Catamaran uneven tank location problem

It is all rather elementary.

The distance from CoG of the tank to the end of the raft structure x the weight of tank = bending moment.

The global loads (which should be a full load displacement used in the calculation, ergo weight of tank included which is pressed up) from the sea into the raft structure = bending moment.

Which do you think has the greatest moment?

 

Keeping it simple for the benefit of all

Do you think that a (your) boat will flex dangerously if one person stood on one bow while another fished from the opposite stern?

Clearly not unless, as I say, it was a very poorly designed boat which I knows yours not to be, or the crew were VERY heavy (ie very big tanks)

But for sure the boat would pitch more with such a poor weight distribution.

Richard Woods

 

Of course Richard,

But lets assume a hypthothetical (highly improbable) situation of a catamaran which had a shell weight of 1000kgs (250kgs per hull and 500kgs for the bridge / cross structure). The raft cross structure consists of a typical sailing cat type twin beam structure with the aft beam 1m from the stern and the front beam 7m from the stern and a flat floor between. This boats designed max displacement is 2000kgs and its 10m DWL. and thus the loads are calculated with such in its design. As there are many different approaches to how the loads are determined in a catamaran, we will leave this open for now and see what comes out in the wash.

When we take this boat sailing, we load 500kgs weights on the forward stb perpandicular and 500kgs on the port aft perpendicular, so its in a seastate at max displacement and seeing max accelerations of 2g`s on the bows in a quartering sea to get the max twisting load.

How are the loads on the cross structure different or the same, if we move the 500kg weights to midships on both hulls - and for arguments sake, the boat still responds with a 2g motion?

 

Once I wanted to jack up my 44 foot 14 ton cat as it sat on the trolley of a ship yard. I had one jack under one hull and jacked . I was expecting a few inches of lift on the jacked hull before the other would lift. This I thought would have been quite acceptable.

In reality no movement good be seen at all the two hulls lifted immediately.

My question of how much weight in the 2 tanks has been ignored I know from personal experience that if I had one water tank full at the very front of the cat that hull would enter waves as the other would lift in a different way.

Not all waves are taken head on. Cats are very sensative to weight and I would avoid any weight on the front more that is necessary.

I like light responsive lift from the hulls to get over waves to avoid a bridge deck clash.

 

Groper, that's a bit too hypothetical

Assuming we are talking about petrol tanks, then (rounding up a bit each time for simplicity) each tank will be about 1cum in size. And assuming each hull is 1m wide then the tanks will be 1m fore/aft and 1m high. It's more likely the tanks will be nearer 600mm wide and high, so would be are over 2.5m long. Thus the CofG will be at least 1.25m from the fore and aft perpendiculars and thus 3.5m apart, not 6m

A better example would be to consider a sailing catamaran almost flying a hull and nosediving thus with a point load near the stern (the windward shroud) and a load at the other bow (the hull buoyancy)

Although an open deck cat may flex in those conditions (not all will, depending on beam size and placement) a bridgedeck cabinned cat won't if properly designed.

Adhoc can tell you if the 2g loadings you suggest are realistic. Seems "uncomfortable" to me, especially in the aft position

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Realistic or not, thats not the point... the point is, according to adhoc, the loads on the cross structure dont change... im looking for an explaination as to why they dont change as his previous post i could not understand...