November 26, 2009

Propellers made easy

A FEW DAYS AGO I was editing an article for a forthcoming issue of Good Old Boat magazine. It said:

“When you select a propeller, you should match every dimension of that propeller to the hull and the engine driving it to attain maximum efficiency. This makes propeller selection and calculation very difficult for those of us who are not naval architects.”

Well, that’s not completely true in my opinion. The experts in too many fields, such as navigation and splicing rope, like to spread the word that it’s more complicated than it really is.

There are two ways of selecting a propeller: theory and practice. And even naval architects often have to resort to trial-and-error after they’ve tried their best with theory.

Let’s say you’re not satisfied with your boat’s performance under power, and you suspect the propeller is the wrong size. First check the diameter. Go to page 45 of my book, The Boatowner’s Handbook, where there’s a handy little graph. Lay a ruler between horsepower and prop shaft revolutions, and see where it crosses the column marked “Propeller diameter.”

On page 47 you’ll find another graph that shows you the pitch you need. This points you in the right direction for your prop. It’s about as good a result as the naval architect will get with all his complicated calculations.

So much for theory. Now we come to the practice. This doesn’t hardly need any brains at all.

Make sure your boat’s propeller is free of barnacles and the hull is reasonably clean. Take her for a run in calm weather. The ideal propeller will allow the engine to reach the manufacturer’s top-rated revolutions per minute (and therefore full power) with the throttle opened fully. And at this stage, your boat should be achieving full hull speed.

Now, if your engine starts to lug, or emit black diesel smoke, before it reaches top-rated rpm, you’ve probably got too much pitch. It’s like trying to ride a bike uphill in top gear.

On the other hand, if your engine reaches top revs too easily — that is, before your boat reaches hull speed — you probably need to increase the pitch. You’re riding downhill in low gear and your little legs are whizzing around but you’re not going very fast.

A propeller shop can alter the pitch of most auxiliary sailboat props a couple of inches, at a fraction of the cost of a new propeller. For boats with the usual 2-to-1 reduction gearbox, a decrease in prop pitch of 2 inches will increase engine revs per minute by about 300 to 400.

It’s unlikely that you’ll need to change the prop diameter, but you might like to know that for roughly equivalent performance, if you decrease the diameter 1 inch, you should increase its pitch 2 inches.

You don’t need to be a naval architect to check your propeller’s actual performance this way. It’s as much art as science — plus a bit of grunt work to get the damn prop off the shaft to which it clings so determinedly.

Today’s Thought
An expert is somebody who is more than 50 miles from home, has no responsibility for implementing the advice he gives, and shows slides.
— Edwin Meese 3rd, White House counsel

A pessimist is a person who builds a castle in the air and then locks himself in the dungeon.
An optimist, on the other hand, is a person who fixes your eyes.


Oded Kishony said...

Hi John,

Any advice on the best way to keep the prop fee of barnacles ?

Oded Kishony

John Vigor said...

Hi Oded, I've heard of lots of ways various people try to keep their props free of barnacles, including axle grease, antifouling paint, and special zinc paint. But none of them will last long if you do a reasonable amount of motoring. The abrasive quality of water against a propeller whirring at 1,000 revolutions a minute or more is enough to wear off almost any coating you can think of.

The only absolutely positive way I know of to keep the barnacles off is the method used by racers in warm water. Every time you come back to your slip, someone slips overboard and ties a black plastic bag around the prop.

I myself use a couple of coats of antifouling paint on a bronze propeller, and it lasts a month or two, but after that I can expect a gradual decrease in performance until the next haul-out. I'm afraid it's one of the facts of life. If you can't dive down and scrape the prop clean every six weeks or so, your prop will slowly become less efficient.

You'd think that using the prop more often, or for longer cruises, would discourage the growth of barnacles, but unfortunately there is no evidence that it's true. Nobody told the barnacles, which seem to thrive no matter how long or how fast they are spun around.

John V.