I am wondering about my hull shape speed wise

Discussion in 'Wooden Boat Building and Restoration' started by sdowney717, Jul 6, 2015.

  1. Easy Rider
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    Easy Rider Senior Member

    At speed it's hard to imagine the bow rising "way" up and the stern down. Looking at the pics the after plane appears to have a "hook" in the bottom. Should act like a huge trim tab and lift the stern up.

    Must be a weight or power issue.
     
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  2. LP
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    LP Flying Boatman

    A quick look at the spark plugs could tell you if your engine are running lean.
     
  3. sdowney717
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    sdowney717 Senior Member

    You mean like white color? True, unless the motor also burns some oil. The plugs are usually a mid brownish color, not blackish.
    To read plugs properly, you have to do a speed run, then immediately turn off the engine.
     
  4. JR-Shine
    Joined: May 2004
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    JR-Shine SHINE

    its the hull shape. There isnt much in that bottom that says "planning"
     
  5. sdowney717
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    sdowney717 Senior Member

    That is true. I keep thinking the hull is more like round bilge lobster type boat with a skeg keel. I have been looking at images of those and down east style hulls.
     
  6. sdowney717
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    sdowney717 Senior Member

    On our recent weekend trip, we ran 50 nm
    I burned 45 gallons
    The trip engine run time was about 8 to 8.5 hours at about 6 to 8 knots
    So I figured about 5.2 to 6 gallons per hour.

    About a third of the trip I ran on one motor 2000 rpm at speed of 6 knots. I was kinda surprised at how well just one motor pushed the boat, although I had to compensate the rudder and pay attention to our direction as it wanted to veer off course. Did that from the CBBT third island to Kiptopeke where the WW1 cement ships form a breakwater and we spent the night.

    Typically the boat ran about 7 to 8 knots with 7 about the mean average.

    Coming back from Kiptopeke, the eastern shore, the weather was bad with bay waves of 4 to 6 foot, so I sped up a little and it helped. Bow was buried with waves coming in at us to hit the windows and stern went almost under at times till about half way across the bay, then it let up, so for that leg about 7 miles, I burned more fuel.
    Does this seem good, normal or bad to you?
     
  7. Jamie Kennedy
    Joined: Jun 2015
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    Jamie Kennedy Senior Member

    Nice work. I suspect she has gotten a little heavy, and soft. So then she becomes over-propped as well, and combined with maybe little less engine torque and power and so even more over-propped, and so maybe all that combines to such a catastrophic drop in top speed. You could estimate the current displacement from how she sits on her lines.
     
  8. Jamie Kennedy
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    Jamie Kennedy Senior Member

    So what's to be done? Well the interest seems to be in fuel efficiency rather than top speed. You could see where you could trim weight. Maybe remove a lot of bottom paint. Maybe replace a few planks. Or maybe leave well enough alone until some serious work needs to be done. Then you could look at some engine maintenance and tuning if there is any potential there. Both of these things should improve fuel economy as well as top speed. Then once that is done maybe look at changing the prop to suit the new circumstances, for the cruising speed you want to run at most of the time. You might not improve you top speed much but you might get better fuel economy. Even 10% is a lot right. That might help pay for all the work.

    It's a nice looking boat I think. We have a lot of converted fishing boats and lobster boats on the St. John River here in New Brunswick. People really do seem to enjoy them. I think this one is even prettier than those, and probably never had fish in 'er. ;-)
     
  9. sdowney717
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    sdowney717 Senior Member

    The OEM scribed the waterline paintline into the planking.
    And it is painted on the same lines, I think it rides either where it should be or even up a little.
    I will post a current picture today later on, dont think it will show much.

    Here is an older photo from the recent haul to bottom paint etc.. done in 2014 which shows the scum line. It is still sitting in the same spot today.
    [​IMG]

    [​IMG]

    I made a small mod to the line at the bow, curved it up a little as I liked the look better. You can see the red dipped into the water at the bow, so it does not anymore. The center of the boat, the OEM drew the line up quite a bit higher.

    Stern view, water scum shows it is sitting about right I think.

    [​IMG]

    I left off the tabs but next time they will go back on.

    [​IMG]

    Side view

    [​IMG]

    underwater bow area shows scum line almost touching the paint up at the bow.
    [​IMG]
     
  10. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Yes it doesn't appear that weight gain, if any, was not too severe.

    Another thing that might be fun to do when back in the water is to see what the difference in speed is with tanks empty and just you on board versus tanks full and however many passengers some free beer might gain. It might be interesting to see how much top end speed drops off with weight. I'm not a power boater but if I was I would have a lot of fun playing with fuel economy more so than top speed. Top speed is a great indicator easy to measure. A 10% increase in weight might reduce speed by 20% perhaps more perhaps less depending on where you are on the speed curve, but if significantly different than expected it might indicate that you might be over propped or under propped without having to change the prop. So where are you at now, in the water or out?
     
  11. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Another thing you could try is towing something and measure the strain on the tow line and drop in speed. At full speed it would not need to be too heavy. Maybe a big old rowboat filled with more beer and volunteers on board. OK maybe not. Just fill it with some water. You just need a way to measure the strain on the line. The speed and strain on the line would indicate a power loss from the boat, so that would indicate how much of a change in horsepower will effect a change in speed near top speed. Kind of like a differential tow test. At 8 knots and 300 pounds of strain that would be a loss of 7.4 horsepower, which might be enough to accurately measure a drop in speed for a given drop in horsepower. Just don't take anyone's head off. If you can release the line safely and measure your immediate increase in speed that might be better than taking two measurements under different conditions. Don't forget to go back and retrieve your friend's row boat that you 'borrowed' for the weekend.
     
  12. sdowney717
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    sdowney717 Senior Member

    The tanks can hold 145 gallons times 2 = almost 300 gallons, water tank about 70 gallons.
    I keep at most not more than 100 gallons fuel in a single tank, the other one I don't use, and water average about 40 gallons.
    So I am not overloaded with either.

    This boat has a lot pulling power even at idle speeds. If I am tied to the 8 by 8 bulkhead dock at the stern, and idle it in gear, I can not deflect the cleat lines. I would estimate over a thousand pounds of force gets generated. It certainly can stir up the mud under the boat. Marina owner would not like to see me do that!

    I suppose little slip with low engine RPM, and more prop slip at higher rpm.

    I am more interested in fuel economy, but you know when you want to or need to go fast, that would be nice. Next haulout get the tabs back on. And soon I plan work on the fuel mixture from the carb, maybe it is too lean.
     
  13. Jamie Kennedy
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    Jamie Kennedy Senior Member

    It sounds like you are getting a lot of use and enjoyment out of the old boat. It really is a beauty I think, even though I'm not a power boat guy. I like that era because it is what I grew up in. Looks right off the cover of an old Yachting magazine. Hope you can improve your fuel economy here and there. Every bit helps. Enjoy your summer.
     
  14. sdowney717
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    sdowney717 Senior Member

    Thanks,for the kind words on the boat, some people don't like what I have, I got a lot of negativity from the wooden boat forum, likely because I dont conform to their extremely pinheaded view of how things are supposed to be done.

    I pulled 4 of the spark plugs on the starboard motor, and they were all chocolate brown. Then I was running it in the slip throttled up to 2500 rpm, and when listening to the exhaust, it does seem to have an occasional miss. It could be a lean miss, it certainly never smokes out any exhaust as if it were running rich.
     

  15. Easy Rider
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    Easy Rider Senior Member

    Your first picture in post #1 shows a lobsterboat planing hull w a warped and hooked bottom aft. If it's still squatting and bow high this hull is a little like a stepped hull but the angle of attack aft will be even greater than the stepped hulled boat. Angle of attack + weight = high drag.
    Also I don't think anyone's mentioned that spray rail getting into the act. One would assume it would reduce wetted surface and drag but if they fail to break the outboard water flow they may actually be a source of added drag. A long shot but ?

    If this was my boat I'd want to get the aft bottom straight or even slightly convex. That, however may be very difficult to do.
     
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