FAQ

I advise boaters to spend as much as it takes to get the right propeller for their boating application. Don't be too concerned with prop costs because the biggest bang that you can get for the buck, when it comes to boat performance, is with the propeller. Remember that a slightly incorrect type or size of propeller can seriously reduce several performance aspects of your boat. Of course this doesn't mean you should spend money unnecessarily. The answers to the remaining seven most commonly asked questions, should help you understand more about propellers and to make a decision as to which propeller could be best for your boat. In looking at these questions keep in mind that propellers are always referred to by two numbers. The first number is the 'diameter', or straight line distance in inches, from a blade tip to the center of the of the hub, times two. The second number is the 'pitch', or theoretical distance in inches that the propeller would travel forward in one single revolution.

Most small dings or bends in an aluminum prop can be repaired for a reasonable price. If you do repair, make sure the shop does not file or grind down the blade edges therefor reducing the diameter. If your damage includes major missing chunks out of the bade, you should consider a new prop. Aluminum propellers as well as stainless steel propellers can be welded back to look and perform like new. Stainless props cost more to repair than aluminum because the material is more difficult to work with, however, once properly repaired, they can be as good as new. Remember , Always seek a National Marine Propeller Association certified facility for the highest quality repair techniques and up to date information on propellers.

There are many factors that can cause a boat to have difficulty coming onto plane. Consider for example, your total boat weight, weight distribution, engine trim, hydrofoil, and transom tabs before investing in a new propeller. To assist in the correction of on plane problems, you may also need a different propeller. Your first choice should be a propeller with more blade area. Keep the pitch the same, but provide more blade area either through a slightly larger diameter or an extra blade. In cases where this would not be enough, you may have to trade some top speed for a better bottom end through reducing the pitch a couple of inches.

In most cases, by simply making a direct conversion to stainless in the same size as your aluminum prop, you will notice an improvement in both acceleration and top speed. The reason for this is very simple, but frequently misunderstood. Stainless is seven times stronger than aluminum. Propellers don't have to be that much stronger so manufacturers can make them thinner and still be two or three times as strong. Thinner props cut through the water easier and this of course translates to better performance. Stainless propellers generally cost a couple of hundred dollars more than an aluminum or composite propeller. There are two situations where I don't recommend stainless over aluminum or composite: smaller outboards in pleasure applications, (because the prop can be stronger than the gear case parts) and for very occasional boaters who are happy with the existing performance of their boat.

Probably none! These are two separate objectives that require a separate specialty prop to accomplish each one. For example, while a five bladed propeller could increase your time onto plane, it will usually knock three to five mph off your top speed.
However, Mercury Marine has just introduced a new five blade stainless propeller that is tested to improve hole shot performance as well as some top end increases.
I will dig up some further data on this new propeller and keep you informed in coming issues.

First determine how the boat will be used, or what the normal load will be. If this boat normally operates with one specific passenger load, propeller size is relatively easy. If it has multiple uses ranging from light to heavy loads, the selection of one or two propellers may be necessary.

The stock propeller with witch 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 that rpm. The engine cover is marked with a certain h.p. rating but in most cases 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 related h.p. is developed This of course, is where the propeller comes into the picture. Outboard engines are designed to run at peak rpm for full efficiency. Excessive rpm with its increased friction and wear is obviously harmful to internal engine components. It is equally harm full to run engines so over loaded that it can not achieve its rated rpm. Since this results in excessive carbon build up with subsequent problems of poor fuel economy, pre-ignition, frequent spark plug failure, scoring of the cylinder walls and even burned pistons.

This can only be checked with a tachometer. There are various kinds commercially available. I recommend a photo tach, this will single out any electrical interference and will give actual RPM of the engine and or propeller shaft when relating to inboard applications.

These are the two common propeller measurements. If a propeller is specifies as 10x12 size, this indicates it is a 10" diameter by 12" pitch. Dimensions are always given in this order. Diameter is determined by doubling the distance between the blade tip and the center of the hub. Pitch refers to blade angle in this example the 12" pitch would indicate that with each propeller revolution the boat theoretically would advance 12" Forward. Due to slip loss, actual advancement is somewhat less.

This is due to differences in lower unit gear ratios. Factory production outboards are geared so that the propeller shaft turns at a slower speed than rpm at the power head. 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 the gear has 21. This means the propeller shaft turns only 57% as fast as the indicated rpm at the power head.
The lower the gear ratio, the larger the propeller that can be used and vise versa.
In other instances, engines of different makes and models may develop their horsepower at different levels. Everything else being nearly equal, higher rpm engines require smaller props to achieve greater rpm.

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 the boats hull 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.

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 cables should have enough sweep so as not to kink or stretch the cable. Cable end should be attaching the engine steering arm should be checked and lubricated annually to assure proper movement. Check boat bottom for warping, such as a hook or distortion, which could cause difficulty in steering.