Hardtop design

Hi,

First post here so go easy

I want to build a composite hardtop for our catamaran.

I have done a rough model and drawing. I am thinking of using 75kg/m³ Closed Cell PVC Foam Core sheets, 25mm thick and using the offcuts to build the edges up for strength, with rounded external corners and fillet to the inside.
I have no idea where to go after this with the buildup or if I am on the right track with the proposed thickness. I want the hardtop to have some solar but also to allow me to walk on it to get to the mainsail.
Do I need to get this designed and if so how do I go about finding a designer?
Is there a Fiverr for designers?

 

Does it need to be flat? Some camber will make it stiffer and will look less boxy.

 

+1 re Gonzo's suggestion re incorporating some camber across the roof, ie from port to starboard.

What material are you proposing to use for the pillars?
If they are aluminium tubes, it might be easier to also build an aluminium framework to support the hardtop.
This frame work could be welded to the pillars.
Alternatively, maybe the framework could have flat plate tabs in the corners to which flanges on the top of the pillars could then be bolted to the tabs - this would be useful if you have to build the structure away from the boat.
Similarly, the pillars would have flanges on the bases for bolting to the cockpit coaming.

Alternatively, have the whole thing built of fibreglass, including the pillars.
I think it would still be useful then to have the pillars detachable, ie have fibreglass flanges on the tops of the pillars for bolting the roof too.
Fibreglass tubes would have to be thicker though than aluminium tubes, so this option would probably look more 'chunky'.

Re the top of the roof, have a flange around the perimeter is very useful for adding stiffness - I built a fibreglass bimini for our sailing boat with a 40 mm flange around it - this also allows for catching rainwater.
Mine has fibreglass angle bars glassed to the roof for supporting the solar panel, and these angle bars also help to stiffen up the structure.
My roof is not cored, as it only has to support the weight of the solar panel, and be able to catch rain.

Re your roof, it might be useful to also run a transverse flange on the top of the roof from port to starboard in way of the aft sides of the apertures shown.
This would help considerably to stiffen up the structure.
Re the apertures, I guess they are for the crew to see how the mainsail is setting - will they just be 'open' or will they be clear plastic to keep the rain and spray out?

I think you have made a good start here - supply some more details, and I am sure that a joint effort by the Forum will have it designed for you fairly quickly!

 

I can offer some advice, but need some discussion first.

There are a couple of good points already made. You need to plan for rain, heavy rain. The way it looks now, the rain will dump through the ?access? holes and hard.

In order to make something 3 meters wide; you will need to do more than four supports and one inch thick. In other words, some frame or camber or both or thicker material is needed.

Any place you have load or wants screws needs higher density inserts. I recommend 20-26 pound coosa. It is always smart to add an insert, where an extra pole, or lights or accessories may end up. The weight is not significant.

Planning for wiring is smart. This can be done with a frame consideration.

Anyhow, camber, inserts, tilt, rain gutters, rain collection?.

The hardtop is part of the lines of the boat as well, so designing the hardtop s/b done with a picture or lines of the boat...if the angle is wrong; a lot of work will look bad. My picture shows how matching the lines looks and my trim issues which are separate!

 

Hi and thanks for all the great information.

I will try and add some more info.

First I have never built anything in GRP this big but have watched a lot of Life on the hulls and Boatworks today!!!
I work in the structural steel buisness but not design.

Boat is a Richard Woods designed Flica 37. She has a very low boom so this is a main deciding factor in the design.
At the moment we have a small motorboat type bimmini and ali frame.

This is design version 7! I tried lots frame and canvas covers but not happy with any of them. Prefer the hardtop for access and solar.
I will add a sunbrella enclosure and clear plastic screen and windows eventually.

I intend to make a plywood mock-up and do a trial fit to make sure I am happy with the design before starting.

The two apertures are above the winches and steering. It would be difficult to crouch under the hardtop and winch.
I do want to be able cover these so was thinking of pop on clear plastic and sunbrella covers which I could roll back. Not sure I have the skills to make hard covers with runners
and also not enough room between the boom and top of the hardtop.

I was going to add a slight camber but limited in headroom. Also easier to model in CAD straight !

The mainsheet track runs across the rear cockpit beam so have to allow for this when positioning the supports.

Front 4 supports I was thinking 40mm dia. ali tube and the two rear supports being 60 dia. ali tube. All with top and bottom plates bolted to boat and hardtop.
Note point regarding adding inserts into the core at any fixing points. Not sure I can get Coosa board in the UK?

The lip around the the edge was for catching rain and stopping drips.

Thinking maybe double up the 25 thick foam sheets to make 50mm thick and then add a 25 thick lip around the edge on top of that?

Attached a few more pictures to help.

Once again cheers for all the great advice.

 

I made the rain edge on mine using 6mm core and it just covered the edge. I figured if I needed a drip edge to add something later. My top is 53" wide only by about 65" deep iirc. It is 18mm thick, but has a frame under it. I did the frame, but mostly as a wire chase. My 6mm piece was thus about 1.5", yours would be good at one inch or 25mm plus the top thickness. I covered mine in two layers of 100g woven, one from each direction after glassing the top. I did minimal fairing on mine.

I still think you ought to consider side to side camber unless you will bang your head on the edges.

Are you planning windscreens then?

I made the front of the top to be supported by an aluminum frame for the windscreen. It could be done with composite, but nice that it takes a bolt.

And your solar panel mounting must be considered, both for inserts and for deflection of the top.

The inserts can be made with something other than coosa. I am not a big fan of screw holding of M200 corecell, but it will. Wood is also possible, say like 3/4" ply, but not a big fan for rot.

The thing about camber is the properties of an arch for strength and the water shedding is to the sides. The way you have it now, all the water is shooting forward.

Solar panels on top adds an entirely new dimension. I have two panels on my rooftop and they weigh 106 pounds combined. It is adding to my trim problems and I may turn to one panel further aft. I digress. For you, making the top flat and adding some mass WILL make it sag. You have really no choice but to add stringers or camber or both.

All flat panel composites will sag under load and supported at 4 points. You must determine the loads, also will you be walking on it. Just think how you'll feel when the thing is all finished and a year later is sagging in the middle...

 

I am just wondering what might happen if the mainsail gybes over accidentally when you are sailing down wind, and the sheet catches on one of the aft corners of the canopy.
If it is a violent gybe, it could damage the canopy support (?)
Would it be feasible to build a goalpost or gantry type of framework across the aft end of the cockpit to support the canopy?
This could also be the attachment point for the mainsheet.
Although I appreciate that you have very little clearance between the canopy and the boom above, hence there probably would not be enough room for a block and tackle on a traveller, re the times when you want to sheet in the mainsail tight.
In view of this, I have seen some cats with two mainsheets, one on each side - an arrangement like this might be possible, with the lower ends of the tackles secured to the top corners of the goalpost mast?

 

A gantry or support may be possible but I am trying to keep it as light as possible within reason.
Not enought room for the mainsheet and really don't like the double mainsheet arrangement.
Its possible the mainsheet may catch the aft supports but I have modelled the mainsheet in its location when the mainsail is boomed out and there is clearance.
A gybe that violent will give me more that just canopy problems I think!

 

If you follow the lines of the cabin and add more camber for the structure; it might also keep the edges of the top further from the sail.. just a thought

My boat is a Woods Skoota in case you didn't know.

 

I thought it was a Skoota. Looks very nice
Didn't Richard have one those?

 

Not anymore. His smaller version was destroyed by Hurricane Dorian in Marsh Harbour.