Flat bottom planning hull speed

Discussion in 'Projects & Proposals' started by bradkitcher, Mar 11, 2010.

  1. liki
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    liki Senior Member

    One more thought, if any standard engine labeled as 15hp is allowed, and they are not tested, you could find a model which in fact produces considerably more power than the advertised 15hp.
     
  2. bradkitcher
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    bradkitcher Junior Member

    Already have. sorry. Yes they are tested. No major modifications allowed to the engine for performance.
     
  3. gonzo
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    gonzo Senior Member

    A lighter crew will make the boat float higher and reduce the wetted surface.
     
  4. bradkitcher
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    bradkitcher Junior Member

    Come on seriously is that your best advice. Everyone here is well aware of the efficiencies of power to weight ratio.
    Weight of boat/engine/fuel/crew member 1 and crew member 2 are not changeable. I need someone to think outside the box.
    You are talking about a sport that has been around for nearly 30 years. All the basics were tried long ago.
    Help
     
  5. SamSam
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    SamSam Senior Member

    If you are close to the max speed, one thing isn't going to do it, a bunch of little things will have to add up. You seem to have an awful lot of windage. Can't one of you lay down flat and the other make yourself more streamlined? Wax the bottom of the boat. All those nicks and gouges slow you down. The screw heads in the strakes look pretty rough. The small deadrise at the sides can't help at all.
     
  6. gonzo
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    gonzo Senior Member

    I am serious. How much sacrifice are you willing to do? A diet and exercise can help you shed a few pounds. Maybe enough to win.
     
  7. Yellowjacket
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    Yellowjacket Senior Member

    The deadrise is there so that the boat won't trip on the chines and flip sideways in a turn if the boat is skidding. One thing that will help for sure would be a small spray rail to get the spray off of the chine. That is, a small lip that is out of the water when at speed, but is just above the waterline so that the spray gets knocked off of the chine. It probably needs to be about an inch thick, again, you can use a piece of aluminum angle, glue it on with RTV and try it in several positions.

    The spray is wetted area and creates drag so you want to get rid of it... Directing it down will create lift and also reduce drag. Both are goodness...
     
  8. Yellowjacket
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    Yellowjacket Senior Member

    One other thought on prop pitch. First you need to KNOW (not conjecture or estimate) at what speed the engine makes maximum power. This means a dyno sheet and not what "everybody thinks" is the speed for best power.

    Second, look at what RPM you are running now. If you are currently at that speed, you have the wrong pitch for where you want to be. For example, (and these are just numbers picked out of the air but are being used for example) if you are turning 6400 rpm now, and that is the engine speed for best power, you want to increase speed by 3 kts. At your current speed of 32 kts, that is almost a 10% increase in rpm's if you use the same prop that you have on it now. If you are at the power peak now, you are going to be turning about 7,000 rpm and that is likely past the power peak for that engine. These "fishing" motors aren't ported and they have a fairly big fall off in power if you go to higher speeds than that of the peak power point. Think about pitching the prop for the speed that you want to go and not where you are now. My guess is that you won't lose much speed with a bit more pitch, and you will have more power as you develop your rig.

    You first need to figure out why you aren't as fast with your setup as your competition. If you do figure out something that helps you, your competition will copy it, and they will be that much faster again. Testing is the key here, and you need to be willing to do some testing to run a variation of props, motor heights, setback of the motor and other things to find out what works. As noted above there isn't going to be a silver bullet that makes you 3 knots faster, it is going to take a good bit of work to get there.

    Finally, are the rules for your racing posted on the net. If we had a set of rules that we could see, then perhaps that might be a bit more helpful.
     
  9. Bigfoot1
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    Bigfoot1 Junior Member

    options to try

    1)The porpoising means that the center of lift and the dynamic center of gravity (DCG) are close to optimum. Ie the center of lift is moving ahead of the DCG in one moment, then behind it the next, this is a good thing.
    2) extra width means more lift than extra length, but the shorter the wetted surface means that the porpoising may get a little worse. Ie say you increase your wetted surface 6 inches in width, the stagnation line of the wetted surface, ie the line where the hull exhibits the highest pressure, highest lift is longer. Rather than extend the length of the wetted surface which will produce some more lift, but not as much on a per square inch of wetted surface.

    3) Someone mentioned that there are rules about running a lift strake down the bottom of the hull. Realize that even though the hull is flat, that there is a component of water moving from the inside of the keel to the outside, not much looking at the video and flat of course, but a 1 inch by 1/4 inch vertical set of runners will prevent some of this movement and increase lift. But if the rules do not allow it, then it is a moot point.

    4) I agree on the prop, if it is the best, can you cup it and or make the blades thinner

    5) you have some porpoising which means that the prop is not pushing the boat in the direction that you want it to go, ie bow up, there is HP lost in the downward direction, bow down, same scenario
    If you felt experimental, then a small trim tab, on each side of the motor extended behind the transom say 12 inches, so when the boat wants to rise, the tabs will make contact, and keep the bow fromm rising, as the trailing edge would be back say 12 inches, the movement caused by porpoising would be effective

    6) I would file the leading edge of any part on the motor that you can.
    Ventilation plate,
    perhaps, if the rules allow it,add a nose cone onto the prop hub and make it pointed. File the trailing edge of any part on the motor that you can to reduce
    drag. Sharpen the edges of the prop,

    7) this would be hard to do, but you need to get an idea of the average angle of rise that the boat works through, say it works out to 4 degrees up. then measure this angle with respect to the cavitation plate angle, the cavitation plate angle should be horizontal as you can make it to the water. otherwise it will cause drag
     
  10. bradkitcher
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    bradkitcher Junior Member

    Big foot great response.
    1)Does that mean the further we move back the better?
    2)Not really sure what you mean?
    3)No rules. We have two 1/3 inch runner on the outside edge of the planning surface from the back forward about 4 foot.
    4)Props are cupped and very thin
    5) Cav plate of gearbox is 5 -7 inches above the water and planning hull as we are semi surfacing.
    When we trim the engine to push the bow down and decrease/stop porposing the boat goes slower because of water friction from the entire planning length being in contact with the water. Porpoising actually seems faster because of water friction??
    Trim tabs dont work. We have tried it.
    6) Gearboxes and covered in impact rubber and hence cant really be sharper than they are. They also have a nose cone made of rubber. Prop blades are razor sharp.
    7) Cav plate above the water by 4-6 inches.

    Keep going great questions??
     
  11. bradkitcher
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    bradkitcher Junior Member

    Great ideas. I have a few questions however. Max torque and max RPM are never at the same place on a 2 stroke is that correct?
    What is the cheapest way to self Dyno? Any ideas?
    If my max torque is at 6500rpm but I run my quickest speed on the water at 7100 RPM then do I;
    Increase pitch to drop the RPM.
    Or
    Adjust tuning to drop max RPM to 6500RPM then try out the same propeller?

    Please help?:confused:
     
  12. Yellowjacket
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    Yellowjacket Senior Member

    Max torque is almost always at a lower speed than max power.

    horsepower = T x rpm / 5252

    Power increases beyond the torqe peak because even though you are making less torque, the rpm is still going up. What happens is that at higher speed the torque starts to fall off faster than the speed increase, so power starts dropping off. Depending on the porting the fall off in torque and power can be pretty dramatic and the speed that you want to run at may be higher or lower than where you are now.

    Most of the guys running the APBA have a number of props that they can switch out and try. Knowing the power and the RPM makes much shorter work of the process. If you can find somebody who has a dyno that can accept your engine, that is, in the long run easier, since it takes the cut and try out of the process. Another problem is that when you change props, you can get more or less lift, and that can muddy up your data. It is always best to make one change at a time, measure your results, document what you did and then make another change. You need to take data carefully and see what each change does. Another thing is to ask the guys who are running faster, what rpm's they are seeing. If the engines are prepared similarily, that's the best place to start.

    Since you don't know where you are, more pitch or less pitch may help. That's why knowing the power you have to work with is important. If you make a change to pitch and the change is positive, that is, you get more speed, you continue to change pitch until you slow back down and fine tune it from there.

    If you are running 6500 rpm as a max torque, and are hitting 7100, you are going to need to go to 7800 rpm to go 3 knots faster, and I would think that you are probably over the hill on pitch already. I would try more pitch first, and see what you get out of it.
     
  13. Bigfoot1
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    Bigfoot1 Junior Member

    1) and 5) because you are porpoising, the center of lift and the dynamic center of gravity is very close together. As you said, if you trim the engine down, you get more wetted green water contact and more skin friction.
    The trim tabs that I was mentioning are not the standard trim tab, the leading edge of the trim tab would be attached to the boat say 1 inch up the transom off the bottom of the hull, and the trailing edge would be say 12 inches back,
    Say your average hull angle is 4 degrees up when running before porpoising starts, you would set your tab angle so that the trailing edge is not contacting the water at this angle, as the bow begins to rise on the up movement, the trailing edge would just begin to impinge on the water, but with little resistance to the direction that you are going in. On some race cars, that could lift the front wheels off the strip, you will see ladders, ie a structure that does not touch the ground when the car is flat or even up to say 5degrees of up angle, but then the wheels on the back of the ladders makes contact, not exactly the same as a boat, but the idea is that the trim tab suppresses the porpoising when the bow is coming up only.

    2) I will try to be brief.
    When a hull is moving, the water hits the bottom of the boat and some of the water has to move away, ie the boat makes a trough in the water. Take a line from the front of the wetted surface to the transom. About a third of the way back the water pressure is the highest of any point along this line. call this the stagnation point from this point forward the pressure dimenishes to zero guage, ie atmostpheric, from this point back, the pressure dimenishes to zero as well.
    If you were to take pressure readings along this line, the plot would look something like a sine curve, So say you have 4 feet of wetted surface, and extend this to 5 feet, ie by increasing the lenght of the boat, the pressure curve plot just extends further, ie the slope from the max point 1/3 of the way back just softens out, This means that for an increase of wetted surface of 1 foot, you get a modest increase in lift. (to the naval engineers, this is not the whole picture but the theory) Ie the stagnation pressure does not really change ( a bit) so with increasing length, you would get some extra lift, but on a lift unit per square inch of wetted surface, it was not efficient.
    But if you add say 1 foot of width.
    now you get a another foot of max pressure, along the stagnation point, which lifts the boat higher, ie less drag, not from wetted surface, but from displacing the water that you make when the hull goes along the water surface.

    When we used to design jet boats back in the late 70's, the first design that we had experience with was a boat that was 21 feet long and about 5 feet of chine width. We put in 460 packajet engines, 330 hp prox. With 3 guys, some dive tanks, and 100 gallons of fuel, the hull could barely lift the boat on step. We would have to run the boat at about 4000 rpm, which was close to the max upper limit that we could get out of the pump. I cannot remember the hp curves, but we would probably be putting about 250 hp out of the motor.



    We did a redesign and made the chine width and went to about 6 foot 8 of chine width.
    With the same hp motor, we can get the hulls on step with the same loading, and cruise at about 3200, and our fuel consumption shows that are using about 150 of engine hp.

    This is not my idea, over the past 40 years, many power boat manufacturers are making the beams wider to accomodate this reality.


    One thing that I missed before is that in the video the air resistance to the driver and nav must be huge. Certainly the driver has to see,, but perhaps the second person could hunker down,

    It is best to keep as much weight back as you can but it looks like you are there.

    good luck
     
  14. Bigfoot1
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    Bigfoot1 Junior Member

    some comments on HP and Torque

    I have read many many boating magazines and it is evident from the expert writers that they do not understand the relationship

    First Axiom
    It takes a fixed amount of horsepower to move a given hull through the water at a given speed.
    Not torque, Horsepower.
    Horsepower is the rate of doing work. say foot pounds per second

    Torque is a static load acting on a moment arm. foot pounds

    As YellowJacket said, you have to find the max power rpm of the engine

    And how will you know that the engine is putting out the max horsepower that it can?

    This occurs when the engine will not turn any higher rpm

    So you put a prop on that limits the engine rpm to say 3000 rpm WOT, ie it will not turn past this rpm as the prop is absorbing this horsepower. But at this rpm, the engine is producing 100 hp. Now you change the prop that allows the engine to turn 3200 max at WOT, wide open throttle, and at that rpm the engine is producing 110 horsepower, and say now that is the max horsepower available from the engine, (if it were dyno'd) but now you change the the prop so that the engine turns 3400 max at WOT, but the engine has fallen of its peak horsepower ability, and is now at 90 horsepower, your boat will not go as fast.

    Dynos, measure torque, (various means, electrical, hydraulic, friction, strain in shafts) in static foot pounds at given rpms ( or at least in the US) then either a computor or a person plugs in RPM and the torque value and the horsepower
    is calculated.

    The only time that an engines actual horsepower output will be the same horsepower as on a horsepower curve is when the engine cannot go past that rpm limit.

    So dont chase the maximum torque point on the curve, You are trying to find the max horsepower point,

    Summary
    change props so that at wot the max rpm the engine is making is at the max horsepower rating rpm point of the engine.
     

  15. bradkitcher
    Joined: Mar 2010
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    bradkitcher Junior Member

    I really appreciate the engine advice. Now all I need to do is find a dyno at home or nearby.
    Does anyone know if you can use a gokart dyno or another type.
    I cant find one Outboard Dyno within 500 miles of my house.
    HP vs RPM is extremely interesting. I cant wait to try it out.
     
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