Size of boat needed to handle coastal cruising safely

[donradcliffe]

You need to make the decision between a go fast boat and a trawler. Both have their advantages; its whether you want to go 7 knots or 20 knots. Then you need to decide how may people are going to live on board. If its 1 or 2, you can get by under 30 ft. If its 4, you probably need minimum of 35 ft. Bigger boats aren't necessarily more seaworthy, but can do more damage if you screw up docking--they also cost more.

[Sailmon]

Important implication/question regarding trawler vs plaining boat question is fuel budget.

[30West]

Originally Posted by Sailmon

Important implication/question regarding trawler vs plaining boat question is fuel budget.

Twin engine 40' powerboats all seem to burn around 4gpm at around 7 knots hull speed. Large gas engines or diesel, displacement or planing hull, I am seeing about the same fuel burn. If someone has different numbers I'd be interested in reading about it, I really expected displacement hulls to be much more efficient. I have very few true displacement hull twins to compare, they don't seem common. Single engine is of course more efficient, but if you are buying to weather big waves, you want a second engine.

A true displacement hull may be more stable in big waves at 7 knots, less likely to capsize, but it can't go much faster than 7 knots. A 40' planing hull can double it's speed and mush through big waves much more quickly, burning about ten times the fuel, but perhaps getting out of a building storm the displacement hull will have to ride out as intensity increases.

[ranger58sb]

Originally Posted by 30West

Twin engine 40' powerboats all seem to burn around 4gpm at around 7 knots hull speed. Large gas engines or diesel, displacement or planing hull, I am seeing about the same fuel burn. If someone has different numbers I'd be interested in reading about it, I really expected displacement hulls to be much more efficient.

Our LWL is 43.875', twin 450-hp diesels, planing hull.

Theoretical maximum hull speed (were it a displacement hull) is about 8.87 kts. If we run at 1200 RPMs, approx 8.9 kts, we burn approx. 5.8 GPM, approx 1.53 NMPG.

7 knots for us is about 900 RPMs, about 3 GPM, about 2.35 NMPG.

-Chris

[Lodesman]

Originally Posted by ranger42c

Our LWL is 43.875', twin 450-hp diesels, planing hull.

Theoretical maximum hull speed (were it a displacement hull) is about 8.87 kts. If we run at 1200 RPMs, approx 8.9 kts, we burn approx. 5.8 GPM, approx 1.53 NMPG.

7 knots for us is about 900 RPMs, about 3 GPM, about 2.35 NMPG.

-Chris

Surely you mean GPH

[donradcliffe]

My experience with a Grand Banks 42 with twin 200 hp engines was 3 gph at 7 knots (2.3 gpm), 4 gph at 8 knots (2 gpm), and 10 gph at 9 (0.9 gpm) knots (0.9 gpm). With a 42 ft express cruiser with 800 hp, it was 0.85 mpg pretty flat from 18 to 25 knots-- we never ran it at 7 knots long enough to check the mileage.

[ranger58sb]

Originally Posted by Lodesman

Surely you mean GPH

Har! Har!

Yep, brain phart. GPH.



-Chris

[30West]

Originally Posted by donradcliffe

My experience with a Grand Banks 42 with twin 200 hp engines was 3 gph at 7 knots (2.3 gpm), 4 gph at 8 knots (2 gpm), and 10 gph at 9 (0.9 gpm) knots (0.9 gpm). With a 42 ft express cruiser with 800 hp, it was 0.85 mpg pretty flat from 18 to 25 knots-- we never ran it at 7 knots long enough to check the mileage.

25 knot cruise with two 200hp engines? I get about .75mpg with two 375hp gas engines, not too far off your numbers. Diesels?