Maths/physics genius required

[Sea-Fever]

I am planning to fit some davits at the stern (obviously). These will be standard chandlery jobs - you know, stainless steel, mount through the deck then go up a bit before bending over the back of the boat with some little blocks at the end off which your ropes dangle.

Due to the makeup of the stern and pushpit i calculate that when the inflatable is attached the ropes will be unable to hang vertically as the dinghy will push them out so that they are about 10 degrees from vertical.

My question.....if the max load was 150kg (presumably hanging vertically) for the pair of davits what effect does having the tender at an "angle" have?

It's far too long since I did my a levels to work this out.

It may be a negligible effect....10 degrees isn't that much.

 

[thinwater]

You haven't given us all the numbers, but basically the leverage on the davit is proportional to the overhang from the mount to the centerline of the dinghy.

If the overhang would normally be 3 feet and you have increased it to 4 feet, the increase in leverage is about 4/3. The increase in stress will be (4/3)^2 and the increase in flex will be (4/3)^3. On the other hand, if your dinghy only 3/4 of the specification, you should be rosy.

The other concern is chafe. It can be really bad unless you take precautions.

 

[Momac]

The reduced gravity load is 150kg x cos 10 degrees = 147.7kg

If you are worried about that i suggest you avoid going over any waves .

 

[RupertW]

I'm not sure we need the distances as the angle of the weight from the vertical should tell us extra force needed as a proportion of the total weight so Force = Weight plus (Tan(AnglefromVertical)xWeight) or to make it simple,
Weight x (1+Tan(AV)), so in this case 189kg

 

[VicS]

I am planning to fit some davits at the stern (obviously). These will be standard chandlery jobs - you know, stainless steel, mount through the deck then go up a bit before bending over the back of the boat with some little blocks at the end off which your ropes dangle.

Due to the makeup of the stern and pushpit i calculate that when the inflatable is attached the ropes will be unable to hang vertically as the dinghy will push them out so that they are about 10 degrees from vertical.

My question.....if the max load was 150kg (presumably hanging vertically) for the pair of davits what effect does having the tender at an "angle" have?

It's far too long since I did my a levels to work this out.

It may be a negligible effect....10 degrees isn't that much.

the load on the ropes will increase to 150/cos10 .. ie to 152.3 kg

 

[Momac]

the load will increase to 150/cos10 .. ie to 152.3 kg

That's what I said but expressed it as a reduced gravity load to result in the SWL not beng exceeded

 

[Momac]

I'm not sure we need the distances as the angle of the weight from the vertical should tell us extra force needed as a proportion of the total weight so Force = Weight plus (Tan(AnglefromVertical)xWeight) or to make it simple,
Weight x (1+Tan(AV)), so in this case 189kg

Wrong

 

[Momac]

You haven't given us all the numbers, but basically the leverage on the davit is proportional to the overhang from the mount to the centerline of the dinghy.

If the overhang would normally be 3 feet and you have increased it to 4 feet, the increase in leverage is about 4/3. The increase in stress will be (4/3)^2 and the increase in flex will be (4/3)^3. On the other hand, if your dinghy only 3/4 of the specification, you should be rosy.
.

Not exactly but very safe

 

[Sea-Fever]

Wrong

Please continue to battle it out ..... I'll get my popcorn.

The thing about horizontal distance from the mounting sounds familiar.....turning moments and all that - it's coming back to me! The original moment would have been .720 x 1500 Nm ..... The altered one (due to the shape of my stern...ahem) would be .920 x 1500 Nm ..... I think that's how the calc goes???

In addition you have the increased load in the rope yes???

 

[GHA]

the load on the ropes will increase to 150/cos10 .. ie to 152.3 kg

:encouragement:

The load down the lines will increase, vertical load remains the same, plus there'l be a horizontal component on the davits away from the boat of 26.4KgF .

 

[john_morris_uk]

I’m not going to enter the maths debate. It’s a long time since I did ‘Applied Mathematics’!

However, I was advised that each davit should have a load max of a least double the weight of the dinghy (and out board if you are leaving it on - and what’s the point of davits if you aren’t leaving the outboard on the dinghy!)

 

[Sea-Fever]

You haven't given us all the numbers, but basically the leverage on the davit is proportional to the overhang from the mount to the centerline of the dinghy.

If the overhang would normally be 3 feet and you have increased it to 4 feet, the increase in leverage is about 4/3. The increase in stress will be (4/3)^2 and the increase in flex will be (4/3)^3. On the other hand, if your dinghy only 3/4 of the specification, you should be rosy.

The other concern is chafe. It can be really bad unless you take precautions.

On your point about chafe...I agree it can be a serious concern. Fortunately I have identified some very comfortable pants from M&S which seem to be holding up well at present.

 

[thinwater]

I'm not sure we need the distances as the angle of the weight from the vertical should tell us extra force needed as a proportion of the total weight so Force = Weight plus (Tan(AnglefromVertical)xWeight) or to make it simple,
Weight x (1+Tan(AV)), so in this case 189kg

The angle doesn't tell you anything unless you know other dimensions. Might as well skip the trig until he posts a sketch.

 

[thinwater]

Not exactly but very safe

Exactly. This was an oversimplification to give him some idea, until he posts a drawing.

 

[richardabeattie]

The angle of dangle is proportional to the heat of the ...

 

[VicS]

The angle doesn't tell you anything unless you know other dimensions. Might as well skip the trig until he posts a sketch.

Exactly. This was an oversimplification to give him some idea, until he posts a drawing.

Dont need a sketch or drawing or any dimensions. He has told us all that is necessary.

The three things we know define the vector triangle. Load, the angle deflected from vertical and a right angle.

 

[Momac]

The angle doesn't tell you anything unless you know other dimensions..

Yes it does
Because the steel rope is always running over the pulley to the davit at the same location.
The dinghy may move away from the base of the davit but the lever arm to the pulley remains at the same distance.

 

[Momac]

In practice the 10 degree angle is of little consequence .
The dynamic affect of the boat going over a w ave is probably far greater . I don't know what that is a but a factor of 2 seems reasonable.

On reflection the base will see a small increase in moment but the lever arm is that from the base to the pulley and not the lever arm to the dinghy.

 

[TOKOLOSHI]

Shirley not a problem if the inflatable is secured with a lashing!:encouragement:

 

[Neeves]

JD - where are you when you are needed