One for the engineers

[valhalla360]

Originally Posted by garyfdl

Having said that, the sides are what support the load (bending monment) and keep the tube from buckling. If the side openings are far enough forward on the roof (if I understand Spot's layout correctly) toward the end of the tube opposite the overhang, then you can probably get away with it.

It's the opposite. The top and bottom of the beam do the bulk of resisting bending moment. The sides resist buckling from overtightening the bold and from shearing along the beam.

What is the spacing? Peak bending moment is at the last point supported by the deck. A lot will depend on the lengths involved, as will the weight of the dingy. Also, it would be good to have the specs on the tubing.

[Spot]

Sketch update ala MicHughV. No warranty expressed or implied. See a doctor if symptoms persist and I am not a legal spokesperson...

Where is that traveler track going anyways?

[Chotu]

Originally Posted by Spot

Sketch update ala MicHughV. No warranty expressed or implied. See a doctor if symptoms persist and I am not a legal spokesperson...

Where is that traveler track going anyways?

I love these drawings and this whole thread!! Wow!!

Really looking robust, and best of all, using the current material with no additional weight added for the increase in strength!

Looking at putting the traveler track either between the davits on the roof or on top of the davits on the roof.

I don’t know how long it is yet. I’m picking it up on Friday along with the rest of the rig.

[MicHughV]

Chotu,

We're making progress here....one step forward....two sideways....but hey...moving forward..
Those rear pulleys need to be as close back the post as possible..wanna make sure it clears the dinghy tubes...

Once the dink is on the davits, I'd like to see some arrangement where you can lead a line from the bow and stern of the dink to a a cleat somewhere on the rear part of the roof.....yes....I know...more stuff.....but need to keep that dink immobilized at sea....

After you've lowered the dink.....you're gonna have this long pulley assemble dangling around...pull it back up, so it is about 3' long, swing it back and just clip onto the rear pulley connection..

I've got some ideas for the traveller.....let me know when you get the bits and pieces..keep in mind, the traveller is gonna see a lot more loading than the davits...let me know the sq. footage of the main....and the length of the boom...the height of the mast, also let me know, where the boom ends, and how high above the traveller it will be........will the boom end over the roof, beyond the roof, etc also, will the main sheet be on the end, or somewhere along the boom..and will the boom have multiple pulleys...also where do you want the tail end of the mainsheet to end up..

I'm just curious....how do you trim the main.....your vision of the main will likely be limited ....will you be able to adjust the main sheet traveller....ie, pull it left or right....ooops...port or starboard...and how do you plan to do this.....again....being on the roof all this cordage needs to be directed somewhere...

I'd really like to know how you've got those posts attached to the roof, and also the base....that's key...an uncontrolled main jibe can induce a stunning shock load on a traveller...

There's a lot to think about here...doing it all by messages is complicated....drawings will help.....I can do CAD work, but would have to swap computers....a pain in the arse.....so hand drawings is what you'll get....

Regarding the upper davit tube.....I'm leaning to having just a small radius here, on the upper rear part of the tube to leave as much meat on the tube as possible. The forward part of the tube can be vertical as it is over the roof, though I would round the edges.

[MicHughV]

Chotu,

Was thinking about this some more....rather than put more cleats on the roof....another method to stop the dink from swaying around is per attached sketch....the attachment point from the dink can be led to the roof support posts, or eyebolts, which will be a strong connection point.....as the dink will be hanging, the support ropes will be aligned in an upward direction.....the rear support rope I tie in a V to some pad eyes I mounted on the dink transom....but some dinks come with this...this allows the dink to be hung in the davits, left to right or right to left
This is how I do it my boat, as I don't have a roof....I also have straps that go completely around the dink, enabling me to cinch it tight behind the davit uprights...another thought for you...

When I'm bouncing around out there...my dink wants to move...and I don't want that...

Last thing.....I buy some inexpensive swimming pool noodles, Walmart and any pool supply carries these.......these are foam noodles about 4' long, about 3" in diameter with a hollow core....I cut these in 12" sections or so, and slit them lengthwise, so I can fit them over the restraining ropes.....so that the restraining rope does not rub on the dink (or in my case, as well as the davit uprights)....they sometimes want to come loose, so I tape the edges with some duct tape, this keeps them on the restraining ropes and uprights...

I typically tie my dink, so that the stern is bit lower than the bow, and remove the drain plug, this prevents the dink with filling with water. The drain plug has a small tether that ties to one of the transom pad eyes....as I've lost this before....now I carry a spare in addition...

Offshore, I carry my dink in the davits all the time, but I always take the engine off, and this might be a wise thing to consider....I remove the fuel tank, and all the other clutter that ends up in there...except for a small bailing scoop, which lives permanently in the dink with a string tether.....

Can't say I've ever had much of a problem doing this offshore...

[MicHughV]

my sketch was not attached...

[MicHughV]

let me try again

[Chotu]

From the other thread, my flanges are in. Hard to tell the size in the pic but they are incredibly beefy and incredibly light.

These go on the aft structural beam and under the roof to hold the support columns under the davits.

[MicHughV]

just to be clear....the bolts attaching the flange to the roof....they go all the way thru'...right ???...4 of them....?? How big are they ?
is there a plate over them or just independent washers ??? how big are the washers ???..or plate...??

what is the beam size ??? what span ?? is the beam just between the posts or does it extend the width of the roof...and what is it made from ???....is the beam an integral part of the roof....in other words.....if you give me the beam size does that include the roof thickness...how thick is the roof there...??

and what about the bottom of the posts...how are they connected to the deck....???

sorry...a lot of questions, I know...

[Chotu]

Originally Posted by MicHughV

just to be clear....the bolts attaching the flange to the roof....they go all the way thru'...right ???...4 of them....?? How big are they ?
is there a plate over them or just independent washers ??? how big are the washers ???..or plate...??

what is the beam size ??? what span ?? is the beam just between the posts or does it extend the width of the roof...and what is it made from ???....is the beam an integral part of the roof....in other words.....if you give me the beam size does that include the roof thickness...how thick is the roof there...??

and what about the bottom of the posts...how are they connected to the deck....???

sorry...a lot of questions, I know...

The response is a little short as was my last one. Because I am running around right now. But I want to be timely in responding.
The response is a little short as was my last one. Because I am running around right now. But I want to be timely in responding.



These bolts are half inch through bolts. 4 of them.


Currently I have not bought the hardware for this part. So I do not have the fender washers yet. Maybe I will put a backing plate. Probably should since the Traveler load will be on these.

There is no beam across between these yet. But there will be something to hold a traveler in there. This part is still in the design process I guess. Because I don’t have the Traveler yet. I will have that tomorrow. Or maybe Saturday.

Everywhere, the roof is 1 inch thick, plus glass skins.

The bottom posts are not connected to the deck. The bottom posts are connected to the main structural crossbeam. The box beam in the back of every catamaran that holds the catamaran together. They are being through bolted to that beam, which is the attachment point for the traveler anyway on the plans.

[MicHughV]

well, before you start bolting those posts to the roof with those four 1/2" bolts you were talking about...I'd hold off until you get the mainsheet traveler...as they could provide a good anchor point for your traveller...you will likely need longer bolts....as they will need to go thru' the roof as well as the traveller support...but we can cross that bridge after you get your bits and piece...

[Chotu]

Originally Posted by MicHughV

well, before you start bolting those posts to the roof with those four 1/2" bolts you were talking about...I'd hold off until you get the mainsheet traveler...as they could provide a good anchor point for your traveller...you will likely need longer bolts....as they will need to go thru' the roof as well as the traveller support...but we can cross that bridge after you get your bits and piece...

I agree completely. This is all on pause a few days while I’m picking everything up.

Getting the mast in the morning

[MicHughV]

sounds like a plan to me, Chotu, I spend w-a-a-a-y to much time on this forum....I need to get my life back again....

I have plans to head out into the ocean next week....but I'll check in..

[Djarraluda]

Originally Posted by Chotu

Oh boy. I guess I’m not being clear. Or no one is understanding this. I’m just looking for advice on what makes a beam more strong. Putting holes in the side? Or putting holes in the top. That’s the question.

Chotu, in simple engineering terms, a flat plate works best to resist flexion in it's longest plane. We then put flanges on that plate to stop twisting or misalignment. T section or I section explains that easiest of course. For your case, the key loading is vertical motion, holes in the sides are directly compromising the beam, holes in top and bottom are only working in compression/tension and can transmit the forces into the sides. Of course, this goes pear shaped when the dinghy lurches side to side, but assuming you lash the dinghy when it gets a bit rough this should be fine especially as you have a decent amount of the member remaining around the hole. As has been said already, round holes are less prone to stress cracks than those with corners. Ideally you would minimise the holes by sleeving the tube and putting a dome head bolt all the way through but that is less visually attractive. Calculation wise, of course there is the additional support of the length of the member where it sits on the roof anyway.
Roger

[zstine]

Originally Posted by Djarraluda

Chotu, in simple engineering terms, a flat plate works best to resist flexion in it's longest plane. We then put flanges on that plate to stop twisting or misalignment. T section or I section explains that easiest of course. For your case, the key loading is vertical motion, holes in the sides are directly compromising the beam, holes in top and bottom are only working in compression/tension and can transmit the forces into the sides.....
Roger

respectfully, I believe you have misunderstood the issue or have been misinformed. The top/bottom flanges on an 'I' beam take the vast majority of the bending (vertical) loads, they are not there to resist twist. The web takes very little loading, in fact zero at the neutral axis for pure bending. It is common to see holes drilled in the web of an 'I' beam to lighten it's weight.