Help - stress cracks and reinforcing old fibreglass hull

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nakufeel
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Joined: Thu Oct 23, 2014 12:08 pm

Help - stress cracks and reinforcing old fibreglass hull

Postby nakufeel » Thu Oct 23, 2014 12:58 pm

Hello,

i've recently acquired an old (early 1980's) 18ft Jurgens Craft ski boat (fro SA) and have a few queries about some signs of age and how to restore her. Im not sure if she is a polyester or epoxy resin hull but she has what looks like a foam mat core. All bulwarks, gunnels, stringers... are plywood tabbed in as photos will show.
Most worrying are
1) The underside has two visible cracks across a port side strake near the stern of the boat. These appeared hairline but on further inspection go clear into the laminate. The cracks are about 2" long, going perpendicularly to the strake. They seem to be stress cracks from "hard spots" as they line up with two of the athwart bulwarks internally. Any ideas on how to best fix this? I have attached a photo of one of the cracks. i know i will need to get at this from the underside but i am worried about it happening again so am after ideas of how to strengthen (stiffen?) the hull at these points? The photo of the interior shows one of my ideas - to put triangular flanges/supports on either side of the athwart members to help spread the stresses at these points. Will this help?
2) The internal strake profiles are glass moulded over (sacrificial?) foam pieces. I have drilled several pilot holes and sanded these away in points (as will be seen from the photos) and found that three of the four are saturated with water inside and the foam pieces have effectively broken down. I expected this where i could see cracks in the gelcoat below but not in the others. Im concerned there are more cracks lurking under the paint on the underside of the hull. In an ideal world i would flip the boat over and sand this back to take a look but i do not have that capacity at home and am after ideas on how to best fix and strengthen the hull from the inside before arranging to have it flipped and fixed on the bottom. Will i need to remove all the plywood members and sand everything back to the hull and start again? Or can i just drill out the waterlogged voids and allow them to dry in the sun before plugging any holes with epoxy?... and then perhaps (as above) add some triangular flanges/supports on either side of each athwart plywood member to help spread the stresses?

Id like to make any improvements to the boat now, before i put a new deck on her and seal myself out!

Anyone out there who has done work on old f/glass boats please lend a thought here. she's a beauty, just got to get her floating again.

dom
Attachments
rear port side.JPG
interior.jpg
cracked strake.JPG

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billy c
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Re: Help - stress cracks and reinforcing old fibreglass hull

Postby billy c » Thu Oct 23, 2014 2:06 pm

do you know if the hull has a wood/sandwich core?
(insert Witty phrase here)
Billy's Belle Isle website

nakufeel
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Joined: Thu Oct 23, 2014 12:08 pm

Re: Help - stress cracks and reinforcing old fibreglass hull

Postby nakufeel » Thu Oct 23, 2014 2:28 pm

hi billy

thanks for the reply.

Im not sure what was used in SA in the 1980s but when i look closely there is a regularly perforated (grid square pattern) whitish material inside the hull which looks to be foam but could also be something else. Sorry i really don't know. any ideas what that would be?

cheers

dom

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billy c
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Re: Help - stress cracks and reinforcing old fibreglass hull

Postby billy c » Thu Oct 23, 2014 3:39 pm

if there are any thru hull fittings you may be able to see if it is a solid layup or if they used wood or foam sandwich in the construction. i owned a sailboat built in the 80's that looked pretty good from the inside and outside but got heavy and areas became weak around the mast step. found that the layup was glass then a wood core and then another glass layer. some of the inner core was full of water from a few stress cracks near the centerboard! was repairable, but required removal the inner layer of wood and glass that got waterlogged, a barrier coat on the bottom to the waterline, then the reconstruction reinforcing areas of stress with wood, cloth and resin much like you propose
(insert Witty phrase here)
Billy's Belle Isle website

nakufeel
Posts: 3
Joined: Thu Oct 23, 2014 12:08 pm

Re: Help - stress cracks and reinforcing old fibreglass hull

Postby nakufeel » Thu Oct 23, 2014 11:36 pm

hi billy,

again, thank you.

there are a few brass plugs that appear to go through the hull which, to be honest, I'm not sure what they are for. Could they be from when the original mould was vacuum bagged during manufacture? you will see one of them in the photo i attached of the interior....just near where i wrote "sanded down and wet foam removed". I have been reluctant to touch these but maybe i will have to. the core being wet definitely scares me.

One question though.....the foam pieces that the internal strake/stringer profiles have been moulded around appear to have been glassed in after the hull was moulded so there should not be any way for water to get from the core into these as there is a GRP barrier between....unless of course the interior glass skin of the hull is also full of cracks?? Is there an ingenious way of testing the core?

anyhow, thoughts would be appreciated

have a good day

dom

Leo2767
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Joined: Thu Jan 04, 2018 2:09 am

Re: Help - stress cracks and reinforcing old fibreglass hull

Postby Leo2767 » Thu Jan 04, 2018 2:23 am

Hi Dom,

I have just bought a 15ft jurgens Craft which has similar issues . Could you advise me on what you did to get your boat right. I also found water in the the foam strake and chine supports. I see your transom has Knee braces which mine does not. I suspect there was work done to mine which was not correct.

Please could you assist with any info or photos. I am basically having to rebuild form the ground up.

thanks

craig

PeterG
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Re: Help - stress cracks and reinforcing old fibreglass hull

Postby PeterG » Thu Jan 04, 2018 8:14 am

Hi Dom,
Sound like your boat has a foam core. There is a common core material made of sheet foam that is cut part way through on a grid pattern so it will flex and bend to lay inside a boat hull. It is glued in and then fiberglassed over, forming a stronger hull. Sometimes as with a vacuum- forming, the hull is made by laying in the hull fiberglass, then the core foam, and the inner layer of fiberglass and a vacuum process sucks it all tight and draws the resin into place. Makes a great strong hull. Complex stiffening design makes it so they are installed after the hull is formed. Some manufacturer's take minor shortcuts and don't install enough fillets at the hull. As far as stiffening your hull, I highly recommend the book "The Elements of Boat Strength" by David Gerr, he is a naval architect and yacht designer who has excellent information on best practices for hull structure design and build. He has info in the book that would help you. Basically, you have cracks at the hard spots due to the hull flexing between the "frames" like you said. The best thing to save this boat is to replace the saturated foam core and provide some additional stiffening where the cracks developed. That stiffening should be installed fore/aft over the length of the port and starboard strakes. It can be made of a batten of core foam about 1 to 1.5" thick and about 3" wide, laid flat, with the edges tapered down to the hull at about a 45 degree angle. This is epoxied to the hull and fiberglassed over with the glass at the same weight and thickness as the hull. A fillet of epoxy fairing at about 1" radius should be applied along both sides of the existing frames and longitudinal stiffeners to distribute stress and reduce the risk of cracks at those places too.
Murphy's Law: Anything that can go wrong, will go wrong.
Griffin's Law: Murphy was an optimist.


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