Series Drogue & Displacement Factor

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September 29, 2020 at 14:40 #26747
Czarina Blue
Czarina Blue
Participant

Hello All,

In preparing to construct and install attachments for a series drogue I am querying the displacement of my Nic 38.

‘Czarina Blue’ is a live aboard and well loaded especially during hurricane season, judging by my waterline, which also replicates conditions for a long offshore passage. In the focsle the wooden plaque above the bunks reads REG TONS 9.39 , which if relating to imperial tons would seem to equate to 9540 kg. Do we know what this wooden plaque’s figure represents?

According to Jeremy Lines the displacement of  the Nicholson 38 is 7.1 tons with 2.65 tons of lead, which I understand is based on half load conditions.

Can anyone shed light on the likely tonnage or kg of a Nicholson 38  starting on a 3 week ocean passage with water and provisions up to the gunwales? I have heard other liveaboards saying ‘add two tons’ to the official displacement for live aboard situation. Also I note mentions on this forum of 20,000 lbs, and 10 tons in cruising trim…

I hope to use this to decide on what length of series drogue to fabricate.

Any clarity on the matter would help me.

Thanks

October 5, 2020 at 20:31 #26750
Van
Van
Participant

We used the numbers from Jeremy. We built a Jordan drogue using the 6,700 kg design, which means 107 cones, more or less.  I think we used a few more cones than that.  It would be quite reasonable for you to use the next size up.

The design from Jordan is not meant to be exact, and includes a generous safety factor. The key is to make sure that the weakest link matches the design goals, which is dependent upon displacement. The displacement determines what the maximum load during a breaking wave event is – in the case of our Nic 38, I took that as 10,000 lbs, but in fact, all the components I used are rated quite a bit higher than that.

The weakest link will not be the length or number of cones, it will be the shackles, chain plates/backing plates, and ultimately the strength of the fiberglass where you mount the chain plates. As you can see from his design, doubling the boat displacement from 10,000 lbs to 20,000 lbs did not make a big difference in the length or number of cones in the drogue – from 100 to 116.

FWIW, we used dyneema lines for the bridle and leader because they do not absorb water, small and easy to handle for their strength, and are easy to splice. Elasticity of the bridle and leader is not important because the drag of the drogue does not create “shock loads”, but instead acts as a shock absorber. We used closed thimbles at all the line ends, and used over-sized soft shackles everywhere except at the chain plate, where we used shackles with 4.5T working load limit.

We are just now getting ready to install the chain plates on the aft topsides, a few inches under the toe rail.

Good luck!

Van

October 9, 2020 at 00:21 #26753

Kari and Lawrence
Participant

Dear All,

 

The displacement figure is the weight of average sea water that is displaced by the insertion of the hull volume into said seawater! The weight of the vessel is irrelevant in this equation.

 

Hope that helps.

 
<p style=”text-align: left;”>Lawrence</p>
 

 

October 9, 2020 at 07:33 #26754

Kari and Lawrence
Participant

PS, I would be interested to discover if anyone has actually deployed their series drogue and what there experience and thoughts are?

 

I deployed ours, whilst returning from the Azores in 2018. The breeze at the time was about Beaufort 7, I believe. Blowing from the north, I was headed in a north-easterly direction and have discovered i dislike sailing to windward in anything greater than 30 knots apparent! Hence my decision to test the drogue, initially I attached the bridle lines to the after deck cleats but found this seemed to drag her down in the stern too much. At times this resulted in white water jumping onto the after deck! I found the motion and snatching uncomfortable. I can not remember the rate of leeward drift, I did record it but the log is in Swansea and I am in London, caring for mother!! Anyway, let’s say it was something like 1.5 – 2 knots. I then attached additional mooring lines, lead from the cockpit winches, this solved the problem of immersing the stern but did not improve the motion. There was an added chafe problem with the mooring lines crossing the rail, also I had left the mizner boom swung out to port, I then found the lines to chafe on said boom and at times managed to ride over and commence chafing on the out haul and other fittings. After a sleepless night I hauled the thing in, somewhat slowly and vowed never to use it again!!

October 11, 2020 at 00:52 #26770

Murray Smith
Participant

I own the S/V Keneskoonech (hull @25 built in 1968).  The registration plate reads 10.93 tons but I have no official reference to whether they are short or long/Imperial tons nor being displacement, net or gross tonnage.  I would oversize it for reasons of safety; especially the hull attachment arrangement.

October 14, 2020 at 21:53 #26775
David and Felicity
David and Felicity
Participant

Hi All,

I have the original owners handbook for Borlowen (hull no 125). This gives the tons displacement (mean) as 7.1 tons (7214 Kg) and the tons per inch immersion as 0.39 tons. These are the figures you should use, increasing the displacement to allow for the extra fuel, water and stores you have embarked.

The registered tonnage is a calculated figure based on the total internal volume of the hull. It is not directly related to the displacement and was really introduced for commercial shipping.

David

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