Waterline: Mark it when lofting

[Éric Bouvéron]

Bonjour,
On a Tiki21, I whish to know if it is possible to have the distance to the waterline from the top of the plywood sheets. It would be easier to trace it at the beginning, for the late painting job. It would be marked on the plywood when lofting the boat.
Thanks,
Éric

[Omar M. Rashash]

When the hulls sides are on the boat, they are curved, and the waterline appears as a straight line. When the hulls sides are laying flat, the waterline will be a curve, and just about impossible to draw. The sheer line on this Hitia hull was a straight line when it was laying flat. With the bulkheads installed, the sheer now dips about 6" in the middle.

Omar

[Budget Boater]

Use a long length (40-50') of clear hose. Tape each end to the bow and stern with a couple of feet extending above the waterline. Use the waterline mark on the stem and stern posts, and transfer the line to the hose. Use a funnel and fill the hose with water until the water touches the marks on the hose, both bow and stern. Plug or pinch off the ends of the hose. Remove only one end of the hose from the hull, and walk the hose in increments that you chose along the hull making sure the water in the hose touches the line and transfer marks to the hull. If you do it right you should have a straight dashed line along the hull to trace for your waterline.

[Omar M. Rashash]

LOL, the ancient art of the water level. It's cheap, it works, and it is at least, if not more accurate, than using a laser level.

Omar

[Shaun]

Hey Guys,

Just a quick thought with the water level, it works brilliantly with one caveat that I found when building my parents house; the surface tension of the water can give a false reading.

I found it occuring when we would move the hose slowly and carefully to each post as we set them in concrete. It appears that the water would kind of stick in the hose and settle either slightly above or slightly below the accurate level.

We used, in the first instance, normal sized yard hose and fitted a clear hose piece to the inside of that hose, my suspicion is that the small I.D. of the clear hose actually has quite an impressive friction level for the surface tension. To rectify the issue we fitted a clear hose to the outside diameter of the yard hose giving the clear hose a larger I.D. and it helped immensely.

It maybe that we had extra thick water Tin Can Bay, or it's something between my ears, but it certainly made a significant difference in our marks.

And Omar, as for the laser level, yes it is extremely accurate,....at giving a perfectly flat line of plane over a large area. However if one has, as I intimated above, something a bit left of center between the ears, that perfectly flat line of plane can have a delightful 5 degree slope if the laser is not perfectly oriented in the first instance. There's an ablutions block in southeast Queensland that can attest to that.

Cheers,

Shaun

[Budget Boater]

Even if you have the laser leveled properly, the subject matter upon which the laser shines (i.e.: a boat hull) should also be level, otherwise it is an exercise in futility. (BTW, the problem is the same with a water level: the hull still must be level for it to work.)

Shaun said:

And Omar, as for the laser level, yes it is extremely accurate,....at giving a perfectly flat line of plane over a large area. However if one has, as I intimated above, something a bit left of center between the ears, that perfectly flat line of plane can have a delightful 5 degree slope if the laser is not perfectly oriented in the first instance. There's an ablutions block in southeast Queensland that can attest to that.

Cheers,

Shaun

[Omar M. Rashash]

IIRC, I was taught in High School chemistry that one must read the meniscus (concave or convex) consistently to get an accurate reading. I'm sure that someone with expertise in hydraulics can better explain why the inside diameter of the tubing, and the length of the tube, would affect a water level's responsiveness to changes in elevation. I'm sure it has something to do with surface tension, wetted area, and viscosity.

Omar

Shaun said:

Hey Guys,

Just a quick thought with the water level, it works brilliantly with one caveat that I found when building my parents house; the surface tension of the water can give a false reading.

[boatsmith]

If your shop floor is flat and smooth you can make a rolling cart with a pen on it and roll it around the boat for a good mark.