JSD Chainplates - how many holes and where

Firstly, I'm new here and have no qualifications in boat building, design or engineering so I hope this is the place to ask, I'm not sure if this is the right section for this so if not Mods feel free to move it and slap my wrist.

I've designed a pair of chainplates for a Jordan Series Drogue for my boat on SketchUp (to learn a bit about 3d design programs during lockdown) and posted them on the Jester Challenge Facebook group, two people who's opinions I respect have mentioned about bolt spacing being too close.

In the interests of learning more, I thought I would open this up to the forum for discussion and hopefully pointed to information on what's best.



It's 500x75x10mm the three centre bolts are 125mm c2c away from eachother, the doubled up ones 37.5mm c2c and in the middle distance of the three central ones. My thought the offset ones would spread the load onto more area od the grp. The plan was to affix with thickened epoxy oversized 10mm G10 to the inside of the hull (approx 12mm thick) and glass over it (so in theory around an inch thick, then use 3mm s/s backing pads, I could also attach the actual plate to the outside of the hull using a permanant type sealant as belt and braces. Obviously this is all man maths which was the principal of it it looks like it's strong... but obviously only a fool would ignore warnings.

Now Jim Jordan's straps are thinner, narrower and shorter but recommends nearly as many bolts.

Are any of you guys smart enough to calculate the loads, stresses etc or offer advice either way/post grp being damaged by being ripped out or cracking from bolts being too close together. IMO opinion the more bolts the more the load is shared and it's down to the glass/backing pad to keep everything together?

Many Thanks.

Tom

 

It looks like an excessive amount of bolts. I see square holes, so you are planning on using carriage bolts. To be able to give you an answer, it is necessary to know what the structure of the boat is and what the magnitude and direction of the forces you expect on the plate. I see the quote by Jim Jordan, which presumable is the designer. The statement that "his arrangement feeds the load directly into the hull and imposes no bending or pullout loads on the hull or deck. For a load of 14,000 lb. a strap 1/4" x 2-1/4" x 18" attached by 6 - 3/8" bolts......." makes no sense. If there are no bending or pullout loads (should say forces), then what is the function of the strap?

 

Hi Gonzo, thank you for the response. These are chainplates to attach a drogue behind the boat in heavy weather so there would possible be quite high snatch loads as the drogue bites. I believe the general thought is you should be able to take the weight of the boat, there are two, one either side but load could be on one temporally if the boat gets pushed about.

The boat is an GRP Albin Ballad 30ft, the thickness of the GRP is around 12mm at the location of mounting, I would guess a weight of 5 tons loaded up for an ocean passage. I was going to add an oversized 10mm G10 backing pad that will be glassed in as well. The bolts (Carriage) would be in "shear", I've attached a picture of a set on a slightly smaller boat with narrower plates then mine to give you an idea but if you google Jordan Series Drogue you'll see pictures of them in action.



I was thinking of mounting mine in the location of the red square (great paint skills!).



I hope this explains what I'm trying to achieve, as you can imagine once I'm at the point of using this I really don't want any failures!

Many Thanks

Tom

 

Tom, it is all very well trying to do an overkill on attaching the plate to the hull with lots of bolts - but look first of all at the single hole where the drogue rope is attached.
This is where all the load will be acting - too much load and you tear the shackle out of the eye (assuming that the bolts in the hull do not tear out first).
Establish what your maximum allowable stress will be on the material.
Now then, Stress = Force / Area
You mention a load of 5 tonnes - you could use this to start with.
OK, you will have two chainplates, but for safety you want one of them to be able to take the full load.
Force = mass x acceleration so for 5,000 kg the force would be 5,000 x 9.81.
Lets round it up to 50,000 Newtons (kg.m/s.s)

Cross section area required of the steel (to resist the material being torn) will be Force / Stress

Decide on an allowable stress, and you can calculate the minimum area of material required between the plate edge and the hole.
You know the material thickness, so you can then calculate the distance of the hole from the edge.

If the fibreglass could cope with it, then you could have a single hole in the other end of the chainplate for attaching the plate to the hull - this hole would need to be the same distance away from the edge.
Or in other words, the plate would look symmetrical.
But the odds are that you will simply rip the bolt out of the fibreglass, no matter how well you think you have reinforced it on the inside.
So you add more bolts.
I would think that 3 or 4 bolts in line would work - in your initial sketch you have way too many holes!
Also, do you really want to have square holes? Surely round holes would be better?

In conclusion, the important bit here is the fibreglass hull structure locally in way of the chainplate - you might want to add some extra fibreglass reinforcing on the inside if this is feasible.
Although it should be pretty strong in that locality, right next to the transom.

Edit - Yellow Ballad also asked his question on the YBW Forum in the UK - some more opinions here.
JSD Chainplates - how many holes and where (stress cracking) https://forums.ybw.com/index.php?threads/jsd-chainplates-how-many-holes-and-where-stress-cracking.560126/

 

Hi Bejansailor, I recognise that username. I found an interesting thread on YBW forums which seems to say a bolt spacing of 120mm in 10mm glass with backing etc. I'll redesign mine so slightly longer and slightly narrower with four bolts.

Drogue Chain Plates Strength? https://forums.ybw.com/index.php?threads/drogue-chain-plates-strength.448943/

 

Personally, I would go look it up in the AIS Steel Construction Manual. Generally, in riveted and bolted connections it is the squeeze (shank tension) and not the shear (shank area) that provides the load carrying ability. This means that head area and spacing are the important factors. So just glancing at my old riveting tables, for 3/8" (10mm) plate the fasteners should be 3/4" with no less than 3 diameters between fasteners.

 

Also the preferred shape for holes is round. Square holes create stress risers that make the plate fail at lower loads.

 

But at that size, there is no hope of pairing fasteners on a 75 mm wide strap.

For paired holes, you could do 12 mm, leaving 8 mm on the edge and 35 mm between. Backing plate should be about 8 mm thick and have 8 mm minimum outside the holes. If the inside is tight, the backing plate may dictate narrower hole spacing, which is okay. You have the thickness to use something like an M 12 x 50 countersunk hexsocket bolt if you want it to look decent. Just eyeballing it, four pairs at 30, 120, 210, 300 mm from front might work if you can get the backing plate to work with that.

Or you could just do like the photo and use single row 17-ish mm bolts. The backing plate should be 10 mm thick in that case, and about 50 mm wide.

 

Yep, and what part about fatigue cracks do you not understand....Holes are a pre-formed crack initiator...<pounding head against wall>...why do I even try.

 

Yes they are, but it can be dealt with. A lot of people would consider the exposed heads of 3/4 hex bolts cosmetically unacceptable. I guessed that was why the OP was changing the arrangement. Not sure what your objection is, too many holes, or too small and prone to upset? The countersunk hexsocket bolt in a tensioned member is not ideal, but how many load cycles is a drogue anchor plate going to accumulate in, say, the next 100 years? The genoa track will see more use, and mine is held by 5/16 countersunk bolts and show no signs of any problems after 40 years an occasional loads up to 20,000 pounds.

I wasn't faulting your answer, just offering one that I thought would work, last pretty much forever with average workmanship and decent materials, be easier to torque properly in that cramped interior space, and offer a cosmetic option.

If you look the specs on a 3" Schaefer pad eye rated at 8500 pounds, you'll see why I'm not too worried about the fasteners.

 

Phil, do you see anything wrong with the orientation of those square holes?

 

I am baffled by the square holes shown - @Yellow Ballad are you really intending to have these?
If yes, how would you make them?
Surely round holes would be better?

 

I wasn't looking at them. Gonzo had asked about the carriage bolts and I thought the matter would get sorted by others. I addressed what was asked about for a change, and handled the bolt pattern question.

Yellow Ballad, were you planning to have those holes made at a machine shop? The holes normally run full depth in thinner material. Companies such as Marsh make specialty marine carriage bolts to spec, but why go to all that trouble? Just do it the normal way with normal round holes.