Diesel/Gas efficiency at displacement speeds

I just discoverd this site - I love it! As an engineer/gearhead/boater/pilot, these forums are fascinating to me. I could use some advice - I am trying to decide on a relocatable 'waterfront condo' - a 40' Aft Cabin Motor Yacht or a Trawler (probably a Sundeck version). I would prefer a fishing boat (convertible), but have to appease some females (enough of that!). Will a 700 hp twin gas powered planing hull operated at the 1.4*sq.root WL (i.e. 8 kts) speed be as efficient as a 'real' trawler? How much different? And how much more fuel hungry will gas engines (twin 7.4L Crusaders) be than like-power diesels (twin 3208 Cats)? I am guessing 20% delta in gph burn. There is a $40K premium for diesels (at the 15 year old era I can afford) - that buys a lot of dinosaur juice. The advantage of the ACMY over the trawler is that if I need to get going in an emergency - I can (albeit at horrendous fuel burn, around 30 gph at 20 kts). So - two questions: 1) planing hull efficiency at displacement speeds with loafing engines and 2) diesel vs gas. Thanks in advance.

 

Gas engines are the best choice for lots of pleasure boats. the main reason is the ease with which they can be kept out of service without a
pickeling" procedure. DD and most engine mfg offer guidelines , for DD its 30 days of nonuse to need the procedure.

Gas engines require far less of this PM and so make better pleasure boat motors.

The hull design of a go fast will loose much of its efficency at trawler speeds , as it drags too much transom in the water. Loss maybe 20% compared to more rounded trawler style . The diesels loose a great deal of efficency and severe underloading may cause wet stacking and a loss of the oil lands in the cylinder walls from low load burnishing.

Most trawlers operate at sq rt lwl times 1.1 as its 1/2 the fuel of 1.4.



"Mid Size Power Boats" by Dave Pascoe is a "must by " as is a site visit to learn a better opinion of gas vs diesel.

http://www.davidpascoe.com/mid_size_power_boats.htm

FF

 

Thanks for the input! I have heard of him - I'll check it out.