Effect of aft warp?

Discussion in 'Boat Design' started by ezombie, May 12, 2007.

  1. ezombie
    Joined: May 2007
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    ezombie Junior Member

    Howdy, ya'll...

    Just a lurker here who has hit a snag in his research. I have a warped plane-style boat hull ( pram with a full keel + skeg ) with following parameters:

    3 stations at 20%, 45% and 80% of the boat length starting at the transom ->

    transom - 0 deadrise
    station 1 - 0 deadrise
    station 2 - 0 deadrise
    station 3 - ~6 deadrise
    bow - ~15 deadrise

    The keel is flat from the transom to about halfway between stations 2 and 3, then rises to about 6 inches at the bow. The draft at the transom is just under 3 inches loaded, with the water line parallel to the keel/chines.

    This has actually worked pretty good in the model, and given the four outer battens located on the underside, the boat planes nicely with very little power, good directional stability and not to harsh of a ride ( for the streamlined jon boat it is ). The CG is well rear of the front of the flat planing area so there has been no tendency to porpoise.

    What I am wondering, and unable to find out in my various searches, is what effect adding about 6-10 degrees of deadrise to the *transom only* would have?

    Now I'm not talking about adding any rocker here, the keel would still be flat in the aft. Just thinking of pulling the chines out of the water at the transom, to help with the slow speed efficiency - in effect presenting the waterline with a strongly tapered exit when not on plane. The goal is to increase battery life while trolling, yet retain the lower power planing characteristics already present.

    The rear buoyancy could be a smidge lower anyways so I could move the CG forward a couple inches, and I was staring at the model and pondering ways to solve both issues.

    Would this present the hull with undesirable handling characteristics or instabilities at speed? Or would it just increase the power requirements to achieve plane?

    The former I am a bit worried about, the latter is an acceptable trade-off. Any thoughts?
     
  2. messabout
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    messabout Senior Member

    Adding some deadrise in the after sections is OK. It will not be likely to affect handling while on a plane. If anything it might become a bit more stable. It is going to use more power for equivalent speed and load.

    I have some doubts that the deadrise boat will be signifigantly better when powered by the electric. Electrics are presumed to run at 3 or 4 MPH. At that speed, your improvement would be marginal. I suggest that you install an ampmeter in the electrical circuit and experiment with weight placement. Moving forward will lift the transom a bit. The ampmeter will tell you if you have done any good.
     
  3. ezombie
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    ezombie Junior Member

    You see the point of my one-two punch thoughts - to lift the keel at the transom to a 2 inch draft with a nudge forward of the seats, and to take the hard chines out of the water for the last foot or so of the boat. Both events are hopefully complimentary ( adding two small effects that reinforce each other ). I'm looking for a 10-20kt top speed in a 8-12 footer, BTW.

    Hrm... overcoming the logistical difficulty of installing an ammeter in a 12th scale model boat, combined with then monitoring it while R/C'ing said boat around the local pond has become a bit of a stumbling block for me. Anyone know of a < 2 oz logging multimeter?

    I was hoping someone else might have some first hand experience on this subject, and there is only time for one more model before I start building the fullsize deal. Given that I already have built non-warped aft version, and there has been only small alterations since then, I will go ahead and make the last model with the modest transom deadrise. If it can't hurt, then it only can help, right?

    Currently, if I push the speed up to high then crank the rudder over hard, the boat will perform a little dance that reminds me of a hockey skater showering you with shaved ice... perhaps the a bit of aft deadrise will mellow her out when pushed to hard, as you suggest.


    I have also discovered that I basically stumbled upon a variation of an existing design:

    http://www.boat-links.com/Atkinco/Articles/BDQ/ChatterBox/index.html

    The main difference is mine currently has a dead flat bottom aft, and a bit finer nose, along with outer and more numerous keel sons and a much more substantial outer keel.

    Given who the original boat wright was, this gives me hope that I am on the right track.
     
  4. Willallison
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    Willallison Senior Member

    Adding deadrise to the aft sections will most certainly be detramental to planing performance! You will essentially be taking a planing hullform and turning it into a displacement one.
    Adding deadrise to the aft sections will result in buttock lines that curve up to the transom. The boat will tend to 'bog down' as speed increases. The gains made at trolling speed will (to coin a great Aussie expression) be bugger all.
     
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  5. ezombie
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    ezombie Junior Member

    Here is a photo of the existing model before it was planked, and some markup to show the changes.

    The side battens(?) are actually level across the top of the frames, the angle of the camera makes them appear to curve up. The frames are bulkheaded and used for the base of the seats giving ample storage and rigidity. The sheer is a 'floating' one, consisting of an inner and outer sheer clamp.

    I am just finishing with the frame-up of the new model. Will post pics before and after planking. If the transom deadrise causes any excessive pitching at high speed, I can just narrow the transom a bit to pull the nose up ( it's currently an inch or so lower then I'd like at rest )
     

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  6. Willallison
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    Willallison Senior Member

    All I can do is to reiterate what I said before. Raising the chine as you've shown WILL be detrimental to the boats planing performance. If you want to have any marked effect on it's efficiency at displacement speeds, make it twice as long & half as wide without increasing the total weight
     
  7. tom28571
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    tom28571 Senior Member

    Listen to Will. He is giving you the straight of it. Any longitudinal upward curvature of the keel or chine or any buttock line in between will require more power in planing mode. It will also tend to lift the bow and make it more difficult to get on plane in the first place. It will improve both handling and speed in displacement mode. This is all fundamental.
     
  8. ezombie
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    ezombie Junior Member

    Thanks for the good advice everyone, this forum is a great resource for those of us 'experimenting' with various ideas.

    Although I wouldn't call it fundamental, Tom. Just about any authoritative source does state that about the *keel* ( what is described as 'rocker' ), but none seem to mention much of the effects of the aft chines.

    For future references, does anyone have any good sources of design info which deals with smaller craft at higher speeds. This paragraph struck a intellectual nerve with me:

    Which appears to be the rut we are still in today. I've owned a jon boat or two, they takes gobs of power to move along at any reasonable speed. I will merely echo these fitting words, which I heartily agree with:

    On a related note, as we start to see 2-5 HP electric drives becoming available in the marketplace, would not these now extinct craft be the perfect home for such drives as these:

    http://www.westmarine.com/webapp/wcs/stores/servlet/product/10001/-1/10001/274855/10001/820/478/10
    http://www.westmarine.com/webapp/wcs/stores/servlet/product/10001/-1/10001/272249/10001/820/478/10

    Hmmm... planing along in almost total silence, with only the rush of the water tickling your ears. Seems heavenly.

    But I digress. I do expect the former model to perform better then the latter, but I have surprised myself before. Better to have built and been disappointed, then to have never built at all, right?

    Thanks for the help everyone!
     
  9. tom28571
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    tom28571 Senior Member

    Fundamental it is. ANY bottom surface that contacts the water that has longitudinal convexity will have the negative effect on planing we are talking about. In the forward bottom sections, we must put up with some convexity (warping) in order to be able to negotiate waves. Fortunately these forward sections are mostly out of the water at planing speed. In the aft sections it will have the results we speak of. Trim tabs or aft downward warp of the chines is another matter.

    Several books are very useful for understanding planing. High Speed Small Craft by Peter DuCane and Naval Architecture of Planing Hulls by Lindsay Lord are two of the best.
     
  10. ezombie
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    ezombie Junior Member

    No wonder I couldn't find anything. :lol:

    Seems the knowledge I seek is contained in rare, out of print books. I should've expected that. I think I may be able to get the former at a non-back-breaking price, but the latter appears well out of my reach.

    I suppose my next question would be: how much of a difference? I'm sure there are too many factors involved to know for sure, and scale models - however accurate I build them ( I'm even using 12th scale, 3-5 ply plywood and timbers of comparable wood densities ) - don't lend themselves to measuring things such as efficiency very well.

    So if the two final scale models act similarly, then I will be building the latter one this spring. That way, I can carve two 'wedges' that will convert it into the former hull shape.

    I know I appear to be spending undo effort on such a trivial design, but I *am* going to be building about a dozen of these little 'skipper' boats this summer. A local ( rustic ) lake resort is looking to replace their rental fleet with something a bit less heavy-slow-plastic and a bit more stable-efficient-oldtimey. Beachability is a must, movable by one person, capable of carrying two adults, used primarily for cruising around and fishing. 5HP 4 stroke outboards are what they have for propulsion.

    The first one shall be the experimental prototype, I suppose... I shall just keep that one for myself. ;)
     
  11. tom28571
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    tom28571 Senior Member

    Actually, Nature of Boats by Dave Gerr and Ted Brewer's basic design book or several other basic books are all you need to present the things you need for simple small boats like you describe. These are still available and may still be in print.
     
  12. ezombie
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    ezombie Junior Member

    Oi...

    Well, here is a basic set of lines for the boat. My first ever :D

    Yes, I know they are rough, and could use some - messaging - perhaps? But the 1/12th scale model *is* starting to look nice. I widened the transom a scootch, and of course immediately doubted the change. But this transom should give me the proper CB/CG setup with the two man bench seat and decent legroom. Being an itty-bitty boat, the battle between nice lines and ergonomics is a constant tug-of-war.

    Criticize away, keeping in mind that the boat is for 1-5HP, calm waters, with one or two adults.
     

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  13. tom28571
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    tom28571 Senior Member

    It's sort of an in-betweener. Not a planing hull but can be driven past "hull speed" with enough power. It won't handle well at greater than hull speed though with the bow up and throwing a big wake. With a small kid sitting forward and about 3 to 4hp, it will do a bit better. A 30 second analysis.
     
  14. messabout
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    messabout Senior Member

    Tom is right. The boat is neither fish nor fowl. Decide whether you want a planing boat or a displacement boat. This one is neither.

    For a planing boat, get the keel line higher at the bow, do not use deadrise at the stern, make the boat longer.

    For a displacement boat, rocker it generously at both ends. Also make this one longer.

    Get Gerrs' , The Nature of Boats as Tom suggests. The book has the whole plan set for a little dinghy that is made in two separate sections for ease of transport and simplified storage.
     

  15. ezombie
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    ezombie Junior Member

    LOL - yeah, I know. I've built boats by plans before, and I wanted to do something different this summer.

    Part of me has always wanted to find a balance between the two, like a seesaw with two equal sized kids on it. As in, not a dog at slow speed, and still able to semi plane. It's not meant to actually plane, just ride nicely on it's bow wake up to a point.

    I didn't mark the center of gravity. It's just aft of the front of the bench seat The previous scale models didn't make much of a bow wake, even when fully loaded to a scale 600lb cargo, so I'm hopefull.

    The weight is ~82lbs empty, the static waterlines shown are at no people, one person, and two persons.

    The ultimate goal is portable stability and ease of construction with decent performance given a certain footprint and weight. The current BOM is at $335 each, with me able to make two each week utilizing the weekends and the evenings. So much has been sacrificed for those requirements.

    I thought about carrying some deadrise down the length, but then it wouldn't beach as nicely. The skegs under the boat have a teflon runner on each one, allowing the boat to slide on the gravel. These will be beached everytime they are used, and the idea is to push the back three-quarters into the water and then just walk on. Egress is the same.

    No comments on the lines? Is it a fugly, or perhaps just homely? I'm the wrong person to judge the looks of a boat, I tend to like unconventional designs anyways ;)

    I found a good supplier for glass and glue locally (finally), so hopefully the prototype will be going together soon. Still waiting on the 1/64 plywood planking for the last test scale model <grrrr>
     
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