GPH vs GPM vs MPH

Discussion in 'Boat Design' started by discovery, Nov 10, 2013.

  1. discovery
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    discovery Junior Member

    I know every hullwill be different, but is there a point where incraesing the speed(mph) actually brings down the economy (gpm)?
    Case in point, my boat, its speed range means it will only just get over the hump and so never really sees the "unload" effect that getting on the plane proper gives. This means that as the speed increases, the fuel used per hour increases also, and so does the fuel used per mile. Is there a point where I will ever see (at a faster speed) where the gallons per mile actually go back down as the boat goes faster?
    I'm asking this more as a curiosity point than to set any sort of idea or target.

    I am in the peocess of serious modifications to the keel area of my hull (still planning but have many of the details sorted) and after a recent trip of approx 100 miles beginning to be able to put a dollar figure on the drag the keel is causing me.
     
  2. Barry
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    Barry Senior Member

    Need more info on your boat, hull form, type of keel, power, etc
     
  3. gonzo
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    gonzo Senior Member

    Every hull has certain speeds that are the most and the least economic. Usually when the boat is just before trimming flat and getting "over the hump" it is using a lot of fuel for the speed.
     
  4. WestVanHan
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    WestVanHan Not a Senior Member

    Mine does this..once she's on plane the mpg varies little from 15 to 30 knots...and in fact is about 8% better at 24 knots than at 18-22 IIRC.

    But of course it's not just the hull but also mostly the engine load/rpm and props being tuned for that particular speed. Plus she is quite sleek with no canopies and whatnot hanging off the top.

    I think it's quite rare to have this though,maybe the Bluewater coastal cruisers and perhaps a Meridian amongst cruising motoryachts,but day boats it may be fairly common.
     
  5. Barry
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    Barry Senior Member

    The reason I asked for more info was that you mentioned changing the keel design for increased economy
    Always refer to your economy as miles per gallon or gallons per mile because gpm or gph does not relate to economy

    If you are barely getting "over the hump" your term not mine, you must be referring to a planning hull so the addition of trim tabs might help get the bow down and help efficiency
    If you are putting tabs on, buy the widest that you can fit across the beam as you will get a higher lift to drag ratio than a narrow set
    "Boating" magazine used to, and maybe still does, give a mpg table for different boats and hull speeds and there is a very significant increase in economy after the boat has reached a speed where the bow is approaching a more flat attitude and then the economy will fall off
    Note that for various dead rises there is an optimum range of bow up angle for the best economy. Generally the flatter the dead rise the lower the angle. One snapshot of this was 12 degrees of V ---- 4-6 up degree optimum angle

    If you offer up some description of the hull which you say needs some keel redesign, a contributor might offer some help
     
  6. discovery
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    discovery Junior Member

    The reason I didn't offer any more info is that I already have. If you searched 3 or 4 pages, you would have found a fairly reasonable conversation about my boat with pictures and a lot of peoples thoughts on my hull.

    The main thought I have with this conversation is im beginning to see a dollar figure on my keel with regards to the extra fuel I'm burning every trip with it in place and my query about the general efficiency of a slow planing hull such as mine(minus most of the keel).

    I will refer to the economy as I see fit because I see different uses of a boat changing the way to view it. If I were trawling, litres or gallons per hour matters more because the distance isnt important, the time the nets are being pulled is, and litres per hour refers to economy in that mode more relatively. When I reef fish, liters per mile is more relative as the distance travelled ismore important than the time (when looking at economy).
     
  7. Petros
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    Petros Senior Member

    several things to keep in mind, the power demand (and the fuel consumption) will always be increasing as you pick up speed.

    the other thing is that almost always, the drag increases with increasing speed as well.

    the big question is does the speed increase faster than the fuel consumption goes up. And that is dependent on the prop design, the engine performance curve, the hull design, and the sea conditions. And perhaps some other factors as well. usually you have to specifically design all of these things to optimize at your chosen design speed (at a given sea state), and take the rest as it comes.
     
  8. Yellowjacket
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    Yellowjacket Senior Member

    That is not necessarily true, and in particular that can fall apart in the area where the hull is just getting on plane. The reason is that the drag can go down as the planing angle decreases.

    For every hull and CG location, there is specific speed that provides the best efficiency. If your CG is too far aft your hull will be planing at a high angle and that is not very efficient. Optimum planing angle is about 3 degrees (for a flat bottom hull(according to Savitsky), but it is also very dependent on hull design specifics such as deadrise and chine shape.

    If you are just getting on a plane, but aren't getting down to a relatively flat planing angle (close to 3 degrees), you don't have a very efficient boat. Think about it as more a matter of trying to find the most efficient planing condition for the speed you are traveling. If you improve the planing efficiency, you will save fuel at that speed, or you can go faster for the same fuel burn. Either one is better than where you are running the boat now, but fortunately, you have a couple of choices, or things you can do to improve the efficiency.

    The first reduce the weight of the hull and move the CG forward to reduce the planing angle and improve the planing efficiency. This will let the boat achieve the most efficient speed for the power available. In your case you have a waterlogged keel that is heavy and it is heaviest in the back half of the boat. You need to get that properly repaired so that it isn't dragging you down, both literally and figuratively. You have the right idea in that keel is horrible and if you really want to go at the speeds you are cruising at now, you need improve the weight and CG location first.

    If you don't want to fix the keel, the other thing you could do is to increase the planing area, and this can be done with what I would describe as "planing flaps". Trim tabs are relatively small area flaps that are used to trim the nose down some. Larger "after planes" are full width flaps that are six inches or more wide. If you set these to be essentially level with the bottom you won't increase drag much and that will provide more lift aft that will improve your planing efficiency by increasing lift aft, and dropping the nose down to an efficient planing angle.

    The first order of business should be reworking the keel and getting rid of the rotted and waterlogged wood. Remember that the original keel also supplied floatation since it was made of wood and was lighter than water, so instead of floatation now it isn't providing any and that hurts your trim problem. If you cut back the keel you will be reducing floatation aft and that won't help your trim problem, although it might help your propeller efficiency. Certainly thinning the keel just upstream of the prop would help your high speed efficiency and if that is what you are looking for that is a good place to start. Just remember that you might need to add lift at the stern to improve your planing angle if the floatation is reduced and an added aft planing surface extension can certainly help in getting the boat back to a proper trim angle. Rounding and improving the leading edge of the keel is a good idea too, since if the keel is square it has really messed up flow all along the keel when you are trying to run at speed.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I doubt removing material from the timber keel would have any substantial effect on bouyancy, it could be neutral or even heavier, if a local hardwood.
     
  10. michael pierzga
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    michael pierzga Senior Member

    A displacement hull benefits by reducing skin friction. A racing sailboat will have its antifoul airless sprayed then wet sanded down. The increase in performance is dramatic. A good antifoul job is easy and cheap..it may be more cost effective than reshaping the underwater profile.
     
  11. CDK
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    CDK retired engineer

    Yes there is, but not as significantly as you'd wish for.
    The curve for speed vs mpg does have a small dip at or near the point of stable planing, but beyond that slopes upward again. Only at speeds where the wet area of the hull has become really small a gain in economy can be expected (if the hull shape allows for it).
     
  12. discovery
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    discovery Junior Member

    Im open to all suggestions. At the moment, I have the prop off getting measured for a replacement. There were no indications on the prop as to its dimensions. The plan at the moment is for the wooden keel to be removed and a small skeg and bearing bracket replace it. I will be going back to a industrial plastic bearing as I can make it myself on my lathe to suit. I still want some propeller protection so I am leaning toward plate steel to make the skeg from and to incorporate a bar rearward for rudder support on the lower pintle bearing. I like the way it steers now so the rudder will stay.
     
  13. discovery
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    discovery Junior Member

    Im not really hoping for huge gains, but I'll take whatever I can get. I see that keel costing me at the pump (fuel) with no real need for it.

    When I get it up on the plane, at around 14kts, the spray starts moving noticably aft from its point in the lower speed range. The difference between 14 and 17 kts is possibly 18 inches to 2 feet rearward at the cutwater point. I had someone drive the boat and ive leant over the side to watch this.
     
  14. Yellowjacket
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    Yellowjacket Senior Member

    You need to determine what trim angle you are running at. Where the spray is coming from is meaningless. The trim angle defines efficient planing performance. Find something on the boat that is parallel to the planing surface and then determine what angle the planing surface is running at versus speed. Then you have something to work with and can improve your performance from that point on. Without it you are flying blind and you have nothing and can't make any intelligent changes to the prop, or anything else really, since you don't know where you are.

    You also really should get a copy of Dingo's Savitsky spreadsheet and figure out what the potential real speed of the boat should be, what it is and figure out what would be an efficient performing hull.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

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