Lifting strakes or a pad for a fast mono hull ????

Discussion in 'Boat Design' started by tunnels, Feb 1, 2009.

  1. tunnels

    tunnels Previous Member

    What is the best for a small but quick power boat stakes or a pad ?? The hull is 14.7 feet long . And what size , how many ?and where should they be placed ??
     
  2. PortTacker
    Joined: Nov 2008
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    PortTacker Junior Member

    First, define "fast."
    HUGE difference between 50 mph and 85 mph. (below 50 is not fast...)

    My opinion, broad generalities:
    I think up to a certain speed it matters not. The vee with strakes probably rides smoother.
    I feel that once you go beyond a certain speed, for a 15' boat, 50 mph-ish perhaps, you need a flat pad.
    Lots of design solutions are probably possible though, including neither of the above.
    Like, how about reducing the deadrise toward the transom?

    Easiest way is look at the bottoms of some boats that are similar to what you are envisioning, NOTE: make sure the ones you look at handle well at "fast" speeds, and see how they were designed.
     
  3. tunnels

    tunnels Previous Member

    Hi
    Speed in the 70 mph bracket !
    A flat keel will give lift but i feel it could also lead to instability as speed increases in the form of possible chine walking and there is no way in this world that i want that .
    Because of the older style of hull it needs a slightly greater vee up forward and the vee carrying on to the transom for longtitudinal stability in the rough . Because of the bigger motor i feel it needs strakes in the after 1/2 if the planing area . one to get the hull up on the plane quicker and two to hold the hull steady at speed . Rather than reducing dead rise i feel the placement of strakes could be more beneficial to achieve stability and lift as speed increases. At the end of the day its a fishing boat and a ski boat but just able to go like stink !! there needs to be a margin of safety thrown in . unpredictability at speed is not to be any where near this excise ! :cool:
     
  4. PortTacker
    Joined: Nov 2008
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    PortTacker Junior Member

  5. u4ea32
    Joined: Nov 2005
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    u4ea32 Senior Member

    Learn from the master. Go measure a Fountain powerboat.
     
  6. tunnels

    tunnels Previous Member

    Sorry non of those boats within 8000 miles of us !
    But i have a idea fixed in my mind of what i has to be done after a run this morning with a bigger prop that gave a good 20% gain in speed at the same rpms on the old prop and i still havent pushed the motor to its limit yet . Need more seat time behind the wheel and trim setting up before pushing it further ! and the strake possition and design is falling into place as to what i have to do !
     
  7. tunnels

    tunnels Previous Member

    strakes vs pad ???

    Strakes and or a pad i aske again the question its possible to get chine walk and thats not where i want to go if i use a pad and it produces to much lift . So does anyone have a magic formular of length x width x speed =amount of lift produced
    Also for strakes ,length and placement x width x speed = amount of lift
    My lean is in favor of strakes but am willing to hear other peoples opinions !.
     
  8. Commuter Boats
    Joined: Oct 2006
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    Commuter Boats Commuter Boats

    "tunnels
    Because of the older style of hull it needs a slightly greater vee up forward

    Because of the bigger motor i feel it needs strakes in the after 1/2 if the planing area "

    Although you don't say it,from your comments I assume you have overpowered an existing hull and have handling problems you are trying to correct.
    You state that the hull is 14'7" long and that the desired speed is 70 mph and then you follow that up with,
    "At the end of the day its a fishing boat and a ski boat but just able to go like stink !! there needs to be a margin of safety thrown in . unpredictability at speed is not to be any where near this excise !"
    Please excuse me if my assumptions are incorrect.
    If I am correct, I would like to express caution as " unpredictability at speed" is probably where you're at.
    To push the the average "V"hull, 14'7" commercial hull to 70 mph is most certainly outside the design parameters as to scalings and handling considerations.
    In a case like this the "there needs to be a margin of safety thrown in ." most often requires quite a lot of seat time.
    I hope you're up to this for your safety and the safety of others.
    Okay caution aside, I have spent quite a lot of time messing around with existing hulls in an effort to push the design parameters. One of the designs I've spent a lot of time with has similar parameters to where you're at, a 15 foot Boston whaler with an intended maximum horsepower of 70 hp.
    Even with a 70 hp, when light it could be a little squirrelly, we've put 90s on a bunch of them and up to 120s on a couple. When loaded and with modest power, the whaler is well behaved with a sharp V that evolves into a rounded section at the transom, one lift strake on each side in the forward two thirds of the hull, and large sponsons. At speed ( beyond what the boat was intended to do) the hull will try to run on the aft 4 feet of that rounded section with the sponsors out of the water. The boat will predictably start rocking port to starboard, from sponson to sponson , with each impact greater than the one before it until the pressure on the sponson is high enough to lift the center of the boat out of the water and "unpredictability at speed" is obtained!
    I think this condition parallels your " chain walking" and I have had success controlling this by extending the lift strakes to the transom while making them slightly larger in section.
    I agree with your assumption that strakes would benefit stability more than a pad would.
    As to a magic formula, there are two many verbals, without somebody with experience looking in your hull, you are left with a need for some trial and error. You should play with aluminum angles or triangular pieces of wood, temporarily bonded or fastened to your hull and observed the effects.
    The closer together, the less wetted surface and the greater the speed potential. Farther apart and more wetted surface but more stability.
    You should try one modification at a time and start conservative in an effort to understand the effect of each modification.
    Although you may have to return to engine height throughout the process, the first thing you should do is try to optimize your engine setup as too low of an engine mounting as well as the wrong propeller will have huge impacts on the handling of a light boat at the speeds you are trying to achieve( propellers can be design to provide bow lift or stern lift).
     
  9. BMcF
    Joined: Mar 2007
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    BMcF Senior Member

    I have an old Champ 17 semi-V hull that has both the strakes (nearly identical in size, location, length and angle to my 16' Donzi...but I understand Aronow and Charley Champ might have collaborated a bit on the Champ design..) and a 'ride' pad. I cannot tell *exactly* at what speed she is on nothing but the ride pad but I'm guessing around 65 mph or so from some of the photos friends have taken from a distance. The fastest I've had the boat was 74 mph on GPS.
     
  10. tunnels

    tunnels Previous Member

    Thanks for the feed back we think along same lines .
    Have messed with boat since i was at school and now want a go fast but also a fishing boat to be able to pull skiers as well . Its does those things plus , but the bottom is in need of a make over from years of abuse from other owners and sitting on trailers that were meant for a differant shaped hull .
    I have spent 2 hours this morning laying under the hull with straight edges and a maker pen looking and see ing what i sould be doing to it come this winter .
    Have been and done the motor height thing and where its at right now suits the hull in the condition its in . The boat is a 1975 and yes its a wee bit over powered . Safety comes first and i am always careful when thinking about pouring on the coals, early mornings when its shiney calm , no other boats about and im by myself ! The boat has show no side affects to the added hp and speed . Strakes are number one in my book and set a little way apart as to keep the balance , they only need to be half the lenght of the wetted lenght as its just the aft end that need to get up and a little way out of the water . By taking the roundness out of the keel area and turning it into a vee will help no end with water entry at speed and keeping in a straight line .
    Il anso ask another question about HOOK AND ROCKER built into the hull . Hook i am not in favor of but rocker yes , The interpretation of rocker can be miss leading . It can be a gentle curve but it can be a point where the keel has a built in ever so slight change in dirrection . The Wally boats have a pronounced version of what i am looking at doing . I totally agree with what they do and can see great merit in the idea and have done something simular myself on small boats .
     
  11. Commuter Boats
    Joined: Oct 2006
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    Commuter Boats Commuter Boats

    tunnels,
    A little background, I will try to keep it short. I missed being born on a boat by 20 minutes, was always around boats growing up , my dad built few, when I was 14 I was repairing/ modifying with fiberglass, commercially fishing and spending all of my free time on the water. When your boat was built (1975) I was working at a marina ( fiberglass repairs and modifications, repowers), in the mid-80s I spend half my time in industrial fiberglass and the rest of my time with boats as owner-operator of a small shop. Through the 70s and 80s I devoted much of my spare time to composite and boat design, 1988 I launched a 25 footer of my own design. I still spend much of my time in repair and modification but will occasionally except a commission for a design-build, the largest to date is 40 feet, all have been successful.
    Most of the boats that I have design-built have a hull shape referred to as" warped" (I don't like that term) or" double wedge" planing hull. As such you could say they have "rocker", from a side view there is an apex on the centerline, 33% of the hull length forward of the transom. Because I use it, I obviously think that it can add something useful to some boats although you will rarely see it in a modern design. My boats are by today's standards, long and narrow with the longitudinal center of gravity is little far forward for a planing hull and rocker is being used for bow lift and to increase dynamic stability. My chins are designed to add lift / dynamic stability but I do not use lift strakes.
    My 25 footer ( 25'7"x 6' 3200 pounds) made 56 mph with a 135 hp outboard and 65 mph with a 175 hp motor. It was very well behaved, a good sea boat, and a joy to drive. Although it was never finished out as it was just a prototype, it logged over 3000 hours and countless miles, one of the most fun lessons of my career.
    From your last post it would seem as though your hull was designed without lift strakes and you are contemplating adding them only in aft half of your hull and you plan to sharpen the "V" and build in some rocker.
    I would feel more comfortable making suggestions if I knew more about your vessel but my intuition is to suggest full-length strakes and no rocker.
    Maybe you could post some photos and we could continue the conversation.

    "tunnels
    I have spent 2 hours this morning laying under the hull with straight edges and a maker pen looking and see ing what i sould be doing to it come this winter . "

    You're planning for the winter, I just want winter to end!!!!
    I'm at the other end of the world from you and it's cold out!
     
  12. Jimboat
    Joined: Feb 2002
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    Jimboat Senior Member

    Tunnels - sounds like you've got a fun project! what are you trying to accomplish with your pads vs. strakes consideration? are you designing a new boat from scratch? there sure are alot of issues to consider in designing a "fast" hull.
     
  13. tunnels

    tunnels Previous Member

    Hi Jim !,have one of your books and followed you postings for year on differant sites like scream and fly !
    yes its a fun thing to make this into a good alround go fast fishing boat .
    Im 65 years ,worked with boats for years , fibreglass is my trade and i love a challenge of getting more speed out of this old clanker! .
    Because the motor is a couple of sizes bigger than what it should be it still has potentual to be faster and more stable than it is now.
    Its a little over weight in the rear with the 115 yamaha so needs strakes to lift with and carry the motor . Have shifted 3 fuel tote tanks up front plus the anchor and chain etc . Just the oil tank and battery in the back thats all .
    I am a tunnel man myself and have built quite a few over the years .
    I would love to have the money to be able to build a new boat just have to make do with what i got for the moment till the state of the world gets back to where it was last years .
    A pad is not completely out of the question for what im looking for, and it could help with the rear end lift i am looking for but Have to do a little soul searching on this one and spend another few hours laying on my back under the hull looking . :?:
    It has potentual to form a nice vee shape in the bow area with out to much hard work or heart ache. The front 1/3 i need to keep clean and a bit sharpen for water entry ,as it goes fatser so the shape now is not suitable and gets a little hard riding ,its just the other 2/3ds that needs a little more lift to get the hull out of the water a little more and shed water at the aft end not out the sides.Its also a reason for the rocker question as well !
    Because i have stepped the motor back 400mm with the new transom the power trim works great and is nice and responsive to the touch of the trim Theres nothing on the planing surfaces to give any form of extra lift so it needs a boost so to speak !.
    I use to make fast surf life saving ridged bottomed inflatables (fibreglass hulls )and all the tricks that i used on them i would like to use in this situation as well .
    The thing i really am searching for is a caliculation or formular to be able to work out the lift thats possible to generate from strakes! what size gives what sort of lift at what speeds -area vs speed = lift ?? put in simple terms :confused:
    .
    Its a old boat (1975 ) and was never meant to do what i am trying to do . Its fun , its a challenge and friends just look the other way or ask when are you going to buy a wheel chair and act my age !:p
     
  14. Jimboat
    Joined: Feb 2002
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    Jimboat Senior Member

    tunnels - you may find helpful an article that I wrote on "vee pad design". There are calcs that will esitmate lift from strakes (email me if you want more info). I expect that there would be more to your "go fast" issue than just adding lift strakes, as this would also change dynamic CofG, and hence handling.
     

  15. tunnels

    tunnels Previous Member

    The go fast is the real fun side and very invigerating just i have to contend with harbour chop and wakes from other boats and venture out onto some rough water so tracking straight and not broaching etc come into all of this as well . Im not asking much just the seemingly impossible from my little old 14.6 power boat . :p :D
    Strakes are just part of what i want to do along with vee in the front sections ,possibly a pad at the rear disapating into the vee as it goes forward alng with introduing rocker (with a differance ) into the keel line .
     
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