by GreenLake » Thu Feb 02, 2023 3:48 pm
There are a lot of interesting variables that make finding an optimal solution a challenging issue.
There's price and range, operating costs and maintenance, and weight on transom.
For a sailboat, weight on transom is an important consideration, not just for structural reasons. Small sailboats are sensitive to any added weight, but specifically to weight on the transom. The latter affects the fore/aft balance in a negative way. A stern-heavy boat will squat and drag the transom through the water slowing you down. While you are sailing, the power you have available is limited. This matters most in the medium speed range, where you are sailing slower than the hull speed.
I've taken measurements with a small motor powering the boat at a bit over 3 knots. In that speed range, moving your seating position from the rear half of the cockpit to up at the cuddy can make a difference of almost .5 knots. Weight on the motor bracket is further back, so the effective lever arm is longer. That's why it pays to really keep the motor weight low.
When using a motor that's sufficiently strong, you may be able to reach hull speed even without good fore/aft balance. And when you are approaching hull speed, the overall resistance increases so rapidly with speed, that adding a small amount of resistance requires only a little bit of the total power required, so you may not observe the difference when motoring, but you would under sail.
I'm not sure what size motor would be required to get a DS up on plane. I suspect it's well above the range you were considering. That means, even with a motor, you are limited by hull speed, which (theoretically) is 5.4 knots for the DS. A 30 mile lake would take you nearly 6 hours to traverse, while sailing you might take up to 10 hours. (An average speed of 3 knots sounds low, but isn't uncommon for a point-to-point average, as you rarely have ideal winds the entire day). Half a knot better sailing speed would save you over an hour.
A smaller, lighter motor would not affect the sailing characteristics as adversely as a heavier one, but given that you are limited by hull speed, it might not be all that much slower.
~ green ~ lake ~ ~