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The
answer to your question is "no."
There, that didn't take long, did it? Okay,
but explaining why the answer is "no" is going to
take a bit longer.
In today's scientifically oriented society, we
tend to live by the notion that there's always an engineer out there
somewhere with a calculator that can figure out the answer to anything.
That's not really the case. Engineering is the result of experience;
no experience, no engineering. Create the thing, the equation that
explains it comes after.
Nobody can tell you what the right size propellers
are for your boat because there are too many variables involved.
There are hundreds of thousands of boats of all sizes and shapes,
as well as power plants. You may be thinking, "Oh, but my boat
is just an ordinary 32' so and so, much like all the others out
there."
Okay, to you it is, but not to anyone trying to
figure out which propeller will offer the best performance. Propellering,
you see, is more of an art than science. All boats are not alike,
not even two of the very same model, at least not in terms of the
possible variables that affect performance. Without writing a book
on the subject, take it on faith that every boat presents an incomprehensible
set of variables which are more or less unpredictable. You and your
neighbor may both have a Trojan F32, but it is entirely possible
that there are factors that make those two boats sufficiently different
that each will need a different propeller. Our problem is that we
do not know what those factors are, and we probably can't find out.
To obtain the technical solution to your propellering
problem, we need a formula for each and every boat. A formula that
is not available.
Engineers have been attempting to develop sure-fire
propeller size selection tables right from the very first propeller.
And right up to this day of computerization, no set of charts or
tables has ever proved completely effective. I'll give you just
one example that will illustrate the dimensions of the problem.
I was doing a survey one day and I happened to
sight down the length of the keel of a 60 foot fiberglass hull.
Something about it caught my eye, something that didn't look right.
This prompted me to take a second look, and then to get out my measuring
tape. The keel of this yacht was found to hook 8" to
the left. That's right, the hull of this boat was literally bent!
You are wondering, of course, how the heck that could happen. The
answer is very easy.
It was not the hull mold that was out of symmetry.
What happened was that the builder had pulled the hull out of the
mold before he added the internal structural components. Builders
sometimes do this to increase their mold turn around time. If he
was going to do that, the hull should have been placed into a jig
that would maintain the shape of the hull as the framing system
was added. The only way this curved hull could have happened was
because the builder did not have a jig, and when the floppy hull
was pulled out of the mold, it lost its shape. When the framing
was added, the misshapen hull then became permanent.
That was a million-dollar-plus boat. Imagine the
prospects for lesser boats.
Now, that 8" hook in the hull had an enormous
effect on boat speed, and it was no surprise that during the six
year life of this yacht, the owners (Yes, the first owner dumped
it after two years) had been constantly playing with propeller sizing
because it would not perform the way it was supposed to. But there
were no propellers that were going to take that hook out of the
hull. The point here is that there are endless factors that go into
determining performance which cannot be anticipated, and which explains
why you may not be able to simply refer to a propeller selection
chart, hit the nail on the head, and live happily ever after.
If you have a problem with inadequate performance,
when all the other boats like yours with the same props seem to
be doing fine, then chances are that there is some peculiarity about
your boat that is causing the problem. The real trick is in finding
out what it is.
The Michigan and Federal propeller companies have
computer programs that can tell you what the right prop size SHOULD
be, but that does not mean that use of that prop will give you the
performance you desire. It is one thing if you are merely attempting
to optimize performance with the gain of a knot or so. Many boat
owners are constantly doing this, playing around with prop sizing,
which can get very expensive.
If your problem is more than a knot or so, if you
are dealing with a major performance deficiency, then it is unlikely
that propellering is the nature of the problem, or that changing
props will solve it. It is more likely that you are dealing with
a major fault(s) that may lie in any one, or combinations of many,
possible factors. Not very encouraging, is it? No, and that's why
it's not a good idea to go fooling around with propellering to try
to solve a major speed problem.
Yes, we would all like easy answers, but there
aren't any, and least of all from someone who knows nothing about
your boat. Builders and designers first choose props from calculations
and charts. Rarely do they hit it right the first time. Back in
the heyday of Bertram Yachts, there wasn't a day go by that I wouldn't
see a Bertram out zooming around on Biscayne Bay at full speed.
What they were doing was constantly testing propellers and speed.
Bertram, you see, used to certify their boat speeds upon delivery.
I don't know of any other builder who does or did that. So when
you think about all those boats that are built in plants that are
nowhere near the water . . . . well . . . it's not hard to see why
there are so many performance problems.
The only way to efficiently resolve the problem
is to hire an expert. But there are not many experts around, and
it may take him a long time to find the problem, especially if it
is a combination of factors, which it usually is. One ends up going
through a process of elimination, which is very time consuming and
costly. The sad fact is that people with major performance problems
usually end up stuck with them, even after spending a lot of money
to find out why.
Some of the responses I get to this web site blast
me for being so "negative." That's true. This web site
is all about what can go wrong. They complain that I don't present
a "balanced" picture. Nope, I don't. If you want a balanced
picture, go read the boating magazines. They will make you happy.
There are all those questions and problems about poor performance
out there because there are so many builders turning out poorly
performing boats, and people paying huge sums of money for them
without checking before they buy.
If you want to know how bad it really is, consider
that there is one popular builder out there that has been turning
out essentially the same poorly performing boat for around twelve
years. The people who buy them don't know any better. They don't
know the difference between good and bad performance. Only later
do they find out when someone asked, with raised eyebrow, or suppressed
smirk, "Does your WaveBaster 39 perform alright?" Only
then do they start thinking about it.
So if your boat is suffering from serious performance
problems, the real question becomes: Why did you buy it in the first
place? The usual answer is that the owner didn't know enough to
realize that the boat had a performance deficiency in the first
place.
Most of the time when I do sea trials on poorly
performing boats, the buyer says, "I'm not really concerned
about the speed." Of course, a new or used boat being sold
is empty. When he loads it up with fuel, water and provisions, that
18 knot boat is now a 15 knot boat, and suddenly the fellow is very
concerned about speed. Albeit a little too late. So now he's going
to play with propellers, when propellers aren't the problem.
Test before you buy because a lot of builders don't.
However the boat performs on the day you pay for
it, realize that it's only going to get slower as you add weight
and the engines get tired.
Stuff grows on the bottom and propellers, slowing
it down. Clean bottoms are the exception, dirty bottoms the norm.
How will it perform with a dirty bottom if performance is so-so
with a clean bottom and empty tanks?
If the performance is marginal on day one, it will
be unacceptable on day 301.
The best solution is not buy into the problem in
the first place.
After the fact resolutions usually fail, and at
great cost whether they fail or succeed.
Be wise; file trust and faith in file 13.
Look before you leap.
If you don't know the right questions to ask, find
someone who does.
Posted November 22,
1998 |
