Sooner or later it happens to all
of us, even the most careful of navigators – the prop gets dinged. Sometimes it’s
a little more than a ding, yet all of the blades are still there, more or less.
Your boat dealer, local mechanic, and the local boatyard will all tell you
the same thing -- replace the prop! (They can’t fix it and want to sell you a new screw.)
But you wonder: “Can’t a good prop shop salvage this with some creative metal working
and make it almost as good as new?” If you’ve faced that dilemma, you are not alone.
Can this prop be repaired?
First of all, was your prop doing a good job before it was damaged? That means—
1. Was your prop keeping your motor in the proper rpm range? Failure to do this
results in poor fuel economy, poor performance, and most expensive of all—potentially
catastrophic damage to your motor.
2. Did your boat accelerate and corner well?
3. Did you experience noticeable vibration from the prop? Nothing good comes from
If there are issues with these items, it might be time to consult with a prop manufacturer
or your propeller dealer and replace your prop. After all, a prop that is perfectly
repaired, but is not properly suited for your application—is perfectly improper.
Can this prop be repaired? Should it be?
If the basic performance factors were okay, next you need to evaluate the feasibility
of repairing the prop. If you have an aluminum prop, you should inspect for excessive
metal loss (more than 10% of the blade area), extended cracks, blades that are too
thin, or cavitation burns. If your prop has any of these issues, bite the bullet.
An additional consideration is whether your prop may need the inner hub that protects
your drive train replaced. This can add another $40-60 to the repair cost, so it
needs to be examined as well. Aluminum propellers are fairly inexpensive to purchase
new, so you don’t want to put too much money into them, unless they are in good overall condition.
The left image shows the results of collision with a heavy chain. As you can
see the bronze blades are badly bent and damaged. The right hand image shows the
finished result. (Pictures courtesy of French Marine Motors, UK.)
What if you have a V-drive or inboard skiboat/cruiser with bronze wheels? The critical
issue here is blade thickness. Even if you made a “pretzel” out of your prop, skilled
prop shops can work wonders. But, if the blades are excessively thin, you may be
sinking money into a lost cause. When you straighten, you need to regrind to smooth
the surface and remove dents and nicks. Each time this is done, critical metal thickness
is lost. Remember that a 20% reduction in thickness may reduce the strength by up
Cavitation has worked its magic on this stainless steel prop.
Stainless Steel Screws
Stainless props need to be evaluated on similar criteria as bronze ones. First of
all, is the remaining thickness okay? Any signs of cracks? How extensive is the damage?
How much metal is missing?
Generally speaking, stainless props are made from two general types of metals. High
carbon stainless is very strong and damage resistant. It welds well and can generally
be repaired unless it is bent excessively. Given its high strength, it does not
like being bent into a pretzel and straightened. Typically this metal is used by
Mercury, Yamaha, Turbo-Stiletto (now a Yamaha company), Michigan Wheel, and PowerTech.
High-Strength or Low-Strength?
Another characteristic of high-strength stainless is the fact that it shows more
corrosion than low-strength stainless. This is a natural consequence of the higher
carbon steel content used in the alloy. Low-strength stainless uses more nickel
in the alloying combination. This provides a significant increase in corrosion resistance,
but also drops the tensile strength, typically by 50% compared to the high-strength
The material used in low-strength stainless props is more similar to that used for
deck hardware. Since the strength of the material is more like bronze than a high
strength stainless, it will “pretzel” pretty easily. The good news is that it can
normally be straightened out again.
In either case, the issue will be to examine for enough blade thickness to allow
the repair to be completed and still have sufficient “meat” left for durability.
Proper repair takes time and the right equipment.
Repairing Props is an Art
If the decision is to repair the prop, look for a well-recommended repair shop.
The proper repair of propellers is an art. It takes a true skilled technician to
do the job you want. There are many good repair facilities available. If you have
a prop shop that is doing a great job for you, stay with them. However, if you are
looking for a shop, you may want to consult the National Marine Propeller Association
(http://www.nmpa.net), which provides
training and workshops to help members keep their technical skills current.
It may be a good resource to assist you in finding a skilled prop shop.
PowerTech! also has a list of dealers by state which can repair your props…www.ptprop.com.
PowerTech! also will be glad to help you zero-in on the right prop for your application
if you think your boat is having performance problems, or if your prop needs to be repaired or
replaced. Simply fill out their diagnostic form…