Hull shape compromises...

[tomherrick]

I've got an '84 C-Dory (not Cape Dory) and have been extending the hull. I've worked with a naval architect who gave my design a thumbs up, but that was before I got interested in the possibility of auxilliary electric propulsion. Unfortunately, the NA passed away, so I'm looking for input here.

To save a thousand words, here's a drawing of the hull as it was planned:

My understanding is that the flat bottom and transom will cause drag requiring more HP to move the boat than a rounded, tapered transition at the transom. The 30" 'bracket' has not been constructed as yet, and I'm wondering if it would make sense to taper the bottom of the hull upward and round the sides to reduce the drag. The boat is expected to draw between 5"-6" so there would still be plenty of bracket transom for mounting the existing outboard main and kicker, and - if this works out - an electric outboard. The flip side of this question is whether the upward tapering would negatively affect the planing surface.

 

[tomherrick]

Something like this:

 

[BlueBell]

Stick with the plan.
Keep the transom "square".

What's your cruising speed target?

 

[DCockey]

The shape you have drawn will cause the stern to squat and will increase drag compared to a straight extension of the bottom shape, particularly at speeds over 5 knots or so. At higher speed as the boat starts to plan there could be handling problems.

 

[tomherrick]

Most of the flat-bottomed C-Dorys top out at around 30MPH. Beyond that and they tend to lose steering control.

 

[BlueBell]

What is your cruising target speed?

 

[tomherrick]

I don't have a target for my cruising speed. Most of what I've read from other C-Dory owners regarding cruising speed varies from model to model. There is no other flat-bottomed 28' LOA, 7.33' beam C-Dory to compare hull characteristics one-to-one, and I don't have any powerboat design software, so I'll just have to see.

If your reference is to desired speed under electric propulsion, I'd say between 3-6MPH.

 

[Mr Efficiency]

Yes, that idea is no good unless you intend to only use electric propulsion, and low speed. Even then, it won't help much.

 

[tomherrick]

One too many compromises is like too many cooks in the kitchen... I had some idea that a tip of the hat toward low-speed displacement cruising would adversely affect the planing hull performance. Now, on to figuring what kind of HP would be required to move the extended hull at 3-6MPH, then how I'd find the power to run the motor... Thanks, folks, for the input.