Thread: Weighing boats

Originally Posted by woodeneye

I weighed my GIS late last night balanced on the gunwale on a single bathroom scale. I got roughly the same reading (47.6kg) as I got lifting each end while standing on the scale, so that's good enough for me!

Any thoughts on where this fits into the weight range for the GIS? I'd like to make an appropriate comment (such as 'wow that's light') but I really don't know.

I too would be interested to see how much weight finishing adds. Sadly, you've probably precoated and added a fair bit of that already but the 'ready to paint' vs 'fully painted' readings may prove interesting.

Richard
used to chasing weight in model aeroplanes where 1 oz is noticeable and 3 oz is the different between a gun model and an average one

Originally Posted by Daddles

Any thoughts on where this fits into the weight range for the GIS? I'd like to make an appropriate comment (such as 'wow that's light') but I really don't know.

I too would be interested to see how much weight finishing adds. Sadly, you've probably precoated and added a fair bit of that already but the 'ready to paint' vs 'fully painted' readings may prove interesting.

Richard
used to chasing weight in model aeroplanes where 1 oz is noticeable and 3 oz is the different between a gun model and an average one

I'm not sure Richard as I don't think there are any recorded weights for the GIS apart from Peter Hyndman's original Gruff. But it depends on whether it is supposed to include fittings or not.

Yes, I've coated inside the tanks and the inside of the two sides, but it's largely uncoated. Construction stuff yet to do includes:

add the gunwale capping and trim down the gunwales
add the bow piece
add the bottom runners
cut the cb slot in the bottom

I estimate maybe 1.5 to 2kg of net weight in wood has to be added before the coating can start.

I guess it will be at the lighter end of the spectrum due to extensive use of Paulownia for the framing and Joubert marine ply.

If it's any help, I used the following method that I found somewhere on the internet:

"The boat stays on the trailer the entire time. Put the scale under the tongue near the end. You can even put a block on the scale to lift the tongue so you can see the dial. Record the weight. Now, slide the boat aft on the trailer 12" to 18". You must measure exactly how many inches you moved it, and record that. With the scale in the same location under the tongue, record the new scale reading. Finally, measure the distance in inches between the trailer axle (wheel centerline) and the point on the tongue where it touches the scale. You have then four measurements:

W1 = first (heavier) scale reading in lbs, W2 = second scale reading in lbs, X = the distance you shifted the boat in inches, C = distance between trailer axle and scale point in inches.

The formula is Boat Weight = C (W1-W2)/X

A couple of notes. The result, like any measurement, is sensitive to the accuracy of the input data. In this case, the most critical is the distance you slide the boat. One inch error out of 12 inches will really make a difference in the result. Slide the boat as far as you can and still have a readable load on the scales. Notice, too, that by subtracting the two scale readings, any constant error in the scale is canceled out.

Note to techies: The formula was derived by taking the sum of the moments around the trailer axle for each case and (since the sum of moments about a stationary object is zero) set the two equations equal to each other and solve for the trailer weight. Things that you don't know, like the trailer's weight and the location of the boat's center of gravity cancel out, leaving just the variables that you can measure, and the boat weight."

Hope this helps, but is obviously only useful if you've already got your trailer

Originally Posted by gsg270

If it's any help, I used the following method that I found somewhere on the internet:

"The boat stays on the trailer the entire time. Put the scale under the tongue near the end. You can even put a block on the scale to lift the tongue so you can see the dial. Record the weight. Now, slide the boat aft on the trailer 12" to 18". You must measure exactly how many inches you moved it, and record that. With the scale in the same location under the tongue, record the new scale reading. Finally, measure the distance in inches between the trailer axle (wheel centerline) and the point on the tongue where it touches the scale. You have then four measurements:

W1 = first (heavier) scale reading in lbs, W2 = second scale reading in lbs, X = the distance you shifted the boat in inches, C = distance between trailer axle and scale point in inches.

The formula is Boat Weight = C (W1-W2)/X

A couple of notes. The result, like any measurement, is sensitive to the accuracy of the input data. In this case, the most critical is the distance you slide the boat. One inch error out of 12 inches will really make a difference in the result. Slide the boat as far as you can and still have a readable load on the scales. Notice, too, that by subtracting the two scale readings, any constant error in the scale is canceled out.

Note to techies: The formula was derived by taking the sum of the moments around the trailer axle for each case and (since the sum of moments about a stationary object is zero) set the two equations equal to each other and solve for the trailer weight. Things that you don't know, like the trailer's weight and the location of the boat's center of gravity cancel out, leaving just the variables that you can measure, and the boat weight."

Hope this helps, but is obviously only useful if you've already got your trailer

Wow, that is amazing gsg, and very inventive! Yes, I have a trailer so I'll give your method a go. I had a feeling that somebody on this forum would come up with something brilliant, so well done

That sounds pretty good.

I generally weigh lighter boats with me on the scales as well and deduct my weight - because you want the scales up in their normal working range.

But with bigger boats like the goat I try and balance them on their gunwale so that very small side load is required to keep the boat balanced.

Main thing is reproducibility - if you can get the boat to weigh the same a few times then you have it sussed.

MIK