Trig Help

[Lodesman]

Originally Posted by john manning

If your bridge clearance is a mapped value then it will be the height of the bridge above the map datum for sea level. This is usually set below lowest tide level.

Not quite! Should have read through the thread mate. Most charts have bridge clearance from highest astronomical tide or higher high water large tide or equivalent. The OP is in the US, where they use mean high water (MHW) - this means there is frequently less than the advertised amount of clearance. Fortunately on most US tidal bridges, there is a tide-board on the stanchion that indicates the actual clearance.

As an aside, while datums are frequently based below lowest tides, in the US not so much - negative tides (ie. subtract tide from datum to get actual water depth) are quite common.

[CaptJPMcGuire]

Originally Posted by Spot

We don't need no stinkin' trig! Or maybe we really do.....

Don't you just love Autocad !!

[john manning]

Originally Posted by Lodesman

Not quite! Should have read through the thread mate. Most charts have bridge clearance from highest astronomical tide or higher high water large tide or equivalent. The OP is in the US, where they use mean high water (MHW) - this means there is frequently less than the advertised amount of clearance. Fortunately on most US tidal bridges, there is a tide-board on the stanchion that indicates the actual clearance.

As an aside, while datums are frequently based below lowest tides, in the US not so much - negative tides (ie. subtract tide from datum to get actual water depth) are quite common.

Thanks for the correction.
I will have to read my charts more closely.

[Spot]

Originally Posted by CaptJPMcGuire

Don't you just love Autocad !!

Yes we do!

[betwys1]

Originally Posted by belizesailor

There you go! The practical empirical approach!

..or not, as the case may be. That weight on the outrigger or ladder had better be water - or you may invite a near-capsize.

[belizesailor]

Originally Posted by betwys1

..or not, as the case may be. That weight on the outrigger or ladder had better be water - or you may invite a near-capsize.

Maybe you misunderstood Jim's original proposal. No "outrigger" involved and the weight only serves to straighten/stabilize the line. Very much like a plumb line rigged from the masthead.

His clarification of the weight:

Originally Posted by Jim Cate

Perhaps my term "substantial weight" is misleading here. The weight isn't meant to help with the heeling, it is to keep the line taught and would not be physically large. The drag from a barrel suspended from the masthead would make steering rather difficult IMO.



I had something like a lead salmon sinker (a kg or so) in mind.



Jim

[betwys1]

Originally Posted by belizesailor

Maybe you misunderstood Jim's original proposal. No "outrigger" involved and the weight only serves to straighten/stabilize the line. Very much like a plumb line rigged from the masthead.

His clarification of the weight:

My note which had pointers to three video clips illustrating various methods of ducking under bridges seems to have got lost.
Onboard: water barrels - needs a plumb line
outboard: water bags can work without a plumb line if the water bags are judiciously hung
outboard: ladder and two crew walking the gangplank probably better with a plumb line and crew ready to jump in

[Jim Cate]

Originally Posted by betwys1

My note which had pointers to three video clips illustrating various methods of ducking under bridges seems to have got lost.
Onboard: water barrels - needs a plumb line
outboard: water bags can work without a plumb line if the water bags are judiciously hung
outboard: ladder and two crew walking the gangplank probably better with a plumb line and crew ready to jump in

Just to clarify this a bit: the subject of the thread was how to determine the required heel angle to clear th e bridge, not how to achieve the heeling.

My post was to describe a method that would assure bridge clearance calculation was in fact adequate. It has nothing to do with getting the boat heeled adequately and safely.

Jim

[betwys1]

Originally Posted by Jim Cate

Just to clarify this a bit: the subject of the thread was how to determine the required heel angle to clear th e bridge, not how to achieve the heeling.

My post was to describe a method that would assure bridge clearance calculation was in fact adequate. It has nothing to do with getting the boat heeled adequately and safely.

Jim

Jim insists that a measured plumb line has nothing to do with getting the boat heeled sufficiently. I think more than enough has already been said on the desirability of a measured plumb line already.
One other point that may be worth stressing, is that an outboard load i.e water bags or whatever, has increasing effect as the vessel heels, so that there are video clips that show inadequate heel, which is corrected by a rapid turn to dip the load. In the worst possible case, a heeling moment may increase enough with heel angle to take the boat past its critical angle, unless load contact with the water immediately unloads this moment, to provide a stable heel, something that the outboard ladder with crew does not.

[Icarus]

Make drawing to scale.

[JuliusAA]

Originally Posted by belizesailor

You make a lot more sense when you do math. [emoji6][emoji253]

ððð

Ioi ,ð