Testing for drag – part 1 towing in static water
There are two ways we can test for drag. Here is the first – it was suggested by Marcus, who saw someone testing in this way.
It comprises hoisting a weight to the top of a 10m (approx.) mast, then releasing it. The halyard runs back through a turning bock and thence to the model where it is attached to a towing point. When the weight falls, the model is towed down a runway alongside my shed.
Here is the towing apparatus (53 grams hoisted to 10m) and the “tow path” at spring high tide.
The idea, as I see it, is that a fixed amount of potential energy (mgh – the weight at its highest point) is released and imparted to the apparatus. After friction losses (which should be approximately constant between trials) the energy is converted into forward motion of the dinghy (kinetic energy) and wave motion. I don’t intend to attempt any calculations, hoping that timing the run will be a sufficient measure of the model’s drag. We will also be able to observe the dinghy’s behaviour under tow, and perhaps the waves it generates.
Using a sail-maker’s thread for a halyard, and tiny blocks – and much trial and error – a 53 gram weight was found to be sufficient to tow each model through the water at a fairly constant speed. (The first metre, while accelerating, was ignored, but after inertia was overcome the rest of the run was at a kind of “terminal velocity” and that part of the run was timed). A crude assumption was made that the time of the run would be a measure of the dinghy’s drag.
(I think this might be the nearest we can get to learning if there is any difference in how “easy” each dinghy would be to row).
Here is Sibling Tender under tow with one “passenger” (a jar of sand weighing 600 grams, equivalent to a 75kg person) – less a factor allowing for the model’s over weight.
Each model trails a retrieval line, adding a little more drag, but the same amount for each.
Timing the runs using a “stop watch” app on a cell phone proved problematic – wet fingers etc – and eventually, after many foul-ups, much bad weather (and bad language) I recruited a grandson as technical assistant, and he simply made a video of each run, enabling rough timing (to the nearest second) between two recorded events (a) passing the start point after initial acceleration – and (b) running into a stop at the end of the tow path. Not very precise – but probably consistent enough.
The video timed-runs have been edited into a clip and placed on Youtube – here is the link
https://youtu.be/M9PFpAM5zUs.
This will allow some observations of wave creation and behaviour under tow, though wind and other conditions probably affected some of the runs.
There were two stand-out features regarding behaviour, which can be observed in the video: (a) the steady course (directional stability) of Oyster and some others with deep or long skegs – and (b) the yawing behaviour of David T.’s three, none of which have skegs. These three were OK when lightly laden, but with two or three "passengers" would tow straight only when trimmed by the stern which probably then added more drag. Note however the “horses for courses” motto – these three were not designed for towing, they were designed to have minimum drag and maximum manoeuvrability for rowing – and as it turned out, they were among the fastest on the tow path. David has agreed that in order to test like-for-like we might add skegs to the models, for future trials, and if I have time, I will do so. KISS also has no skeg – but this dinghy has its centre of buoyancy much further aft, trims down by the stern anyway, and does not seem to need one.
(As a digression, directional stability is the inverse of manoeuvrability, so you can’t expect lots of both. And it may not just be a matter of skeg or no skeg. I think that if the centre of buoyancy is further aft, with respect to the centre of mass, a vessel might tend to have more directional stability. [Edit: I have since realised that is nonsense. The distribution of weight and reserve buoyancy might, however, have some effect on directional stability when the dinghy is overloaded.] By design, baby SILBIM and particularly Sibing Tender both have full forward quarters – whereas KISS has her main body fullness aft).
Here are the numbers for the first trials of “drag while under tow”.
Some of the trials were repeated, in order to check consistency, and where there seemed to be a range of results they are shown here. The asterisk refers to tows in which the vessel yawed, and the trial was repeated with a down-by-the-stern trim.
The next set of trials will be with the dinghies stationary, on a tow line, in moving water.
Down at the launching ramp, where there is a proper pontoon. When the weather improves a bit.
There will also be some static testing for stability, posted on this thread in due course.