Techie structural question

[zoidberg]

Needing to stiffen up the foredeck due to fitting a hefty U deck-eye for an inner stay, I queried my pet shipwright who recommended a chunk of underslung 1/2" ply as likely sufficient. Being me, I found two slices of 13mm grp sheet and have emplaced those, with more than adequate 'lots of sticky gunk'. The U-bolt legs protrude through. So far, so good.

The next step involves the attachment of a 'spreader' plate of 3mm stainless underneath the above lot of grp plates and sticky stuff. This is about 35 cm x 18cm and has a 50mm lip, and I can fit this onto the screw-legs of the hefty U-bolt, either way up. Here's the question...

Is it likely to matter significantly which way the lip faces - up or down?

 

[europe172]

Needing to stiffen up the foredeck due to fitting a hefty U deck-eye for an inner stay, I queried my pet shipwright who recommended a chunk of underslung 1/2" ply as likely sufficient. Being me, I found two slices of 13mm grp sheet and have emplaced those, with more than adequate 'lots of sticky gunk'. The U-bolt legs protrude through. So far, so good.

The next step involves the attachment of a 'spreader' plate of 3mm stainless underneath the above lot of grp plates and sticky stuff. This is about 35 cm x 18cm and has a 50mm lip, and I can fit this onto the screw-legs of the hefty U-bolt, either way up. Here's the question...

Is it likely to matter significantly which way the lip faces - up or down?

If the lip is to stop the 3mm plate deflection then it will do that no matter which way it faces, but your reinforcement might give a hard stress raiser where it finishes your foredeck might crack at the interface

 

[bedouin]

I'm no expert but my understanding is that steel is better in tension than in compression - so that would indicate having the lip upwards. In practice I am sure it will work fine either way up.

 

[Fr J Hackett]

I apologise if I am being obtuse and not understanding what you describe but your "spreader plate" sounds like a large U section are you sure about the dimensions? In any case if the 2cm lip ( 50mm) is facing upwards it will not give a good contact area as the only contact will be with the 3mm edge of the lip. The function of a spreader is to distribute the loading over a wider area, so if you are going to use this lump of stainless then lip down would seem to me to be the obvious way.
As for the GRP deck stiffening why didn't you laminate with GRP mat or oven cloth, securing two plates of GRP with sticky gunge seems odd as unless the sticky gunge forms a hard bond the deck will simply flex with the two sheets, they would have been better bonded imo with thickened epoxy.
A picture is worth a thousand words.

With all the stuff that you are adding to your pocket cruiser you are going to have to redefine her marks, any more and the sea will be lapping the gunnels.

 

[zoidberg]

I apologise if I am being obtuse and not understanding..... A picture is worth a thousand words.

With all the stuff that you are adding to your pocket cruiser you are going to have to redefine her marks, any more and the sea will be lapping the gunnels.

Point taken - and picture taken!

I agree that using lotsa epoxy bog would likely prove even stiffer - once the stuff was all in place and hardened off. There are several real physical difficulties with me wriggling on my back into what is a very tight space while balancing a heavy and unwieldy grp panel some 60cm x 18cm x ~3cm thick with 'gloop'..... then pressing that up against the deckhead just 6 inches above my nose - with the U-bolt legs fitted through - and persuading it to stay there while I fumble for washers, nuts and a ratcheting spanner..... without it falling on me and covering everything in epoxy 'gloop'.

I used C-TEC's 'Grab And Bond' which, when cured, will provide a very solid bond between the extra grp plate and the underside of the foredeck. Crucially, it 'grabbed' instantly as advertised so I was able to wriggle about to find, locate and spin up the necessary 'ordinary' hex nuts 'n washers, then wind them up with said ratcheting spanner to ensure good contact.

Today, I'll remove those nuts, emplace the steel sheet and wind on a pair of locknuts with large washers.

I'll have to 'suck it and see' whether deck deflection is minimised, or whether it'll be 'back to the drawing board.

As for Colin Chapman's 'simplify and add lightness', proportionately lots of weight has been saved by replacing the antique Kubota tractor engine c/w humungous flywheel with a modern Beta/Kubota unit without flywheel, several generations of retired 'lecky wiring, unwanted gas cylinders, piping and gas oven and various other antiquities.

But.... my shiny new boot topping is rather wider than recommended and thus should accommodate a few kilos of muesli, MarsBars and meals-in-bags.