Hull design - sail and power

John Teale (How To Design A Boat) explains that when referring to a drawing of a 20ft sail boat that - "Without the rig and with a different layout she could equally well be an outboard powered vessel or even a motorboat with a low power inboard engine. The hull form would be the same in all cases."

I am assuming he is generally referring to most boats? I had always imagined that the hull shapes would differ between sail and motor due to the position of propulsion - sails are from 'above' and motor is from behind.

 

He means that for a low power installation the coefficients of form will be very similar to the coefficients that best suit the relatively small sailplan.
If it were to be high powered and operated in the higher resistance regime of the drag curve it would have different ideal coefficients.

Longitudinal stability is so great in a run of the mill hullform that the position of the driving force makes little difference ( creates very little trim).

 

I am familiar with one such example. A local MA designed and built a fast sail boat hull and later converted it to OB power about 26-30ft long. it did well with a modest OB. This mind you was a modern light sailboat hull with planning potential not a classic clam crusher.

 

That's not true in general for all boats.

A highly-optimized hull form for each case would look a lot different, especially for a high-performance vessel. (I'm sure you've seen the crazy designs that come from extreme optimization.) Most hull forms are pretty boring and only roughly optimized and it is believable that these different powering configurations could use the hull form in question with comparable performance.

 

John Teale is referring to displacement hulls in that quote and he's correct, a sailboat hull will make a fine putt putt, though it will be a bit cramped, compared to a boat designed for this service from the outset. He's not referring to "most boats" just ones shaped like this and within the limited performance envelop they offer. Mike is correct in assuming he's talking about modest Cp's being the focus of John Teale's comments and that much deviation from the target Cp derived speed potential and you pile up an unacceptable wave train pretty quickly, which in low power applications will be insurmountable.