One of our readers had some questions about going cruising now with a small engine on his sailboat, or waiting until he could afford a larger engine:
There is no magic formula for engine size per se – and this needs to be looked at in terms of the total propulsion package. This includes transmission reduction gear and propeller size/type. The subject itself is quite involved (you will find a large section on this in Offshore Cruising Encyclopedia), however, there are certain basic principles.
First, the prop efficiency is more important than engine size. This can run as low as 20% for a fixed, two-bladed “sailing prop” or as high as 60% for a slow-turning (with a big reduction gear) three-bladed wheel (like the controllable pitch Hundested prop we use on BEOWULF). Your 18hp engine will do a better job with A good prop than the 27 with a bad prop.
The other issue is where and how you cruise, and the handiness of the boat. Our old INTERMEZZO is a good example. Her engine/prop setup was pretty inefficient and not very reliable. We always tried to stay out of situations where loss of the engine would endanger the boat, and would sail her into and out of a lot of tight spots just for fun (and to stay in practice). She would not power to windward against any sort of a wind or sea-so we always sailed uphill. This didn’t seem like such a big deal as prior to this boat, we’d always had engineless vessels. Of course, if you are dependent on your rig to keep you out of trouble, you will need good sails, and will want to practice your close-quarter maneuvering as much as possible under sail.
Coming back now to your original question, our philosophy has always been to go now, rather than wait to accumulate more money so you can have more stuff. So, as long as you have a reasonably efficient prop/gearbox for the new, smaller engine, I’d be inclined to do what got me cruising sooner rather than later. On the other hand, you will want to consider how this will affect resale value and if the noise/vibration factors are significantly different (often the smaller engine will be much harder on your nerves than the larger – especially if there is a difference in the number of cylinders). Good Luck – Steve