Outboard Boat Propeller Help


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	General Sales Department (800) 454-6960 (954) 444-6259 sales@deepblueyachtsupply.com

	Inboard Propeller Questions Use our 'Inboard Propeller' Form Or call us at: (800) 454-6960 (954) 444-6259

	Outboard Propeller Questions Use our 'Outboard Propeller' Form Or call us at: (800) 454-6960 (954) 444-6259

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Outboard Boat Propeller Question and Answers

Below is a list of questions and answers about outboard boat propellers that we have come across and have been asked throughout the years. Simply click on the question below to read the answer.

How do I find the proper pitch for an RPM range?

3 Blades or 4 Blades?

What's the advantage of a left-hand prop?

Aluminum vs. Stainless Steel?

Why do outboard motors of the same power sometimes take different prop sizes?

How can I be sure my motor is operating within the recommended rpm range?

What is the purpose of the rubber cushion hub in an outboard prop?

Will a different prop correct bad torque action (listing and hard steering)?

I have a twin engine outboard. Can I get a propeller of opposite rotation and run one motor in reverse?

What is the correct transom height for my outboard engine?

Is it advisable to have outboard props repaired?

What is the best tilt-setting or shaft angle?

Why change propellers?

What about plastic propellers?

My outboard seems to vibrate excessively, yet the propeller hardly shows signs of use. Why?

Is an adjustable pitch propeller possible or satisfactory?

The rubber hub in my propeller seems to be slipping. Is this possible?

Why do I shear so many pins without apparently hitting anything?

Can a prop change help me in water skiing?

Can I troll better with a different sized wheel?

How do I find the proper pitch for an RPM range?

Each engine is geared to operate within a specific range specified by the manufacturer. Taking your current propeller pitch, and the RPM's that are being generated, you can calculate which pitch to move to by the following equation:

(Current RPM / Desired RPM) x Current Pitch = New Pitch to achieve desired RPM.

Example: If you have a 21 pitch propeller, and are turning 4900 RPM, and want to go to 5350 RPM, then
(4900 / 5350) x 21 = 19.25, which is the new pitch.

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3 Blades or 4 Blades?

We recommend 3-blade propellers for recreational boats with 3, 4, and 6 cylinder outboards and I/O engines. These propellers provide good "hole shot" and top-speed performance.

We recommend 4-blade propellers for bass boats and boats with high performance hulls running high horsepower outboard engines. Compared to 3 blades, they provide better "hole shot" performance with less steering torque and less vibration at high speeds.

When switching from a 3 blade to a 4 blade, remember to drop 1 inch in pitch to keep the RPM's as close to the same as possible.

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What's the advantage of a left-hand prop?

Two propellers spinning the same direction on twin engine boats will create steering torque. In other words, two right-hand propellers pull the stern hard to the right and the bow to the left.

Two opposite-direction propellers on twin engines eliminate this sterring torque because the left-hand propeller balances out the right-hand propellr. This results in better straight-line tracking and helm control at high speed.

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Aluminum vs. Stainless Steel?

Most pleasure boats are factory equipped with aluminum propellers. Aluminum propellers are relatively inexpensive, easy to repair, and under normal conditions can last for many years.

Stainless Steel is more expensive, but much stronger and durable than aluminum.

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Why do outboard motors of the same power sometimes take different prop sizes?

This is due to differences in lower unit gear ratios. Stock outboards are geared so that the propeller shaft turns at a slower speed than the rpm at the powerhead. This is usually expressed as a ratio such as 12:21 or 14:28, referring to the number of teeth in the drive gears. In the first example, the crankshaft gear has 12 and the propeller shaft gear has 21. This means the propeller shaft turns only 57% as fast as the indicated rpm at the powerhead.

The lower the gear ratio, the larger the propeller that can be used and vice versa.

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How can I be sure my motor is operating within the recommended rpm range?

This can only be checked with a tachometer. There are various kinds commercially available.

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What is the purpose of the rubber cushion hub in an outboard prop?

It is not intended to prevent blade damager as is sometimes assumed. This device protects lower unit parts by cushioning the shock of propeller impact. Its prime purpose, however, is to prevent excessive breakage of shear or drive pins that otherwise would occur due to the jolt or shock that is experienced in the process of shifting gears.

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Will a different prop correct bad torque action (listing and hard steering)?

Usually not. Most likely it is the result of any of several irregularities in the hull, the steering hook-up or the engine mounting. Steering wheel must be properly located relative to propeller rotation. If an engine has a right hand rotating propeller, steering wheel should be on the right or starboard side. This side normally would tend to lift as the result of torque action and the driver's weight offsets it. Modern outboards have built-in features in the lower units to compensate for torque. Engine tilt should be such that the prop is horizontal when underway. If it is up or down, the propeller can have a definite pull to one side. See that engine is at exact center of the transom and is setting level. Steering linkage should have enough adequate sized pulleys, properly swiveled and with the right cable tension. Check boat bottom for warping, distortion, which could cause difficulty.

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I have a twin engine outboard. Can I get a propeller of opposite rotation and run one motor in reverse?

No. This is a frequent question, and we would caution anyone against attempting to run any engine in reverse continuously at high speed. The power units are not made to withstand the reverse thrust and this type of operation can only result in lower unit failures. Some lower units have been available with reversed gearing so counter rotating propellers can be used in a dual installation.

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What is the correct transom height for my outboard engine?

On average boats, it is best to mount the engine so the cavitation plate is approximately 1" below the bottom of the keel, or 1" below the bottom of boats without keel.

For racing boats, better speeds can be attained by raising the engine to reduce lower unit drag and exhaust back pressure. Best transom height can only be determined by experimenting . . . get the engine as high as possible, or to the point just before propeller cavitates excessively.

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Is it advisable to have outboard props repaired?

Depends on the material. Those made of sand-cast aluminum are repairable at about 1/3 to 1/2 the new propeller price. Die-Cast propellers generally are not repairable. The material is very brittle, breaks off easily in the straightening process and will not weld satisfactorily. (Original equipment propellers are usually made of die-cast). It is advisable to discard such propellers and replace with the more durable sand-cast aluminum.

Extensive service is offered by authorized propeller repair stations located across the country.

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What is the best tilt-setting or shaft angle?

Proper tilt is extremely important, and is determined only through experimenting. In any boat, the tilt-setting can change as the load changes. Tilt adjustment determines the planing angle and if improperly set, the speed loss can be substantial or the boat may not plane at all. Tilting the engine in toward the transom pulls the bow down. Tilting it away from the transom pulls the bow up. Vary the anle to find the point where the boat assumes the best planing position.

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Why change propellers?

The stock propeller with which most outboards are equipped is a compromise. Since it has fixed diameter and pitch, it is really limited in its use and it does not provide satisfactory performance for all the combinations of hulls and loads that will be encountered once it is installed. One important fact to note is that the propeller moves the boat through the water at a specific engine rpm, and h.p. is directly related to the developed rpm. The engine cover is marked with a certain h.p. rating but in most instances the full benefit of the possible h.p. is never realized. Along with the h.p. rating equal emphasis should be placed on the rpm at which the rated h.p. is developed. This, of course, is where the propeller comes into the picture. Outboard engines are designed to be run at peak rpm for full efficiency. Excessive rpm with its increased friction and wear is obviously harmful. It is equally harmful to run the engine so overloaded that it cannot achieve its rated rpm since this results in excessive carbon buildup in the cylinder with subsequent problems of poor fuel economy, pre-ignition, frequent spark plug failure, scoring of the cylinder walls and even burned pistons.

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What about plastic propellers?

Nothing to date has been developed that has all the qualities of propellers made of metals. A good propeller must be durable, repairable and above all, it must perform well. So far the available plastics fail in these important requirements.

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My outboard seems to vibrate excessively, yet the propeller hardly shows signs of use. Why?

This is not unusual. Propeller blade damage very often is not discernible to the naked eye and blades can become bent or distorted without showing signs of impact or abrasion.

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Is an adjustable pitch propeller possible or satisfactory?

An adjustable pitch propeller permits more flexibility of operation than the standard props supplied with the outboard. However, it is only efficient at one setting since the blade is a true helical surface only at one particular pitch angle. Two or three well designed rigid props of pitches needed for different loads will give better performance than an adjustable propeller.

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The rubber hub in my propeller seems to be slipping. Is this possible?

It is a definite possibility, but it does not occur too frequently. Take a look at the propeller. If the blades are visibly bent or distorted, you very likely are experiencing cavitation — and cavitation is often mistaken for a slipping bushing. Have it checked by a reliable prop service station. The bushing can be replaced if it needs it or the blades can be restored to proper accuracy to eliminate the cavitation.

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Why do I shear so many pins without apparently hitting anything?

The engine may be turning too fast in the shifting range. The shear pin hole or slot may be sloppy or oversized. The shear pin may not be of improper strength for the amount of power involved, or the cushion hub may be frozen tight.

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Can a prop change help me in water skiing?

Yes, in most cases. Original equipment propellers are pitched a little on the high side. Not knowing the boat the engine will be used with, the manufacturer pitches the prop a little high so the engine does not exceed top rpm if placed on a light boat. However, on a heavier boat, or with water skiers, this propeller tends to overload the engine, resulting in poor speed, poor acceleration and sluggish performance, making it difficult to get a skier up. This is corrected with a lower pitched prop.

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Can I troll better with a different sized wheel?

Low pitched propellers are always best for trolling. The lower the pitch, the better. Standard propellers with relatively high pitch troll too fast and in throttling down to extremely slow speed, they tend to overload permitting the engine to idle faster while moving boat slowly.

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