Are these structural stringers?

Tim Good

Well-known member
Joined
26 Feb 2010
Messages
2,797
Location
Bristol
Visit site
Photo below. See long built up section running through the back of this locker. This is one of two in this lock that run the length of the boat to the transom. Are they structural stringers and most likely a large section of Plywood covered in lots of GRP?

11817143_10153461112698162_3432424274026316724_n.jpg
 

30boat

N/A
Joined
26 Oct 2001
Messages
8,558
Location
Portugal
Visit site
Photo below. See long built up section running through the back of this locker. This is one of two in this lock that run the length of the boat to the transom. Are they structural stringers and most likely a large section of Plywood covered in lots of GRP?

11817143_10153461112698162_3432424274026316724_n.jpg

They look like the ones on my Westerly Fulmar. They should be foam underneath. And yes they're structural. Lloyds insisted on those stringers. Wauquiez also had them.
 

Tim Good

Well-known member
Joined
26 Feb 2010
Messages
2,797
Location
Bristol
Visit site
They look like the ones on my Westerly Fulmar. They should be foam underneath. And yes they're structural. Lloyds insisted on those stringers. Wauquiez also had them.

Foam ok. So it is the shape of the GRP that gives it strength rather than what is inside.

Reason for asking is I'm trying to find a good place to mount chain plates for a series drogue. They should be capable of taking up to half of the boats displacement. In my case 18/2 = 9T on each side. So I thought I could drill straight through the stringer and allow the forces to be distributed more evenly along the hull. By drogue chainplates i mean like this:

cordova_tnw_ppl-60-edit.jpg


Although if it is foam then maybe that is a bad idea as it may crush. Or do you think if the backing plate is large enough then it might be ok?
 

Martin_W_Brown

New member
Joined
17 Nov 2013
Messages
12
Visit site
Reason for asking is I'm trying to find a good place to mount chain plates for a series drogue. They should be capable of taking up to half of the boats displacement. In my case 18/2 = 9T on each side. So I thought I could drill straight through the stringer and allow the forces to be distributed more evenly along the hull.
Although if it is foam then maybe that is a bad idea as it may crush. Or do you think if the backing plate is large enough then it might be ok?

Personally I'd probably mount the chain plates just above or just below the stringer - you'd still benefit from the extra strength and you wouldn't have the problem of the foam crushing. Otherwise you'd need to support the skins to prevent crushing.
 

Tim Good

Well-known member
Joined
26 Feb 2010
Messages
2,797
Location
Bristol
Visit site
Personally I'd probably mount the chain plates just above or just below the stringer - you'd still benefit from the extra strength and you wouldn't have the problem of the foam crushing. Otherwise you'd need to support the skins to prevent crushing.

Yes good point. Maybe I could build up an area just above them with a few layers of rovings and then tie the strengths in together.
 
D

Deleted member 36384

Guest
You need to glass in a large backing pad of significant width to spread the loads over a wide area, the backing pad should be bedded on glue as well. It is possible on a two legged bridle for one leg to take the whole load, the last thing you want is a strip torn out your hull.

The width and thickness of the backing pad is a recommendation from Ian Nicolson's book 'The Boat Data Book' for U bolts where he gives a rule of thumb that the thickness is at least 4 times the U Bolt threaded leg diameter and width is 10 x (hull thickness + backing pad thickness + nut thickness). Ian Nicolson has an excellent reputation as surveyor and yacht naval architect. While at first the dimensions appear to be huge for the backing pad you are designing something that will be loaded in survival conditions; don't skimp on backing pad area. He recommends gluing/bedding in on a non-hardening waterproof compound.
 
Last edited by a moderator:

William_H

Well-known member
Joined
28 Jul 2003
Messages
13,594
Location
West Australia
Visit site
I find the photo of the "chain plates for drogue" rather strange. In fact I wonder if they are not an attachment for backstay given the rigging wire attached.
I would suggest for aesthetics alone you beef up the mooring cleats on the deck for the drogue. If you have doubts about the strength of the deck you could add stays under deck running forward to "chain plates" glassed into the inside of the hull. A piece of SS plate can have large holes drilled through so that GRP over the top can bond through the holes. Turn up one corner or edge to attach a shackle or rigging wire with a bottle screw so that it can be tensioned to take some of the load to the base of the cleat. It should be possible to get a cleat strong enough to take 9 tonnes.
If you really want to put bolts through the hull just where the stringers are. I suggest you cut out a large circle of foam where the bolts will go an fill with resin. Then lay up more GRP and epoxy over the stringer so that you increase the thickness of the hull over an area like 30cms by 30 cms with another 6mm of GRP. Use twill weave cloth on a diagonal to get the bend in the cloth into the curve from stringer to hull.
good luck olewill
 

savageseadog

Well-known member
Joined
19 Jun 2005
Messages
23,301
Visit site
Foam is used so the fiberglass can be laid on.The shape of the stringer doesn't lend itself to being made separately and then stuck on the hull.

I was only pointing out that the foam wasn't necessary and might not be there, my boat has plain stringers and ribs, I've no idea how they were made.
 

Strolls

New member
Joined
9 Mar 2015
Messages
740
Visit site
Foam is used so the fiberglass can be laid on.The shape of the stringer doesn't lend itself to being made separately and then stuck on the hull.

I was only pointing out that the foam wasn't necessary and might not be there, my boat has plain stringers and ribs, I've no idea how they were made.

My understanding is that the old-fangled (and perhaps some would say "proper"?) way is to use the foam to form the shape of the stringer during layup - this way the stringer and hull cure together and form a strong bond.

If you try to apply new fibreglass to an already cured hull then the bond will be relatively weak.

With the latest generation of adhesives (last 10 - 15 years?) it has become viable to form the stringers separately and glue them to the hull. This is how all current Beneteaus, Bavarias and Jeaneaus are made, I think.

I was going to post some photos here, but realised that doing so would distract the topic further. A good description of modern manufacturing process is on pages 21 - 24 of the Cheeki Rafiki MAIB report.

I hope this comment is helpful. I'm not an expert and the above is only what I've understood from casual reading, so I welcome correction.
 

30boat

N/A
Joined
26 Oct 2001
Messages
8,558
Location
Portugal
Visit site
My understanding is that the old-fangled (and perhaps some would say "proper"?) way is to use the foam to form the shape of the stringer during layup - this way the stringer and hull cure together and form a strong bond.

If you try to apply new fibreglass to an already cured hull then the bond will be relatively weak.
You are correct.These days liners with all the reinforcements moulded in are glued to the hull.The advantages and disadvantges of the method have been widely discussed here.On this particular case I assumed it was an older hull with conventional stringers.

With the latest generation of adhesives (last 10 - 15 years?) it has become viable to form the stringers separately and glue them to the hull. This is how all current Beneteaus, Bavarias and Jeaneaus are made, I think.

I was going to post some photos here, but realised that doing so would distract the topic further. A good description of modern manufacturing process is on pages 21 - 24 of the Cheeki Rafiki MAIB report.

I hope this comment is helpful. I'm not an expert and the above is only what I've understood from casual reading, so I welcome correction.
 
Top