Help me pick a lower unit!

[echoes]

Hey everyone, this is my first post here. I'm very new to boat design, and I'm in a group (of 10 senior mechanical engineering students) designing a solar powered boat. The boat will be a catamaran, around 12 feet long and 5 feet wide, with a single person onboard.

We have to compete in a short distance sprint, as well as a two hour long endurance race. The propeller guy tells me he wants to use a propeller with an 8" diameter for the sprint, and a propeller 12-14" in diameter for the endurance race.

We considered buying a totally integrated outboard motor, such as this Briggs and Stratton unit, but it takes only one propeller size. We don't have enough money to buy a different outboard for each event.

We are going to use the Briggs and Stratton Etek electric motor used in that outboard, but we need to find a lower unit that can either accomodate both propeller sizes, or could be easily modified to do so.

We might build our own lower unit, as discussed here, but that'd be much more labor intensive and we may not have the time. Still, if there are strong advantages to that, we wouldn't rule it out.

If anyone has any advice or could point me in the right direction, I'd really appreciate it. Thanks.

[marshmat]

Does it have to be an outboard engine? How much thought have you given to a straight-shaft inboard arrangement? You'd want to have some sort of fairing around the shaft of course, and then you need a rudder too, but you'd get your prop flexibility and probably less drivetrain losses than with the outboard.
What model solar cells are you intending to use? Stock sailboat cells, or are you going for something like an SP A300 or (gasp!) Emcore? With A300s you would have perhaps 800-900 watts or so to play with on a sunny day if the whole deck is covered, so about 1.1 hp. Perhaps 20% more than that if you want to spend $40k on GaAs cells.
Surely some of the more experienced mariners here will weigh in....

[echoes]

Thanks for responding.

I should have mentioned in my first post that the rules specify that solar cells are limited to a rating of 480W, so we have very little power to work with. We're probably going to get panels from BP Solar. Our total budget is around $9,000. If I remember correctly, we'll get those 480W for around $2,100.

During the sprint, you don't even have the solar panels onboard, just the batteries. Since we have a low budget, we're only using a single motor, and we don't expect to be competitive in the sprint. That's fine- this is the first year we're even entering the competition and we really want to focus our resources on the endurance race instead.

Here's a picture taken from Inventor showing the rough shape of the hull with the solar panels installed at the front of the boat.

What about mounting the motor underneath the solar panels and having a straight shaft going down at an angle into the water? So the drive shaft would be rather long... and then the propeller would be at a small angle. I know very little about propellers so I don't know if that's acceptable, but right away it seems much less efficient to me. If the propeller has to be horizontal, could that angled drive shaft connect to a constant velocity universal joint (which would be submerged), in order to make the propeller horizontal? I feel like having such a long drive shaft might cause problems with vibrations, although the electric motor itself should generate little vibration.

I looked awhile ago and while I found "marine universal joints", none were designed to be submerged.

I think the easiest solution may be to place the motor so that the drive shaft faces down and connects to the drive shaft of a lower unit. But I need a lower unit that can accomodate both propeller sizes as mentioned in the first post.

[gonzo]

If you want to compete, the Briggs &Stratton is a poor choice. They don't make super efficient engines. What voltage do you plan on using? Or is it directly fed from the cells?

[Robjl]

I have some experience with the type of project you are into..
Can you post all the rules.
Are there limits on battery power/capacity?
Is there a voltage limit?
Is there a motor limit?
Do you have to carry all batteries all the time?
How long/far are the sprint endurance components?
In many of these events you are really designing for two separate events.
In the sprint you want massive battery capacity that you will discharge quickly to a maximum power motor (usually high efficiency brushless DC)
In the endurance you will aim to use all the power the PV (photo voltaic) cells put out plus you will aim to drain any batts you have on board in the time of the event...the motor/PV cells/batteries should all match voltage (simpler that way).
You will probably be better off using a large prop....
efficiency is the winner with plenty of trials in prep for the races. You need to know exactly how to get the most out of your unit.
In my experience many entrants will have assembled their car/boat the night before... they always come second.
Cheers
Rob