Propellors getting it right
Propellors can make the difference a great boat with a good performance and economy and a dog of a boat. Rick Huckstepp explains how you can get the best from your boat.
The "in" word in the new boat sales industry has for the past five years, been, "packages'.
A package allows one to walk into a showroom or yard and purchase a complete unit, hitch it onto the vehicle and go fishing. Sounds easy, doesn't it?
The add ons to dress up the base product will be as many as one can afford. Things such as built in fuel and water tanks, canopies, auxiliary motors, rod holders and anchor winces are just a few of the extras that may find their way onto the new purchase before it leaves the showroom floor. And then there is the myriad of marine electronics. Given an unlimited budget, the average family boat could get to look like the cockpit of a space shuttle!
"Packages" are all the range in the boating industry these days. It is a fantastic way to buy a boat, however it may not be "propped" for the best all round performance. Many things can influence what prop your boat needs'.
The engines that are bolted to the transom are often those that the boat dealer is franchised with. If you're lucky, he might have a couple of brands that he deals with, but often there is only one. In many cases, especially with engines around seventy horsepower or less, a standard propeller comes fitted on the leg, in the crate.
Not too many years back a standard propeller was feasible. Most runabouts back then had a forward steer, set seating arrangements and pretty much similar weight distribution.
Nowadays things are a whole lot different. A particular hull may be optioned to have a center console with overhead hard top or forward steer with windscreen or a simple tiller steer. Seating is optional in many cases on factory run hulls and with anglers fishing further afield, a number of fuel and water tanks are available - above or below the floor.
The option list given to you by the dealer nominates the goodies available to the buyer and also the engine size and propeller specifications. Nowhere does it take into account, the added payload of extra iceboxes, extra fuel capacity or even the fact that mum and dad might weigh in 100 kilos or more, apiece.
What might have been a brilliant match of motor to boat for the average family fun about in some cases turns into a floating nightmare for other buyers.
To a certain extent, a lot of the problems generated by an increase in payload, the change of that weight distribution and other factors such as wind resistance on canopies and consoles, may be assessed at the propeller. A significant difference can also occur with altitude and the different floatation properties of fresh and saltwater.
If a boat is not performing well due to excessive weight, or some of the other factors pointed out above, stepping down the pitch may get the engine revs back to where they should be, lift the vessel out of the water more easily and improve the over all fuel economy.
Depending on the amount of change needed required to get the vessel to perform to a particular level, some other part of the performance is often sacrificed. Usually though, some of the top end speed is lost which is often not as bad as it sounds.
Running across choppy seas flat stick is great if you are into chiropractors and other forms of pain relief. Unless you have a vessel designed to do it continually, that sort of boating has a minimal following and the toughest of them will eventually crack, passengers included. One is usually better off to have a comfortable cruise speed, the ability to get "out of the hole" quickly and maintain a slower speed while still on the plane.
An engine does not attain its rated horsepower unless the correct RPM is reached. A change at the propeller often will address this lack of power.
My tri hull had a problem getting out of the water cleanly and quickly. The three bladed prop was 15-inch diameter with a 19-inch pitch on a 90 horsepower motor. The extra weight of a four stroke sitting on the transom and nothing much mid ship kept it in the hole too long and it was bad news in big seas if I had to duck and weave around the breaking swell, or carry extremely heavy loads such as ice boxes and bulk fuel on extended trips along the coast. That prop gave me a top end speed of 29.5 knots on the GPS. Most of the time, short sharp chop prevents vessels travelling at that speed.
After getting some advise from Steve Evans at Solas Propellers, the problems were identified and I was advised to change to a four bladed propeller with reduced diameter and pitch. The four bladed model would give me more lift at the transom and keep the vessel on a lower altitude at a slower speed. This change over did just that.
I trialed a four blade, 15 inch pitch which when given a fist full of throttle, nearly snapped your neck! This prop gave me around 25 knots top end speed and at 4500 rpm a cruise speed of around 19 knots.
I decided on a 13.25 inch diameter prop with a 17 inch pitch. This prop reduced my top end speed back to 26 knots from the original 29 given by the three blade prop and offered a 4500 rpm cruise speed of 20 knots. The vessel can easily carry all my supplies for a week down the coast, a passenger, tackle and three hundred liters of fuel gets on the plane nicely.
When making the purchase of a new engine or replacing a worn or broken propeller, a little forward thinking might save you a lot of wear and tear in the power head and heaps in fuel both of which equate to bucks left in your pocket.
It may be that you will need to have two propellers. I always carry a spare in the boat anyway, so it may as well be one that earns its keep whilst on the motor.
The days where I do lots of river work with small chop and only a small payload, the 19-inch pitch gives the tri hull long legs. It's handy for covering the miles quickly.
A trip up the coast for a few days sees the four-blade back on. It's a great excuse to grease the spline and release all the tangled fishing line off the seal at the same time.
On the next page I run through some of the terminology on props and some of the options open to the new prop buyer.
Propellors - the finer points
Understanding some of the finer points such as pitches and what they mean to the way your engine behaves is a little more in depth. I will spell out some of the terminology as explained to me by Steve Evans of Solas Propellers, shortly.
In the mean time though, one of the most asked questions when the subject of propellers arise, is weather to go the added expense of buying the stainless steel propeller or stick with an aliminium type. Most outboards under 70 horsepower are supplied with a "standard" propeller (if there is such a thing these days) and many over 70 horsepower are supplied without any propeller.
When a propeller that is spinning at any rpm comes to a complete stop against a rock, floating log or the concrete boat ramp, some part, somewhere, will be on the receiving end of some amount of damage. An aluminium propeller blade will bend, break off or wear down. The rubber bushing holding it onto its hub that fits over the outer of the female spline might slip. This is what it is supposed to do, to prevent damage to the gearbox in the leg.
A stainless steel propeller is up to double the weight of an aluminium model of the same type. They tend to be thinner in the blade gauge and the extra weight acts as a flywheel, allowing the engine to rev. out harder. Stainless steel propellers are great if you can afford the extra price and have generally clear water to operate in. They wear less in sand and will chop through sticks and branches without too much damage to their cutting edge. Hitting the same hard objects as explained in the scenario with the aluminium prop and there could be a few problems.
Only you know what the chances are of damaging the prop or leg of the outboard motor you are operating in your particular waters. Stainless may be the way to go for you. Personally I like them. My years of professional guiding in shallow sandy country took its toll on many aluminium propellers and a lot less stainless steel models.
My favorite propeller is a 13 inch four blade as explained on the previous page. Unfortunately at this point in time I cannot get this model in stainless, but was assured by Solas Propellers that the aluminium used in this Alcup 2000 propellers was the next best thing to stainless. They were right again. I have hit some horrific submerged objects that have stalled the 90 hp Honda, chopped through trees, carved through gravel beds and hit logs in the Daly River, traveling in reverse, that has put everyone on their bums! I have given it only one panel beating job and the blades are still well shaped. You can tell the difference in the tensile of the metal used, just by flicking it with your finger. It sounds high tensile and gives of a "ping" rather than a dull noise.
Now to the technical side of things.
A propeller has information stamped on it, some of it being a set of numbers such as 10x12. The "10" is the diameter of the propeller and is always the first number of the two. That measurement is the overall width of the propeller, from blade tip to blade tip.
This refers to the angle of the blade. A propeller with a pitch of 12 means it is a 12 inch pitch and in one revolution, the boat should move forward 12 inches. Slippage in the water means that this is not exactly correct and that distance would be less than stated.
The correct combination of diameter and pitch may mean the difference of running an absolute pig of a boat that is an incredible fuel guzzler to one that is a pleasure to be in and doesn't keep you poor. It is that important.
Your out board engine is designed to run at its best, developing its stated horsepower at a particular R.P.M. If this R.P.M. is not reached, the engine cannot give you the return that it should.
Running a propeller that allows the engine to over rev. can result in premature wear from excess friction.
On the other end of the spectrum, over loading the engine will choke it up with carbon, which will reduce plug life and may damage the cylinders and pistons.
If you are a small boat operator and your vessel does not have a tachometer, your local dealer might be able to assist you at the boat ramp with a vibration tacho. This little instrument is held against a solid part of the engine. Not exact mind you, but it should get you with in the ball park and at least go some way towards ensuring that your out board engine is running at its optimal performance.
Should you damage your propeller, all is not lost. In many instances, welding may repair aluminium and stainless steel propellers. A trades person who understands pitches and diameters should carry out this type of repair. The end result should be that the propeller is balanced. Imaging breaking the tip of one blade only and the repair resulted in that part being replaced by a heavier and or larger piece of metal that throws the whole shooting match out of balance. The wear and tear to the seals and bearings inside the gear case will be accelerated dramatically. I found that repairing the aluminium propeller by welding resulted in a soft blade, having lost some of its temper around the weld site and the additional metal being soft to start with. These re-welds usually did not last long, but rather, got me out of a predicament while a new propeller was ordered in.
Bear in mind though that I demanded a lot from my equipment and the average boater may be a lot easier on gear than I.
I have not had to repair a stainless steel propeller with welding. In they past they generally wore down and often lasted the life of a two-stroke motor, which was somewhere between 800 and 1000 hours, which equates to a years work for me. The reduced diameter meant that the engine would over rev so one had to be mindful not to run at full throttle on a worn down propeller.
On occasions when the propeller contacts a solid object and stops dead, the rubber bushing between the hub and the spline tears away and will allow the unit to turn separate to that spline. This might not be easily recognized as that bushing is pressed in under high pressure and it may be that the problem will only show under hard acceleration from a standing start. It will be noticeable, when the engine is revving and it appears that speed of the vessel is not picking up as the motor increases its revolutions. Sort of like wheel spin of a cars tyres in gravel. This over revving should not be confused with cavitation, which may be caused, by any number of factors which will be looked at, at a later time.