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 Looking
For The Right Prop
by
Steve Horsfall
I have a 1989 20-foot Baretta sport
boat with a 260 horsepower motor with Alpha One drive with a 21p
stainless prop, and thru-prop exhaust. The boat will run 57 mph
at 4,500 to 4,600 RPM and I'm trying for 60 mph. I was told that
a Bravo Three exhaust tube would help when running at wide open
throttle so the exhaust bypasses the prop when trimmed up for speed
(a couple of mph) because of less exhaust restriction.
I am having a bit of a problem getting the proper
part number. Could you also tell me if a 2 mph increase from this
tube is possible? I am told my boat should run 60 to 62 mph the
way it is, so I'm wondering if the prop is outdated (Laser #48-11070-A40-21p)
or is the Mirage Plus 21p a better prop to run on this application.
What is the maximum diameter prop I can run on this boat, and what
would the effect be?
On another topic, on the bottom of the hull
I noticed that, on one or two of the ribs, there is about a foot
or two that has been scraped on something hard because it has removed
some gelcoat and now shows black fibreglass. It doesn't look too
bad, but I'm told that the hull will eventually start to soak up
water, increasing its weight.
Is this true, even if the boat is not left
in the water? And do you know of a reputable repair facility in
the Oakville area?
K. Mellis( via E-mail)
Part number for the exhaust tube is 78458A1. I
doubt if it will make enough of a difference to reach 60 mph, but
it is the cheapest thing you could try. I would bet a real good
wax job on the hull each time before the boat is launched would
get you over the 60 mph hump faster than anything.
My experience is that the Laser prop is pretty
good. RPM being what they are, you are in the right ball park, prop-wise.
I would suggest you check out www.marinasontario.com
for an Ontario Marina Operator Association member marina in your
area to do the fiberglass repair. The boat will soak up water only
when it is in the water, but will be slow to dry out when removed
from the lake. Best to keep all gelcoat marks repaired to be sure.
RPM and propeller size
We own and operate a houseboat rental business on the Trent-Severn
Waterway. Our houseboats are 42 feet long and weigh about six tons.
We are running 140 hp MerCruisers with Alpha One drives and 1:98:1
gear ratios.
The mechanic who rebuilds our engines tells
us we are not getting enough RPM. He would like to see a maximum
of 4,400 to 4,600. Our current maximum is 4,200 RPM with a 16 x
11 prop and less with 16 x 13 prop.
Although we do not recommend our customers
exceed 3,000 RPM, our main concern is that the higher RPM at wide
open throttle would be better for the engines even at 3,000 RPM
(i.e.: engine would not be working so hard and would last longer
between overhauls).
We would like your opinion on our mechanic's
advice and your recommendation on a good size propeller for us to
try.
Happy Day Houseboats (via E-mail)
It is always best for your engines if they are
able to obtain the manufacturer's maximum recommended RPM, even
if you never drive them at this speed. When the propeller is too
large, it is the equivalent of driving a truck in too high a gear.
The engine is lugging or working too hard.
You can figure that a one-inch pitch reduction
will increase top engine speed by about 200 RPM, therefore, I think
a 16 x 10 or a 16 x 9 would be about right. If you can be sure that
your customers will not disobey your instructions (re: 3,000 RPM
limit), I would go with the 16 x 9. Mercury used to make a 16 x
9 propeller (part number 48-36024 a4), but I don't know if it is
still available. A check with your dealer's parts history price
book will let you know if it is and/or if the number has been changed
to a new part number.
Otherwise, you could have your existing stock
of 16 x 11 propellers re-pitched to 16 x 9 or 16 x 10, if you like.
A propeller repair station, such as Kawartha Propeller Repair or
Bayview Propeller can handle this for you. The reason I am not suggesting
you get the 16 x 13 props re-pitched is because it is not recommended
changing the pitch of an existing propeller more than two inches.
Premature blade breakage is the usual result.
Also note that changing the diameter of the propeller
has a similar effect on the RPM (i.e. smaller diameter equals higher
RPM), but I am not recommending smaller diameter because on slow
moving, heavy boats, such as yours, larger diameter propellers usually
provide better gear shift response for maneuvering, especially reverse
thrust for stopping. Enlarging the drive cavitation plate with bolt
on plate extensions also helps these boats by reducing cavitation
while cruising and when running in reverse.
Gelcoat crack
I own a 1996 Maxum 17-foot runabout with a 1996 continued from
page 20 Mercury 135 hp V6 outboard. As I boat in cottage country,
this is a wonderful boat. My problem is that the cottage is 12 hours
away, therefore, this boat has seen an incredible amount of trailer
miles over the past five seasons.
I have two problems that concern me. The boat
has started to develop a small hairline crack about four inches
long in the starboard corner inside the splash well. It is a very
fine crack and hard to see. The boat also has two vertical lines
on the outside of the transom where the splash well drain holes
have been installed by the factory. These lines are just dimples
that can be seen in the right light at the right angle. There are
no cracks at all in this area and the lines are in the identical
position from the engine on either side. The inside crack does not
extend to the edge of the well and did not get any wider or longer
during my last two trips to the cottage.
I need to know if this condition is somewhat
typical of a boat of this type or am I developing a problem which
should be of some concern? I have always trailered the boat using
only the factory support lever on the engine. Should I be using
a transom saver bracket from the trailer to my engine lower unit?
My dealer feels this problem is normal for these boats and other
than fixing the gelcoat crack, I shouldn't worry. What do you think?
My second problem concerns the engine. It produces
an incredible amount of power and runs quietly and smooth compared
to what I have seen in other brands, but it can be very difficult
to start when I first arrive at the lake. Once it has been warmed
up, it runs flawlessly. I tried everything and it just seems to
be a hit and miss problem for that day. I have taken it to two different
dealers and they say they can find nothing wrong.
I have changed the gaskets between the air
box and the carbs twice a summer because gas evaporates during trailering
and the oil drips past and into the splash well. On two occasions,
I have found a puddle of gas in the air box. Is this from over-squeezing
the primer bulb or over-choking before start-up? I am a little confused.
Any help you can give me would be appreciated.
B. Holland (via E-mail)
Firstly, tilting the engine all the way to the
top, at any time, but, especially when trailering, is definitely
not a good idea. It is probably the root cause of all your problems.
When it is tilted this high, the angle of the carbs is such, that
gas drips from the fuel bowl vent hole and into the air box. It
is more than the air box can hold, so it drips into the motor well
of the boat. The gas in the motor well evaporates, leaving the oil
behind. When you arrive at the lake, the carbs are empty. If you
do not completely prime the engine with the primer bulb (squeeze
again and again until it feels firm), the engine will have no gas
to run on and the choke will do nothing to help this. Therefore
the engine will be very difficult to start.
Some gas is expelled into the air box under normal
usage when the choke is applied. There is a small drain tube that
is supposed to recycle this once the engine starts. It cannot handle
the volume of fuel that collects in the air box when the engine
is fully tilted. The only time the engine should be fully raised
is to change propellers or to work on the lower unit or to clear
weeds from the drive.
The use of a transom saver bracket is a good idea,
just make sure the engine is no higher than it needs to be, i.e.
skeg does not need to be higher than the trailer axle. The name
"transom saver" is a bit of a misnomer as it really does not save
the transom. When you think of the enormous amount of stress placed
on the transom when the engine is pushing at full thrust, the weight
of a few hundred pounds of engine bouncing on the transom when trailering
seems pretty insignificant. However, when the engine is fully tilted
up, a good portion of the weight of the motor is now teetering on
the other side of the transom and a seesaw motion is set up. The
probably accounts for the stress cracks in the gelcoat.
I would not be overly concerned about the gelcoat
cracking unless it occurred where the engine fastens to the transom
or where the motor well and transom and deck meet. I would suggest
you repair the gelcoat after installing the engine support bar and
if the problem with the gelcoat returns, I would investigate it
further. Otherwise, enjoy your boat!
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