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+--- Thread: Propellers 101 (/showthread.php?tid=323)
Propellers 101 - Chris Burkard - 03-04-2013
I get a lot of questions about props and I see a lot of posts about props, so I cover some information about proper propping of a vessel in this post.
1) First, props require maintenance just like every other component of your vessel. Even if you have not grounded or hit anything, props can change pitch and balance over time. This is just normal wear and tear. My suggestion is have your props pulled, scanned, tuned and balanced every 3 to 5 years. Of course if you ground or hit an object, they should be checked at that time. Out of balance or damaged props, they can cause vibration which can damage other components such as cutlass bearings, transmissions, shaft logs (and related seals), engine mounts and even the engines themselves.
2) Brand new props can be out of pitch or balance. Over the years I have seen brand new props with pitch and balance issues. These issues are usually not visible to the naked eye. My guess is that during shipping and handling the props are either dropped or stacked improperly causing a blade or blades to be put under enough stress to change the pitch. It is rare, but it can happen. If your vessel has a vibration issue, or a performance issue and you have eliminated all other possible issues, it may be time to check the props.
3) Propping a vessel is not an exact science. There are pros and cons with every prop design based on hull design, vessel loading, atmospheric conditions and water conditions. Did you know that salt water is more dense than fresh? Did you know that planning vessels tend to be slightly faster in salt water when temperatures and water conditions are similar? Manufacturers such as Tiara use base figures for windage, temperatures, and vessel loading when selecting the factory props. If you operate your vessel outside of these base parameters most of the time, you may need to adjust or in some cases replace the factory props to optimize your performance. Here are some examples: Vessels equipped with aftermarket fishing towers (which add weight and windage) generally require re-pitching or in some cases may require a completely different prop. If you regularly tow a tender, you will most likely have to re-pitch or replace the props. If you typically operate with heavier than normal loads, you may need to tweak the props in order not to overload the engines which can lead to engine damage or premature engine failure.
4) How do you know if your props are operating within design specifications and not overloading or under loading the engines? First, make sure your vessel bottom and running gear are clean. Make sure your engines are properly tuned and things such as your fuel system (filters, injectors, fuel tank and carburetors) are clean. Make sure your vessel is loaded as you typically operate (full of fuel, water, people and gear). Note: mechanical tachometers are notorious for being inaccurate and can be off by over 100 RPM. If your engines use mechanical tachometers, you may need to use a tach gun (should only be used by a professional as it is dangerous to operate a vessel at speed while in the engine room or with the engine hatch open) or a plug in electronic tachometer. Newer electronic engines have digital meters, which are very accurate. Check your engines maximum RPM stats under load (check your engine manual or call the engine manufacturer). Make sure your temperatures both water and air are typical. Only operate your vessel at WOT if the sea conditions are calm and any passengers are braced and or seated. You should also only operate your engines at WOT for less than 60 seconds. If your engines are rated for 3000 RPM, you should be able reach 3000 RPM at WOT. It is normal for the engine to go over the Maximum rating by up to 50 RPM. So any reading between 3000 and 3050 for this example would be considered normal. Everything should be smooth with no out of the normal noises should be heard. If your engines do not turn to their proper RPMs, then you need to create a speed/RPM graph.
Graph your RPMs and Speeds:
You will need a long straight away where you can run straight (safely and legally) for at least 7 to 10 minutes at speed. Starting at 1000 RPM, graph your speed. Increase to 1200 RPM (allow to stabilize) and graph your speed again. Do this every 200 RPM all the way to WOT. Then conduct the same test in the opposite direction. This will negate the effects of wind or current on the readings.
You will need your vessel information (model, size, weight, engines, fuel capacity and prop dimensions). Contact your local prop shop or distributer with this information and your speed/rpm graph and they should be able to help you either tweak your existing props or order you new props for your vessel.
Should I change from 3 bladed props to 4 or 5 bladed props? This is a common question I get. Tiara does a good job of selecting the correct props for the model. If however you operate your vessel well outside the norm such as adding a fishing tower or towing a tender, you may benefit by changing to a prop with more blades.
a. 3 bladed props are generally the most efficient for a typical inboard Tiara (29' to 41').
b. 4 bladed props are smoother (less vibration) and have quicker hole shot (quicker acceleration to planning speeds) than their 3 bladed siblings, but they also tend to have more surface area and therefore more drag which usually means lower cruising and WOT speeds. There is however, a point at which windage, drag and weight can get to a point where a prop with more blades and surface area becomes more efficient. Fishing towers, towing and larger heavier vessels commonly use 4, 5, 6 and 7 bladed props depending on size, weight, beam, load and engines. This is something your prop shop or distributer can go over with you based on your vessels specifications and your performance graph.
Good Luck. I am always available to answer questions.
Sincerely,