Propulsion and Steering

Greetings from Venice.

I'm a small boatbuilder and I often design my own boats, but I've just started a project which is larger than any I have ever undertaken and would like a little advice. I'm designing a house cruiser that I intend to live on some day and use for cruising on a large river.

The LOA is 20 meters and beam is 5 meters. I've given her a flat bottom to make her as stable as possible, but she does have a double chine, giving her some grace as she parts the water. She also has a very nice rounded stern, so the drag should be minimal. The draft, not considering deadwood, rudder or anything you will hopefully advise me on, is 40 cm. There is almost no rocker. The LWL is 18.65 meters and whetted surface is 78 square meters. I'm hoping for a cruising speed of 7 or 8 nauts and top speed of 10 nauts.

What I would like your advice on is the propulsion and steering of this craft. Since I build mostly boats for oar or sail and have never built a boat with an inboard motor, I have a lot to learn. However, I have been surrounded by boats for much of my life and so I do have a general idea about what is needed. Naturally, I am thinking of a diesel with a shaft, a prop and a rudder, to keep things relatively simple. I suppose the thing to start with is the prop, since that will determine the placement of the deadwood and the size of the motor.

Being such a long flat bottom with no keel (unless someone can convince me that there is an advantage to attaching one) the she will be rather slippery on the curves, so I was thinking of putting either a bow thruster or a bow rudder in. I like slippery, as it can be advantageous once you learn to handle the craft, but I think that a bow rudder would be the easiest way to keep her under control in all situations (including loss of power). But what do I know?

I'm also thinking that a nozzle might be a good thing to put around the prop, since I'm not going for high speeds, but I'm concerned that my stopping power might not be adequate. I also think that it might be really easy to screw up on the design of the nozzle and make things worse. It is kind of low budget, so I really don't want to take that much of a risk. If it will mean that I can save a significant amount by using a smaller engine and less fuel, than I'd be really happy to try.

So, the things that I need to know are: prop diameter, pitch, number of blades, rake, material, etc.; engine specifications; shaft inclination; rudder size and location of rotational axis; bow rudder/thruster information. Please tell me anything that I may have forgotten to ask and if you have an idea which is completely different from mine, I'd be happy to consider that too.

If you give me really useful information, then I'll take you for a cruise.

Grazie mille,
Thom

 

Welcome aboard, Thom!
A single inboard diesel, on a boat like this, will want a large-diameter, shallow-pitch prop. Your draught will get a lot deeper if you go this route- it's probably the best option for overall propulsive efficiency, but will make river navigating difficult. Single inboards aren't known for great handling (yes, some are OK, but twins are nicer in tight situations).
On a boat as long, wide and flat as this, I'd be looking at twin diesel inboards. Yes, that's a bit more money up front. But it'll be more manoeuverable, and will draw less water because of the smaller props.
A bow rudder is an option; I think I'd prefer a thruster though. With twins you might not need either.

 

Dear Matt,

Thanks for the warm welcome!

Remember that this is a house cruiser. Mostly it will be a house and sometimes it will be used to move around on 2 or 3 day trips. The river I am thinking of is in South America and very big, so I'm not so concerned about the draught.

Since I'm not trying to move very fast, I don't think that I will need a lot of power, so twin screw I think would be going a bit overboard. It is true that it is more maneuverable, but when I compare the cost of a bow thruster with an extra engine and the fuel that it will consume, it seems a bit lavish to me. Usually twin screws are used on boats that need a lot of power. I'm thinking that 50-60hp will be sufficient (and I'm hoping that someone can tell me if I'm in the ball park or not) so it seems that two 30 hp engines would be a little silly. If we were talking about hundreds of hp, than it would be different.

I just was looking on the internet and came across a really cool diesel-electric sail drive system. I think that a shaft that conects the engine directly to the prop is the most efficient solution, however, I was thinking that one could mount two of these electric saildrives up to one diesel generator..... Probably not the best solution for me.

The advantage of the bow thruster is that I don't have to be moving forward in order for it to work, and I can just spin on my tail if I want. If I were to put a fin towards the middle of the bottom, I could even spin on my axis, which is pretty nifty.

Thanks for your input!
-Thom

 

Au contraire. This could work pretty well for you. Could you post a link to the drive you're talking about? Diesel-electric can be a very efficient option for a boat with a high hotel load (that is, lights, appliances, everything that isn't propelling the boat). Saildrives that spin on their pods (azipods, they're sometimes called, when they're seen on bigger ships) give amazing agility. If you don't move often or fast, you'll find your engine serves mainly to charge batteries for running your gear. Put in enough batteries for a week at anchor, and an inverter and charger, and your main engine will top them off whenever you move.

 

No problem:
ohttp://www.fischerpanda.de/products/eng/182
They offer up to 75kw, which must be something like 90hp. I agree that it is a good idea in theory, but I am curious to know about cost: purchase price, operational costs, and maintenance. The nice thing about a Volvo Penta is that you can get parts if you need them, which you probably won't. I was thinking of putting some solar panels on the roof for when I'm not plugged in, dockside. Getting electric from the grid is usually cheaper and more environmentally friendly than generating it yourself (lots of hydro-electric down in those parts) and I'm still convinced that a shaft delivers the most efficient power from the engine to the prop. There is a lot of heat in these motors and even if you run double wiring there is still a lot of waste. I have a little experience with electric boats. I like them very much in theory, but I'm not convinced that they are very efficient.

-Thom

 

Here's a screen shot of the house boat so far.

-Thom

 

I like the diesel/ electric pods idea it would be great for control and efficiency. Also consider no motor at all on the house and use a tender to move it from place to place.

Gary

 

Do you mean like a tug boat or an outboard skiff? What's the advantage to operating two crafts in stead of just one?

-Thom

 

Just found this very informative article which answered a lot of my questions regarding Diesel Electric vs Direct Drive:

http://www.yachtingmagazine.com/yachting/electronics/article/0,24579,1081484,00.html
According to this article, the overall cost is comparable to a twin screw direct-drive installation. As I've mentioned, I think that is a bit more than I need or will be able to spend.

My experience with electric boats was a small pleasure craft powered by a MinnKota submersible motor running on rechargeable batteries that got charged dockside. There were many advantages and many disadvantages. I think that at that level, the technology is no where near as efficient as with large generators and powerful electric motors.

The problem is that I don't know how much power I need to move this craft at the speed I am hoping for. So the first thing I need to know is how many hp or kw will be needed to push this boat at 10 knots. From there I can understand what type of propulsion and power-train to use.

One other possibility is to use a hybrid power-train: a shaft driven prop connected to an electric motor AND a diesel engine that can be disengaged. I could even build the boat with just a direct drive installation and leave space for the electric motor to be inserted at a later date. The advantages of this would be fuel efficiency at cruising speeds and silence at trawling speeds running off the batteries which are topped off while cruising, at dockside, or by the solar panels.

I recognize that having a duel azipod arrangement would be more maneuverable, but I still feel that a bow thruster and a single prop is every bit as maneuverable on a flat-bottomed boat. Twin screw is a waste of energy because you use have the two props working against each other in order to make a tight turn. It might be cool, but it is an unnecessary waste of energy, and the same result can be obtained more efficiently. It is like two strong men who lift a long plank from the same end: one pushing up close to the end and one pushing down at the very end. Two weak men could do the same work by simply lifting at each end of the plank and use much less energy.

-Thom

 

Another good article:

http://www.ossapowerlite.com/tech_library/fuel_efficiency/fuel_efficiency.htm