Chine and strake design

Discussion in 'Powerboats' started by meren, Dec 14, 2006.

  1. tunnels

    tunnels Previous Member

    A 4 inch wide pad is next to usesless to give a hull 20 foot long any sort of lift!, you wouldnt even notice any differance unless you were travelling at 100mph :confused: !, it would need to be 10 inchs wide at least!! :rolleyes: . Depending on how fast you are able to get the boat to go . anything below 60mph ,10inchs would be a advantage but its not going to climb out of the water and fall over ! Think about a pair of ski's how wide are they ? and thats just to hold your weight up ! a boat is a lot heavyer than a person !! put things into prospective as to how fast and how heavy then think about the amount of lift you are lookin for and do the sum's. Dont forget the trim angle of the hull and how much hull is actually in the water at speed . you could be amazed at what you find:?: . I always take video from the shore at as close to water level as you can without getting wet. The boat traveling at differant speeds and differant trim angles . Put the tape through the computer and freeze frame and analyze what you are looking at . You can learn heaps from just doing simple basic things . :p
     
  2. Jimboat
    Joined: Feb 2002
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    Jimboat Senior Member

    I agree with tunnels suggestion that a pad for this hull would likely need to be wider than a few inches in order to contribute any significant lift. A flat or low deadrise pad will be more efficient in lift generation, and can be designed to support a large portion of the hull's weight. The best way to size the pad is to know the amount of 'lift' desired to be contributed to the performance of the hull and calculate the pad dimensions to suit. Weight and power of hull are strong contributing factors to the required design of the pad, as are the deadrise of vee portion of hull, since this determines other hydrodynamic lift. There is no good 'rule-of-thumb' that can provide for the required dynamic balance of a performance hull, since the ever-changing balance between pad lift/drag and vee lift/drag has a strong impact on dynamic balance.

    You may find helpful an article that I wrote on "vee pad design". There are also calcs available that will estimate lift from strakes (email me if you want more info).

    Note that as others have suggested, the addition of a pad and/or lift strakes will also change dynamic CofG, and will generate different handling characteristics for your hull.

    check here for other threads on pad design.
     
  3. FlyingTime
    Joined: May 2010
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    FlyingTime Junior Member

    Well, I said that's where I generally start. I didn't mention going wider from there depending on the situation, my bad... Yes, if you're not expecting to go very fast a wider flat would be better. The ski analogy is a good one but obviously the flat isn't the only lift variable. If you put a 10" wide flat on a fast 20' hard chine boat it will be faster but in rough conditions you will feel it in a bad way. Boats are a series of compromises.
     
  4. tunnels

    tunnels Previous Member

    No speed have been forth coming so its all just speculation and speed in rough water is a hard one to work around whats good in smooth water maybe not good in the rough so how about telling us what speeds you are working at !! . Everything ends up a balancing act ! what works for one person is not suitable for another so its finding whats best suited for your needs . :D
     
  5. FlyingTime
    Joined: May 2010
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    FlyingTime Junior Member

    I was just responding to Wayne's question... The boats mentioned are all very fast. Very generally I would say around a 10" flat is what I would use on a 30 to 40' offshore go fast capable of speeds of about 60mph. Again, many other factors to consider. The 1" for every 5' proportion mentioned earlier was told to me by someone much wiser than myself...
     
  6. Wayne Grabow
    Joined: Aug 2003
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    Wayne Grabow Senior Member

    FlyingTime, tunnels, Jimboat,

    I appreciate all your comments, and they make a lot of sense. In asking about keel pads, I simply wanted more information because I had found little so far. In boating terms, it appears to be a fairly recent design feature. As yet I have no definite project in mind, but thought that it might be an interesting feature to incorporate in a future hull design project. As a hobby I have created a variety of personal boats and am now doodling (not sketches, mathematical calculations) which might result in a future project. I am currently finishing a variable deadrise hull meant for more modest speeds.

    I understand that there are no hard and fast rules, and a variety of factors need to be considered. Just wanted to make sure that I wasn't missing any important factors or conclusions.
     
  7. tunnels

    tunnels Previous Member

    Hi
    I understand what you are about ! I have a sheet of board painted white up on the wall in my shed and thats where i do my doodling , that way i can draw full size, because some things in small scale look ok till you get them actual size and then you can see problems that are not possible to see on paper . The pad thing has been aroung since the 1980s here . as it was some where in that era that Motors became more efficent and more powerful etc etc .
    Myself i have built boats with a combination of pad and strakes to get to where i wanted to be . The pad was to help lift the hull but only as far as the strakes were and no further . Example !,The pad was 6inchs wide and the 2 strakes 18inchs away from the pad on each side and they were 4 inchs wide each one with a 1.5 degree turn down . the pad was 70% of the length of the hull from the chine at the front and the strakes 50% of the same measurments . Its was a 14 foot INFLATEABLE and used to really move and turn and go back the other way in its own length if you were game enough to hang on and not get thrown out , I also had 1 inch of rocker built in to the keel but the rocker was more near the front than anywhere else . The high point of Rocker can be shifted anywhere along the lenght of the keel to perform differant tasks depending on how you want the boat to sit at speed and how quick you want it to turn . this one needed the bow up a little but to track true and straight in rolling waves and breaking surf and be able to bowlift quickly and ride over rolling water and not loose any speed or via off along the face of a wave when running 45 degrees to the wave ( Broach or bow steer sort of) . :p
     
  8. FlyingTime
    Joined: May 2010
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    FlyingTime Junior Member

    Variable deadrise as in Carl Moesly's Seacraft designs?
     
  9. tunnels

    tunnels Previous Member

    A boat designer and boat builder friend and my self used to design with the though in mind always of what will be the max speed this boat could do ! not what we wanted it to do . we always went a couple of steps further ! As i said used to always draw out full size even up to 25 feet. Its so easy and you can stand back and look at your creation and veiw it from all angles . As a kid i drew boats and made models as a hobby till i left school and them started work and started building my models full size . i Learned a lot in a very short time .
    The Carl Moesly's design principles are good and his methods relavent to todays wants and needs , speed is the thing people are wanting more and more . I am no exception to this i want both ends of the chart , a good sea boat in moderately rough choppy seas and reasonable big swells to a fast get up and go fast in smooth water , This is the hard one to satisfy ,whats good for one situation is not good for the other so after building race boats i do a combination of pad and strakes combinations and have been able to get what i like . The wide pad can be not stable at hight speed but can get the hull to high out of the water and a balancing act , to me The starkes you see people use are always in the wrong place and little and do nothing , Theres no use having something that doesent work or is in the wrong place and causes problems .Strakes in the first 1/3 of the hull are usially causing more problems than they are meant to solve . A well designed chine will difflect water and spray but strakes should be there for the purpose to help lift the hull and keep the hull stable ,so they need to be much bigger and more powerfull in there design , placement and use . And as for starkes hindering the hull turning etc there ways round that and you can have all the sideways slip you want in the rear end yet still have a wide strake with capabilities and the lift for straight running .:p

    Lots of things have changed over the years but everything stays the same!:confused:
     
  10. Wayne Grabow
    Joined: Aug 2003
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    Wayne Grabow Senior Member

    FlyingTime: "Variable deadrise as in Carl Moesly's Seacraft designs?"

    No, variable deadrise as in early 1900's hull design. I spent some time at the boat museum in Clayton, NY, studying what designers did, back when engines were heavy and of modest power, to get the most speed out of a hull. They were all variable deadrise designs. Since I was designing for modest speeds, 10-20 mph, I thought it might be applicable.

    tunnels: You obviously have a long-time interest, extensive experience, and an enjoyment of boat design. Your comments are a shortcut to my learning, although you are looking at different criteria than I have. Each boat is a compromise to suit specific conditions. Here, I am designing for the US mountain west lakes. I have switched from sail to power because the reservoirs in mountain valleys have uncertain, swirling winds that can make sailing frustrating. At the same time, waves have little chance to build up, and the size of the lakes are modest, but the scenery is beautiful, forests and snow-capped mountains. So I don't need a sea boat, and high speed just gets me to the other end of the lake sooner before we turn around. Still, it might be nice to get home quick when the weather changes.

    It will be interesting to see how the variable deadrise handles, and then I may try a higher speed design exercise. Sculpting a hull in wood is satisfying.
     
  11. tunnels

    tunnels Previous Member

    If you are wanting to see something that evolved a long time ago .See what you can find out about the older Eskimo canoes and check out the bow sections and there design and see if you can find the story about them .
    We think we are really clever but are we? maybe the ancients were the clever ones . :confused: :p
     
  12. stuartc24
    Joined: May 2010
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    Location: Australia

    stuartc24 New Member

    Investigating dragging strakes

    I am a green boat owner, looking to solve a challenge with some strakes and dragging at planing and seeking some more informed input. Pictures attached

    I have found that the boat will plane quite quickly (40 hp) and is relatively stable at 16 kts at approx ½ throttle. Any faster and the boat will drag dramatically if I get any lean on it or am in chop – I presume as these strakes dig in and the boat pulls. I have not yet been game to try 100% throttle.

    The boat is stable at rest but not the quickest turner.

    I am a little bit suspicious that these strakes have been added post manufacture when comparing the welding. If you look closely, there is a spray chine running the length of the boat and these strakes fit just inside the chine.

    I am trying to get a handle on the best solution – persist and forget about flat out boating, reduce the length of the strakes or cut them off altogether.

    I will be interested in what you think.
     

    Attached Files:

  13. tunnels

    tunnels Previous Member

    With things that look like that the only thing i can see they would do is hold the hull in a straight line when its going . Must be a pig of a thing to turn at speed !!!:confused:
    My choice woild be to simply chop them off and smooth the welds off , it will make the hull turn quicker ,running at a angle to the swells it could drift off line if you are into big waves . :p
    People do things without much thought most of the time .if you want tracking you add a keel not those kinds of silly things . :p
     
  14. FlyingTime
    Joined: May 2010
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    FlyingTime Junior Member

    I agree that they should be removed... I'm wondering why someone went through all that trouble to put them on in the first place. After removing them if you find the boat to be unstable while running or not planing very well you could add speed rails. Basically, aluminum angle about 1/2" tall max for a boat this size to correct any issues. But first, definatly take those things off!
     

  15. tunnels

    tunnels Previous Member

    These things are not really strakes but stabilizers and the drag will be quite high ! CHOP THEM OFF:D

    The other thing also a 40 hp will be struggling with not only pushing the boat but a couple of stoppers along the bottom . If they had been a proper strake they should have slightly enhanced the performance not slowed it down and also not made a lot of differance to the turning of the boat at speed . If its tracking and wanting to go in a staight line then adding to the keel would have been a better option , theres only one keel so half the problem to add to .
    I didnt read anywhere the length of the boat but more hp like 50 or a big foot 60 and those thingies wouldnt have been such a problem . Small boats die very quickly when they are underpowed and loaded with lots a heavy gear and they can be dangerous to if you get caught in bad weather because they simply struggle to make head way in punching into head sea and the possabilities of getting swamped in a following sea and not being able to stay ahead or a rolling /breaking sea when things get bad like they do sometimes . .
     
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