Can anyone please confirm my Pythagorus workings ?

[Alan de Enfield]

Trying to confirm total height of the boat for the potential trucking company to sort out routing.

 

I was informed Broker (when we bought her) that the water draft was 4' to 4' 6"" (depending on fuel levels in the tanks)

 

I have no problem with measuring the air draft - I simply put a ladder cross the roof (with the radar-arch folded down and the solar panel removed) and measured 3.09m (10' 3" ish)

 

The problem with the water draft is - she is in the water.

 

I took a length of rope tied onto the railings, under the keel, up the other side and tied onto the railings.

I hung upside down gripping with my toes and tied a cable tie around the rope (on both sides) to mark the water line.

Slid the rope back from under the boat and measured the length between the cable ties as 5.35 mts.

 

Halved the rope measurement to get the 'hypotenuse ' (2.675m)

 

Measure the beam of the boat (as best as I could at the waterline (not easy !!!) and measured about 3.75m.

Halved this to get the 'base' of the triangle  giving 1.875m

 

Hypotenuse C² = Height B² + Base line A² 

 

So wanting to find the height B² = C² - A²

 

So √ B = √ (C² - A²)

 

I estimate / measure the draft as 1.9m

Deduct 50mm for the width of the keel (approx.100mm) gives a draft of 1.85m (6 feet ish)

 

Anyone see any errors ?

I(f it is correct - its maybe another lesson to learn about brokers).

[WotEver]

3 minutes ago, Alan de Enfield said:

Anyone see any errors ?

If the hull is curved from keel to waterline your hypotenuse measurement will be longer than a straight line. Therefore your calculated depth will be deeper than actual. But at least it’s going to be less than you measured, not more. 

[Alan de Enfield]

7 minutes ago, WotEver said:

If the hull is curved from keel to waterline your hypotenuse measurement will be longer than a straight line. Therefore your calculated depth will be deeper than actual. But at least it’s going to be less than you measured, not more. 

I'm struggling to follow that.

The hull is basically a barrel shape with a keel hanging down, (Curved 'inwards' a - concave curve) 

 

I need to get the height of the boat from the bottom of the keel to the water line.

I weighted the rope and pulled it up to the water line and it only touched the hull at the waterline (the hull continues to get wider above the water line)

 

The straight line from the keel to the water line 'must' (?) be the hypotenuse, and the 'straight line' (in section) vertically from the bottom of the keel to the waterline is the draft.

 

Were you thinking that it was 'convex curve' ?

 

An old picture :

Edited August 5, 2019 by Alan de Enfield

[MtB]

I've just measured it for you on my screen and the water draft is 2"

 

Hope that helps.  

 

 

34 minutes ago, Alan de Enfield said:

Measure the beam of the boat (as best as I could at the waterline (not easy !!!) and measured about 3.75m.

Halved this to get the 'base' of the triangle  giving 1.875m

 

But being more serious, I can't see how this bit can be right. You need to have measured the distance between the two cable ties you afixed at water level, then halve that.

 

Although you say you only affixed one, so I'm not sure how any of this can possibly work. 

 

 

[hovrin]

I think your calcs look OK and my windows calculator comes up with 1.907 metre so 10/10 and you can go to the top of the class!?

[Alan de Enfield]

6 minutes ago, Mike the Boilerman said:

But being more serious, I can't see how this bit can be right. You need to have measured the distance between the two cable ties you afixed at water level, then halve that.

 

Although you say you only affixed one, so I'm not sure how any of this can possibly work. 

 

 

I did fix two, and then halved the length.

 

39 minutes ago, Alan de Enfield said:

I took a length of rope tied onto the railings, under the keel, up the other side and tied onto the railings.

I hung upside down gripping with my toes and tied a cable tie around the rope (on both sides) to mark the water line. 

Slid the rope back from under the boat and measured the length between the cable ties as 5.35 mts. 

 

Halved the rope measurement to get the 'hypotenuse ' (2.675m)

 

[MtB]

1 minute ago, Alan de Enfield said:

I did fix two, and then halved the length.

 

 

Ok that's fine then. 

 

To get the length of the base of your triangle you need to have measured the distance across the water surface from one to the other, then halve it. Did you do that? 

 

Not easy given there would have been a boat in the way. 

[beerbeerbeerbeerbeer]

29 minutes ago, Alan de Enfield said:

I'm struggling to follow that.

The hull is basically a barrel shape with a keel hanging down, (Curved 'inwards' a - concave curve) 

 

I need to get the height of the boat from the bottom of the keel to the water line.

I weighted the rope and pulled it up to the water line and it only touched the hull at the waterline (the hull continues to get wider above the water line)

 

The straight line from the keel to the water line 'must' (?) be the hypotenuse, and the 'straight line' (in section) vertically from the bottom of the keel to the waterline is the draft.

 

Were you thinking that it was 'convex curve' ?

 

An old picture :

Keep it simple: Work it out by counting the steps on the ladder. 

[MtB]

1 minute ago, Goliath said:

Keep it simple: Work it out by counting the steps on the ladder. 

 

There is 12. How does that help? 

[WotEver]

43 minutes ago, Alan de Enfield said:

Were you thinking that it was 'convex curve' ?

Yes

13 minutes ago, Mike the Boilerman said:

To get the length of the base of your triangle you need to have measured the distance across the water surface from one to the other, then halve it. Did you do that? 

 

Not easy given there would have been a boat in the way. 

A good point well made. @Alan de Enfield how did you do that measurement?

[WotEver]

1 hour ago, Alan de Enfield said:

Measure the beam of the boat (as best as I could at the waterline (not easy !!!) and measured about 3.75m.

This bit - how did you do that? Measure the beam at deck height then deduct the distance from a plumb line to the hull at the waterline?

Edited August 5, 2019 by WotEver

[MtB]

1 minute ago, WotEver said:

This bit - how did you do that? Measure the beam at deck height then deduct the distance from a plumb line to the keel?

 

Even that would be bloody awkward as there is a cabin in the way!

[WotEver]

Just now, Mike the Boilerman said:

 

Even that would be bloody awkward as there is a cabin in the way!

Yeah, drag the boat out onto hardstanding with a crane then drop some plumb lines from the deck edge, then put the boat back... oh, wait...

[Neil2]

 

Just now, WotEver said:

This bit - how did you do that? Measure the beam at deck height then deduct the distance from a plumb line to the keel?

I was about to ask the same question.  I reckon you could get a half the width measurement from the swim platform which is pretty near the waterline.  

[TheBiscuits]

Just use the picture.  Measure the cabin window for scale,  and work it out from there.

[Graham Davis]

53 minutes ago, Alan de Enfield said:

An old picture :

From that picture measure a known height, such as the height of the stern rear cabin section, and then scale it. That should give you a fairly accurate distance.

[WotEver]

22 minutes ago, Goliath said:

Keep it simple: Work it out by counting the steps on the ladder. 

Roughly 5 ft then. But could be anywhere between about 4’ 6” and 5’ 10”... depends on the ladder

[MtB]

Looking at that photo, I reckon assuming Alan's bit of rope was under the bote right at the skeg, and given the depth and shape of the hull, the cable ties would have been on the water surface pretty damned close to the hull anyway.

 

So using the beam would give a pretty reasonable estimate of the dimension we need. 

 

 

[Chewbacka]

Or count the rungs on the ladder

[MtB]

 

 

 

2 minutes ago, WotEver said:

Roughly 5 ft then. But could be anywhere between about 4’ 6” and 5’ 10”... depends on the ladder

 

Also depends on the angle the ladder is leaning at. 

 

Pythagarus could help here.....

 

 

 

Oh hang on, no. Trigonometry!

 

 

[Ray T]

"Tis all Greek to me."

[Alan de Enfield]

24 minutes ago, WotEver said:

Yes

A good point well made. @Alan de Enfield how did you do that measurement?

I measured the o/a beam.

I dropped a line down from the deck (into the water) and measured from the line to the hull where it entered the water, and deducted this measurement from the o/a beam.

Repeated on t'other side.

So had the o/a beam less the two 'curvatures' to give 3.75m

 

1 hour ago, Alan de Enfield said:

Measure the beam of the boat (as best as I could at the waterline (not easy !!!) and measured about 3.75m.

Halved this to get the 'base' of the triangle  giving 1.875m

 

[Alan de Enfield]

24 minutes ago, Neil2 said:

I was about to ask the same question.  I reckon you could get a half the width measurement from the swim platform which is pretty near the waterline.

 

The boat tapers in at the stern. I did consider this point to measure the draught because the platform was nearer to the water and easier to measure the beam - but had the Props and Rudder to contend with. 

 

The widest part is roughly just astern of the ladder.

This is where I did all of the measurements (to avoid getting the rope caught around the prop or rudder).

[Yellowback]

Weed hatch?

[RLWP]

1 hour ago, Mike the Boilerman said:

I've just measured it for you on my screen and the water draft is 2"

 

Hope that helps. 

 

And of course it does. You can measure the height of the real above water bit and scale the under water bit off the photo

 

Richard