cm) are required for the hull and the interior structure -- hard to top that.

14. ## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

I am building the Spindrift out of nidacore and a mix of carbon, glass and vectran. Only some parts will be 6mm plywood as per the plans, but not enough to qualify as a wooden boat I'm afraid!

The Spindrift would have required 4 sheets of 2.44 x 1.22m, so to build a boat that is 2.5ft longer, has full side seat tanks and a generous foredeck out of 5 sheets is indeed impressive.

15. ## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

Originally Posted by flo-mo
I use Prosurf for hydrostatic calculations. So yes I can provide these numbers.

For 225 kg displacement:

Prismatic coefficient: 0.56
LCB (center of buoyancy from x=0): 214.4 cm
LCF (center of flotation from x=0): 231.4 cm

I created the model of the hull in Rhino. What program do you use for your calculations? Maybe we could share files.
FWIW, to others reading, you can also do the same calcs in Rhino. The hydrostatics command does not calculate prismatic coefficient for you, but it gives all the numbers you need to make the calculation.

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## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

Originally Posted by canoe_sailor
A very good visualization, flo-mo! Thanks a lot.
I am not able to imagine what hydrodynamic advantages such a hull shape might have. I do suspect some added drag due to eddies at the chine, and some loss of secondary stability, and I think that the wave pattern of such a hull might be worth a close look. Even when heeled a lot, this chine seems to be too wide aft to have an effect on the entry angle. I wish somebody of the experienced folks could tell us more about the hydrodynamic behaviour of such hulls.
I'm not expert, but no one who sails a Sabre or 125 has ever mentioned that the shape has any major benefits or issues. The first use in a popular class was apparently the Gwen 12 of 1943, designed by Charlie Cunningham who with his son Lindsay went on to fame for designing the first modern catamaran class as well as C Class Little America's Cup winners and the world sailing speed record holder, Yellow Pages Endeavour.

Gwen 2.jpg

The rising chine does seem to inherently create a fairly wide angle of entry. The Gwen was a VERY fast boat for its day, almost certainly the fastest 12 footer in the world, but was always more of a reaching machine. The 125 and Sabre have small rigs and are designed as cheap, economical boats that are easy to sail in the very rough conditions of Australia's southern states, and they excel at that. The small rigs mean that the behaviour of the hull is pretty hard to separate from other factors but it would seem fair to say that the chine shape does not excel when going upwind in light airs.

17. ## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

Originally Posted by Chris249
... it would seem fair to say that the chine shape does not excel when going upwind in light airs.
Chines in general or these designs in particular?

I read the entry on Sailcraftblog about the Finn and how its bow sections made it good for going upwind in waves. I was surprised to find that the Cp of the Finn hovers around only 0.55, regardless of pitch trim. Could it have been faster on reaches or plane in lighter conditions if the hull had a higher Cp?

18. ## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

I have to wonder whether a hull with its LcG as variable as a dinghy operating in relatively huge waves is at all sensitive to stuff like Cp and entry angle?
Those are relevant for displacement and semi displacement hulls, but when planing?
National 12's go from this in light air

to this with the bow out and a wide entry angle

19. ## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

Originally Posted by whiskeyfox
I am building the Spindrift out of nidacore and a mix of carbon, glass and vectran. Only some parts will be 6mm plywood as per the plans, but not enough to qualify as a wooden boat I'm afraid!
Nonetheless a very interesting project:

20. ## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

I was not aware of the Gwen 12.

There are drawings of the original plans and also a plan from 1977.

It is interesting to know that in the 60's Iain Oughtred built and raced several Gwen 12.

With the information from these plans I am now able to compare the hulls of the 125, Gwen 12 and the Mirror 16:

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## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

You might also be interesting in the 'Pathfinder' series developed by Colin Cummings:

22. ## Re: Design Exercise: 3D Model of the Hull of a 125 Sailing Dinghy

Or his later iteration

I'm glad Clarkey posted that, It ws in my mind but I could not remember the guys name.

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