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Captain Konz Services
Underwater Hull Inspection


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Underwater Hull and Machinery 

With correctly running under water machinery your (boat) vessel will last longer and run better.  Their are no gas stations on the water, many breakdowns result from problems with these parts!

What is Underwater Machinery?

Underwater Machinery consists of propeller, struts, stern drive units, bushings, rudders and various other items necessary for the operation of a boat.  

These always consist of metals that have corrosion resistance.  Aluminum, stainless steel and bronze are extensively used.  

Noble metals on the Galvanic Scale resist corrosion better that less noblemetals.  Stainless steel will resist corrosion better that aluminum.

Underwater machinery  must be inspected for collision damage and(corrosion) galvanic or stray current corrosion damage and wear.  

The type of underwater machinery varies from sail and motor boats to jet skis as does the method of inspection.   


Propellers are carefully machined, rotate hundreds of times per minute (RPM) and must be in good condition to give the best performance.  Inboard boats have propellers attached to propeller shafts that (usually) rotate in a cutlass bearing set in a strut which consist of a bronze sleeve and a rubber lining. 

Propellers are inspected for the correct size.  The incorrect size propeller will impair the boats operation.  An incorrectly size propeller can result in poor performance, transmission and engine damage.  

Propellers must be free of nicks, wear and corrosion. Struts must be securely fastened to the hull and free of leakage at the point of fastening.

Underwater machinery must be properly grounded to the vessels grounding system. Cutlass bearings must hold the propeller shaft snug with out wear or end play.   

Stern Drives

The Aluminum casing of stern drive units must be free of corrosion, it must be without collision damage and must pressure test to 15 psi.  

Aluminum casing are most susceptible to corrosion damage and must be protected by zinc.  Stern drive units are essentially boat transmissions and exhaust systems that are submerged underwater.  

Stern drives, continued

The stern drive is filled with gear lube (oil) and must remain free of water or damage to the internal gears of this unit will occur.

A stern drive unit replaced new can cost as much a $6000 or more.  This can be an expensive item to replace.

Keel & Rudder

Careful attention must be paid to the keel, rudder & rudder linkage, in addition to the propeller strut and propeller shaft on sailboats.  (The rudders on sailboats are sometimes delaminated). 

Wear in not uncommon at the mechanical steering mechanisms of rudders.

Drop centerboard keels must be inspected for corrosion at their pivot pins.  A sailboats keel is most exposed to collision damage.  

A collision of the forward section of the keel will produce extensive aft damage to the keel.

Problems associated with each of these units remains the same, collision damage, corrosion, "note; electrolysis is what happens to the fluid during corrosion not the metal "corrosion and wear.   

Key Benefits of a survey

Since underwater machinery drive a vessel loss of its use will surely ruin your day and probably cost much more then if the problem had been corrected prior to use.

Maritime Vessel Surveying, Inc.  can provide a detailed assessment of all the underwater machinery in your vessel.

This inspection is included in our pre-purchase condition survey.  It may  also be carried out as a separate assignment for a vessel owner who just wants his boat checked out. 

The pre-purchase survey  includes an underwater inspection of all machinery, using our industry standard equipment. (See Diagnostic Tools Used)

Please contact our office if we may be of service to you:
Captain Stanley G. Konz, A.M.S.
Maritime Vessel Surveying, Inc.
If you require immediate support on your vessel,  please fill out the Auto-Responder located on the  Contact Us page and we will get back to you as soon as possible.