Chainplates for Drogue

[invictus301]

Hi all,
I own a '73 C&C 30 MK1 with a weight of 9700 Lbs loaded. I would like to put a set of chainplates on the port and starboard aft sides to attach a Farentino shark drogue. I'm not sure of an adequate size and was wondering if I could get some advice. I have been reading on the forums for info, I have read it is a good idea to have;
1. One inch of material between the shackle hole and the end of the chainlate,
2. Chainplates made from 316 Stainless Steel,
3. 1/4 inch thickness of chainplate,
4. Mounting holes of chainplate to vessel are staggered.

I thick these suggestions are good ideas, my questions are;
1. How long and wide should the chainplate be to accommodate 9700 Lb vessel,
2. Number and thickness of mounting hardware from chainplate to vessel, i.e. would 3/8" be adequate or go with 1/2".
3. Would it be advantages to have a slight bend in the end of the chainplate to line up the shackle to the bridle, if so, what degree of bend.

Any help would be greatly appreciated, one thing I wish to avoid is ripping the chainplates from the hull due to forces placed on the drogue.

Cheers,
Tom

 

[dlochner]

Welcome to SBO. Those are fairly sophisticated engineering questions.

The question not asked is about reinforcing the hull. Typically hull thickness get thinner as they near the hull/deck joint. Will your hull be thick enough and strong enough to support the load and spread the load out across the hull?

 

[Scott T-Bird]

A sketch would help understand the layout. You don't seem to indicate how many mounting holes will be used - that would define your length & width of chainplate. I'm completely lost on your description of #1. I see that you are asking about the number of fasteners. You need to know the loads and I suspect that drogue manufacturer should provide that info - and probably tell you the number of fasteners. Thickness of bolts depends on shear loads or loads in tension? It sounds like you are fastening chainplates to hull sides right at the corner of the transom - so that would probably be shear. It really just depends on the number of fasteners and the total cross-sectional area - and the crushing strength of the surrounding fiberglass. The strength specifications should be available in publications. I think that anybody would suggest backing plates epoxied to the inside of the hull. I wouldn't expect anybody to do these calculations for me - unless I paid a professional who would then take responsibility.

The bend is a good idea, I think. I would lay out the drogue on the ground and measure the angle the lines make with the hull.

 

[rgranger]

I agree with @dlochner. I think the weak point in this is the hull. Do you have access to the backing plate area? Can you put a strong (and relatively large) backing plate in place? Will you need to add a few layers of fiberglass cloth around the backing plate area?

 

[Scott T-Bird]

I agree with @dlochner. I think the weak point in this is the hull. Do you have access to the backing plate area? Can you put a strong (and relatively large) backing plate in place? Will you need to add a few layers of fiberglass cloth around the backing plate area?

It seems to me that the way he is describing it, the attachment of the chainplates is to the hull sides (right at the corners with the stern), not the stern itself. The fasteners would be in shear against the fiberglass. I agree that a backing plate is still advisable. If attached to the stern directly, a robust backing plate to prevent pulling out is absolutely necessary.

 

[dlochner]

It seems to me that the way he is describing it, the attachment of the chainplates is to the hull sides (right at the corners with the stern), not the stern itself. The fasteners would be in shear against the fiberglass. I agree that a backing plate is still advisable. If attached to the stern directly, a robust backing plate to prevent pulling out is absolutely necessary.

The loads from a properly sized drogue can be exceedingly high and there can be a lot shock loading. Think about the boat accelerating down a wave in a gale and being stopped short by the drogue at an angle with it happening repeatedly for hours.

If this were my boat, I would be finding a naval engineer to run the numbers and then I would build up the hull around the mounting points to achieve enough thickness and to spread the load out over a large area of the hull and use a substantial backing plate. The backing plate will keep the bolts from pulling through, the beefed up hull will spread the load over a wide area reducing any flexing in the hull from the loads, especially the dynamic loads.

 

[All U Get]

It would be proper to ask this question to someone from Australia or New Zealand since they routinely go out in this type of weather.

The Pardy’s would use their bow into the wind and heave-to, I enjoyed reading their book…

 

[jssailem]

The loads from a properly sized drogue can be exceedingly high and there can be a lot shock loading.

The fasteners would be in shear against the fiberglass.

Most chainplate applications on masts utilize the chainplate attached to a tabbed bulkhead. The bulkhead on my boat is doubled so as to spread the loads over a broad area of the hull. I do not think the loads on the mast chainplates would be anything like the loads of a drogue in a storm ( the normal time you would use a drogue).

I agree that in sheer the structure would be better, but I do not believe that without calculating the actual dynamic strengths of the materials and the structure needed to sustain continued shock loading forces of the drogue could you feel safe taking the rig out into the ocean. It would be just guessing.

 

[Scott T-Bird]

If this were my boat, I would be finding a naval engineer to run the numbers and then I would build up the hull around the mounting points to achieve enough thickness and to spread the load out over a large area of the hull and use a substantial backing plate. The backing plate will keep the bolts from pulling through, the beefed up hull will spread the load over a wide area reducing any flexing in the hull from the loads, especially the dynamic loads.

Yes, I agree. I wouldn't do this without somebody taking responsibility for calculating the loads & designing the connections ... somebody other than forum respondents!

 

[rgranger]

Yes, I agree. I wouldn't do this without somebody taking responsibility for calculating the loads & designing the connections ... somebody other than forum respondents!

What are you trying to say!?!?!?

I can do math...

 

[invictus301]

Thanks for all the replies, much appreciated. I posted over at design boat and have appropriate inquiries/replies from a couple of engineers. Cheers

 

[dlochner]

Thanks for all the replies, much appreciated. I posted over at design boat and have appropriate inquiries/replies from a couple of engineers. Cheers

Would you share those replies or a link to the thread? I'm curious about what the engineers said. Thanks.

 

[JamesG161]

Would you share those replies or a link to the thread?

and manual...


One page shows the forces, but most use the aft wenches.
Jim...

PS: NASA did the design and force work.

 

[invictus301]

Would you share those replies or a link to the thread? I'm curious about what the engineers said. Thanks.

Will do

 

[invictus301]

and manual...


One page shows the forces, but most use the aft wenches.
Jim...

PS: NASA did the design and force work.

I have used the drogue in the past delivering boats equipped with aft winches, unfortunately my boat does not have aft winches, I'm not a fan of high shock loads on winches. I highly recommend the manual as a good read/understanding of the drogue. I failed to mention I was going for a 2:1 safety on the chain plates. My drogue could develop 2200 Kg I'm looking for a strength of 4400 kg on the plates. My hull material/ thickness is not the weak point, hence I did not mention it. Cheers.

 

[kappykaplan]

most use the aft wenches

 

[All U Get]

Yep, my problem was with the aft wenches.

 

[jssailem]

This is a series of tests from 2012. Interesting recorded results using different drogue's while motoring.

Of note for optimal performance you want the drogue to set up in the second swell behind the boat. That way there is less chance of the drogue pulling out of the water as the boat careens down the wave.

 

[Project_Mayhem]

I haven't dug into specifics but take a look at SV Sparklemuffin. The owner put a TON of research and work into his chainplates

 

[thinwater]

This is a series of tests from 2012. Interesting recorded results using different drogue's while motoring.

Of note for optimal performance you want the drogue to set up in the second swell behind the boat. That way there is less chance of the drogue pulling out of the water as the boat careens down the wave.

That is, in any practical way ... less practical than it sounds. When you need a drogue it is because the waves are irregular. This is one of the major advantages of multi-drogues--some of the braking force will always be on the backside of a wave and farther underwater.

---

Two more things to think on:

• The load is NOT evenly shared by a drogue chain plate, like a rigging chain plate, because the angle to the rode varies up and down as the waves pass. Most of the load is on the last 1-2 bolts.
• Shock loads are NOT like those on wire rigging or a hammer against an anvil, where brittle fracture effects matter. They come on over a period of a few seconds at least, the rope stretches, and the drogue also pulls through the water (unlike a ground anchor). The load can reach significant amounts, but not more than the breaking strength of the rode, and actually quite a bit less than that if it is expected to last through a storm. Probably no more than 1/3 BS.

I've heard of drogues failing, but I don't think I have heard of a drogue chain plate pulling out. Follow Jordan's advise and you will be fine. As an engineer, I could add a string of caveats, but his advise has been working so far, unless your boat is very lightly built.