Power boat design for economy.

i don't think common rail engines would bother you, they are so reliable i doubt they would ever let you down. carry a spare ecu if you think you need one. i drive common rail engines everyday and have never had a breakdown in 2 million plus km. the fuel economy makes them worth while. who is going to service a conventional injector pump in a remote region anyway.

 

I agree.

And carry a couple spare injectors,the necessary sensors and just go.

 

Well boys you can kick that old Carnation can around as much as you want, there is no magic bullet. You want the big toy you gotta pay the bucks, thats capitolism. A 36ft. cruiser, power or sail is not a small piece of equiptment. If well designed and built it is an expensive piece of gear and more expensive by by the truck load if you are not extremely careful when tweaking it for maximum efficiency and economy. As has been pointed out fuel is just one cost and quite possibly not the most important one, when the whole cradle to grave cost package has accumulated. If one breakes the vessel down into cost stages and bases the construction choice of each on the law of diminished returns (on what you can honestly afford) the result will be no more and no less than the best you can achieve. The economy of that machine is one of scale which in reality is the only one that anyone of us as an individual can hope to sustain--Failing that, cost is not a factor

 

Cost is nearly always a factor... All the big splendid polish and varnish vessels proudly gracing the front row of the marina near where I park have NOT been out in over a year - Exhaust pipes on most of them are big enough to take a large pumpkin... so I guess at around 65litres/hour fuel burn? Two are second hand from USA...

 

I know what you mean Masalai, I've a lowly 52 year old, cedar over oak, carvel, power cruiser, 60 HP Cummings and I feel bad she's not been out more then twice this year. I've had some health stuff which has prevented my puttering around more, as she's usually out a few times a month if for nothing else, then to blow the carbon off her piston tops. She's near the "big boys" which don't ever seem to move. My box keel, double ender is very efficient and barely makes an appreciable wake, at anything less then WOT. I do love motoring past as every so often, someone is aboard those monolithic beasts, usually cleaning up after a teenager's brothel, the previous night, so I guess they have some use.

 

Um, Masalai... The point wasn't the exact outboard model, it was that you can retract props with the touch of a button at the helm. The 9.9's in the example picture are currently made and available from Yamaha with good ratio and large prop.

However. I have recently discovered we are all way too Hung up on these "high thrust" models. There is another Kurt Hughes (40 ft) that just arrived where I am building. It is powered by a single Honda 40hp outboard set up like in the pictures, but centrally mounted. It's just a regular old 40 HP not high thrust. He is quite happy with the setup and it works just fine. That surprised me after hearing I needed a pair of high thrust 60 HP motors at 300 lbs each.

You can get a lower ratio by changing the prop pitch and save a lot of weight.

 

I did mention outboards but I was thinking of some horse power in the 300 400 range into a sea going shaft.

 

I had this that I sold a while back

It's a Yanmar leg tiltable and steerable and were extended for use in a pod on Fastback Catamarans in Australia

You can just make it out here

http://yachthub.com/list/yachts-for-sale/used/sail-catamarans/fastback-43/96574

 

That's a more difficult engineering problem I gave up on myself due to lack of time. Although, 300hp outboards also have power tilt.

 

Seems we all have different goals when acquiring a new boat. I already have a boat so it's not urgent to get a new one as quickly and cheaply as possible. Fuel efficiency is important to me, and I intend to build my new boat anyway so I don't have to factor in what's available on the boat market.

I've created and to some degree optimised different hulls for up to 15 knots in full displacement mode and calculated the power needed to move them as a function of speed. All hulls are as narrow as I could make them while still having a righting arm of 200 mm at 30 deg tilt. I enclose a graph showing a 12 m / 4 tonnes catamaran, a trimaran and a box keel type hull. For the box keel hull, the box keel displaces 3 tonnes and the upper part of the hull 1 tonne.

It's clear to me that the catamaran can achieve better than 2 NM per liter at 12 knots (corresponds to 32 Hp at 12 knots). The necessary width of the catamaran is a drawback for me so I'm looking at what can be done with the box keel type hull.

Erik

 

I don't know how you calculated the graphs but I think it is unlike that in practise you will only need 5-6hp to push a 4T 12m boat at 7-8 knots. I'd double it at the very least, (unless the graph is actually per ton rather than total displacement)

At this size catamaran I've found that, very, very roughly, you need an extra 10hp for every knot of speed over about 8 knots. So the slower your top speed the more economic the boat.

As most motor boats typically only travel for 5-6 hours a day you have to decide whether arriving at your destination 30 minutes earlier is worth the extra costs involved

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Richard,
The graph is for the total 4 tonnes displacement. I've use Michlet for the calculations. Michlet has a reputation for being quite accurate for long and slender hulls at speeds where wavemaking is the primary source of resistance, but I have no practical measurements to back up the results. The 70% efficiency I have assumed may be too optimistic, especially if outboard engines with small propellers are used. Reduce the efficiency to 40% - 50% and the result will match your experience better.

The catamaran hulls I've used are double-enders with a L/B of 13:1.

Erik

 

No doubt Leo can explain your figures. But I would expect a minimum of 20hp to do 8 knots on a 12m/4T multihull in flat water and no wind

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

2 litres per nautical mile @ 15kts with twin 160hp diesels and 9 tonnes displacement. Max speed = 22kts. 9 tonnes is quoted as heavy ship, and is rather heavy because its fitted out with american cherry interior and all the other mod cons, bells and whistles and a full load of fuel. 6 tonnes should be quite achievable in a similar boat boat with bare essentials.

The efficiency could easily be improved with much lower displacement and cruising at a more moderate 12kts...

These kinds of boats are popping up everywhere, to say they dont exist is naive... do some searching and you will find...

 

Hi Richard Woods,
I have 2 x 20hp nanni saildrives with fixed 3 blade 15" x 12" pitch propellers that appear slightly under-propped and give just over 10 knots at 3000 engine rpm... CNO is 40 ft LOA and 21ft beam and weighs somewhere between 4.5tonnes and 6tonnes I would have been above 4.5 tonnes at the time of testing with about 500 litres of fuel and 200 litres of water...

- - 2 N Miles per 1 litre on one engine @3000rpm = 6knots . . . . 1.7 N Miles/litre both engines @10knots