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The 200 rpm per inch rule...
by
Steve Horsfall
We purchased
a 1986 Chaparrel 28 foot. When we bought her, the prop was a 19-inch
pitch. For one entire summer we had a very hard time to get her
to plane off.
This summer, after asking several people for
advice, we were told to try an 18-inch pitch stainless steel propeller.
Well, a very expensive mistake! There was no difference between
the two props. Late in August we tried and brought a 15-inch pitch
prop. She planes off a whole lot faster. But we still never really
got our questions answered. So finally here are my questions.
Would she plane off even better and still have
good gas mileage with a smaller pitch prop? Is a five blade prop
better than a four blade?
V. LeBlanc Via E-mail
Generally speaking a smaller pitch propeller will
always get the boat on plane faster, however, you must be careful
not to go too small. The trick is to select a propeller that will
keep the engine within the manufacturer’s recommended rev. range
for the specific engine (usually around 4000 to 4500 rpm.
Each inch decrease in pitch, is good for about
a 200 rpm increase in engine speed. You should take the boat out
with a normal load of people and gear and run the engine up to wide
open throttle (being careful not to exceed your engine’s recommended
maximum rpm). (Top)
If it turns, say 4500 rpm, at wide open throttle,
you have the best propeller for the boat. If it turns less than
4500 but more than 4000, I would leave it the way it is. If you
need to bring the rpm’s up, you go down in pitch, keeping in mind
the 200 rpm per inch rule. If it turns more than 4500, you go up
in pitch following the same guide lines.
If the boat is still slow to plane off when you
have the right propeller, I would recommend the installation of
hydraulically adjustable transom mounted trim tabs. These have the
added advantage of allowing you to level the boat side to side while
underway if the load is unevenly placed within the boat. As far
as the 4 or 5 blade propeller is concerned, it is more a function
of smoothness than efficiency in most cases. For your boat I would
stick with a three bladed aluminum propeller.
One final note, if you find the optimum size is
around 16-inch pitch for your boat you can have a propeller repair
station re-pitch your expensive stainless prop by up to 2 inches
in pitch, either way. Personally, I like the aluminum prop on larger
boats, as it tends to transmit less shock to the drive line if it
hits a submerged object and is cheaper to repair or replace. (Top)
A real head-ache...
I am writing with regard to Evinrude oil injection motors. In
the summer of 1997, I bought a 25 hp Evinrude motor and a Princecraft
boat and the head blew on the first tank of gas. Parts delivery
was delayed for the repairs, so the dealer gave us a loaner boat
and motor.
We liked the loaner better than our boat so we
bought the brand new loaner. It was a Princecraft 162 with a 35
hp Evinrude with oil injection. The head on this engine blew on
July of this year (2000). The bill to repair the engine, which was
no longer under warranty, was expensive, but the factory covered
about half the cost.
The units were equipped with an Engine System
Check monitor, but in both cases, the monitor did not alert me to
a problem before the head blew. I am wondering if there is a problem
with these engines and if they should have been recalled to correct
the problems?
The factory and the dealer also recommended adding
a bit of oil directly to the gas as a good insurance measure. What
do you think? (Top)
S. Forbes Bristol, Que.
If the cause of your powerhead failure is that the
oil unit was empty, but, the alarm did not tell you that it was,
I would think that it would be prudent to check the oil more often
and not wait for the alarm to tell you it is low. However, I feel
that if you write OMC Service Dept. with the same information that
you sent to me, they would consider some kind of customer relations
adjustment. The fact that they paid half the cost of repairs is
pretty generous, on their part, as the engine was three years old
at the time of the problem and you are basically getting a brand
new unit with no hours on it.
If the cause of the failure is lack of proper maintenance,
such as lack of proper winterizing or dewinterizing, running the
engine out of the water of water inlets plugged with weeds. I would
be less enthusiastic about your chances to recover any of the costs.
I think adding oil to the gas is a good added insurance
policy also. (Top)
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