Hybrid sailboat

[prp25brad]

I have a hairbrain idea. I want to ask the idea here first before going to meet with someone face to face and maybe get laughed at. If it does seem possible I will go to a professional to discuss all the details. With that said. I'd like to get a 55-60' catamaran with 200hp engines. Simple enough. Here is the twist. I would also like to install smaller, prefereably, A/C electric motors 20hp. This creates many problems, but lets stick to only prop sizing and ratio problems for the moment. Maybe later we can discuss the numerous other problems that are obvious too. So why use electric motors? So I can recharge batteries while sailing and maybe charge batteries with solar sails (which again is another problem but lets stick to props and ratios.) So here are my issues related to propulsion:

1) Adding another motor will cause an extra 2 props (its a catamaran). This is obviously undesirable. So can I attach the motor to the inboard 200hp shaft? Assume I only want maybe 5 knots possibly 6 knots from my 20hp motors and knowing that the torque from a electric motor is constant throughout its entire rpm range (max rpm 1800 rpm), is it not possible to achieve 5 knots or so? I also understand there are more issues, but assuming that 2-20hp will could give me 5 knots or so with the proper prop, will those same motors potential give me 5 knots if attached to a prop for designed and sized for a 200hp motor? So to say it another way...Is there a combination of prop size and pitch that would give me something close to top speed for 200hp motor and a desired speed for 20hp?

2) Going along with the above, if I had a transmission on the 200hp motors, turining the shaft with them attached is unwanted and detremental to the transmission. (Also I do not need reverse either because the electric motors will be used for that) So I propose to "direct connect" the drive to the crankshaft with a PTO or some sort of disconnect when using the electric motors to keep the electric motors from turning the diesel motors. Again lets not worry about he specifics of how it is attached, but if I'm using a Yanmar that is running 3600 rpm and "direct connect" would give me a 1:1 ratio, I think this is unwanted. Or is it? Most sailboats have a 2:1 ration but I think this is for efficiency. Well, on my boat I would be using the smaller 20hp motors for effiecency and the 200hp for getting from point A to B fast and not as concerned about efficiency. So assuming I just want a decent speed out of the 200hp motors, is a smaller prop turning at 3600 rpm too fast?

I hope I've over explained my situation. It's an odd situation. Also, I know that there is prop theory and what not and other issues such as boat size and wieght. My main thrust is to find out if I should even talk to an engineer about the idea or if it is so hairbrain, just to leave it at the drawing board.

Thanks,

Brad

[prp25brad]

After reading other threads I'd like to point to other issues as to why this this option is something of interest for me.

1) I would likely be using a diesel generator to produce enough power for the electric motors when the batteries are not enough (which is basically all the time because battery technology........well sucks.)

a) Now factoring in diesel genset, I've lost 10% or more. BUT, here is the kicker. I would be using solar and wind generators to supplement the power. Of course those have costs too. Overall, would those offset the cost of just running a diesel engine? Maybe, most likely not, but I value costs to the environment and other costs. I also am looking to the future when I expect fuel costs to begin to rise MUCH higher than they are now.

2) I also like the idea of "quietly" motor sailing. Every sailing log I've read complains of motor sailing because of the sound and vibration. I imagine a genset providing power in a "sound proof" enclosure making it more comfortable. In the real world, I know you would be able to still hear and feel the genset, but in realiaty it SHOULD be quieter and with less vibration. Also in reality, it should be more efficient than an engine hooked up with a transmission. I read a post that talks about AC gensets being more inefficient. I disagree. If you look at the torque curve a diesel engine most of the power is achieved around 1800 rpm and that is where most engines are the most efficient. Factor in the inefficencies of the transmission and high speed of the engine with that of the genset and they nearly even out. (One might be 2% more effcient...that's peanuts) That is another post for another day but I had to get on my soap box atleast once.

So take these factors and those that follow in deciding on your post.

I expect to more "sailing" more than the average sailer. I read logs of people describing long 30 day passages and pissing and moning about them. To me, spending long days sailing with no one around is a blessing. Once you arrive at shore at you next destination your excited to see land and other people. I also plan on doing artic sailing as well. I know wind in these regions blow longer and harder. This leads to my other result I failed to mention. I also plain on using a heater and A/C (only in the master bert) when needed. Taking all this together, I expect my power consumption to be in the area of 4 times or more than the average sailor. So I need a system that could give me a substantial power replenisment when sailing. (and this is accomplished by this type of system)

The reason I post this supplement to what I posted earlier is to show why I would go through all this trouble, and it is a lot of trouble. In the end it would be more economical to design a boat with two 20hp and two 200hp? Or, make a compromise such as only two 100hp total? To answer the question, would it be more economical to design one of the two above? The answer is always yes. In fact the option I explain is almost alway twice as expensive, but in the end I think there are advantages. I think my potential circumnavigation will be 60,000 miles instead of the usual 30,000 miles. So basically the extra time, milage, and money MAY be worth it to me in the end, but many issues have to be worked out and the ones I posted above are the first I'd like to discuss.

[mackid068]

Sounds interesting. Post some drawings and prelim specs please! I like the solar/wind idea. Very nice idea.