Islander 28 Refit (propellor)

[bobtourtillott]

I did get the book, thanks. The tranny is a 2.62 reduction. The prop that came off was a 12x13 3 blade and looks like it might pre-date the Yanmar. From my reading it looks like a significant "overprop" in terms of pitch anyway. I need to re-measure the clearance, it looked like there was room for a 14". The folks in BC recommended a 13x8 Campbell Sailor and now I worry about getting to hull speed!

[bobtourtillott]

So true, this is my third fixer upper in the last decade, so I should know better!

[bobtourtillott]

It does help, thanks. Do you know which of the three gear ratios you have behind the Yanmar? What is your max rpm under load?

[Paul Kotzebue]

Originally Posted by bobtourtillott

The folks in BC recommended a 13x8 Campbell Sailor and now I worry about getting to hull speed!

Powering a sailboat at "hull speed" with an auxiliary motor is over-emphasized IMHO. You can run at 75% of hull speed with about 1/3 the power.

[westwinds]

Probably 2.21 that is normally specified for a 13" propeller by Yanmar. The 2.62 reduction is for a 15". The engine is rated at 16 horsepower continuous duty at 3400 RPM. That gives a propeller RPM of 1357 for a 2.21 reduction, and 1298 for 2.62 reduction. Lets use a 14" diameter propeller as the OP thinks there is enough room for it. Using the graphs in Propeller Handbook by Dave Gerr, it looks like a pitch of 10 inches would do it. That would be a 14X10 propeller. However, the Propeller Handbook has another chapter that gives more accurate results so maybe later. It's getting near 11:00 PM so not thinking very well. That would be for two blades. For three blade pitch, divide by only 1.01, or just ignore difference. Three blades are needed if there is a problem with cavitation.

[eyschulman]

As I suspected you already have bad advise to over prop by 10%. So when the boat gets a little bottom dirt you will probably be over 20%. There is no benefit and significant down side to over propping. It is one of those dockside old wife tails that die hard. No doubt there will be some retort with supposed benefits. You want to know whats really right read the articles I mentioned on previous post. On older none turbo diesels on sail boats you could get away with lugging the motor not good for it but you could get away with it. On modern lighter higher rpm turbo motors lugging is more destructive. Overload is overload at any rpm so any argument stating that if run overloaded at lower rpms is ok bypasses the whole point that the motor would do better if correctly set up with WOT load at rated RPM +100

[westwinds]

Originally Posted by westwinds

Probably 2.21 that is normally specified for a 13" propeller by Yanmar. The 2.62 reduction is for a 15". The engine is rated at 16 horsepower continuous duty at 3400 RPM. That gives a propeller RPM of 1357 for a 2.21 reduction, and 1298 for 2.62 reduction. Lets use a 14" diameter propeller as the OP thinks there is enough room for it. Using the graphs in Propeller Handbook by Dave Gerr, it looks like a pitch of 10 inches would do it. That would be a 14X10 propeller. However, the Propeller Handbook has another chapter that gives more accurate results so maybe later. It's getting near 11:00 PM so not thinking very well. That would be for two blades. For three blade pitch, divide by only 1.01, or just ignore difference. Three blades are needed if there is a problem with cavitation.

I see a previous post for the OP states the reduction is 2.62. Can you have a diver clean the bottom and propeller and then do a run on a calm day with no current to see what kind of speed you get at maximum RPM? You could use a GPS for speed. Do a run in both directions and average the speed. From that info, easy calculations can be done about the best propeller, but there are other ways that can be used, but not as accurate.

[westwinds]

A 12X13 propeller, using a rule of thumb conversion if one inch increase in diameter of the propeller is the same as two to three inch increase in pitch. If the diameter is increased to 14 inches, then the pitch would be decreased to 9 to 7 inches so the propeller used would be a 14X9 to 14X7. This assumes the propeller tip clearance from prop tip to bottom of boat is at least 2.1 inches or 15% of diameter of the propeller. That really is not much pitch so thinking that original propeller was sized for a different engine and transmission. For a 2.62 transmission reduction, the propeller shaft speed is 1300 RPM at an engine speed of 3400 RPM at 16 horsepower and the pitch is estimated between 9 and 11 inches by the graphs in the Propeller Handbook using Couch's Propeller Method.

[westwinds]

There are some 14X9 and 14X11 propellers on ebay with a shaft diameter of 1 inch. If you buy a propeller with a pitch that is too low, the Yanmar I believe has a governor that will keep the engine from going beyond the redline of 3600 RPM, which would damage the engine. You would also get less than the maximum horsepower available. The Yanmar should not be operated continuously beyond 3400 RPM for extended period of time. The Propeller Handbook states that the continuous RPM of 3400 RPM can be used as the design speed.

[westwinds]

Did a Google search a while back and book marked this place as a supplier of propellers: PROPELLERS, Used, New, inboard and outboard

[Paul Kotzebue]

Originally Posted by westwinds

You would also get less than the maximum horsepower available.

I would say "you would also use less than the maximum horsepower available". The engine is still capable of producing maximum power, which gives you some reserve power for adverse conditions.

[westwinds]

Originally Posted by Paul Kotzebue

I would say "you would also use less than the maximum horsepower available". The engine is still capable of producing maximum power, which gives you some reserve power for adverse conditions.

What I meant is the engine will not produce maximum horsepower if the propeller does not have enough pitch though the engine is at redline of 3600 RPM. The torque the engine produces is less than what the engine is capable of producing. Torque with RPM gives horsepower. If you can increase pitch and the engine can run at 3600 RPM, then you get more power to move the boat. Now if you increased the pitch and the engine would not go above, lets say 3200 RPM as an example, then you would have maximum torque, but lower RPM so less power to move the boat. You need that fine balance between maximum RPM and maximum torque. Myself, I would design for 3600 RPM, but would never exceed 3400 RPM except in an emergency because of excessive engine wear.

[westwinds]

Here are the power curves for the Yanmar http://www.sailingboatefaki.gr/yanmar%20gm%20series.pdf Right click on the image and them zoom to get a larger picture.

[Paul Kotzebue]

Originally Posted by westwinds

Now if you increased the pitch and the engine would not go above, lets say 3200 RPM as an example, then you would have maximum torque, but lower RPM so less power to move the boat.

You would also move the propeller demand curve up and overload the engine at full throttle, unless the engine is governed to turn at no more than 3200 rpm with no load. The propeller demand curve, which is unique for each boat, should be below the power curve for the full rpm range of the engine.

I'll admit I'm looking at this from the parallel universe of large commercial vessels where fitting a propeller is much more involved than what is described in this thread. The consequences of overloading an engine of several thousand horsepower that operates for 2,000 hours a year are more severe than the consequences of overloading a small sailboat auxiliary for brief intervals. That said, your approach of pitching the propeller for a few hundred rpm above maximum continuous rpm will work fine for a typical sailboat auxiliary, but I would suggest a more rigorous analysis for a motor vessel that runs at maximum continuous power for 24 hours or more at a stretch.

[westwinds]

Originally Posted by Paul Kotzebue

You would also move the propeller demand curve up and overload the engine at full throttle, unless the engine is governed to turn at no more than 3200 rpm with no load. The propeller demand curve, which is unique for each boat, should be below the power curve for the full rpm range of the engine.

I'll admit I'm looking at this from the parallel universe of large commercial vessels where fitting a propeller is much more involved than what is described in this thread. The consequences of overloading an engine of several thousand horsepower that operates for 2,000 hours a year are more severe than the consequences of overloading a small sailboat auxiliary for brief intervals. That said, your approach of pitching the propeller for a few hundred rpm above maximum continuous rpm will work fine for a typical sailboat auxiliary, but I would suggest a more rigorous analysis for a motor vessel that runs at maximum continuous power for 24 hours or more at a stretch.

Do take a look at Propeller Handbook. Dave Gerr does cover up to 10,000 horsepower applications. For continuous operation, in general an engine has to run at 80% of maximum power for long life. For a large commercial boat with a controllable propeller, then engine overload can ruin an engine very quickly says Gerr.