few questions about rake of a boat??

Discussion in 'Boat Design' started by chris3298, Nov 5, 2012.

  1. messabout
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    messabout Senior Member

    Chris; you said that this is to be a drag boat. You also inferred that you are looking for top speed. Those are two different animals. If drag boat is the aim then acceleration rate is the criteria. If top speed is what you want then the boat will have different design characteristics.

    Another thing. You said no, that is too long. Not so! Do you know what a blowover is? A longer boat can be superior to a shorter one for all kinds of reasons, not the least of which is safety. If you blow the boat over or it gets skittish you will not win many races. You may also end up in the hospital or the morgue by having a too short boat with a too much power.

    Build the boat of quality plywood not aluminum. You can make it lighter and stronger with wood. DIg around the internet and see what the pro drag guys are running. USDBA (US drag boat association) for example. Also delve into the various classes of APBA boats. There is a class that uses LemMans starts. Those boats are spectacular fast and would make one helluva drag boat. Unfortuneately they are very pricey like maybe 50 grand upwards. In your case you are probably thinking in terms of what is affectionately called the runabout class which separates the type from three or four point hydro classes.

    None of the respondents here are demeaning you. We welcome you to the forum but we do not want a member to indulge in suicidal practices with screwed up boats. That is the reason that we are quick to shoot down some of the notions that less experienced people may entertain.
     
  2. tunnels

    tunnels Previous Member

    ya got to think a little from here on !!

    No mater what design you pick on once over the 35 to 40 mph barrior you are getting into a place where air at speed is going to have a big influance on you boats shape and as speed increases then so does the effects of aerodynamics need to be looked at very seriouly !!,air gets under the hull and it wants to lift the front because its just a turn up and forcing the front of the boat to lift and get airbourne ,
    Now its time to use the brain ya mother gave you when you were born and look at the wing of a aeroplane and see how it works !!
    The air going inder the hull and the air going over the top, both are wanting to lift and when it reachs that magic point wooosh your swimming !!happend in the blink of an eye !!
    Remember the air going over the decks surface and the shape of the very front of the hull are the keys to woosh and swim or pour on the power and go even faster
    Now thats got ya thinking !!! look very carefully at the front of the wing shape its really important and think hard about the decks surface as well . The bigger the deck the more effect it will have and the greater the lift possible . You could change the hull bottom shape to help stay in the water and go in a straight line !! or just keep on the path you on and reach a point where you cant go any faster unless you wearing a parashute !!:eek:
     
  3. chris3298
    Joined: Nov 2011
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    chris3298 Junior Member


    The way the guys I have raced with race is two guys basically will run side by side on a plane and once you both reach the first buoy the race is on to get to the finish line. which is about a quarter of a mile. This type of racing is not from a dead stop like normal drag racing is. I have learned most of the boats are in the 14-15 foot range while there are a few that are running home made 10 ft wooden boats with 70 hp stock pushing 130 hp outboards on them which are freaking insane and I wouldn't have the balls to even run something that short. These guys literally have to sit on the bow to get them going and once they hop up on a plane they just back in the seat.

    So YES I want top speed is what I am after. I know what a blow over is yes, wind catches and well over you go.

    I understand you guys don't want me to die but reality is I'm gonna build a boat, so hopefully I don't die and I don't want to die so that is why I am here asking questions. Hell if I wasn't concerned about dying I wouldn't have even posted here and I would have gone with trying to build a boat.
    If I was in yalls shoes and someone posted on here well i'm not gonna sit back and not say anything I'm gonna say what I felt was right and also tell that person well look this could happen and this could happen if you do this and I don't feel this is what should be done and so forth.

    If you think by not telling me certain things that I'm just not gonna try and build a boat well you're wrong... I'm one determined SOB when I put my mind to it.

    My father is a trim carpenter an is all for helping me with this project. I know welding I've been doing it for over 7 years now. I have even thought about making some stuff out of 1/4 inch plywood and if I like it well use that as my template to build out of aluminum. Wood is probably a better way to go about this but I can't help it I want a challenge I want to make it out of aluminum. I have a way of getting aluminum cheaper then a lot of people so why not make it out of it.
     
  4. chris3298
    Joined: Nov 2011
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    chris3298 Junior Member


    I know what you mean my current boat will run a little over 38 at the moment and i have had problems when it is windy on the river that the air wants to lift the boat and actually has made me drift to the right when running. This was a time i back down on the throttle a lot to keep me from blowing over.

    I have actually kinda thought about this as well like you have explained you want lift for the speed but not as much lift to get blown over. I have wondered how could I build some kinda wing of some sort to mount or build into the deck to keep the front end down and keep me from blowing over, kinda like a spoiler on a car.

    Ok you got me really thinking here. I could actually make part of the deck like a wing like you say so I could actually manually adjust it like a wing to find that sweet spot for lift and speed hmmmmmmmm

    Thanks
     
  5. tunnels

    tunnels Previous Member

    Now ya thinking !! have a look at really fast boats like as in quick . take carefull note of the shapes and the designs and figure out how they all work together as one . There is a thing called surface effect where a flying object just above the surface can actully stay there on that layer of air and its stuck there !! its hard to discribe .
    Study surface effect how it works and what its all about and thats where you need to be !!:confused: windy or not wont affect you if its aerodynamicly stable and thats ya key to wining races!! :D when others have to button off you can just keep going as fast as you can go !!!!
     
  6. chris3298
    Joined: Nov 2011
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    chris3298 Junior Member

    I've seen on TV what you describe, years ago seems like it was something about hydrofoils or something along that effect how once it rose up it would stay there.

    The video I posted of the flat bottom boat on youube. I have personally seen this thing run in person and the original builder built this boat and one other that is a V hull with a pad and like you have said about surface effect this guy was running around the 80 mph mark but still very very stable this boat looked.

    I can say this, that flat bottom boat looks wide on video but in person it's super narrow compared to the other guys in there 15x48 inch bottom boats. I have been told this guy has a 42 inch bottom but God thats narrow but i guess narrow is maybe also the key to keeping it streamlined and aerodynamic kinda like the guys who run in China on the skinny skinny boats you see on youtube.

    I have seen this on two boats this same builder design which was the look of like a nitro or ranger bass boat. Where the transom would be straight across for some aluminum flat bottom boats or any boat really that has a outboard motor on it. These two boats the designer made it so the transom would extend further back and in a upward angle to help get that motor even further back kinda like a jack plate you can add you can get that front end to fly higher for less drag and more speed but at the same time you gotta be careful going to far you can get the front end up more and end up with a blow over.

    I'm experimenting now with jack plate on my current boat seeing how it affects the way the boat performs and it completely changes the way the boat runs. I am able to jack that motor up way higher for less drag but am learning I will need to have cup added to the prop or repitch it soon as the rpm's climb. My poor little fishing boat well has become a pretty serious test boat the last 5 months or so.

    Thanks Tunnels for explaining some of this to me
     
  7. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    That boat shown in in the video is a hydrosled...a single point hydroplane. "Rake" as you wish to define it (like a bargeman, really it is "rocker" as rake only applies to the bow transom ahead of the flat bottom) for one of these things can be a straight line from bow to stern as long as weight is kept well aft..but forward of the Cp at full speed. In these type of hulls, there is no bow rise to step on plane as the AoA is designed in (I'd bet ~5-7 degrees). Notice the extrenely large bow down trim of the motor, that is needed to keep the boat from blowover. Also notice the extreme porposing caused by a shift fo the lift Cp about the longitudinal CG. I'd move/add some weight forward to stabilize the boat...a little slower, but much safer.

    Again this is a hydrosled, but this time there is a "flat" bottom reather than continious rocker from bow to stern. Reaslistcally, this is a more useful design than the previous one... not so extreme

    The video show the truth of this...look carefully at the trim of the motor...way bow down.
     
  8. tunnels

    tunnels Previous Member

    So now you seen sone of the boats have a look at Formular one tunnel hulls !! i used to work in a place making and repairing race boats the F1 tunnels is the optimum of boat design it literally flies and they are so fast and the acceleration is head wrenching like a dragster . tunnels use all the things i discribed aerodynamics at the top end and Hydrodymanics at slower speeds and cornering etc . take care !! learning never stops ,even at my age !! :p
     
  9. chris3298
    Joined: Nov 2011
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    chris3298 Junior Member


    The video and well several videos i have seen of this boat makes me want to say it is a flat bottom boat but you're saying it is a rocker so if I understand this right there is no real flat bottom towards the stern of the boat?? it is a continuos rocker from the bow to the stern but as you put it just a little bit of a rocker at the stern? ok this is starting to make some sense as to what I have wondered about but did not understand the term which you call a rocker. I have dealt with small rockers and hooks on the bottom boats before but they were right next to each other which caused a boat to porpoise.

    My current boat has basically a long rocker and has small rockers and hooks along the bottom but this because the way they welded the braces in it which should have been welded length wise instead of width wise.

    So in building a boat you should add rocker to it but not so much because it could give to much lift ?

    Thanks guy for all the info here I got a lot of thinking and learning
     
  10. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Do not misinterpret the term; rocker. Draw a rough sketch of a boat in side view. Starting at or near the deck draw a curve down toward the bottom of the boat. The curve will intersect the bottom at some point not too far from the front of the boat. From that point aft, the bottom will be dead straight.

    Displacement boats are different. The bottom will not be straight and it will curve upwards toward the transom. That is the generally accepted description of "rocker". If you have even a small bit of aft rocker, on a planeing boat, it will porpoise like mad and will be slower than you would like. Too much aft rocker and you'd never get it on a clean plane.

    You wann'a go fast you need a very light weight boat. You wann'a go fast you need a flat or very near flat bottom with no longitudinal undulations whatever. You will be hard pressed to achieve both those aims with aluminum. If the boat is really fast you might do well to design in some "non trip chines" with a dutchman. You can do that with aluminum but it will be a pain to build. Another thing...... Since you want to maximize speed then the running surface of the bottom must be absolutely smooth with no protuberances like rivet heads or weld beads.

    If I wanted a fishing boat I'd be smart and happy to use aluminum. The allie boat is rugged and durable and will withstand abuse and being rode hard and put away wet. On the other hand, If I want an all out race boat then I'll use okumee or meranti along with some sitka spruce framing and very judiciously applied epoxied glass.
     
  11. chris3298
    Joined: Nov 2011
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    chris3298 Junior Member

    Thanks, I was thinking of having some chimes rolled into the bottom plate to help it track better but also give it some strength.
     
  12. tunnels

    tunnels Previous Member

    And what else will it help with !!! can you guess ?? you looked at the material side now think what else its going to do !! The shape and size of the ridges is working for you so think seriously about how it will help you even more than just the tracking !! You tell me what the extra benefits could be??

    Rocker can be just a curve from the low point of the bow all the way to the back ! right?? or you can shift the same amont of rocker and change its shape !! ARE you with me ?? Yes or no ?? .
    The placement and shape of the lowest part of the rocker can change the whole boats performance , lift the bow !!make it plane quicker!! and out of a hole in super quick time!! can also make the boat turn better !!and almost turn it its own length (not at really high speed mind you) !!
    Understanding what its doing what and how it works and how you can use it to your advantage takes a little thought and lot of doodling on paper !!:idea:
     

  13. chris3298
    Joined: Nov 2011
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    chris3298 Junior Member


    pm sent to you
     
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