Single Engine Cat Pod Depth

Wondering if anybody has some thoughts on the varying depths of sing engine cat pods. In the examples below some go all the way to the same depth as the hulls keels... (livingstons come to mind). Others are a few inches above the keels, while some are even higher.

Obviously the higher up the pod is the less drag and better efficiency. However too high and too fast and the prop will be grabbing air...

If the shorter pods work why so some builder extend the pod so deep?

An example that comes to mind the the blue gulf cat 17' (the last picture). Even above 50 mph the short pod keeps the prop at a good running depth.

 

The pod is part of the overall design. What may work in one, won't in other. There are too many variables.

 

A big variable is the surface conditions of the water.
The rougher the seas, the deeper the drive will need to be to prevent ventilation.
Longtail motors are an ingenious means of variable depth drive.

 

The decision to use or not use a center-pod in a tunnel hull/catamaran design is determined by the objectives of the intended use/performance of the boat. If designing with a center-pod, then a pod that is 'higher' than outboard sponson bottoms will give performance dramatically different than a pod that is 'lower' than the outboard sponson bottoms. These performance characteristics are also quite different throughout the operating velocity range of the hull. Height of center-pod will affect when, at what velocities, the pod lifting surfaces will be wetted and load carrying. Depending on relative deadrise of pod vs outboard sponsons, the center-pod may or may not benefit desired performance at the speeds that you are most interested. As you point out in your research of different boat designs, there's lot's of variations, and some of these different designs are intending to satisfy alternative performance objectives. It's quite a study in catamaran performance design. what are you actually looking for?

 

Yes I do but it depends on what you want to know.
Am I to guess you're trying to decide on an engine mounting depth?
What does the designer call for?
It's all just guess work if you don't give some context.
You could mount your outboard on an electric jack plate, adjust it on the fly to whatever works best,
remove the jackplate and mount it permanently, and sell the jackplate.

What is the context of your question?