Originally Posted by sailingfool
No need for a bridle at the boat end. Two points to make.
Add a third line from the dingy bow handle/tow attachment ring to the bridle. Adjust this line so it is just long enough to take up the force of a pull directly ahead. It will then also take up any non-horizontal pull. A problem with the bridle lines to the side D rings is that if you tow the dingy through rough wave action for a day or so, the bridle lines will rip the fabric tabs holding the D rings right off the inflatable tubes - these ring attachments are not designed to take loads off the horizontal. A short center line ensures the only pull on the side D rings is generally horizontal, whether to left or right, but not up/down.
The second is to run the dingy tow line to a secondary winch, if you have one. You can then easily adjust the length of the tow line if necessary even when underway. You should shorten the line whenever you anticipate manovering or stopping, such as when entering a harbor.
Sailingfool, I respectively have to disagree with your analysis. The D-rings are held on with adhesive patches. I'm assuming no additional sticthing. The patches will hold the most force when they are loaded in shear as opposed to tension. Shear loads are the loads parallel to the adhesive surface. It doesn't matter if the shear load is horizontal w.r.t. to the water or vertical. The strength will be the same.
The reason for a bridle, with the tow line fixed at the center of the bridle, is to cause the dingy to straighten when it turns to one side or the other. Let's say the dingy turns to starboard. The starboard half of the bridle takes more load and it is reduced on the port side. When this happens the larger load, on the starboard side, becomes even more closer to pure shear.
When the dingy goes up and down in waves, the load remains a shear load.
By adding another line, in your case from the bow handle to the center of the bridle, you are simply distributing the load between more points. This reduces the stress on the D-ring attachments. That will reduce the chance of failure in rough weather, but it is does not have anything to do with non-horizontal forces.
Aside from wanting to use the bridle to help keep the dingy tracking straight, there is another reason to use it. Attaching a tow line to the handle or bow D-ring puts that patch in pure tension. That's the worse thing you can do. I suspect that most handles and bow rings are attached with stitching as well as adhesive to take into account the high tensile loads here. But maybe not; I haven't looked closely.
In short, failures of the D-rings in rough seas is purely due to the fact that they are "rough seas".
It's interesting to note that if the D-rings on the bow sides are meant for towing, then they are not installed in the optimum direction. The standard is for the straight part of the "D" to be horizontal. It should be vertical for the purpose of towing. This will allow the D-rings to lie near flat when towing. This helps ensure shear as opposed to tension.
I've towed a dingy with a bridle and hauled it with the dingy bow on the stern of the boat. I prefer davits!