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quidproquo
03-10-2007, 07:23 AM
Here's the issue

The dinghy I am building has a thwart across about the half way point of the boat length. It is supported by risers screwed to the inside of the planks and a strut from the keelson.

It also has side seats running aft from the thwart to a bulk head, These seats are supported by risers on the planks and a cleat on the bulkhead as well as an inboard riser

The inboard riser is notched in the aft cleat and runs under the centre thwart

Should this riser be attached by screwing down through the thwart or up into the thwart

I'd prefer not to show screw heads or plugs on the thwart but does it make any difference to the strength of the structure which way it is done?

Bob

Jay Greer
03-10-2007, 10:02 AM
Since the thrust on the riser is downward and little, if any, twisting forces will be applied to it, you can, most likely get away with under fastening of the component. Fastening through the thwart, however would be the strongest form of tying the pieces together. If under fastening doesn't work out, you can always refasten later.
JG

quidproquo
03-11-2007, 07:05 AM
Thanks Jay
I wasn't sure whch way screws worked in relation to the forces

Bob

Hwyl
03-11-2007, 09:33 AM
Jay is suggesting a screw up and I agree.

Paul Girouard
03-11-2007, 10:49 AM
It would also depend on the thickness of the materials , Your discription lacks info IMO. Screwing up , in this case , could work:D

quidproquo
03-12-2007, 08:52 PM
Good point , Paul. The thwart is 3/4 inch thick (10 in wide). The riser is twice that thickness and runs almost to the front of the thwart.

Bob

Bruce Hooke
03-12-2007, 09:04 PM
In general it is strongest to screw through the thinner material into the thicker material. That gives the threads the greatest length of wood to bite into. So, it would be stronger to screw down into the riser. However, in this case I think you can probably get away with screwing up into the thwart.

It should be noted that either way the screw is in tension, so the up versus down part really does not matter, what matters is the relative thickness of the two pieces.

Another point to consider is if the thwart is softwood and the riser is hardwood then even if the two were the same thickness it might still be stronger to screw down. However, this gets more complicated because there are two likely ways for a screw to fail in tension: by the threads pulling loose from the wood or by the head pulling through the board. The former is less likely if the threads are biting into the harder wood and the latter is less likely if the underside of the head is bearing on hardwood. Still, unless the thwart is very soft I think it more likely that failure would occur because of the threads pulling free from the riser than from the head pulling through the thwart.

Thorne
03-12-2007, 11:03 PM
I always heard that you should screw from softer woods into harder woods, so you might consider that.

Also, plugs and 'screwing down' give you the option for an easier removal of the thwart -- which might be for any reason from damage to the thwart to a need to access the hull or keelson area.

quidproquo
03-13-2007, 05:31 AM
Thank you all. Good information here
Bob

donald branscom
03-13-2007, 08:59 AM
Here's the issue

The dinghy I am building has a thwart across about the half way point of the boat length. It is supported by risers screwed to the inside of the planks and a strut from the keelson.

It also has side seats running aft from the thwart to a bulk head, These seats are supported by risers on the planks and a cleat on the bulkhead as well as an inboard riser

The inboard riser is notched in the aft cleat and runs under the centre thwart

Should this riser be attached by screwing down through the thwart or up into the thwart

I'd prefer not to show screw heads or plugs on the thwart but does it make any difference to the strength of the structure which way it is done?

Bob

I would screw downwards so if i wanted to remove the screws for any reason it they be easy to get to.

Lucky Luke
03-13-2007, 11:58 AM
.... I ve been screwed sooooo many ways...!!!!

(Sorry, couldn t resist!)

jimmy
03-13-2007, 12:12 PM
Yes, I don't know how people resisted the obvious jokes. Anyway, in similar situations building canoes, seats and thwarts are often attached by through bolting with carriage bolts. The head of the bolt shows on the top, but if you are worried about strength, this is what I would do.

Charles Burgess
03-13-2007, 12:29 PM
Given the 3/4 thicknesses of both pieces, why not use a trunnel fastening? Looks great and spreads the stresses better.

paladin
03-13-2007, 02:05 PM
Darn, Luc...I was being nice....and I had first chance......:D

Ross Faneuf
03-13-2007, 02:32 PM
I used SS flat head screws and trim washers, top town, for my Pooduck skiff. I don't remember if this was the suggestion from 'Build the Shellback', but I believe so. Strong, removable, looks pretty good.

Bruce Hooke
03-13-2007, 03:50 PM
As far as looks go, a key point to remember here is that because these screws are holding on the cleat the holds up the inboard edge of the fore-and-aft seats between the center thwart and the stern sheets, these screws will not be right near the ends of the thwart but rather well in from the sides. So, I can understand not wanting to have visible screws out in this area, and I would also be inclined to avoid anything raised, like oval head screws in trim washers.

quidproquo
03-18-2007, 04:48 AM
Thanks for all the replies

Lots of good info for me

Bob