I have seen intakes machined for ride plates not including a shoe, but I think you are really missing out on the performance gains. First, I don’t think the transition from the intake to the ride plate is ideal especially on a rounded keel intake. More importantly, you loose out on tunability of the biting edge, opening size, and ride plate height.
The ability to place the biting edge height of the intake relative to the keel is paramount to properly feeding the jet pump with water. Insufficient intake pressure will cause unrealized bowl pressure (thrust) and therefore unrealized speed. Insufficient intake pressure can also result in starving the impeller for water, resulting in pseudo-cavitation and possible damage to the pump. A custom shoe can be made to place the biting edge in a good starting point. Shims can also be made at the same time as the shoe and give you some adjustability.
The opening size is relative to the front edge of the loader pad (rock grate) recess. 13-1/2” inches is the standard opening size. A larger opening will charge the intake harder. For example, if a boat is generating too much lift, the biting edge depth can be reduced and the opening size can be increased, thus maintaining sufficient intake charge without unwanted added lift. Ultimately you can drive yourself mad (and to the poor house) trying to discover the optimum lift to drag ratio while maintaining proper pump loading. Roughly, 1/2” (.500) of intake opening size is equivalent to .060 (~1/16”) biting edge depth
Varying the height (or depth) of the ride plate, relative to the keel, will affect the amount of lift generated by the hull. Most hulls ride too wet and can benefit from added lift created by lowering the height (or depth) of the ride plate by means of a back-cut shoe. The small angled surface of the back-cut in the shoe is generating a small percentage of lift (and drag), but the majority of lift is generated by a lower ride plate position. The net result is that the boat planing higher in the water reduces the wetted surface area and therefore total drag on the hull is reduced.
Some hulls inherently generate too much lift and become unstable. Net lift can be reduced by means of a flat shoe or in more extreme cases, a tapered shoe.
Attached are pictures comparing a 3-degree back-cut shoe (top), a flat shoe (middle), and a 1.5-degree tapered shoe (bottom). All three of these shoes have the same biting edge depth of 1/2” (.500), from the intake surface and therefore, on the same boat, all of these shoes will pick up the same amount of water. The difference between the three is the ride plate depth and the amount of lift generated by the ride plate. The second picture is a comparison of an actual tapered shoe (left) and a back-cut shoe (right).
These comparisons assume that the shoe surface of the intake is installed parallel relative to the keel. Intake angles vary for different reasons, and therefore the angle actually cut into the shoe should actually be measured relative to the keel. Typically, anything beyond 3-degrees back-cut or more than 1.5-degrees taper relative to the keel is pushing it. The actual angle cut into the shoe will be determined by the installed angle of the intake (at the shoe mounting surface). This angle can be either positive or negative and occasionally an extreme shoe design will be required to get the angle to the keel correct. Hull size (weight) and type as well as horsepower and top speed also play a roll in what you can actually get away with as far as shoe design.
Maximum performance will not be fully realized without tuning. The choice for a custom shoe with shims depends on your desire to tinker or if you just want to get in your boat and go. If the latter, the expense of the shims is not worth it, but if you get off on tweaking on your ride, I think it is worth getting one or two shims to start. If your cordwainer is skilled, he will design your shoe so that another shim can be snuck in without the need for rework. Custom shoes are expensive (relatively) but are worth the investment over a generic shoe based on fit and finish. My experience has been that the biting edge depth and opening are not even close and you will not be able to take advantage of the lift characteristics of back-cut or taper.
Cheers,
Joe
