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Series drogue attachment

Series drogue attachment

by berwick » Sun Mar 15, 2009 12:54 pm

I am interested in going with a series drogue at some point in the future. I am not crazy about having a big chunk of stainless steel sticking a few inches out the past the stern on the hull to attach the drogue to. My engineering question is - if I attach the drogue to the stern cleats then take tension on it from there to the main winch and then carry the tension from the winch to the mid or front cleats, would this spread the load out? If each device could take 2000 lbs individually would all 3 maybe split the load so that if the drogue pulled 5000 lbs I would be ok? Maybe the problem would be getting the tension close enough at both sides of the pull to make a difference?

Thanks, Richard
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Re: Series drogue attachment

by Jim Ferguson » Sun Mar 15, 2009 11:18 pm

I do believe you can go in that direction. I have never used the series drogue, but I have used the same setup to protect my cleats. Once in Santa Maria de Leuca, Italy, we were stuck on an articulating dock in a horrible force 10+ storm in a lousy harbor. I it did this after our stern cleats with their 3/8 bolts folded and crushed their teak pads. Since that time We have used that technique when towing boats. I like to use sheets, less stretch and run them up to my secondary winches. If you can control chafing and you have enough winches to keep your sails under control I think it should work. Hopefully we will get some real imput from those that have used the series drogue.
Jim
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Re: Series drogue attachment

by Jack Tyler » Mon Mar 16, 2009 7:07 am

I'd suggest you visit bethandevans.com as they have extensive experience with - and very methodical, cleanly articulated thoughts on - drogues. While reviewing their articles, don't miss viewing some of the pics in their gallery as you'll see their own drogue rode techniques nicely illustrated. E.g. note http://bethandevans.smugmug.com/gallery ... 1629_QgoUd

Jack
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Re: Series drogue attachment

by Donal » Thu Mar 19, 2009 10:44 am

The point of having a chain plate-like attachment point is to be able to use metal to metal connection, thus eliminating problems with chafe. Metal plate, metal shackle, metal thimble for the line.

We had a pair of bronze pad eyes cast by Port Townsend Foundry. The eye is placed at one end of the mostly square base. All round soft shapes, almost impossible to snag, but massively strong for taking directionally changing loads.

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Re: Series drogue attachment

by Jaga » Thu Mar 19, 2009 12:12 pm

The other point of a chainplate type attachment (and I think the primary reason) is to spread the load, which can be in excess of 1/2 the weight of the boat if you're falling down a wave [ok - it's spread over probably two attachments]. Donal's approach is fine as long as the point the padeye is attached to the boat is strong enough to bear those kind of loads.

One other approach, assuming your cleat is strong enough, is to beef up the backing plate, possible a stainless (or bronze) strap leading away from the pull and with multiple bolts attaching it. This would spread the load and reduce the chance of the cleat pulling out. All depends on how strong your deck is. Most aren't THAT strong.

Scot
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Re: Series drogue attachment

by Donal » Thu Mar 19, 2009 9:03 pm

Good point, Scot. Ours is a composite wood/epoxy hull and the drogue attachment padeyes are going on the hull (double ender) soon with substantial backing plates that will connect to frames to share the load. The strength rating on each pad eye is 18,000 lbs., a little more than needed, but the pattern was in stock.

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Re: Series drogue attachment

by Evan.gatehouse » Fri Mar 20, 2009 3:43 am

My answer to the original poster, as an engineer, is nope. It would be very hard to guarantee that load is distributed evenly among the fittings by applying tension. What is more likely is the first fitting takes most of the load.

I'm going to build some carbon fiber strap chainplates for parachute sea anchor of the bows and some sort of drogue from the sterns of my catamaran. Then I'll cut slots in the hull and bond them to the insides of the hulls. Not much protruding from them.

I am not as fond of padeyes for taking big loads like this. The padeye is usually strong enough (Spinlock and Harken make some very strong 17-4 s.s. ones) but the deck or hull below the padeye is likely not to be able to take the load, even with big backing plates. You really need to cut away the core if through cored construction and add many layers of additional glass to the underside of the deck or the sides of the hull. The bearing loads on the bolts are pretty substantial.

If you go with a strap type chainplate then the number of bolts is increased and you can have them further apart so the bolt loads don't interact as much.
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Re: Series drogue attachment

by Jaga » Mon Mar 23, 2009 9:49 am

Evan,

Thanks for the engineering perspective. It's easy to look at the picture of Evans Starzinger, with his drogue hung off his winch and deck cleat, and think that's adequate. One tends to forget they have a metal boat and those winches and cleats bolted to the deck are effectively bolted to one massive backing plate. Fiberglass (and wood) boat aren't so fortunate in that regard.

Scot
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Re: Series drogue attachment

by Baraka » Mon Mar 23, 2009 11:09 am

Metal boats sure are nice that way. I figure our cleats are through-bolted to the steel deck so I don't have much to worry about. If I need a stronger point of attachment, I'm currently upgrading our primary winches, which are also through-bolted to the steel deck, but also have 1/4" stainless backing plates. I didn't know they were there, but in replacing the winches I figured there is no reason to get rid of them. My last boat was fiberglass, and I remember spending a weekend installing backing plates on all the cleats. A couple dollars for a bunch of scrap plates and a few hours to drill and install, I imagine it would be just as easy to do for drogue attachment points.
Jaga wrote:Evan,

Thanks for the engineering perspective. It's easy to look at the picture of Evans Starzinger, with his drogue hung off his winch and deck cleat, and think that's adequate. One tends to forget they have a metal boat and those winches and cleats bolted to the deck are effectively bolted to one massive backing plate. Fiberglass (and wood) boat aren't so fortunate in that regard.

Scot
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Re: Series drogue attachment

by Jaga » Mon Mar 23, 2009 2:46 pm

Yeah Greg, but I'll still keep my woodie :wink:

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Re: Series drogue attachment

by bsadler » Sat Apr 11, 2009 5:30 am

does anyone have any idea what sort of loads are drogue sized for a 42ft/15ton boat would impose on the attachments points ??
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Re: Series drogue attachment

by estarzinger » Sat Apr 11, 2009 6:18 am

bsadler wrote:does anyone have any idea what sort of loads are drogue sized for a 42ft/15ton boat would impose on the attachments points ??


The truthful answer to this question is no - but there are some guestimates around. You say 'drogue' and there are two quite different kinds, single element like a galerider and series, the series typically have much higher drag & loads.

We have measured (very crudely) the max peak loads on a galerider on our 47' boat (the designers say its 15tons but its really more like 20 tons while cruising) - 3000-3500lbs max peak load.

For the series drogue the best source is http://www.jordanseriesdrogue.com.

They say for your boat the 'lifetime single peak load in a worst case storm and worst case wave in that storm' is 19,000lbs, but they also say the 'working load in a severe storm' is only 10% of this' (eg 1900lbs).

It's not too hard to design the attachments to meet 2000-300lbs, but it is rather more difficult to meet 19,000lbs.
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Re: Series drogue attachment

by Louis Riel » Sat Apr 11, 2009 5:13 pm

I wouldn't go offshore without mooring bits that will take the full load. Mine are good for 90 tons. I lost two 12 inch cleats in New Caledonia by sheering off 8 half inch bolts. Friends broke two brand new one inch nylon springs in Rarotonga on an Islander 44. Mooring bits , chocks etc should be stronger than the biggest line that will bever be used on them. If you ever have to worry about spreading the load , they are not strong enough. The outside world doesn't have such sheltered marinas.
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Re: Series drogue attachment

by bsadler » Mon Apr 13, 2009 4:26 am

there is something a little contradictory here

from the ace sails site the max/peak load for a 35,000lb boat from the series drogue is given as 22,000lbs

the offered drogue construction for this size boat is based on 3/4inch nylon double braid (sail-rite, ace sailmakers)

tensile strength for nylon double braid is approx 19,000lbs (samson XLS, new england DB)

given that most people when designing wouldn't utilise beyond 80% of strength this suggests that the drogue construction is for a maximum tensile load of 15,200lbs (i.e. not the peak load of 22,000lbs that is stated)

looking at the coastguard report fig. 26 it would appear the use of 3/4inch double braid was only recommended for boats of about 15,000lbs displacement or less (where the drogue design load is around 12,500lbs)

i'd be interested in other's take on this

whichever way you go i would have thought there was little point in designing the boat attachment points to cater for much in excess of the ultimate tensile strength (breaking strength) of the tow line
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Re: Series drogue attachment

by Louis Riel » Thu Apr 16, 2009 3:11 pm

If you get stuck on a lee shore and a tug is available to tow you off, expect the tow line they offer you to be huge.
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