Propeller problems

[Peter Lynch]

Hi all

We have a four year old Hunter 27 on Sydney Harbour. We love it!

However, last year we discovered the propeller had been eaten by erosion because the boat had not been wired to avoid electrolisis.

The dealer claimed Hunters didn't need this. But guess what: they do.

We had to replace the prop - but our problems haven't ended.

Last November, we were advised to fit a Gori 2 blade folding r 15x12 rh shaft drive pilot bored propeller.

We had some power problems with the boat afterwards, eventually leading to a total loss of power.

We have now had the engine inspected by a reputable Yanmar mechanic who tells us that the propeller is too large for the boat, and as a result the exhaust is being blocked by unburned diesel. We have had to replace the pipe on the exhaust.

We have spoken to a propeller supplier who tells us that this propeller is almost twice the diameter and the wrong pitch for a vessel of our size.

Our problem, having paid for the exhaust to be replaced on a vessel with just 150 hours on the engine, is that this will re-occur.

Can anyone shed any light on what we should do?

Thanks all.

 

[Ed Schenck]

There are several "prop sizing" calculators on the web. This site has both a calculator and a table of boats: http://mysite.verizon.net/resqp86c/performancepropellers/id37.html . But still better to hear from some satisfied owners I think.

 

[Ken13559]

We don't know what engine you have. 1GM10 ro 2GM20F? Question is can your engine reach 3400rpm with the Gori prop? If yes, then Dia/Pitch is probably right, if no (and smoking black) then too high pitch and/or diameter. Its normal for the U-type mixing elbow to foul over the years so probable annual preventive maintenance is to remove it for cleaning the rust/foul and replace it after sometime (depends on how badly corroded the inside is).

 

[Rick D]

However, last year we discovered the propeller had been eaten by erosion because the boat had not been wired to avoid electrolisis.

The dealer claimed Hunters didn't need this. But guess what: they do. Is your boat in a slip, moored or dry-stored? How old are your docks? Do you have shore power? Is the shore power normally left on? Is there a galvanic isolator installed? Is your shore power wired for 220 volts throughout or is it converted to 110 volts? Is there / was there a shaft zinc installed? What was done to 'properly wire' the boat? The point is that this is not normal and the electrolysis source needs to be identified.

We had to replace the prop - but our problems haven't ended.

Last November, we were advised to fit a Gori 2 blade folding r 15x12 rh shaft drive pilot bored propeller.
Who made the recommendation and how was the sizing determined? What engine do you have? Have you contacted the manufacturer directly? You might want to try calling Seahawk in Melbourne to cross check the sizing and pitch from their records. Although it is a different manufacturer, their information for a similar prop may give some helpful information.

We had some power problems with the boat afterwards, eventually leading to a total loss of power.
By that I assume you lost RPM over time?

We have now had the engine inspected by a reputable Yanmar mechanic who tells us that the propeller is too large for the boat, and as a result the exhaust is being blocked by unburned diesel. We have had to replace the pipe on the exhaust. That could certainly be correct. Also, what are your motoring habits? Does your engine get a chance to warm up fully and run at 80% of recommended maximum RPM, or do you only use it to get out of and into the marina and then set sail? Do you occasionally run it at wide open throttle for five minutes or so?

We have spoken to a propeller supplier who tells us that this propeller is almost twice the diameter and the wrong pitch for a vessel of our size.
I would start by contacting Gori directly, and maybe cross checking with Seahawk. You may be over propped, and it makes sense what the mechanic told you, but 'twice' the size? What size was the fixed prop that came off?

Our problem, having paid for the exhaust to be replaced on a vessel with just 150 hours on the engine, is that this will re-occur. It most certainly will unless it's corrected. Exhaust elbow maintenance is a normal item but at no where near this frequency.

Can anyone shed any light on what we should do?
If you can provide some additional information, maybe some people here on the boards can help point you to a problem.

 

[Don12364]

More details needed please for "Propcalc".

Peter,
As RickD has already pointed out there are many factors in matching a prop to an engine to a boat.
If you could supply the make & model of the engine plus the gearbox ratio - this will be on a small plate on the gearbox itself - then it is possible to calculate the nominal size and pitch which would be a first approximation to the optimum prop.
I have done many of these calculations and find the best site for this is http://www.castlemarine.co.uk/pitch.htm. Try looking at this and you will see the factors involved.
This will give a 'nominal prop' and is often all that is needed for general running; but some owners like to slant their choice towards motor sailing at minimal RPM whilst others want to make their boat into a tug and to plough into head seas and strong winds.
In all cases the engine should be able to reach the makers recommended maximum RPM with the boat not tethered to the dock.

Also, if the supplier recommended this prop rather than being given specific instructions, then any reputable prop shop should stand by its recommendations and either alter or replace the prop if it is found to be unsuitable.
If you care to let us know the boat/engine/gearbox details including waterline length, displacement, engine HP plus max RPM and the gearbox ratio as above, we may be able to help.

 

[Steve Dion]

Here is some information so you may be able to answer some of your own questions.

http://www.olds.com.au/marine/propeller_faq.html

Typically the clearance of the prop tip to the hull should be approx. 15% of the diameter of the prop (i.e.: a 15" prop should have "about" 2.25" of clearance). There is a lot of things that go into determining the "propper" prop for a boat and is best left to a professional that has the knowledge beyond what a computer program may tell you.

I would contact the mfg and have them give you some advise. You may have some other issues too.

 

[Maine Sail]

Can anyone shed any light on what we should do?

Thanks all.

Get the right size prop so you're no longer overloading your engine.

With the right prop your engine should be able to hit max rated RPM with a clean bottom and clean prop in flat current free water. If you can't hit max rated RPM, let's just guess 3600 RPM, then you're going to do this all over again. A prop that is slightly under pitched is more often better than one that is over pitched/sized.