[tommygee]

Can anybody tell me how to find out the right size props for my 1993 tiara 260 sportboat it has duo props and a 7.4 volvo. when I bought the boat off the last person the had told me the lost the original props and had been given used props from a dealer. The props are now getting holes in them and would like to know if there is a better prop out there. The stamp on the ones I have are C

[Uhehner]

If you have holes in your props (I assume they are aluminum) you have a serious electrical leakage problem. Putting on stainless props will not solve that problem. It's time to consult a marine electric expert. Props are sized by diameter and pitch and need to be sized to your boat and engine so you run at the proper RPM. Talk to a propeller expert. It's too expensive to guess wrong. Capt. George

[tommygee]

Thanks, Capt. George will do in the summer

[trpntj]

Uhehner - 12/12/2012 8:07 PM

If you have holes in your props (I assume they are aluminum) you have a serious electrical leakage problem. Putting on stainless props will not solve that problem. It's time to consult a marine electric expert. Props are sized by diameter and pitch and need to be sized to your boat and engine so you run at the proper RPM. Talk to a propeller expert. It's too expensive to guess wrong. Capt. George

While a bonding issue is a possibility, I would look more into cavitation burn/erosion, especially with a duo-prop and possible mis-matched props. It happens a lot at the root of the blades, but could happen anywhere over the entire surface or edges.

heres one pic I found of a duo prop case. See the discoloration then the pitting.

[trpntj]

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C indicate the SS series propellers for DP280/290 drives. Are there anymore stampings on the props?

[tommygee]

I have to check for the stamping as they are not to clear to see. I have a mild amount of pitting but does look like the picture you posted

[Chris Burkard]

I do not know the correct prop size for your boat, however you mention you are getting holes in your propellers. Yikes! Cavitation could eat away at the propellers, but another more likely cause of holes in propellers is electrolysis.

Are your props stainless steel or aluminum?
Do you keep your boat in the water full time?
Are your sacrificial anodes (zincs) in good condition?
Is the electrical wiring on the engine in good condition?
Are all of the bonding wires (green wires connected to all metal parts exposed to water) in good condition?
What is the rated WOT rating in RPMs for your engine?
What RPM are you able to reach at WOT?

If you answer the above questions, I may be able to solve your holes in the props issue.

Sincerely,

[Chris Burkard]

The picture posted above looks more like cavitation, but it may also be electrolysis.

Cavitation:

If the issue is cavitation, then you likely have the wrong props on the boat.

1) Call a good prop shop and have a discussion with them about your boat. They are going to need; Make, Model, Hull design, Length, Beam, Engine, Drive and Weight. From there they can probably tell you what props you need. Aluminum props are cheap, but they have a much shorter life span and they are subject to much greater damage when grounded. Stainless props are what I suggest.

2) Make sure you go over your zincs, bonding system and electrical system so you do not damage your brand new props.

3) Once you have them installed, make sure your bottom is clean, engine is properly tuned and run the boat. Your props should allow your engine turn within its is rated WOT range. I believe the 7.4L is designed to turn between 4000 - 4400 RPM (please check as I am not 100% on this). Ideally, your engine should turn in just over the minimum in the range when fully loaded (fuel, water, gear, stores & people). When your vessel is light, it should turn towards the upper range, but not over. Engine damage can occur if your engine is unable to turn to the minimum WOT range. This is known as Over-Proping. Under-Proping can also be harmful if you exceed the maximum Rated RPM of the engine.

I hope this helps.