Best construction method for a coach roof

[Captain Coochie]

I nipped down to see Forbsie today and we where deciding on the best way to construct the coach roof and forward cabin . We came up with two options
1 use plywood following the curve of the carling and fix a beam shelf to the top of that .
2 make a frame up like a ladder following the carling and the top of this will be the beam shelf .
What say the experts ?

 

[Blueboatman]

The plywood will splay along the top unless you wrap it around a predetermined set of cabin top carlines and (laminated?) beams.
One of the most watertight ways is to construct double carlines at deck level so that the (18mm?) ply cabin sides will be fastened from the outside in rather than trying to wedge and brace them and fix from the inside out at deck level.Though others may well frown on this..
Are the cabin sides to be vertical or cambered inward at all? Lots of fun.Should I buy shares now in Sika and West systems ? /forums/images/graemlins/smile.gif

 

[Captain Coochie]

Yes the sides will be cambered inwards a little . My thinking was if a timber is cut to copy the curve of the deck and then made up into a ladder with the studs having a bevel of 6*? on the stop tenon shoulders this could be dropped into place and support the coach roof . The 18mm ply could be then fixed on afterwards .

 

[Blueboatman]

That will do it.
If you also fit some cabintop beams and some permanent (or temporary) hanging knees as well,before actually attaching the ply sides,then you can wrap the ply around one side without fear of distorting the new cabin's profile.(I dont know how curvaceous is this babe!)
Its well worth using 10mm mdf or sh-t ply to mock up the cabinside (template it),cos as you know you can bend and twist ply but it will not follow any sort of compound curve,best to 'test' and ensure that 6 degree camber throughout its length..
How are you going to secure the verticals where they meet the deck carlines-throughbolts?
And the most vulnerable area for water ingress is going to be the only place where movement between the new rigid structure and the existing boat can occur..-the lower edge of the ply,of course. Especially if the existing carlines are not pretty substantial..

I think that is why I like double carlings although it makes the decks narrower effectively by the width of the carline.

Or you consider securing the ply to the inner face of the existing carlines(bit tricky to stop splaying at the top without temporary framing and tricky to get that 6 degree bevel along the carline face without a compass plane...and then frame it and attach the cabintop beam shelf after the fact..More work but a very strong arrangement because the ply is bracing and tying together every part..and the exposed plywood end grain is now inside the boat..
It is easy to overengineer stuff but there is nothing worse than going through all that work and then having a floppy structure that refuses to remain raintight....
Gonna post a piccie ,Jason?

 

[Captain Coochie]

I was thinking of either putting a base plate down to hold the verticals or birdsmouthing them over the carline , i think Forbsie is off to take a pic of the area .
Fig 1 and 2 are my thoughts and fig 3 is what i think others are talking about .

Sorry for the poor drawings .

 

[Forbsie]

Thanks for the advice, Blueboatman. This is the job as it stands this evening and a shot of how she was originally.

I'm coming round to Jason's ideas as long as he does it. These weird angles are doing my head in. /forums/images/graemlins/confused.gif

 

[Blueboatman]

Hi Forbsie and Jason (nothing wrong with the drawing quality there!)

The boat looks very pretty-is it a naval pinnace conversion ?

I am now going to have to apply myself to posting a scanned sketch too aren't I !

Thanks to Dogwatch this might indeed be possible, meanwhile if either of you pm me an email address I can zap an attachment over to you guys..tomorrow am(pub time now)

I see that the cabin/wheelhouse structure is quite rectangular,so there is less curvature to apply to the new cabin sides than with many..that will make it easier of course...

IMO (and there will be boatbuilders reading this thread with far,far more experience than I).....

All 3 drawings would work but the 3rd has best strength and is vaguely boatbuilding...so long as your longitudinal 'stringers'shown are turned from horizontal to vertical , and now say 50x50 or 40x40 oak 'beams'@400centres. These need to be notched at their top and outer faces to accept the cabintop beam shelf,which I would set VERTICALLY,say 25w x 75-100 h in size.

...This is then in turn notched to accept the new cabintop beams flush with its top INNER edge.the new cabin beams can be spaced to not coincide exactly with the cabin sidebeams....
If there is any camber to the cabin top then the offending 'proud'outer edge of the cabintop deckshelf can be simply planed down flush.

The ply roof can then be laid and fastened to both beams and shelf,which stiffens everything up furthar still and if you allow an overhang to the ply(as indicated in your drawings)you may then 'wrap' any -fiberglass?-covering down and over the endgrain to create a drip edge...

Meanwhile the lower ends of the new vertical 'beams' can be throughbolted through both ply and carlines and this will make a strong and secure attachment.

I can't quite see in Forbsies pics,-there are bolt heads showing along the iner face of the carlines-are there hidden tiebolts going horizontally through the carline and across to and through the beamshelf at the hull side? That is a standard way to strongly ensure that the carlines cannot separate from the deck half beams (instead of just relying on the deckbeam half dovetails and new ply decking)...

Your drawing Number 1 is indeed the double carline method but because there is no way to bolt the vertical beams to the new upper carlines you can make the carline a little more square in section ,say 75x75 and laminated if curved) so there is more attachment surface for the structurally loaded ply cabin sides to attach to...

 

[Captain Coochie]

Yes it is a Naval Pinnace conversion and the wheel house is comming down too aswell as the aft cabin . I think the shape is staying roughly the same but its Forbsies boat im just there helping out and having fun when i can find the time .
The reason i was thinking Fig 1 was a good idea is the boat is going to be a liveaboard so we can fit insulation into the void aswell as leaving space for airflow . Its standard practice in house and loft construction . But on boats /forums/images/graemlins/confused.gif
All help and advice is welcome please .

 

[Blueboatman]

This would be what I had in mind.
You can still insulate between the upright beams and between the cabin roof beams and then line with 4mm ply.
The only thing I would say is paint the bejasus out of everything that is going to be covered up,it is inevitable that some condensation will form behind the linings...
Another way to create the 6degree angle on the inner face of the carlines is to put the cabin sides in place loosely and then fill the 'gap' with epoxy followed by an epoxy/filler runny mix..It will be very strong and never leak.
The strength issue is worth considering imo because there is a lot of old boat with little tieing the two sides together at deck level except for the quality of the new deckhouse construction.
I am sure others can add to these advises /forums/images/graemlins/smile.gif

AND A VERY BIG THANKYOU TO DOGWATCH FOR HIS EXCELLENT PHOTO TUTORIAL-I CANNOT THANK YOU ENOUGH,THAT WAS A PAINLESS AND STRAIGHTFORWARD FIRST PICCY POST HERE ! (Apologies for the capitals)

 

[Captain Coochie]

Ahhhhhhhhh that makes more sense now , thankyou .
Would it be a good idea to steam the new Beamshelf to the shape of the Carline then fit the ply and uprights notched out and slip the top Beamshelf in afterwards ? Or am i going the wrong way about it .

 

[Blueboatman]

Hi yup .Working from the bottom up(!),with some temporary braces across bewtween the two new cabin sides ,clamp the top beamshelf in(could be made in laminates if the curvature is a bit much..get it to the right horizontal angle,fasten and trimoff any excess ply on the top edges of the sides,fit the new deck beams and then finally add the upright beams..You can add some hanging knees if you feel it merits it too..

....And finally.Make all the new deck beams on the same template and just cut each to the correct length,so that you have a constant camber along the length of the cabintop and the ply can bend in one direction only.
Sorry if this all is bleedin obvious to the chippy,just a question of deciding on the most suitable technique really-wot is wot you asked
Surprised any practising boatbuilders haven't joined in yet though...

 

[Forbsie]

Thanks Tim, I really appreciate your input.

I like the simplicity of your design, but would the fact that the wall is coming down the inside of the carline not invite leaks. What if the upright beams were more substancial and the walls laid above the deck. I'm having shaped and routed 3"x3" pillars in the corners anyway.

 

[Blueboatman]

Hey Forbsie,you are welcome-I am really only trying to pass along what others have patiently tried to teach me....plus I have done a couple of these way back when.
You could indeed fix the ply cabinsides over the decks,with 3x3 in the corners routed to receive the ends of the cabin sides and front/rear panels and with further uprights say 2x2 every couple of feet that should be fine...you will need to attach the base edge of the cabin sides to something though,for example a second set of carlines set ontop of the existing ones...this will make the cabin wider.
If there is anyway that you can keep at least one full deckbeam going right across the hull at deck level(eg under the floor of the wheelhousedeck?)it will stiffen up the hull tremendously and make whatever you do for the cabin sides far less likely to flex and leak,thats all.
From what I have seen in your pics you certainly have the skills to do it all good and strong,its just a matter of drawing it out,getting organised and making each piece (the cabintop beams,any new carlines,the routed corner posts etc in the most straightforward way and ideally off the boat if at all possible)
Cheers,Tim

 

[Captain Coochie]

Just an update , The Carlines are now doubled up and chamfered to give the sides a 6*ish slant . The ply has been put in place so Forbsie can decide on how he wants her to look .

Sorry but another question , is there a rule of thumb for the slant and camber on the coach roof or is it just what looks good ?