Convert 60/70 older "fast" motoryachts to displacement speed

Question is also at which percent of hull speed the square transom really start to drag. If the disadvantage over a displacement hull (double canoe not sure if this is right term) is already huge at 50% hull speed then we may replace engines and all but still find it burns 4lt / mile at 80% hull speed.

 

I can tell you from rowing experience that some planing hulls are really hard to row, compared to proper rowboats. Some people think that only those tin motor skiffs with planing hulls are rowboats. A guy argued with me that my rowboat, which had oars, oarlocks, and even a modest transom, had to be a canoe, because it didn't have a motor.

I imagine that some of those mega yachts might be narrower, in proportion, which should help.

 

Hi all, thanks for all your replies.

As expected there is no magic solution, moving this kind of boats around just cost a lot of energy. Replacing the engines with smaller ones still will use the same amount of fuel (more or less) and the engine swap is not for free.

A hull conversion adding the "missing" part to "remove" the transom step and greatly reduce the drag - I have no idea if this can be calculated beforehand and how big this would be, if this is a 20 foot extension to a 60 foot hull or a much smaller thing that can be hidden under the swim platform. Has this ever been done ?

 

Some years ago,I read a recommendation for a fascinating book "Locus of a Boat Designer vol2" by Kotaro Horiuchi.It is a fascinating account of the latter part of his working life,which was primarily spent working for Yamaha.The reason it relates,just a little,with this thread is that one of the featured designs had a feature that might point to a way forward with this type of project.The design in question was intended to work both at planing and no-planing speeds and the concept involved the use of a step around the underwater region of the transom.The concept was that the step determined the point at which the flow broke free when planing and when in displacement mode,the tapering stern section eliminated a good portion of the transom drag and extended the waterline length.
I have attached a screenshot of a 5 minute CAD doodle to illustrate the principle,if not precisely the proportions.As you may notice,the top surface of such a stern would make a useable swimming platform.

 

So basically having a dry transom even at low speed and a longer waterline to boot. How would someone dimension such a thing ? Software or physical model (water tank etc ?).