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  1. #16
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    McGinnis, I'm white haired and have been chasing this speed thing a long, long time now. Horsepower is only part of the equation.

    You must "fly" not "float" your boat to go faster. Air is 814 times less dense than water, so going faster means getting the boat up and out of the water. I'm not a fluid dynamics engineer, but I've learned by asking, listening, trying hundreds of changes, and careful documentation and study of those changes.

    There was a fantastic article that I read 20+ years ago on surface area and drag. I did not save that article (wish I had), but it explained the affects of drag in water. Using a standard sized gearcase and a modern performance 20 foot hull, the surface drag of the submerged lower unit was more than the boat's entire frontal area being pushed through the air. As I remember, that speed trade off was in the upper 70's. The point here is that you must focus on getting your boat's hull to perform it's best at the "magic" angle of attack of 4 degrees positive to the water's surface. Just as a waterski, this angle produces great hydrodynamic lift while minimizing surface drag of the hull. Moving weight to the back near the transom and careful use of your power-trim!

    Getting your gearcase up and reducing the amount of it sticking down in the water is also very important. A water pressure gauge is absolutely required and don't be surprised if you'll need to plug the upper holes of the water inlets to prevent them sucking air. Don't jump to add LWP as extended gearcases generate even more drag than a stock/normal gearcase due to the longer surface (increased surface area). The real purpose of the extended (nose coned LWP) gearcase is to open the hole (V) earlier so that it closes ahead of the prop to prevent blowout of the propeller. Normally this occurs at 82~83 mph so you'll be fine with the stock gearcase as long as it can feed water to the powerhead as you raise. While you concentrate on the hull's angle, you'll need those high-rake props like the before mentioned Rakers and Trophy's. The Raker 24~26 pitch would be my choice unless you need the quicker holeshots of the 4 blades. My #1 prop of choice for all-round duty is the Trophy, but our boats are different.

    If you want to get a better idea of water's affect on drag, do this simple experiment. Using an old, manual boat speedometer with a bourdon tube mechanism, apply different air pressures to the tube feeding the speedometer and you'll see the required pressures go up exponentially - nothing close to linear……If memory serves me, going 40 MPH took some 22 psi, but 82 MPH took 125 psi - 4 times more pressure to go 2 times faster, so it's not linear. Another quick illustration of drag - stick your hand out the window at 60 with your palm vertical. Now imagine that air was water and 814 times more dense - your arm would be ripped from your torso. Keep the fact that "water drag" is way worse than air drag then make sure you set-up to minimize it. Never think some trim is good, so more is better. You want 100% of your prop shaft thrust pushing forward and the hull surface at 4 degrees, so setting your trim gauge to monitor neutral trim +4 is worth memorizing and getting right. A hull at 8 degrees and your prop thrust pushing the stern downward more than doubles the amount of wetted surface and the load you're trying to push through the water. You want to go over it - not through it. I've made a little guide to finding neutral trim and you can use it to help you center your gauge around it.


    Write down all the changes and study every result. There is no 1 propeller, 1 height, 1 setback dimension, 1 horsepower number that makes a boat fly. Be it 60, 70, 80, or 90+……it all takes careful set-up.

    Click image for larger version. 

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  2. #17
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    I'll add this too - those high-rake props like the Raker will hold water very well at elevated heights as you fine-tune to reduce drag.
    Be aware that you will have to maintain leverage in order to convert that forward thrust to bow lift. Trolling motors, storage lockers, gear, and passengers forward of the balance point all add to the weight that these props will need to lever. More setback will help to regain leverage lost when raising the outboard, but structural concerns may exist if 8", 10", or more are used. I've told a lot of new boat owners that it's addictive. Once you start chasing speed, you'll find yourself looking for that 1~2 mph more......next thing you know, you've bought yourself an Allison, STV, or Bullet, and some hotrod outboard to push it. Gordon

  3. #18
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    Quote Originally Posted by Gordon02 View Post
    I'll add this too - those high-rake props like the Raker will hold water very well at elevated heights as you fine-tune to reduce drag.
    Be aware that you will have to maintain leverage in order to convert that forward thrust to bow lift. Trolling motors, storage lockers, gear, and passengers forward of the balance point all add to the weight that these props will need to lever. More setback will help to regain leverage lost when raising the outboard, but structural concerns may exist if 8", 10", or more are used. I've told a lot of new boat owners that it's addictive. Once you start chasing speed, you'll find yourself looking for that 1~2 mph more......next thing you know, you've bought yourself an Allison, STV, or Bullet, and some hotrod outboard to push it. Gordon
    Lots of good information here thank you very much Gordon. That all makes a lot of sense and makes me think of how often I have over trimmed just to get the lower unit out of the water and reduce the drag without a jackplate. Althought I did gain speed from over trimming I was really just transfer the drag from the lower unit to the stern of the boat. I'm really excited to get the motor mounted and start testing props.

    One question I have is how do I find the neutral trim +4 degrees? Although I'm sure I will end up playing with the trim to find the sweet spot of my setup/load that day, knowing where neutral +4 is will be a good starting point.

  4. #19
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    The little drawing above should open up to full-size with a click. Chock your tires of course, then use your trailer tongue jack to level the pad - much easier if you have a helper, one to hold the level on the pad underneath and one to raise/lower the tongue jack. When your pad is level, then go to the cavitation plate above the prop and use the trim to level it. This setting is "Neutral Trim" - meaning that the prop shaft is neutral to the hull. This is where I like to set the center-line of the trim gauge. Anything above that while running is positive, anything below is negative.

    4 degrees is not much and if you've ever installed door jams or kitchen cabinets, you've probably used those thin wooden shims from your favorite home store. Those shims are probably 4 degrees. One of the coolest and cheapest tools you can buy yourself is a plastic $2~$3 incline meter. These are useful for just about everything and every man cave needs at least 1. I'll post a photo of mine next time I walk to my shop. I started using one back when I was making exhaust and set-up for my race bike. I find that it's got 100 uses around the boat too. I also found "Angle Pro" for your phone in the app store and it's free.

    Keep in mind that this 4 degrees positive is the theoretical perfect hull angle of attack and a 4 degree positive trim would have the propshaft sending 100% of it's available thrust forward - not up where it would push the bow into the water - not down where it would push the stern/transom down further both of which increase wetted surface and of course that horrible anchor called "drag". I'm glad I could be of help! Gordon

  5. #20
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    Quote Originally Posted by Gordon02 View Post
    The little drawing above should open up to full-size with a click. Chock your tires of course, then use your trailer tongue jack to level the pad - much easier if you have a helper, one to hold the level on the pad underneath and one to raise/lower the tongue jack. When your pad is level, then go to the cavitation plate above the prop and use the trim to level it. This setting is "Neutral Trim" - meaning that the prop shaft is neutral to the hull. This is where I like to set the center-line of the trim gauge. Anything above that while running is positive, anything below is negative.

    4 degrees is not much and if you've ever installed door jams or kitchen cabinets, you've probably used those thin wooden shims from your favorite home store. Those shims are probably 4 degrees. One of the coolest and cheapest tools you can buy yourself is a plastic $2~$3 incline meter. These are useful for just about everything and every man cave needs at least 1. I'll post a photo of mine next time I walk to my shop. I started using one back when I was making exhaust and set-up for my race bike. I find that it's got 100 uses around the boat too. I also found "Angle Pro" for your phone in the app store and it's free.

    Keep in mind that this 4 degrees positive is the theoretical perfect hull angle of attack and a 4 degree positive trim would have the propshaft sending 100% of it's available thrust forward - not up where it would push the bow into the water - not down where it would push the stern/transom down further both of which increase wetted surface and of course that horrible anchor called "drag". I'm glad I could be of help! Gordon
    Okay perfect, I guess as long as I know where neutral trim is I can adjust from there, Thanks or the help!

  6. #21
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    Here is a pic of the cheapo incline gauge. You can see that I've added a pencil to one end and got the 4 degrees showing. This one is my long-time favorite because it will fit over tubing with the V edge while the other is flat and has a large magnet. I use this thing all the time - even

    making brackets for my favorite wallpaper! Gordon


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  7. #22
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    I have an incline gage as an I phone app

  8. #23
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    Start a separate thread. But first and foremost all motors need to be proped to at least get into the peek power curv which startes at 5500 so prop your boat to rev up to 6000 then work jack plate an trim after that and every prop size up is aprox 200 to 300 RPM
    Joshaua A. Dolfi

  9. #24
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    Quote Originally Posted by joshuah40 View Post
    Start a separate thread. But first and foremost all motors need to be proped to at least get into the peek power curv which startes at 5500 so prop your boat to rev up to 6000 then work jack plate an trim after that and every prop size up is aprox 200 to 300 RPM
    Good to know, thank you. I would love to be able to spin a 26 pitch prop to 5800rpm but I just don't think that's going to happen. I'm thinking my 24 will do the trick, might have to get it worked to a 25 or something but I won't know until I get the boat in the water in the spring.

  10. #25
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    I have an update, after spending some time running the boat with the new motor. I set the jackplate at 3 1/4" below pad to start and went up from there. The best I found was 3" below pad but I was only able to get 62mph at 5600 rpm. I tried going up another 1/4" and I gained a little bit more speed and rpm but I did loose some bow lift. It feels like I need a lot of positive trim to get my rpms up but I'm also surprised I can't spin the 24 Raker more than 5600 rpm.

    I'm hoping some of the experts can guide me to where I need to go next as I am fairly new to setting up bass boats. I will say I run pretty heavy, 3 batteries, full tank of gas and 2 guys with a fair bit of tackle. I am considering maybe a new 4 blade prop for the stability or dropping to a 23 Tempest but I thought the 24 Raker should spin more rpm. I would be very happy if I could get 65mph out of my boat.

  11. #26
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    I would try a Trophy, start with a 23 they carry weight well even at higher heights. May take some playing with the PVS vents and ring for the best holeshot. But that can be done after you get the top end dialed in.

    Dave
    1980 Cougar 19 tunnel,90 2.4L Bridgeport EFI in middle of restoration.
    1988 BAJA Sunsport 186, 96 225 Pro Max
    79 12' Auminum, 95 Merc 9.9
    RIP Stu
    "So many idiots, so few bullets"

  12. #27
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    Quote Originally Posted by Dave Strong View Post
    I would try a Trophy, start with a 23 they carry weight well even at higher heights. May take some playing with the PVS vents and ring for the best holeshot. But that can be done after you get the top end dialed in.

    Dave
    Okay thank you for the suggestion, I was thinking either a 23 Trophy or a 23 Tempest. I'm just very surprised I can't turn more rpm with the 24 Raker.

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