Diesel/Gas efficiency at displacement speeds

[jeemboNC]

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.

[FAST FRED]

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

[jeemboNC]

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