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Visualize the back of the motor at WOT. If you look at the prop, it is turning clockwise from your perspective. At low motor heights, the entire prop is underwater, the blades are in contact with the water for 360 degrees of their rotation, and the forces are equal in all directions and there is no torque at all. As you raise the motor to an extreme point, say where the propshaft is exactly even with the surface of the water, things are different. Now the prop is only in contact with the water through the lower 180 degrees of its travel, the blades on the top half of the circle are in air. As a blade contacts the water, the prop exerts an equal and opposite reaction to that direction of travel, which tries to push the propshaft to the right, as the prop tries to push the water to the left (and back also). Given the chance the motor will swivel to the right by itself, in reaction to this force being applied. Thats why, when you try to turn right, it is very easy, when you try to turn left, it is harder. Both of these effects are reduced as the propshaft is lowered. It is also why a true “blow-out” results in an instant left turn. The prop loses the bite with the water, and now acts like a wheel that tries to run across the top of the water in the direction the prop is rotating. Which makes you turn left “right now” when it blows out. Normally this wont happen until you reach speeds in the 80s, but it can be _very_ dangerous, or at least very exciting, to say the least. In theory you want as little of the gearcase in the water as is practical. In reality, going too far greatly increases torque and the potential for serious blow-out if you can reach higher speeds… So the lower the motor, the lower the torque and the lower the speed potential. The higher the motor, the higher the torque, and the higher the speed, up to the point where you have too little blade area in contact with the water, where slip starts to increase and speed starts to drop again. There is a “sweet spot” for every boat, motor, prop, load and weather combination. It varies if any external influence changes. Hi temps lower horsepower, which limits speed, which affects optimal motor height. Taking “bubba” fishing adds to the load, which will probably mean a lower motor height to avoid too much slip, etc…2008 Pantera Classic2014 Mercury Pro XS 200
Just a little apprehensive to go dowm more as I would be at 1/2″ lower than when I received the boat. But if the torque is what dictates the height its what Ill do. This will have me 1/4″ below flush on inner plate. I also see posts here and other forums with this boat/ motor combo where people say they run down the lake at WOT at 77 mph and can let go of the steering wheel. No way I could.Last edited by KerthPuma on July 17th, 2008, 5:20 pm, edited 1 time in total.
KerthPuma wrote: Just a little apprehensive to go dowm more as I would be at 1/2″ lower than when I received the boat. But if the torque is what dictates the height its what Ill do. This will have me 1/4″ below flush on inner plate. I also see posts here and other forums with this boat/ motor combo where people say they run down the lake at WOT at 77 mph and can let go of the steering wheel. No way I could. Dont be apprehensive…if you have torque you need to lower that motor. JL
I also see posts here and other forums with this boat/ motor combo where people say they run down the lake at WOT at 77 mph and can let go of the steering wheel. No way I could. You can let go of the steering wheel but I would never advise anyone to do that. You have to pick and choose conditions to do it! Just because one can let go of the wheel doesnt mean there isnt steering wheel torque to the left. Hydraulic steering will take care of the engine wanting to go right. In most cases, ones boat isnt balanced on pad to be able to do it and chinewalk will set in. In addition, you cant worry about where “Joe Blow” has his jack plate set. You have to set yours up where it runs best with your load, water conditions, and your desires. Some like to get every last tenth of 1 mph out of one. Some like a nice hole shot with decent top end speed. Some could care less if it takes 100 yds to get out of the hole. Some like to carry mega loads. Some dont carry squat. Some even have to deal with altitude and others have to compensate for high air temperatures and high humidity. Some run large resevoirs where most runs are in a straight line, others may be running river systems where there is a turn every 1/4 mile. As you can see there are multiple scenarios to deal with and there is no one set up plan that works for all. You throw in the differences between props and the actual horsepower engines deliver, then the situation becomes even more difficult to determine. One can even take a Puma with 20 gallons of fuel and just by changing the location of the fuel from the belly tank to the rear tank pick up 2-3 mph. Moving that 50# bag of plastic worms from the front storage box to one of the rear boxes may be another mph. Choosing a partner that weighs 185 vs. one that weighs 285 is about 2-3 mph or more. Running with no partner may be 5 mph. Running a MG trollingmotor vs. a Minn Kota may be another mph or two. Again, there is no way to duplicate setups. You can get close as Bass Cat does when the boat leave the plant, but you cant duplicate everyone of them. So to be apprehensive about moving a jack plate down makes no sense to me. You cant compare what others are doing if youre looking for 1- 2 mph or your torque at the wheel is a little stiffer than another or your 26p prop is only turning 5600 rpms where “Joe Blow” is turning 5800. Lastly, I prefer some torque at the wheel. Gives me a better feel of what the boat is trying to do.
Trimming the engine should reduce the torque at other speeds as it cleans the hull and removes bow steer.
Just total drag out of the whole?
That is what the PVS plugs are for, start removing them. One at a time.
when I have gone too low…..Steering at lower speeds is getting a bit harder, hole shot not seeming to improve meaning RPMs on takeoff not going up. I am thinking I have hit bottom on the low end…For this prop… I may go back up a 1/4 to where the steering seemed a bit easier and the RPMs on takeoff jumped to their highest point… Just trying to understand a theory that I have never learned before. I am a very hands on person with a HUGE thirst for knowledge. Not much I dont try and learn to do. Suspect at times I might make a choice to pass it off but most times at the least I want to understand and know how to do it…. The best part of all this is my 13 year old son learns along the way, asks allot of good questions and this boy is going to have a whole lot of hands on experience with a lot of things by the time he flies the coop…..May never do any of them but will never be left feeling helpless over them either…. Of course my 12 year old daughter gets a pretty good helping of it along the way as well.. Mom and Dad wont be around for ever…. Has anyone ever written any good articles on this type of boat set up and the principles of setting up outboards? I like to read as well.. Thanks again…..
Put the inside and outside plates at about level to start, then you can go up or down and fiddle with the hole shot and PVS plugs once you get the best top end possible with a load. Once you hit the best speed, lock it in and start with PVS plugs.
I moved it down another 1/4 and gained on the hole shot but then lowered it again another 1/4 and I dont think it helped but steering torgue at cruising was harder. I will try to slip in a plug or two large holes but that shoudl drag down the RPMs correct? I am going to try it again but will probably move it back up. Should try a 25 Fury if I can get my hands on it as well. Need to order a Detwiler from Phil and fooling around will be that much easier… I had an R boat go by today definately new or at least very new, he looked a few times….almost cut off another boat….funny to see….they dont see many cats in this part of the country…..Then we hit it and it was impressive….
Actually when we set one up, we never worry about hole shot adjustments till we find optimum height on the plate, then we begin adjusting the PVS plug sizes and the blow out ring. BCB
Yeppers, lower the engine. I run my at cowl high at wot.
Youre probably experiencing the learning curve. Typically lowering the plate will reduce steering torque, but over trim can affect your wheel torque too. At WOT your rooster tail should be below the top of the engine cowling. Very hard to tell someone what they need to do behind a computer screen and not behind the wheel, especially when a hydraulic plate is in the works. What is WOT speed? RPMs? prop? Lastly, you need to be aware that high engine heights and turns at WOT can be asking for trouble if you dont know what youre doing!
Phil is dead on. I gave this quote in another post, but it bears repeating: “Good decisions come from experience. Experience comes from bad decisions.” If you are lucky, the experience you get from a bad decision relating to engine height and abrupt steering input will be a drop in water pressure and a burned powerhead. If you are unlucky, you can cause what is commonly called a “blowout”. What happens is that the prop loses bite, but it will be turning something in the 3000 rpm range (about 1/2 crankshaft RPM). The prop will act like a wheel on the ground, except the “ground” is the surface of the water, and the prop will tear along the surface the same way as if you replace the prop with a wheel and sat the boat on the ground. This results in what we used to call a “bat-turn” back in the old days of the original batman TV show. Which is essentially a 90 degree turn _right now_. Problem is that physics includes inertia, and while the back of the boat swings sharply right, you continue to move in a straight line, which is over the side of the boat, and possibly into the path of that spinning “wheel” that is going to be a major problem. This is an extremely abrupt thing that is totally unexpected. And it is beyond dangerous. With the prop buried, this is impossible. When you get up to the extreme height area, such as propshaft aligned with the pad, then things get ripe for this event if you are not extremely careful. Bass boats are not especially known to do this, but they will. Believe me. This is a good place to push the envelope slowly and carefully. Trim angle is not as big a problem although once you get the motor “up there” then trim angle changes can cause the blowout also.2008 Pantera Classic2014 Mercury Pro XS 200
I am not a rookie as this is my second big boat, but we always have things to learn. There is quite a difference between my 200 and 250. Since it does not feel right to me even though it may be right I will do the safe thing and drop the motor a inch. I never experienced this phenomenom on my 200 with a manual plate.
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