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How to set engine height & select the right prop!

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  • How to set engine height & select the right prop!

    We have had a few threads recently where people are asking how to select the "best" prop for their boat/engine combo.

    So I thought I would put up here the way my mates and I in our fishing club go about doing this. I'd also acknowledge Ken from the for info in other threads, amd Glen E who wrote a most useful article and instructions about this some years ago on the Verado Club website, which all of us have followed and which has greatly informed and influenced the info provided below.

    Rule #1:
    Rule 1 is that before you start playing around swapping props, you need to get your engine height set correctly. Now you are asking: wouldn't the dealer/boat builder set the height correctly? Well the reality is that many of them don't! I would say from what I've seen that at least 1/3 of boats have their engines buried too deep.

    This is bad. It alters the angle of thrust from the engine to the hull, and it can make an otherwise great boat handle like a dog, and a fantastic engine perform like a slug. We've got an engineer in our club and he can bore the life out of you with the technical reasons as to why this is so! So take this as fact.

    A motor set too deep increases drag on the lower unit, which increases fuel consumption and reduces revs. It can also cause the boat to porpoise.

    Setting engine height correctly DOES NOT involve rulers under the hull etc. So, forget that!

    It DOES involve a water test where we put the boat onto the plane and reach a good fast cruise speed, engine trimmed out to achieve optimum revs, and someone goes to the transom and looks over the back to see where the anti-vent plate is running compared to the water surface.

    It SHOULD be skimming the water surface. It SHOULD NOT be submerged under water and creating unnecessary drag.The anti-ventilation plate is the large plate immediately above the prop. It is sometimes referred to as the cavitation plate (wrong name!)

    We have seen gains of 300-400rpm and 10-20% improved economy and speed just by getting the engine up to where it belongs! I am not joking! So it's pointless playing about with props if the height is wrong to start with.

    So do the above check, and if your engine is set too deep, lift it up and test again, and repeat until you get it right.

    Make sure that your water test includes some moderate turns at cruise speed to check that the prop doesn't "let go" or lose grip too easily in turns. You may need to trim the motor in (down) a little on some boats to keep a good grip. If the prop does slip a little bit, that's ok, it can be fixed later by adding a little cup to the prop blades or selecting a better prop. Stainless steel props typically grip heaps better than ally props.

    Rule #2: Your Suzuki 4 stroke needs to hit its max recommended rpm or close to that to perform its best and have a long, happy life!

    Many Suzuki 4 strokes have a stated rpm range of 5000-6000rpm. Our Suzuki engines just love to rev, and indeed, they NEED to be able to rev to perform to their optimum.

    It is a complete falsehood to run a bigger pitch propellor than the optimum in order to get greater top speed or a better cruise speed!! It is just wrong!

    You may rarely run your Suzuki at WOT (wide open throttle) but it needs to be capable of reaching max revs (or within 100-150rpm of that) at WOT to perform well - to achieve the best balance between speed, acceleration, fuel economy and engine life.

    If your engine will only reach the lower end of the rev range, say 5100 on an engine with a 5000-6000rpm range, then you are lugging the engine, putting too much load onto bearings and rods, using too much fuel, and you will reduce its serviceable life significantly. Not a good idea.

    Rule 3: You need facts and data!
    W Edwards Deming said: "Without data, you are just another person with an opinion".. This applies 100% to testing props.

    Once you've got the engine height set to optimum, the best idea is to establish a good relationship with your local Suzuki dealer or a prop shop, who will let you trial a couple of props until you find one that performs to the optimum on your boat.

    The procedure to test props
    Once the engine height is set right, we then test the current prop by finding a smooth bit of water, using the standard load the boat usually carries, and at every 500rpm from idle to wot (trimmed out) we record Speed and Fuel economy. We make up a table on a sheet of paper like this:

    RPM Speed. Fuel ec (mpg)
    1000. 6knt. 7.2mpg
    1500. 8knt. 7.0mpg
    And so on, all the way up to WOT.

    When we get to max achievable revs, we trim the engine up in increments until eventually the prop loses grip on the water, then quickly trim down a touch until it grips again. We record that as the max rpm at WOT. Don't worry, these great Suzuki engines all have rev limiters so you can't hurt the engine when the prop lets go.

    We then do the following other tests:
    1. a "hole shot" test where we time how long it takes to hit 5000rpm when we hit the throttle hard from idle speed. Use the stopwatch on your mobile phone!

    2. We also record the minimum speed and rpm that the prop will hold the boat on the plane at. This is important for rough offshore weather where you can't go fast but want to keep the boat on the plane.

    3. Finally, we find the optimum cruise speed for fuel economy and record the revs, speed and fuel economy at that point.

    This data gives us the baseline we need to compare new props to. It is also incredibly useful data to provide to your Suzuki dealer or prop shop, as it will massively help them to recommend a better prop for your boat.

    Selecting a new prop
    The rule of thumb is that if you stick with the same brand and model prop as you have now, generally 1" of pitch = 150-200rpm change.

    If you change to a high-end prop like a Intertia Eco or another model, then you can expect less slip and thus achieve higher rpm from a prop of the same pitch. Some advice from a prop expert using your data can help the process of selecting a suitable model prop for your boat/motor. Also, as a general rule, 4 blade props create more stern lift and better mid range fuel economy.

    We then test new props exactly the same way as above, and then compare the data. It soon becomes obvious if you've got a good prop or not.

    The final choice between the props that perform best on these tests can then come down to other characteristics such as which prop gives the lift characteristics you are looking for - some props provide greater stern lift than others, for example. You might be prepared to sacrifice a little bit of top speed for a prop that holds your boat on plane better and gives better economy at cruise speed. Different makes/models of props can deliver such benefits.

    Although I will say that in my experience, OEM Suzuki props do work very well on our Suzuki engines.

    So, in summary:
    1. Check engine height and adjust if required. Many are set too deep which greatly compromises performance. Do not proceed to step 2 until you've done this!

    2. Do the prop tests as described above and record the data so you can compare it with other props.

    3. Select a prop that gives the best results and gets you up to, or within 100-150rpm, of the max stated rpm for your engine.
    Last edited by Moonlighter; 10-12-2018, 01:25 AM.

  • #2
    Using the boat trailer method to lift engine height

    How to lift outboard engine height at home using your boat trailer.

    We have used this method to lift 4 cyl motors and even some smaller V6's. (The bigger motors are best done using some form of lifting device. A block and tackle attached to the motor lifting points is one way.)

    Tools and supplies:
    - You will need yourself plus 2 people to help. Some strength is required so choose accordingly!
    - Tools- wrench or sockets of correct size for nuts on transom bolts. Plus a paint scraper to scrape old Sika off.
    - Supplies - sealant such as 4200 or Sika 291 to seal the bolts where they penetrate the transom. Some alcohol based solvent to clean up excess Sika. We use methylated spirits.

    1. Put boat/trailer on a flat piece of ground, and chock the wheels so trailer can't move. Disconnect from tow vehicle. Trim the motor down so it is vertical.
    2. Place a block of heavy wood under the outboard skeg so it is an inch or so below the skeg. You might need to use some bricks etc to get it to that height, just make sure they are very stable as they will need to take the weight of the outboard and be stable.
    3. Wind the jockey wheel up - this will lower the stern of the boat. Wind until the skeg is sitting firmly on the block of wood you put there in step 2. It should be clearly taking the weight of the motor. The skeg is easily strong enough to take the motors full weight in this position.
    4. Loosen the bolts holding the motor on. Some motors have a slot on the bottom bolts, if so, they just need to be loosened off so the motor can easily slide up or down. If not, all bolts have to be loosened right off and the nuts removed.
    5. At this point your helpers need to be on each side of the motor, holding it steady. They can now carefully tilt the motor slightly back so it comes off the bolts.
    6. While they balance the motor, you go back to the front of the trailer and wind the jockey wheel up some more, and your helpers tell you when it is enough so the bolts line up with the correct holes in the motor bracket that you want to use.
    7. Go back around to the motor and clean the old Sika from around the bolts, use the scraper to clean it up, and then apply some fresh Sika to the bolts and adjacent area so it will all seal up properly again.
    8. Slide the motor back onto the bolts and put the nuts on and tighten up. Clean up excess Sika as required.
    9. Wind the jockey wheel back down so the bolts are once again taking to weight o ft he motor, and check that it is all good and hasn't moved.