BoatBanter.com - View Single Post

Richard Casady

On Fri, 13 Feb 2009 13:24:49 -0500, wrote:

On Fri, 13 Feb 2009 18:17:35 GMT, (Richard
Casady) wrote:

On Fri, 13 Feb 2009 07:29:13 -0800 (PST), wrote:

On Feb 13, 9:43*am, HK wrote:
Richard Casady wrote:
On Thu, 12 Feb 2009 20:46:13 -0500, D K
wrote:

The one thing I know for sure is the F90 is best
suited for a three-blade prop rather than a four.

Three is always more efficient than four. Four blades is a way to get
the blade area in a smaller diameter, if you lack space.

Casady

My admittedly limited experience with Yamaha taught me that the company
is pretty damned good matching up engines, props and boats, and that a
good dealer is also a valuable resource in prop selection. When I took
delivery of my 2008 model boat, the boat factory had just changed the
prop it spec'd for the boat and engine combo, upon a suggestion from
Yamaha. The factory sent me copies of performance print-outs for several
props. I was able to almost exactly duplicate the performance on the
factory print-out.

Almost every outboard boat I considered has performance sheets available
* from the engine manufacturer, and those sheets specify props.

Really? How did they know your exact requirements as far as weight,
loading fore and aft, etc.?

They give you one in the middle of the range of props that are
individually ideal for one of the varying conditions. My sprint car
has a quick change rear axle. You can change the gear ratio in a
couple of minutes. There are three brands around, and they all have a
magnesium case. Stuff is brittle, but there are no impact loads. They
have variable pitch props for ships and planes.

Variable pitch props and adjustable pitch props are two different
things.

What does that have to do with anything? You ever see a stern drive
trailer boat? You can change props in five minutes. It's only trouble
when they have been on the motor for forty years, like the neighbors
at the lake. The guy to the east has a pull start 100 hp Merc. He's
big, a auto mechanic, and he pulls it easy. One of those inline six
"tower of power" motors. They ones to the West has a plywood homebuilt
with a 35 hp Johnston, that dates to when that was the biggest motor.
Merc came out with a forty and the hp race was on. The auto makers
were having a hp race at that time. The fifties when everything had to
be bigger and better. I remember when about one boat in fifty had a
battery. Three inboards, one of which was the cop, on the biggest lake
in Iowa. Most motors were 5 1/2, 6, or 7 1/2. I remember when all the
outboards were too small to pull a skier well. The biggest was a 25,
and Kirk weighed 200. All the fifties and sixties boats and motors are
still around. Motors don't wear out 14 weekends a year. You can fish
all day every day and run the motor less than 3 hours a week. I may
put two good sized trolling motors on my boat, with two tillers and a
tie bar like sailing scows, and never run the engine at all. 5 mph is
a good speed for a cocktail cruise. Charge a couple of golf cart
batteries at the dock. They make a 2 hp electric outboard, 48 volt,
for three grand. Electric outboards start a 135 bucks. Must be
something suitable. One hp is about 60 amps at 12 volts.

Adjustable is passe for planes. Quite a few ships and most planes have
props not only variable, but governed to a constant RPM

On an trailered outboard or sterndrive changing pitch with a wrench in
a couple of minutes is all you need. That's all it takes to change a
prop. Maybe ten if you have help. Someone came out with a two speed
gearbox for inboards. Have your holeshot and top end both.

Casady