Planing Hulls with Twin Diesels...Economical?

[u4ea32]

Good questions, even for a newbie ;-)

At speeds below the square root of waterline, it just takes very little power to move any shaped hull. At 1.2 to 1.3 times the square root of the waterline, then wetted surface and wave making start to matter. At speeds a little over 1.34 times the square root of waterline, wave making dominates, but surface friction still increased by the square of the speed.

Most people drive their powerboats at a little over hull speed, say 1.4xsqrt(LWL). At this speed, the lowest wavemaking comes with a very high prismatic hull -- nearly blunt. That's why you see transoms, and why you don't see deep fine knife like forefoot on nearly any powerboat.

Many trawlers have a LOT of wetted surface in the form of deep forefoot, long keel, and then often a wide flat afterbody. Its hard to come up with a higher wetted surface shape! Therefore, the resistence goes up rapidly, and the fuel burn also skyrockets.

Many planing boats have the transom, but a more blunt bow too, and usually a deep V instead of a long keel for the directional stability. While nowhere near as low wetted surface as possible (a semicircular cross section), its much lower wetted surface than the typical trawler hull form. Hence, at 1.4 or 1.5 x sqrt(LWL) a planing hull may be less resistance than a displacement hull.

If its not a planing hull, you really want to go about 1.2xsqrt(LWL) or lower for efficient cruising, and you really never want to exceed hull-wave speed (1.34xsqrt(LWL)). That is pretty slow unless its a pretty long boat. But potentially VERY low resistance.

To get the efficiency, you also need to size the engine properly: as small as possible. Engines that are running off their torque peak are MUCH less efficient than when they are running at torque peak. Besides people going for records, you'll never find a boat set up to run at the engine's torque peak.

[Watercolor]

This may be a little late, but here's some info to ponder. I'm not a power boat owner but have always liked good boats of any type.

1. Bayliner actually did make some very good boats. There top end larger boats (I don't know the minimum size) weren't made by Bayliner and were of high quality. My guess is that the boats you're looking at with diesels are of the better group. I'm sure someone can tell you if it's one of the better ones.

2. I know several cruisers with twin diesel planning hull boats (around 40 ft) who really like their boats. They tend to run them around 8-12 kts.

3. Twin engines are nice in a pinch.

4. I'm going to guess that there are a lot of the high end Bayliners for sale that have had little use.

5. Having said all the above, I would concur with the other comments, my choice would be to go with a diesel powered boat and a trawler design. However, if money's a concern and your cruising is on the coast and the Bahamas, the Bayliner type boat may work just fine.

[cat man do]

Originally Posted by u4ea32

To get the efficiency, you also need to size the engine properly: as small as possible. Engines that are running off their torque peak are MUCH less efficient than when they are running at torque peak. Besides people going for records, you'll never find a boat set up to run at the engine's torque peak.

I reckon mine is

1600 rpm for 158 ft/lb giving 50hp (x 2)

[u4ea32]

Originally Posted by boatpoker

My Dave Martin designed "Trendsetter 40" with two heads, two staterooms, saloon, dinette and galley will cruise at 8 knots at 1.4 imp. gallons per hr. with a top speed of 20 knots at 4.5 gallons per hour with a single 235 Volvo TAMD 60B.
Power boats do not have to be fuel guzzlers !

I can imagine that 20HP (1.4gph at an off-peak efficiency low power) would push a 40 foot boat at a bit below hull speed, or about 7 knots.

I don't believe these numbers for fuel burn at top speed. A 235 HP diesel should be burning 12 gph, not 4.5 gph: 20 HP per gallon per hour is VERY GOOD for a diesel engine, but 60 hp/g/h is simply impossible, that cannot happen.

However, even at 12 gph, that's a very efficient 20 knots in a 40 footer!!