Hull dynamics/theory question

[ivylikes]

Our first motorboat is a Landau 20 WA with which we've had a very enjoyable first season messing about.
This has a modified cathedral hull (actually called a sidewall trimaran).
It has a 2003 Honda BF75 (carburetors) in great nick and my query is whether the power/consumption figures are radically different from a regular semi/planing hull?
I seem to be getting about 1nm per litre/4 to 5nm per gallon, at all speeds.
For example, a 25-mile trip at 18 knots in smooth water used the same fuel as the return journey with a bit of a beam chop at 7 knots.
Acceleration seems linear,with a slight hump at about 4,000rpm.
Cruising at 2,000rpm gives 7 knots. 3,000rpm gives about 11 knots, 4,500rpm gives about 18 knots, but they all seem to have roughly the same fuel consumption...
It seems better keeping the outboard trimmed in and trying to keep the hull level - is this to do with the lift dynamics of a cathedral hull?
I have two questions: can anyone explain what's happening and how this works, (preferably in lay terms!) and do those figures seem reasonable for a motorboat weighing about a tonne?

 

[victoraspey]

Without looking into this in too much detail and not being very familiar with the boat .... Maybe the boat is behaving like a planing catamaran. These generally have poor fuel consumption at low speeds but very efficient top end. Without a decent planing surface there can be little discernible hump. Getting the air flowing between hulls and creating lift will be helped by trimming the boat. If I am on the right lines then importance of skin friction and keeping a clean bottom is likely to have very big implications on the boats performance.

 

[boatmike]

Very interesting and I think your title has the hint to it. The resistance of a hull in full displacement mode (not planing) will be relatively low until it meets what many would call "hull speed" then it requires more power to lift it onto the plane until again, once on the plane it requires less power and you may well find that you can actually throttle down to keep it there. The actual characteristics depend on the design of the hull but generally the "worst" or least economical speed for a planing hull is at the point where it is in transition from displacement to planing. This may well explain your "hump"

 

[Jessthedog]

My Orkney 20 has an 80hp outboard and weighs the same as your boat. It has a major hump to climb over, and is sensitive to trim (one big bloke moving around can make a 2- to 3-knot speed difference when on the plane), but once up and running at about 18 knots I've worked out very approximately that it does about 4.5mpg in smoothish water and about 3.9mpg when it's lumpy.

I'm no naval architect, but if I recall the Landau correctly it's not a planing shape. Although the central hull is flat-bottomed it's narrow, so at speed you've effectively got a long thin hull stabilised by outriggers. There might be some element of lift involved (the central hull must be flat-bottomed for a reason) but as you experience no 'hump', the most efficient way to operate will probably be with the outboard trimmed right in, as you've found.

Maybe someone more qualified could chip in now...

 

[Wiggo]

From what I remember of the reviews of the Landau when it was first launched, its hull shape is a bit of a curiosity. As you have spotted, it has a pretty linear fuel consumption and that was one of the things the reviewers noted about it. ISTR it is also a very low wash hull for any given speed, too. A pretty boat, if I remember.

Back to the original questions.

1. No. How hulls work is a black art, and impossible to explain. A PhD in computational fluid dynamics might help, but possibly not.
2. I'd expect that motor to be burning about 4.5 gallons/hour at full chat, closer to 3 gph cruising. From the numbers you give, I'd estimate 'cruise' is about 4000 rpm and 15-16 knots, so your figure of 5mpg looks spot on.

 

[ivylikes]

Thank you very much for the quick responses/thoughts...