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From: Bob Dobbs (no email)
Date: Sun May 15 2005 - 11:39:00 EDT
Here is a copy of an article that I found a while back
to answer that question ..... The rest is at the
following link
http://www.geocities.com/alberg22/motor.html
Good Luck!!! Rick
s/v Boatzilla
1967 Cal 25
5.- What propeller?
And last the propeller... which is by far the most
difficult question to answer. Although it is a
questions seldom asked which nevertheless is central
to an efficient auxiliary power particularly for
displacement hulls with low maximum hull speeds. It is
also important because most outboard motors are sold
with standard propellers that cater to the motorboat
larger market, and motor boats are usually planing
hulls capable of higher speeds.
The propeller has several parameters to be considered
and in particular 2: its diameter size and its pitch.
The pitch is the distance that a propeller with no
slippage would advance in oue full turn. These two
parameters are usually expressed in inches and as
"(diameter) x (pitch)". Thus, an 8 3/4" x 7" propeller
would have a diameter of 8 3/4 inches and a pitch of 7
inches.
The diameter is determined by the hp of the motor and
the need to minimize drag particularly in sailboats
when operating on sail.
The pitch, on the other hand, is a function of the
maximum hull speed of the boat and the recommended
operating rpm of the engine at maximum throttle. A
pitch below optimal will cause the motor to deliver
less than its maximum power at maximum throttle while
a pitch above optimal will cause the motor to operate
below its intended rpm which will fowl sparkplugs and
fail to charge the batteries [1]. It will also cause
the boat to accelerate to maximum speed even at low
throttle. This makes difficult the maneuvering of the
boat at low speeds, which can only be attained by
bursts of power at low throttle interspersed between
intervals in neutral.
Ideally, at maximum throttle the boat should reach
maximum speed and the pitch * rmp (prop) should be
covering the same distance as the hull speed - or a
fraction above it due to slippage of the propeller.
Here are the calculations for an Alberg 22 and a Honda
BF75 7.5 hp:
HS = 1.34 x sqrt waterline = 1.34 x 4 = 5.36 knots
1 nautical mile = 72,913.39 inches
Distance covered at HS in 1 hour = 72,913 x 5.36 =
39,0814 in
Distance covered at HS in 1 sec = dist in 1 hr / 3600
= 108.6 in
For a Honda BF75 4-stroke the full throttle range is
of 4,800 - 5,200
rmp. with a gear ratio 2.08:1 :
motor RPS = 5000/60 = 100 rps (motor)
For normal cruising 2/3 throttle is recommended.
Hence, the motor RPS at normal crusing will be ~ 75
rps (motor)
prop rps = 75/2.08 = 36 rps (prop)
optimum pitch without slippage = 108.56/36 = 3.0 in.
Even assumming 50% slippage:
optimum pitch assumming 50% slippage = 3.0 /(50/100) =
6 in.
The BF75 was fitted with a standard propeller 9 1/2" x
8 5/8", a pitch more than 2 5/8" larger than the
optimal pitch assumming 50 % slippage.
The standard propeller in the BF75 would tend to
overpower the Alberg 22 at low speeds as it would tend
to accelerate the boat at full speed even ad a
fraction of the throttle. A propeller of shorter pitch
would be advisable.
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