Prop Damage, Repair or Replace?
So, you looked at your prop and it isn’t looking so great. Should you repair or just replace it. Here are a few tips to help you decide.
First of all, was your prop doing a good job before it was damaged? It is critical that your prop can keep 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. Second, did your boat accelerate and corner well? 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.
(Note: If you are experiencing noticeable vibration from the prop, you should immediately repair or replace the prop. Nothing good comes from vibration!)
However, if the basic performance factors were OK, we will need to evaluate the feasibility of repairing the prop.
If you have an aluminum prop, you should inspect for excessive metal loss, extended cracks, too thin blades, or cavitation burns. If you have more than 10% of your blade area missing, or other problems, it may be better to replace the prop. An additional consideration is whether your prop may need the inner hub that protects your drivetrain 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 pretty good overall condition.
What if you have a V drive or inboard ski boat/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/nicks. Each time this is done, critical metal thickness is lost. Remember that a 20% reduction in thickness may reduce the strength by up to 50%. These wheels are more expensive than aluminum, so if the thickness is OK, repair is a very cost-effective decision.
Stainless props need to be evaluated on similar criteria. First of all, is the remaining thickness OK? 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. Another characteristic of high strength stainless is the fact that is 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 material. The material 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.
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, let me recommend the NMPA. We are members of the National Marine Propeller Association (https://www.nmpa.net), which provides training and workshops to help its members keep their technical skills current. It may be a good resource to assist you in finding a skilled prop shop. We also have a dealer list on our website www.ptprop.com.
In any case, let’s get your prop taken care of, so you can get out on the water and have some fun!