I’m looking for some help and ideas from the forum on an engineering question. I’m trying to think through an approach for attaching a series drogue to the stern of Whisper and wonder if a single attachment point at the apex of the boomkin might serve. A substantial SS strap could be welded to either or both of the plates and provide a stern anchor point as well. I’m thinking about an attachment, not just a fairlead, though I would like to incorporate a fairlead for anchoring use. Seems to me it would take advantage of the forces involved in the standing rigging and would eliminate any chafe or fouling issues. As I look back through the evolution of series drogues, it looks like the trend is toward a peak load of about 10,000 lbs. for a BCC. Breaking strength of the backstay is about 10,000 lbs, the boomkin shrouds slightly more. We are about 2/3rds finished with the drogue (110 cones)and have to make some decisions on the bridle and attachments. Let the brainstorming begin. Ray
Having deployed a jordan drogue from the stern of an island packet. i would be concerned about the difficulty in retrieving the drogue from the apex of the boomkin. you can not easily get to the point of origin.
Jo Anne:
Where was the attachment point for the Jordan drogue on the IP and what size IP?
If a Jordan drogue can generate a peak load of 10,000 lb. I am reluctant to suggest an attachment point without consulting with Cape George, a naval architect or yacht designer.
Jo Anne:
Where was the attachment point for the Jordan drogue on the IP and what size IP?
If a Jordan drogue can generate a peak load of 10,000 lb. I am reluctant to suggest an attachment point without consulting with Cape George, a naval architect or yacht designer.
Hi Jo anne, I’m planning on a stern arrangement that will make it safer to work there. That said, I’m going to splice a long strop into the drogue lead line that can be taken to a winch and loaded, leaving slack for disconnecting the drogue and winching it all in. That means I shouldn’t have to spend much time at all at the stern disconnecting the drogue. Rod, I totally understand, we’re just spitballin’ here. I’d still take any thoughts you have on the idea. Cheers, Ray
Ray:
My initial thoughts are to install too tangs on each side of the hull that overhang the stern. The inside of the hull needs to be reinforced in such away to distribute the the load on the tangs to as much of the hull as feasible, perhaps an elongated radial pattern similar to a teardrop. The inside reinforcement of the hull should have no hard points. The built up section should feather into the rest of the hull. The large the feathered area, the more the force is distributed though the hull.
If I were doing this, I would use three to four 1/2" diameter SS bolts or five 3/8" diameter bolts. The design of the tangs would be similar to a tear drop, such that the forward apex of the teardrop would overhang the stern and the wider part of the teardrop would be forward of the transom. The “teardrop” would be in the form of a triangle where one side is radius.
A bridle would need to be used to attach the drogue.
That is my “spitball” idea. Damn, I should be an engineer.
R
P.S. I would research the force needed to shear the bolts. I also would call West System and talk to one of their tech people.
Ray:
My initial thoughts are to install too tangs on each side of the hull that overhang the stern. The inside of the hull needs to be reinforced in such away to distribute the the load on the tangs to as much of the hull as feasible, perhaps an elongated radial pattern similar to a teardrop. The inside reinforcement of the hull should have no hard points. The built up section should feather into the rest of the hull. The large the feathered area, the more the force is distributed though the hull.
If I were doing this, I would use three to four 1/2" diameter SS bolts or five 3/8" diameter bolts. The design of the tangs would be similar to a tear drop, such that the forward apex of the teardrop would overhang the stern and the wider part of the teardrop would be forward of the transom. The “teardrop” would be in the form of a triangle where one side is radius.
A bridle would need to be used to attach the drogue.
That is my “spitball” idea. Damn, I should be an engineer.
R
P.S. I would research the force needed to shear the bolts. I also would call West System and talk to one of their tech people.
An excellent contribution from Iduna.
My offering: no need to re-invent the wheel. Unless you want the mental exercise to, for instance, stave off Alzheimer’s.
I know that Roger Olson invested considerable time thinking and testing a drogue for Nereus. Roger focused on several steps:
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how to arrange and stow the drogue so it could be deployed (ie when you’re in a survival storm, you are not going to want to put yourself or crew at even more risk deploying the drogue, so it has to be ready and easy to deploy)
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recovery or abandoning the drogue.
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dealing with the forces from the drogue. As you know, it’s going to be transient snatch forces that are important, not a steady pull of so many tens of thousands of pounds force.
From memory, Roger worked his way through those (and probably other) questions. I’d suggest tapping his expertise. To say that Roger knows the BCC is superfluous. And he likely documented his work with quality drawings.
Bil
Hi Ray and Bil , Gee gosh, I just finished re-reading the book, "Venture some Voyages of Captain Voss on “Tilicum” , (SP-? Title - ?)
He mentioned many times in his book and writings about using a drogue and demonstrating to beach crowds in N Z , how well it worked, when crossing bars into harbors.
Re-reading this book refreshed my interest in adapting a drouge to use onboard BCC Calliste.
Of course I do have onboard the 12’ diameter parachute sea anchor, as was recommended in the L & L seminars and videos, but haven’t tested it yet! (just equiped and ready)
So ,I too am looking over Ray’s shoulder for his solution and why he chooses a Series Drogue ?
rod,
it was an island packet38. the bridle was attached to the port and starboard stern cleats. both cleats were thru-bolted with backing plates.
Jo Anne:
Thanks for the info.
Rod
I appreciate the discussion here, folks. It leads to better understanding of the issue. Make no mistake, staving off Old-timers is ever present in my uh…uh…mind. The loading with series drogues tends to be cyclic and progressive (at about 10% of the peak load), not “snatching”, one of the reasons I like the concept. When you look through the accounts of drogue use, it’s sometimes hard to tell whether a single-unit or series drogue is discussed. That would account for the varying accounts of retrieval issues, some saying they cut away, some saying ‘piece of cake’. The 10,000 lb peak is considered to be from a once in a lifetime ‘freak’ wave in a OIAL ‘freak’ storm. I admit I set my risk radar at a lower threshold than that. I’m researching the possibility that a compromise configuration would accomplish 90% of the goal. We have a unique transom, boomkin configuration that would seem to mitigate the loading addressed by the tang at the corners setup. If you put drogue loads at the center of most transoms, bye bye transom. My thinking was that with the load distributed to boomkin legs through-bolted to the deck, the backstay, and two boomkin shrouds attached at the lower corners of the transom, it might serve without having to fabricate a separate structure. In every account I’ve read, a constant comment is that the drogue keeps the stern “aggressively” oriented into the waves. Very little yawing going on. Makes me think that the forces to be dealt with will be (mostly)directly astern. I welcome any and all input from Roger if he’s reading this, if I don’t see anything for a while, I’ll PM him for feedback. We’re always looking for the product of other peoples’ thinking to fill in the gaps in ours so thanks for helping us kick this around. Cheers to all, Ray
Hi Ray , somewhere I heard or read that our midship cap shrouds, the both of them together while attached to their chainplates, are structurally engineered to be strong enough to lift the entire boat !
Now I wonder if the head stay and back stay together are engineered structurally to lift the whole boat as well ?
I will say, without any doubts, that our boomkin and it’s attachments are unbelieveably strong.
My boat came with a Monitor wind vane attached to the boomkin. The boat was inport when that 2004 Asian Tsunami paid our marina a visit.
The boat was of course damaged, but two things impressed me.
First, that the boomkin being knocked from all sides and from all sorts of runaway boats and pontoons, showed almost no sign of damage, eventhough the two lower boomkin stays gained permananate kinks in them.
Second, that Monitor, probably acting as a shock absorber, was twisted off completely from it’s attachment tubes, left hanging underwater by it’s rope control lines.
I would like to suggest you write to L & L and ask your attachment question to them, as I am quite sure that Larry has done this research before, and can answer it with knowledge.
Please share with us, his reply, if you do ask !
Dwkayaks:
Good suggest regarding L&L
DW, I agree that the boomkin structurally is amazing(Monitor too). We were once in a tiny fishing harbor in Greece, tied to a stone wall and had left the boat for a hike in the hills. We came back hours later to find our anchor had budged and Whisper was pounding the Monitor against the wall. The Monitor needed some work with a pipe vise and hammer, the boomkin was unfazed. We’ve also tied stern to in a great many Med locations, some with sizable swells, and watched the loads being dealt with quite well. Other boats around us were not faring nearly as well. I also remember reading somewhere about the cap shroud ratings. At 10,000 lbs. apiece, a pair certainly would lift a BCC clear. BTW, I had sent an email off to Larry Pardey, I’ll post any response here. Ciao, Ray