A Few Thoughts And Facts About Propellers

People think of speed first, but usually acceleration is most important.  You want to feel the acceleration when you push on the throttle.  When you want a little more speed, a mile per hour or two faster so you can pull ahead of your buddy's boat or just a quicker hole shot…  You could get more speed or a better hole shot by switching props!  But…  Most of the time a good prop shop can tweak the prop that you are using for less then a 100 bucks instead of spending 500 bucks on a new prop… But…There's a trick to it.

Everybody always wants an ironclad answer on what prop to use… But finding the answer is always a trial-and-error process.   Well with that said…  There are a thousand props and a thousand different variations in pitch, diameter, rake and cup.  The question is;  How do you find a good starting point for improving your performance?  One suggestion is to go online to find test reports for your boat and motor.  Look for a similar hull of nearly the same weight and horsepower and use their performance reports for a baseline propeller.  But you still have to buy new props to experiment with.  Another suggestion would be to contact your local propeller repair shop and get their suggestions. 

Some Terms You Need To Know

What is "rake?" A prop has positive rake if its leading edge leans toward the stern of the prop hub's centerline. Negative rake means the blade edge leans forward.  Positive rake tends to grab the water and pull it toward the center of the prop. Then what does "cup" mean? A prop is cupped when the trailing edge of the blade is curved backward. Cupping tends to pull water to the center of the prop, improving bite on the water and helping with the acceleration.  How does "Lift" help with speed?  Lift comes from the downward pull of the prop turning in the water.  With enough "bow lift" from the prop in order to get as much of the boat out of the water as possible can give you better speed and performance.

Specified rpm:  Every engine has a specified rpm range. Choose the prop that gives the performance you want without exceeding either the high or the low end of that range.

Pitch:  Theoretical distance a prop will move through the water in one revolution if there is no slipping.

Diameter:  Distance from the center of the hub to the tip of the blade times two. Large diameters sometimes give stronger hookup for slower-moving boats.

Cavitations:  Water passing over the leading edge of the prop vaporizes from the pressure change, then de-vaporizes while striking the trailing edge of the blade or the gear case. Cavitations can erode the alloy much like a sand blaster.

Ventilation:  With-In turbulent water, the prop can grab air pockets, causing it to lose thrust. Larger-fluke props with increased diameter that have less rake and cup can be more forgiving on turbulent hulls like pontoons and tri-toons.  

Ventilation holes:  On performance craft like bass boats, ventilation holes can increase the acceleration by causing air to form around the hub, letting the prop spin up faster.

Aluminum props:  Less expensive metal is more forgiving on impact, but it flexes, slowing acceleration.

Stainless-steel props:  Thinner, stiffer blades give better acceleration but are less forgiving in impact. 

Some Ideas:

Even with more information about props, there are a lot of variables to wade through…  Even when the experts agree on a few guide lines it is still hard to understand all of the ins and outs of propping out one’s boat.  So…  Here are some good starting points for maximizing your boat's performance.

Pontoons:  Pontoons boats create a lot of turbulence ahead of the prop, and because of the hull shape, prop design doesn't impact lift much. To keep the prop performing to the max, try these suggestions; Use a wide diameter prop with a positive rake and little or no cupping.

Bow-Riders and Deck Boats:  These boats are about fun, so acceleration to pull skiers and tubes takes priority over top speed. Use cupping to improve hole shot and or a positive rake for added lift and a moderate pitch for acceleration. 

Bass and Performance Boats:  Are The Need For Speed;  Go long on pitch and forget a snappy hole shot. The goal is to get the most mph from rpm. Use cupping to improve acceleration rake for strong hookup with long pitch for maximum speed and ventilation holes in hub for better hole shoot. 

Offshore Boats:  These days it’s all about fuel economy and mid-range control are most important.  Saltwater is dense and tends to slow the boat down.  Use the following, lower pitch to handle load and fuel.  A four-blade prop to assist in lifting the heavier boat rig on a plain with cupping for acceleration and some positive rake for better top end speed.

The Proptologist 

Keeping your outboard in tip-top shape can be done even if you're not a mechanic. There is nothing worse than trying to run to shore to flee an incoming storm and hearing that all-too-familiar cough and sputter from the outboard. More often than not, just a little TLC and preventative maintenance could have prevented the fact that you are now bobbing around like a cork, pounded by waves and soaked by the torrential downpour.   If you are not a mechanic, get a reliable one and start every season with a professional tune-up. You should have the mechanic test the spark, run a compression test, pressure test the lower unit, check the seals and water pump, test the warning alarms and, in general, go over the motor in such a manner as to eliminate most causes of breakdown. After you have had your annual check-up there are many things that you can do to help assure that you make it through the season without being towed home.

After every outing, flush out the engine. This doesn’t just apply to salt water outings but to fresh water as well. If the lake or river you operate on isn’t as clear as the water from your garden hose you need to flush. If you ran aground and kicked up some sand, you need to flush. It won’t take long, ten to fifteen minutes, and during that time you can check out some other items.  

To flush the engine you will need a set of "rabbit ears" (two flexible rubber seals connected with a metal clamp). Simply slip this apparatus onto the lower unit where the water is picked up and attach a garden hose. Start up the engine and let the water pump do the rest. (Be sure to stay clear of the prop and make sure no one tries to shift into gear)

While the motor is being flushed, check the water pump to make sure you have good water flow. Carefully put your finger through the stream of water. It may be warm, but it shouldn’t be hot. If the output is not strong, you may have some debris stuck in the outflow tube. You should immediately shut down the engine to prevent overheating and damage. A small piece of wire or similar object can be inserted up into the flow tube and worked back and forth. Start the engine again and check the output. If that doesn’t solve the problem you may need a new water pump.

IF you have a carburetorated outboard engine you should disconnect the fuel line and allow the engine to burn all the fuel in the carburetor. DO NOT DO THIS IF YOU HAVE A DIRECT INJECTED ENGINE or IF YOU HAVE AN EFI ENGINE such as an Evirude ETEC.   By the way, make sure you always use fresh fuel. You should not use fuel that has been sitting around for over 60 days. That means; at the end of your season take the fuel in your tanks to the proper recycling authority or just use it your lawn mower. Don’t save it up for next year. That's an invitation for disaster. Other fuel related items you should check are:

·         Check the fuel line for cracks and worn spots

·         Make sure the fuel primer bulb is not cracked and is pliable

·         Make sure the fuel line fittings seat properly and don’t leak

·         Check the clamps on the fuel line for rust or corrosion

·         Check the fuel tanks for damage and corrosion

·         Check the tank vent to make sure it aspirates properly (many a mechanic has handed a boater a bill for services after he simply opened the tank vent)

·         Check for water in the fuel

Once you have finished the flushing and run the engine out of fuel, be sure to turn off the key and, if you have a battery switch, turn it off. Open up the engine cover and check for fuel or water leaks. If you find leaks you should consult your mechanic. Be sure to wipe everything down and spray with a anti-corrosive like WD 40 or Quick-lube. Be sure to lubricate all the moving parts such as the shift and throttle cables, carburetor valves etc. Check your owner's manual for details.   Once you have performed your post-trip preventative maintenance program, replace the engine cover and wipe the outside down. If you have a canvas or plastic cover for the engine you should keep it in place between trips.     Hope this helps and decreases your chance of engine failure in the future. 

The Basics of Gas/Diesel Fuel and Boats

How much do you need?  How much fuel you need to carry depends on the design of your boat and the intended use. In a sailing vessel, for instance, a 50 gallon tank of diesel feeding a 25 hp auxiliary engine cursing at 80 percent of top speed could propel a 33 foot sailboat 300 miles at 7 knots in calm weather.   That same amount of fuel would take a twin engine 40 foot sport-fisher only about 33 miles at 40 knots. This boat would need 450 gallons to cover 300 miles at that speed.   Plaining hulls need much more power than displacement hulls and use more fuel per mile. However their ability to carry fuel is limited because their plaining ability is affected by weight.    Many naval architects build in as little tankage as possible taking into consideration the design of the boat, reasonable usage and the distance between refueling stops in and around its normal area of operation.

How much does it weigh?  Gasoline weighs about 6.1 pounds per gallon. Diesel weights about 7.1 pounds per gallon. Both gasoline and diesel engines use about 0.6 pound of fuel per horsepower per hour. On average, diesel fuel contains about 140,000 BTUs per gallon or 10 percent more energy than the same volume of gasoline.

Where does it go when it is consumed?   Your engine uses the fuel you purchase in several ways.

·         35 percent is given up to the atmosphere in heat

·         25 percent is given up in heat and vibration absorbed by surrounding water

·         10 percent is given up to overcome wave resistance

·         6 percent to overcome wave formation and prop wash against the hull

·         7 percent to overcome skin friction

·         2 percent is wasted in friction at the propeller shaft

·         1 percent to overcome air resistance

This leaves about 13-14 percent of the original energy to turn the propeller.

How much do gasoline and diesel engines consume?  Diesel engines consume about 1 gallon per hour for every 18 hp used. You can estimate the number of gallons consumed per hour by multiplying horsepower used by 0.055.maximum available horsepower. Most marine engines are designed to run continuously at between 60 and 75 percent of maximum speed. Diesels tend to be more toward the top of the range.     Gasoline four stroke inboard engines need about 1 gallon per hour for every 10 hp used. The number of gallons consumed per hour can be estimated by multiplying horsepower used by 0.100. (see note above)     Outboards might use considerably more since two stroke motors seem to have a greater thirst than four stroke motors with exception of the Evinrude ETEC. 

What about fuel economy?  Any attempt to force a displacement hull beyond its maximum theoretical hull speed brings exponentially higher fuel consumption for minimal gain in speed. On the other hand, slowing down a little almost always is rewarded by gains in economy, and thus range. This same principle applies to high-speed plaining hulls as well but only up to the point at which they start to come off plane. Once off plane they tend to lose efficiency.

How about fuel reserves?  Use the old rule of thumb; Use one-third of the fuel in your tank to get there, one-third to get back and save the last third as an emergency backup.

Spring is here and now and you want to take your boat out for a day of fun.  Last winter you did your winterization on your boat and motor, but what about the trailer.  Follow this simple routine to make sure the trailer transports your boat this spring, this summer, this fall and beyond this season.    Wash and Lubricate: Clean your trailer with an all-purpose cleaner that includes a degreaser to remove corrosive salt water, chemicals and dirt before putting it to bed for the winter. Even a galvanized trailer should be rinsed thoroughly to prevent rust if it's seen salt water. Lubricate moving parts such as rollers, winches and other components with lightweight oil to ward off rust.

Remove Rust Spots and Repaint: All rust on the trailer frame should be sanded and painted to stop the deteriorating process before it gets worse. If a structural component appears to be badly rusted, repair or replace it. Have it inspected by an expert if you're uncertain.

Protect Your Tires: It's best to put your trailer on blocks and remove the wheels, especially if you're storing the trailer outdoors. Otherwise, your tires could develop perpetual flat spots due to prolonged inactivity. By storing the tires inside, away from sunshine and moisture, you'll also keep the rubber from deteriorating. Another advantage is that you'll avoid trailer theft.   If you store tires off the trailer but still mounted on rims, inflate them to 10-15 psi. If you leave your tires on the trailer outside, cover the tires with wood or plastic, and move the trailer periodically to avoid those flat spots.

Check Wiring and Connectors:  If wires are exposed, either replace the wire or tape it with electrical tape; exposed wires left untreated during the long off-season will deteriorate, requiring more work in the spring. Put a small amount of electrical socket grease on plug contacts and light bulb bases to prevent rust and corrosion.

How to prep your outboard engine from top to bottom.

How thorough were you when winterizing your outboard last fall? If you did it right, you should be able to start up and go.  But if you didn’t follow proper winterization and preventive maintenance procedures, your spring start might sputter. So before you launch, here’s a mix of fixes that should catch any autumn oversights so your engine emerges from winter unscathed.  Let’s start at the top. Pop the cowling. Depending on your storage situation, you never know what’s underneath—check for nests made by critters that crawled in for a winter home. Make sure the wiring, valves, and vents haven’t been compromised.

Due to the volatility of modern gasoline, fuel begins to break down after 30 days. So if you didn’t drain or run out most of your gas, you may need your injectors or carburetor rebuilt. It’s best to store your boat with a full fuel tank to keep condensation to a minimum. But if you forgot, fill the tank to the brim now to dilute any moisture before running the engine for any length of time. Fill the tank and add a fuel conditioner. Be prepared for multiple water separator changes until you’ve burned that tank.

Next, start the engine on “earmuffs,” or take a quick ride. Turning over the engine and running it for a few minutes should burn off any remaining lubricant and protectant you squirted into the cylinders to prevent rust. Your plugs should be changed after this, as they will be fouled.  While your engine is running, check for a steady stream from the water pump. You might have flow interrupted from debris clogging the outlet or from an impeller whose blades have taken a set during storage. Clean the hole with a paper clip to unclog any detritus, and replace the impeller if necessary.  Elsewhere, if you didn’t do it already, grease the pivot and steering joints to provide lubrication for free movement of the tilt and trim and steering. You should replace the lube in the lower unit, too, if you didn’t do it last fall. Check the sacrificial anodes and check the skeg. And, finally, pull off the prop and grease the prop-shaft and check the hub.

What every smart boater should bring aboard, besides a first-aid kit.

REQUIRED EQUIPMENT. Has a nice ring to it, right?   But when you think about it, required safety equipment is short for “minimum required equipment”—the bare necessities. Consider flares. Sure, the required three flares are better than none, but they provide just six minutes of attention-getting fire. And, of course, those Type II and III PFDs meet regulations, but wouldn’t you rather be strapped into a Type I device if you found yourself overboard offshore? That’s why experienced boaters don’t stop with the required Coast Guard package. They beef up their boats with extra defenses. So besides more flares and upgraded PFDs, what should you bring aboard? Find a place to stow these seven smart safety upgrades.

[1] JUMPER PACK. This battery with hard-wired jumper cables can get you started if your boat’s batteries decide to quit. Choose a model with a 12-volt receptacle so you can power a handheld VHF or GPS from it, too. And, not that you’d forget, but keep it charged.

[2] BACKUP ELECTRONICS. Handheld VHF radios and GPS receivers are cheap insurance against disaster. Keep the batteries fresh and purchase a 12-volt power cord for each. For the VHF, buy an adapter that converts the radio’s TNC or BNC connector to a PL-259 connector ($15 at a marine electronics store) to connect to your boat’s antenna and achieve additional range. Even if you have the most sophisticated electronics suite around, bring paper to back up the paper-free. Paper charts don’t require power and can help you give better information to the tow company or rescuers.

[3] FILTERS AND BELTS. Two of the most common causes of breakdown are the cheapest and easiest to fix. Adjustable emergency belts ($40 at West Marine) can save your bacon. So can a pair of panty hose: Just stretch them over the pulleys and knot them off. You may be embarrassed when the guy at the drugstore says, “Sure, they’re for your wife,” but they’ll get you home. Carry at least two backup fuel filters. Even with a highly contaminated tank that stalls your engine, you can swap out filters as they clog. You may not make it all the way home, but you’ll at least make it somewhere. And don’t forget the filter wrench. Gluing or taping a strip of sandpaper on the inside of the metal strap will give the wrench a better grip on slippery canisters.

[4] FOOD AND WATER. Even if you’re not reenacting Master and Commander, something to eat and drink sure makes waiting for the tide to float you off a sandbar more tolerable. Stow bottled water and some foil-wrapped high-energy or protein bars aboard. They keep forever and will stave off hunger in a pinch.

[5] SLIPPERY STUFF. Having extra engine oil with you is a no-brainer. But don’t forget transmission fluid (gear lube for stern drives) and hydraulic fluid for steering systems and trim tabs.

[6] RADAR REFLECTOR. It’s comforting to know that when you’re adrift, dead in the water, your boat is lighting up the screen of any passing container ship or tug like a Christmas tree. Plus, it helps rescuers and towboats find you quicker. A serviceable one costs about $50.