[Stumble]

Looking at Bob Perry's design for a PSC 6x'er I was struck by the proximity of the keel to the drive shaft exit from the hole. And it got me wondering what the consequences would be of basically adding a sail drive to the aft end of the keel.

As I understand it one of the limitations of how deep a saildrive can go is the lever effect of the distance, but if you can rely on the structure of the keel that should be minimized. So the leg could go much deeper, allowing for a larger prop, more bite, and maximum protection for the running gear.

Any thoughts?

 

[bobperry]

Stumble:
Not sure I follow you. Can you do a quick sketch of what you have in mind?

 

[ggray]

I deal with the advantages and disadvantages of a SD, and am fine with them.

But mine are behind bulkheads that are virtually watertight, and well above the waterline, on an un-ballasted catamaran.

I might think twice about cutting that big a hole that deep in a keelboat.

But perhaps a trunk could be built around the engine, extended to well above the waterline.

 

[Stumble]

Bob, I could give it a try, but my drawing skills... Let's just say I rarely know what I was trying to display.

Instead let me try it with a better description.

Conceptually not from an engineering standpoint. From the trailing edge of the keel cut a section out large enough to create an aperture that a sail drive unit could fit into. Then drop a saildrive into this aperture.

In practice however instead of two appendages the 'sail drive' would be encased by the keel. So there is only one protrusion, with the drive shaft sticking out of the trailing edge of the keel. Because of the structure of the keel the drive shaft could be supported on one side and the bottom thus allowing a longer drive leg, and therefore a larger prop, as well as decreasing the loads applied to the hull mounting.

i can see a few results of this pretty easily, but would be interested in your thoughts.

1) the prop would be pushed forward to the trailing edge of the keel. I can see benefits and negatives of this. Pro- less prop walk from the reduced lever arm. Con-no prop walk for low speed maneuverability.

2) Horizonte prop
3) Limited engine placement options.
4) more support for the prop shaft.
5) highly protected running gear.
6) larger prop (if two bladed it could be tucked very closely behind the keel trailing edge.

 

[bobperry]

Stumble:
Interesting but I can see some problems fitting the mechanism down into the last few percentages of the foil I like to use. I also do not like the idea of mucking up the geometry of the foil. It's a sailboat first. As currently drawn we have no problems. Can't imagine chasing a solution to a problem I don't have. But, I will keep your idea on a back burner for a while. It's an interesting thought.

 

[ggray]

I think what you are proposing would be to immobilize the saildrive "lower unit". Which would also tend to immobilize the engine.

My engine(s) and sail drive(s) move around a good bit, together.

Wouldn't that present a problem? I mean, doesn't the engine need to move on the motor mounts?

 

[Stumble]

G.Grey,

I don't know that the best way to build this is with an actual saildrive. It was just to demonstrate the point. Ideally would be two right angle gearboxes, one at the engine shaft and the other mounted on the step cut into the keel. With a cutlass bearing on the trailing edge of the keel. In theory this would allow the engine and the down saft to be located with some flexability anywhere from just behind the keel to just in front of it.

And generally no, there is no good reason to allow for flex in an engine. As a practical consideration some flex needs to be allowed to keep from breaking things when stuff happens. Flexable mounts also help cut down on transmitting vibration to the hull.