Re: [AtkinBoats] Digest Number 413
- Hey Curtis,
Yeah, you do need some skills to put together Robb's transmission. Kenneth (I think it was Kenneth) made some suggestions for simplifying, but as I see it you wouldn't get a reverse gear that way. Anyway, it is undoubtedly true that a "real" transmission will be good for resale.
But your math is off. 1.5:1 at 3600 rpm gives you 2400. Atkin specifies the prop for 2000 rpm and Robb did his at 2700 rpm. I'm pretty sure you could easily make it work with the 1.5:1 transmission.
Not being mechanically adept, and unable to find a local machinist who
can get enthused about this, I am just about to spring $1,100 for a
marine transmission - but the lowest (highest?) gear ratio available
seems to be 1,5:1 (ZF 10-M). This will result in a relatively slow,
high-pitch prop, and possible cavitation. ( Robb used a 1.2:1, and a
10x8 prop. )
Yahoo! Messenger with Voice. PC-to-Phone calls for ridiculously low rates.
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- On Mon, 29 May 2006 17:43:03 -0700 (PDT), Rob Rohde-Szudy wrote:
> Yeah, you do need some skills to put together Robb's transmission.You're right Rob, there is no reverse gear in my suggestion. But reverse could be added with a little ingenuity and without the expense of a transmission ...
> Kenneth (I think it was Kenneth) made some suggestions for
> simplifying, but as I see it you wouldn't get a reverse gear that way.
I designed a small "utility car" years ago to be powered by an air-cooled electric start 16 HP industrial engine with a charging circuit for the starter battery. The car was driven forward by a CVT system which was nothing more than a belt-drive torque convertor taken from a go-kart or a snowmobile ... but this cheap yet effective forward drive system did not provide reverse.
So I used the starter battery to power an automotive starter motor (because it has a throw-out bearing) for reverse. It was a simple system that avoided an expensive geared transmission, yet it gave me reverse whenever I needed it simply by pressing and holding down a momentary-type electrical switch.
The reason I mention this is because reverse is not needed any more often in a boat than it is in a car, so maybe the same type of system could be used in a boat? The problem of course is that unless you have a 12v charging circuit on your engine, you won't have 12v available on the boat like I did on my utility car. But there's another way to get reverse without the 12v power, just use a different drive system:
Put a flat aluminum disc on the engine shaft, then put a rubber wheel on the prop shaft. Install the engine in the transverse position on a height-adjustable platform. By varying the engine height and speed so the rubber wheel contacts the aluminum disc at different distances from its center, you have a CVT system that provides the full range of speeds -- in both forward and reverse.
I want to do more research into a system like this because I think Toro uses it successfully on some of their lawn tractors and they have a reputation for quality at reasonable prices. But once again it takes some ingenuity to build such a system, thus is is still faster and easier to just "spend the money" for an off-the-shelf transmission.