Jet boat hull chine/strakes questions

Discussion in 'Boat Design' started by YTPaul, Apr 5, 2015.

  1. YTPaul
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    YTPaul Junior Member

    Hello folks
    New to the forum and have some questions.
    I have a 20' Wooldridge Sport IB jet. It is a 2000 model, and on this model, the had an extruded lifting strake welded on just inboard of the chine, extending about 8' forward from the stern. They worked well from a lift standpoint, but on a turn, they would grab when you had any sort of load, and when they did, hold on, because you were turning NOW! Also threw up a mean bow spray. So I cut them off ( as others have done) and it did improve the handling.
    About the mid 2000s, Glen and the lads evolved the design to integrate a fin type profile to the extruded chine, which improved the handling of the boat. I know this as the boat I had previously was an OB jet version of the same boat, but a 2010. My current IB jet Sport still doesn't handle as nice as the newer versions, as it still tends to grab a bit on turns as opposed to a predictable slide/carve. My intent is to pull tank and engine and flip the boat and weld some angle Al over the present chines to simulate the newer version Wooldridge uses. It would end up having the same look as the chines on the North River jet hulls.
    Before I embark on this project, I wanted to get some feedback on chine designs etc. I'm just on my remote right now and I'll post some images soon.
    Thanks for any help/thoughts/sage advice.
    Cheers
     
  2. YTPaul
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    YTPaul Junior Member

    Here are some shots, one of the chine as it is, and with 2 options, 3/4" and 1" angle. These would be carried to the bow.
    Goals: to avoid grabbing, decrease bow spray on hard turns.
     

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  3. Barry
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    Barry Senior Member

    Our 21's, 12 degree dead rise, 84 inch chine width
    We ran 4 strakes, 2at the chines and 2 midway between the chine and keel
    For the mid strakes, we took 3x5 x 1/4 angle, cut the 3 inch leg so the 5 inch flat was 12 degrees turned down from horizontal, ie 24 degrees with respect to the hull. The were ran up the hull until the bow curve came into play

    Much the same as the chines

    They must be fully welded longitudinally, not stitch welded

    I do not understand why you would want a predictable drift. We run a lot of rock gardens and we prefer that the boat goes where you point it
     
  4. YTPaul
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    YTPaul Junior Member

    Perhaps predictable drift was a poor choice of words.
    Here are a couple of (poor) shots of Wooldridges new extruded chine. Id like to mimic this with some angle Al. The new design allows for a very predictable turn, no sudden grabbing, no bow spray coming back at you. I am assuming that the increased size of the extrusion keeps the chine from losing lift and dropping the whole side down, as well as deflecting spray down.
    As this wouldn't be a structural member, why the necessity for a full longitudinal weld? Stress cracking issues?
    Thanks for the input.
     

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  5. Kevin Morin
    Joined: May 2013
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    Kevin Morin Junior Member

    Rail Welding Remark?

    Barry,

    could you take time to expand on your very interesting remark about the welding of the bottom rails? I was unaware of the need to continuous weld outside hull longs and would enjoy learning what your experience has taught you, if you care to take time to share that information.

    I've generally stitched these types of hull elements for along time and was not aware of the need to continuous weld. Admittedly these angles were not on jet powered (read; not nearly as fast) boats.

    Paul, I'd say which ever extrusion you choose (?) just jack up the boat, weld the outside horizontal and the underneath overhead. The lower leg's thickness will form a 1T fillet groove so the overhead will have a fine weld zone prep built by the edge of the extrusion and the bottom plate?

    Seems like more work than needed to pull engine and tanks, roll the boat, just to get position on a weld you can do sitting on a creaper, and if needed put a mast on the creeper as a hand brace? Not a hard weld, in fact an easy one for MIG; especially if the hands are held up on a T brace on a mast mounted to the creeper?

    Cheers,
    Kevin Morin
    Kenai, AK
     
  6. YTPaul
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    YTPaul Junior Member

    Hi Kevin
    Thanks for the info. I was a bit leery of welding overhead and having the puddle drop out. Any suggestions on settings, or just set up as per an in position?
    Any thoughts on how the boat would handle with the 1" angle welded over the chine?
     
  7. Kevin Morin
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    Kevin Morin Junior Member

    Overhead Welding Aluminum MIG

    Paul, I see the concern but if your couple of extrusions are in their relative true position then I'm calling that more horizontal than true overhead. My meaning is the gun angle is pointing upward at say 15 degrees but horizontal at 75 deg. So I don't think the puddle is going to drop out, further, if the chine already has the existing thick extrusion then the main reason for dropped puddles is gone.

    To me thin (<0.100") material overhead has a potential to drop if and when it burns through. But (to me) it's only overhead welding when the torch is vertically directly up -so the torch's gas cup/contact tip is pointing upward; say within 10deg.s of vertical at the gas cup?

    So I think a normally set up weld would be fine, because of the original extrusion's thickness as a heat sink I'd use 0.045" dia 5356 wire in order to carry the amperage to fuse into that heavy bar stock. IF the angle used were wider legged, as Barry mentions, - say a 3" x 2" with a wider reach under and inside the existing chine extrusion (?) THEN, I might look at 0.035" wire but in general I'd want the bigger wire to carry the higher amperage due the big heat sink of the chine and all that appears to be built into that area?

    I've only done a couple pump powered boats and that was a long time ago, I used an 1-1/2" x 1-1/2" x 1/4" to wrap a sheet edge to sheet edge chine (I was thinking rocks would bang up the extrusions and leave the chine alone?) and I stitched the extrusion. I didn't know any better but I'd like to learn more about what Barry has found in his builds?

    My reason for stitching the chine cover extrusion was because I figured it was 'disposable' and would become bent, dinged, and otherwise torn up by rocks so I thought it best to come easily.

    I do not know how the boat will handle in comparison to not having the chine cover you show, and I've built into hundreds of skiffs, but I do not think it will harm the performance by protruding slightly from the chine's edge -as shown.

    I wasn't sure how the original description fit into my limited jet boat experiences but I have heard of chine tripping or shapes external to the hull as the chine where some performance issues arose. MY experience is with mostly outboard powered skiffs, and most with reverse chine flats, but the cover you show has aided in turning tighter, and not shown any problems to the skiffs.

    The Woolrich chine extrusion sure does look beefy enough to take all the structural loads it will see, but I prefer to weld inside and outside all hull seams and use a sheet edge to sheet edge type of joint- then "cap" it as you show.

    cheers,
    Kevin Morin
    Kenai, AK
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Kevin,

    Basically it's fatigue. These running strakes are in the slamming zones and thus experience many cycles. Stop-starts are notorious for fatigue cracking at the best of times if not done properly. In a high impact slamming zone and under water, we too always fully weld these never stitch them.
     
  9. Barry
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    Barry Senior Member

    Kevin
    Re fully welded lift strikes vs stitched
    The water moving from the front of the boat to the rear moves from the keel to the chine.
    When the water hits a lift strake that has a flat or downward orientation to the dead rise, it changes direction which produces an increase of pressure, or lift.
    The maximum pressure point occurs when the water just begins to turn which is right where the weld line is.
    If you have stitched this weld, the potential to maximize lift is reduced because the unwelded area will bleed some of the pressure off.
    Additionally, turbulence will be increased due to the water impacting on the abrupt non-welded edge of the angle
    Before we weld the angle to the bottom, we also grind the inside 5 inch flat at the the edge to reduce the height of the flat to allow the 1/4 inch to lie closer to the hull
    When fully welded, we take a Pferd grinding flap disc, designed to grind inside fillets, and smooth out the weld
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    It is surprising that you do see quite a few alloy boats with stitch welded strakes, they don't look that neat either ! Not production boats, typically.
     
  11. Kevin Morin
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    Kevin Morin Junior Member

    Chine Welding Schedule

    Thanks for the clarifications, gentlemen.

    I built commercial fishing boats, that did not, do not go the speeds that would bring these (explained) considerations into play. They don't go fast enough to slam and flow on the bottom at mid to low speeds is not critical (?) and so the few jet sleds we built (long ago) might have been 'impacted' by this but weren't around our area long enough to show up?

    If I read the posts correctly then.... the unwelded outer chine seam shown above is not good practice for the topsides to chine extrusion?

    I'm assuming this chine extrusion is Y shaped with slots to accept the topsides lower edge? I'm sure it has to be welded continuously inside (??) and that skipping the outside weld is to reduce the cosmetic weld clean up needed for the welds involved?

    Following Barry's fairing guidelines then: YTPaul should plan to weld the angle 'cap' on his chine fully gouging starts and stops to insure full tie-in, (Ad Hoc's structural considerations) and then plan to Michelangelo Grind (sand fair overhead) the weld so there is as smooth a transition from bottom panel to added extrusion as possible?

    Barry, in your building, has the opportunity ever come about to compare one hull fairing with the other? Has a side by side, same hull, same power test comparision ever arisen? It would be informative of a reason to do the weld and fairing, and only a builder working to the higher standard of bottom flow would have the chance to make an apples-to-apples comparison?

    AT what rough speed does one begin to make these (continuous welds & faired for flow) considerations? All boats greater than 25knts? >30? >35.... >X speed?

    Ad Hoc, Barry, I'm a little confused about the force vectors involved in lift strakes (regardless if added to the fairbody or the chines). If there is some force of lift from overall planing body's forward motion over the fluid, isn't the relative force (localized lift) reduced by the increased speed of the fluid over the 'lift strakes'.

    I realize they vector the water downward when viewed in Body Section, but it would seem they'd actually be low pressure areas in relation to the entire surround area as the water has to 'speed up' relative to the overall area. (Bernoulli's principle as principal force analysis?)

    Just trying to resolve the lift-strake's roll in 'lift'. Any remarks welcomed, include text references needed to explore this flow event. As long as we're discussing this question; can any statement of optimal chine flat angle to the waterline be made? (this could be the subject of other posts; I've not searched here.)

    The last question is in relation to YTPaul's optimal angle of installation of any potential angle extrusion at the chine. Where he may be best served with a 10deg down, 12.67deg down angle or some other accepted or 'known' deflection angle at his chine?

    Thanks for taking time to reflect on this issue of weld schedule of the metal chine cover/lift strake on the bottom of welded aluminum boats.

    Cheers,
    Kevin Morin
    Kenai, AK
     
  12. Barry
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    Barry Senior Member

    Kevin
    Re the fairing, continuous welding etc
    The theory behind this extra effort comes from the same principles as you would use when designing a vane for a turbine or even a propellor blade (ignoring surface piercing props)
    In a short explanation it is difficult to cover all aspects of the design components so I will use a basic equation for illustration.
    I will take one cubic inch of water that will have a specific mass. This particular unit will impact the hull say behind the stagnation point on the hull,at the keel and will move outward toward the chine as it moves down the hull. There will be a horizontal component of this unit of water. Looking forward from the transom, this water will move from the keel say to the port side and up toward the chine.
    Without lift strikes, the horizontal component of the velocity will just exit at the chine.
    Instead we will turn it from the 12 degree up dead rise angle (our hull) and turn it down 12 degrees (due to our strake design) at say the first strake
    For simplicity's sake we will use a basic formula of
    F=ma. Or Force = mass (Note you could also use mass flow rate but this is easier to deal with for the explanation) times acceleration
    The equation says that you need a force to accelerate a mass. Acceleration is a change in velocity. Velocity has two components, speed and direction. If the speed is the same as we will assume for this unit of water, but we change the direction of the unit of water, then
    we have accelerated the mass and this will produce a force on the flat of the chine, ie lift

    To maximize this lift, a vane is normally a smooth curve that changes the direction with the least amount of turbulence.

    So we go the extra effort to keep the transition as smooth as we can to maximize lift.
     
  13. Barry
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    Barry Senior Member

    Kevin
    Re quantifying the benefit.
    We started out building boats off the same jig without strikes then graduated to 3 inch by 1 inch, then worked our way up to the modified 3x5. We gained in steering control, hardly had a victory roll, ie swapping ends, and what we perceived to be a higher riding hull.
    As we made changes we ran fuel curves with a Flowscan fuel meter and from the start to the last strake modification we dropped close to 200 rpm for the same given speed.
    Of course, and I will be the first to admit it we did not always maintain the same base line, ie same air temp, the same weight to the pound, used pitot tube speed gauges, but overall at say 30 mph, the rpm came down a few hundred rpm
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Kevin,

    I’m not in the office right now, so don’t have all my references to hand. But straying down into the hydrodynamic side may not be wholly useful to you. The reasons for the change in shape from a classic displacement hull to a planning hull are very well understood and documented in man text books. However, you will get many who “see” or “feel” the water flow and convinced they understand the hydrodynamics behind it. We have seen this plenty of times on such threads on this forum alone. Despite very good research suggesting the contrary to such “beliefs”. And wont be told otherwise. The thread then deteriorates into the my dog is bigger than yours…sigh. Thus if you really want to explore this, I would recommend a few classic references for background reading, which is objective and succinct and not trying to promote itself!

    I won’t labour on the point but ostensibly the flow of water is transverse from keel to chine and you need a sharp but-off to encourage the flow to break free from the hull. This is the difference between a round bilge and hard chine planning boat. The round bilge at higher speeds becomes unstable because the water flow does not fully detach from the hull…again a lot of over simplifications and generalisations, for brevity. The chines/running stakes assist in this flow detachment. The higher the speed the higher the L/B ratio which is also where the additional chines help on the hulls bottom surface, since it assists the hull form in becoming more efficient, again many simplifications for brevity.

    You will also find many references/papers to chine down angle. The one we have used and confirmed ourselves with model testing and sea trials, is that a down angle of 5 degrees is sufficient and all you really need. That is 5 degrees to the static waterline.

    The force vector on the chine is easily withstood by a simple staggered welding arrangement. However it is not as simple as the actual magnitude of the force. With stitch welds, you get localised regions of no weld which is subjected to high localised loads owing to the no-weld pocket that has been created between them. It creates a pocket which magnifies the load. Just like any sea shore breakwater/groyne . This increase in pressure, where no weld exists, works on the ends of the weld close by. Which goes back to my point of fatigue. It is more of a structural issue than anything else. Since if you calculate the required shear area of the weld, it easily satisfies. But taking into account fatigue and the fabrication and the aforementioned, it promotes cracking often very fast indeed!

    Ideally you want to “seal” the chine strake. Since, if you get static water inside the chine angle (just like a crevice), it will slowly corrode as the water slowly becomes more alkaline. If you can’t seal it, make sure you can promote the free flow of the water from inside to out to be self-draining, to prevent this.
     

  15. Kevin Morin
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    Kevin Morin Junior Member

    Hull Externals

    Barry,
    thank you for the report and explanation of the benefits of both your hulls' design evolution and the reasoning behind it.

    I can see pretty exclusively that my building in the low speed end of the market, commercial net boats is a big contribution to my lack of experience in the matters we've discussed.

    I wonder if a cusp shaped (vane section simulation) extrusion used on the bottom instead of a completely flat (90 extrusion planes) lift strake was used if there would be any increased improvement? IF a cupped, long, (5" wide) changing curved face extrusion were used- will the 'vane-like' surface reduce drag while increasing lift? Such an extrusion could have its short leg trimmed in a taper from the after prismatic bottom form forward so the cup of the downward shape (concave in section ) adjusting the vector of the released flow as the body section changed?

    Just speculating about the idea of a vane section versus the flat shape slowly twisting with the body section of the V?

    Ad Hoc thanks, I see much more clearly why the weld zones would have more reliability due to the uneven loading that could be easily overcome by continuous welding. Again, it appears these conditions are for boats faster than my fleet of net skiffs and net fishing boats so I've not seen any stitched weld problems mainly because all of them travel so much slower.

    I'd heard that 5 deg was an agreed angle of deflection but given how much about these higher speed aspects I was ignorant, I wanted to ask to confirm the accepted knowledge. I've also heard countless other adages that turned out to be just 'wives tales' not facts.

    If time permits, could you reflect on the concept of a vane shaped, or "cusp break-off lip" forming extrusion (or roll formed ) hull longitudinal as compared to using a more uniform 90/angle extrusion?

    Also, if either of you gentlemen would take a minute to reply about the ideal outer edge of such and extrusion or edge forming strip? I understand the basics of Ad Hoc's explanation of the need for a separation point along the hull, but I notice in the photo above, the jet pump powered sled has a radius to the chine extrusion?

    Is there any detriment to the radius- given the goal of this shape is to 'release' or break free the flow? ( I realize Ad Hoc's remarks about round bilge designs means that at large radii there are limits so my question is in regard a few mm=R or fractions of an inch=R?)

    Is there any optimum radius (versus sharpness)- again I'm somewhat asking for a summary of educational and experiential knowledge?

    Thanks for the posts, I enjoy learning when information is so succinctly summarized and I don't' have to purchase 200.00$(US) texts and try to recall math that I haven't seen in decades.

    YTPaul, I'd adjust my original answers to reflect some of what we've learned here. First, as to which extrusion to use? I'd now say that holding up the added angle; I'd want to have the horizontal leg reach far enough inboard so that the lowest vertical depth seam could be welded. In the previous pictures that is the longer legged extrusion (1"?)

    This will result in the smallest cross section weld and therefore the easiest to put in continuous and then to dress down the easiest. Those goals seem consistent with the remarks we've seen in the thread.

    I'd try to put the after 1/3/-1/2 of the hull's chine cover looking down at 5deg and to do that I'd level the hull and set up a small tacking gauge that allowed the chine cover to be tacked on at the 5deg down.

    Cheers,
    Kevin Morin
     
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