Modern planing hull/top speed trim tab requirement. A question for hull designers

Discussion in 'Boat Design' started by William George, Mar 4, 2013.

  1. William George
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    William George New Member

    A question for sportscruiser hull designers out there! The question is in relation to a boat approx 11.5 m long planing hull with displacement approx 7500 kg with 500+hp

    I am told by the major international boat builder of this craft that their brand new hull designs require full negative trim on the twin sterndrive installation(-5') and most of the negative trim on the QL interceptors to achieve it's quoted top speed, even with very light load and fuel aboard.

    I am a qualified engineer but not a naval architect/designer, and my instinct is that if this is true then the hull design is not achieving an adequate pitch position to obtain maximum speed with it's recommended power installation, without drag inducing full negative adjustments on the sterndrive trim and interceptors.

    My understanding is that these device adjustments should be for assistance in uneven or stern boat loadings, early achievement of planng speeds or to effect more comfortable ride in differing sea conditions.

    In which case if the builder's statement is true it would look to be assisting for inadequate hull design or/and underpower installation. Is this correct? Do modern boat designers build in this way? If so, why?

    I'd appreciate some qualified opinion on this issue.

    Many thanks
     
  2. gonzo
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    gonzo Senior Member

    Trim tabs are often used to compensate for a bad design. What you are describing is a typical attempt at making a badly designed boat run in trim. Most probably the boat is stern heavy, which is common in boats that have excessive amount of fancy gear added on.
     
  3. Ilan Voyager
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    Ilan Voyager Senior Member

    It's not always a bad design...It's difficult to have a planning hull with good characteristics over all the range of speeds.
    For example the NA optimizes the rather heavy -7.5 tons for a 11.50m- boat for the cruising speed (normally 0 trim and 0 angle flaps) to get a good mileage and a sweet good behavior, as it's the situation the boat spends most time.
    At top speed as the planing surface is reduced and goes aft, the boat has maybe a tendency to sink (after all it has more than 1000 kg of engines at the stern, a common problem with sterndrives) and to have too much angle of "pitch", making it very sensitive to waves. Sterndrive negative trim is one solution, it just diverts a bit of power, as it forces the boat to "bow down". Flaps are another solution.
    You must see what trim is needed at cruising speed. If the trim is zero or very close to zero at this speed, there is only a small nuisance as full speed is not the longest situation. It's common on deep vee hulls.
    Do not forget that with fixed propellers the "negative trim" is about 8 degrees, and there is no mean to change it. The sterndrives offer the advantage to adapt the trim for every situation.
     
  4. tunnels

    tunnels Previous Member

    poor design or poorly loaded call it what ever you like !

    In the case of the company i been working in China all the moulds came from Australia and were much lighter built,the boats built here absolutly needed trim tabs because of the extra machinery that filled the engine compartments ! air conditoners , a gen set that could power a small city and a bank of batteries that could last for a week without having to be recharges . oversized fuel tanks , so naturally all the boats had a huge hole to get out of when the throttles were pushed down and the bow goes skywards .Everyone that drove the boats didnt use the trim tabs ever in the two years i been here .
    All the hulls and decks are way over weight and internals made of ply and solid timber way oversize . when i saw what was happening in the begining i pointed it out and got the cold shoulder that they knew what they were doing !!and eventually because of poor performance and cost of over building the sales of boats feel drasticly and now the company is no more !!.
    So trim tabs are just another tool to add to the control of the boats performance along with the stern drive trim .
    Some times we load our boats how they were never meant to be loaded , everyone loves to stand in the back and bee cool !!! trim tabs were a replacement for introduced slight Hook in some of the older boats that did the same thing . powertrim on the outboard does the same thing and even they get extra fins added to the leg to make them work better !!.:eek:
     
  5. William George
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    William George New Member

    Thanks for the input Ilan Voyager,Tunnels, and Gonzo. I note you all have crudentials, but with Ilan giving the designers the benefit of the doubt, with them maybe striving to achieve a cruising optimisation which prejudices the top speed trim ideal. Unfortunately, even at cruising speed the craft doesn't run best with 0 trim and 0 flaps.

    Ilan, you are right when you comment about the displacement/length ratio, although with 500+ horse power my engineering experience would expect the designer to either adjust mass distribution and/or provide sufficient power to be able to cruise without device intervention(0 trim and 0 flaps), at least for low loading situations. The boat's design horsepower has no problem getting the boat onto the plane.

    All of my previous sportscruisers have always needed positive(leg out) sterndrive trim with no tab trim to attain top speed, and the original manufacturer instruction for this boat was only to use maximum negative sterndrive trim. I never envisaged for a moment that it would also need near maximum negative trim tab with low loads! Consequently I have not tried to improve top speed with full negative trim tab.

    What I do know is that at cruising speed(24/25 Knots) if you trim out from full negative on the sterndrives, the boat puts it's nose up further. It would follow that if it has been optimised for cruising speed, it doesn't work, and the situation is worse for top speed.

    Given the above information, do you still think it's an acceptable design status for your industry?

    Thanks, in anticipation of further input
     
  6. bertho
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    bertho bertho

    planning hull not planning...

    Guys,
    think you all right.. for me, heavy planning hull will be to the past soon, it's a hydrodynamic aberration for cruising vessel.... get to planning.. and stay ..so full speed or nothing.. hope we will be back soon to more pragmatic approach..
    and let the planning hull for racing ! go back to displacement hull, longer is better ..:D !!
    see so many planning hull who can't pass anymore the ..planning stage, so they remain for ever like the worse wave machine ever , and full gaspi machines !! (especially in asia , where boat imported, fully loaded (commercial never said no to a customer who want more dinghy, more stuff on board..= more $$$$ !! not silly ! ) , hot sea water cooling, hot temp air intake, engines never reach they theoretical power already limit.. ... = disaster..
    many for sale her if you want !!
    best
    bertho:p
    www.fusionschooner.blogspot.com
     
  7. tom28571
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    tom28571 Senior Member

    I agree with most postings although I think the design or the building/equipment is faulty for a planing boat. Not knowing the design particulars, I estimate that the planing bottom loading is very high, like maybe 144#/sq ft, which is extremely high. That probably leads to a heap of stern sinkage underway and the negative trim is necessary in a attempt to correct this. Trim tabs are the universal answer in either broadening the range of speed the boat is useful at or in making it workable at all, as I suspect is the case here.

    Negative trim of a shaft drive is not the same as in a stern drive because of the difference in point of thrust application.
     
  8. tunnels

    tunnels Previous Member

    Trimming !! Its not rocket science its just common sense !!

    Setting trim angle on almost any boat the boat whill tell you where it want to be . get on the plane !, set the engine tacho dead on a figure 3000rpms as a example not adjust the legs and all time watching the tacho . if the rpms come up and if you keep adjusting the tacho will start to fall so trim back to where you get the highest reading . Leg set !!Now do the same with you trim tabs untill you get the highest reading . that means the trim angle is at it optimumtake particular note od where the chine spray is coming out the side of the boat . bow could be slighly up and spray could just over 1/3 of the chine length .
    Same with an outboard and only power trim . set tacho to 3000rpms and then adjust the trim in and tuck the leg under the rpms drop gradually trim out and the rpms come up the spray out the side moves back aways, keep trimming and the bow goes up and rpms drop so back off the trim untill you find the sweet spot where the rpms could be 3200 rpms that where your boat is at it optimum .maybe differant for every boat but some where theres a place it like to be !! Its completely up to you to find it . just trim slowly and one thing at a time or you will never find it !!
    My own boat i set 3000rpms and tweek the trim up and watch the spray out the side of the chine ,i can hear the sound of the motor change and sure enough 3150 rpms and thats it , hammer down and going for it . :D
     
  9. tabman
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    tabman Junior Member

    Interesting conversation!

    Trim Tabs on a well designed hull allow the hull to run at its optimum running attitude over a broader range of speed, sea conditions and weight distribution.

    Far from band-aides, they allow the good characteristics that a designer has developed to work in a larger envelope than a "static" design.

    For example if the boat can plane at lower speed and still keep its bow down cutting an oncoming chop it is essentially running on its lines and the good characteristics of its hull design are working.

    Calling Trim tabs a crutch for bad design (if it is a good design to begin with) is like faulting an aircraft designer for including flaps for low airspeed performance.

    Tom McGow
    Bennett Marine
     
  10. Ilan Voyager
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    Ilan Voyager Senior Member

    As I do not know the boat and haven't tried it I would not make any judgment on this design...
    Heavy planing boats are always "problematic", more when the engines are on the stern, making almost impossible to get a good center of gravity. It's even worst with big heavy outboards, than with sterndrives.
    It's the price to pay when you want an apartment with all amenities in a "small" boat. It results to be short, very wide, too high and awfully heavy. Not the best for good speed cruising boat, as it will be by nature unstable. And when the planing surface is too loaded, nope there is no remedy.

    A good designer likes to put all the heavy weights like engines, batteries and also possibly tanks close to the center of gravity, thus eliminating the effects of polar inertia, and to keep a good planing with no more than 6 degrees trim of the hull at full speed. That leads to boats of medium displacement, not very wide (about 1/4 of the length), 12 to 18 degrees of keel angle (after that eats too much power, with not true advantage), and as long as possible with fine entries. No flybridge, or double store sperstructures as the aerodynamic cost is horrendous. Unhappily for the marketing service, such a boat is not roomy in the short boats, so it has no selling arguments for the profane, who wants an floating apartment just a bit less spacious than his house, and with all the amenities (air conditioning, microwave, pressurized water, etc) but does not want to pay the price of a boat able of that (at least 60 feet needed, not 40 feet)

    Trim tabs and trim sterndrives or outboards are very useful even on the best of designs. That permits a fine tuning of the boat in any condition of load, sea, wind and speed. Tunnels gave a good method for tuning them.
     
  11. tomas
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    tomas Senior Member

    A clear and excellent summation.

    It helps me to better understand what I've been seeing on vessels like the latest sport-fishers with enormous weight and power hanging off the transom.
     
  12. tabman
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    tabman Junior Member

    It's also worth noting that the use of power trim on an outboard or inboard/outdrive (I/O) and Trim Tabs together can achieve better results than the use of power trim alone to get the boat's best running angle and maximum thrust at the same time.

    If the boat is running at a bow high attitude and you correct it using the power trim on the drive unit alone the propeller's thrust is now angled down, resulting in a lose of efficiency. Additionally since the propeller is now at an angle to the water flow its slip is increased, further reducing its effectiveness.

    If Trim Tabs are used to adjust the boat's attitude and the power trim is used to position the propeller "squarely into the water flow" the best economy will be achieved. It is quite easy to accomplish this by trimming the boat's attitude with the Tabs, and then using the power trim while observing the tachometer for best RMPs.

    On straight inboards keeping the propeller shafts closer to parallel to the water flow using Trim Tabs at lower speeds will also result in increased efficiency.

    Tom McGow
    Bennett Marine
     
    Last edited: Mar 5, 2013
    1 person likes this.
  13. Ilan Voyager
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    Ilan Voyager Senior Member

    That's the worst, the nightmare. With towers that gives the aerodynamics of a 1918 truck and more loaded by sq foot than a X15 rocket plane...I have seen one with 2x300 HP outboards!!! even in the marina it seems ready to sink by the stern, at sea that makes lots of spray, nothing more.

    12 years ago the nephew of a Mexican friend bought at Cancun a used 30 feet sport fisher. With 2 Ford V8 7.2L it was unable to go over 20 knots. The consumption of gas was horrendous. He consults me, desperate as he wanted to used it for chartering, needed to get back the investment and earn a living.

    Chartering, ie just day trips...just need of a good fighting seat, shadow and good full cooler.

    With bit of work, and the shipyard took out: the double berth, the cabins, the kitchen with microwave and electric oven, the big 120 gallons water tank, the big 80 gallons used water tank, the air conditioner, the genset, the big gas tank of 200 gallons -which was just at the stern-, the 250 pounds marine battery, the 2 engines plus a lot of useless things. Two full trucks.
    Empty, the engine room looked very spacious...How all that was inside a 30 feet boat, it's a mystery. I have no explanation.
    A 25 gallon water tank, chemical toilets, dorade aeration boxes, 2x25 gallons diesel tanks, and one engine, a Cummins 5.9L rated 320HP, one 4 blades propeller, twin small rudders with clark Y asymmetric profile (the original rudders were just a steel plate...) and electric trim tabs.
    Morality 30 knots and a cost of exploitation divided by 3, with a boat as gentle as a purring happy cat.
    Not applicable on a cruising boat but that shows the ill effects of overweight...
     
  14. tomas
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    tomas Senior Member

    Then you'll really like these from the recent Miami boat show.

    Three engines, with 557 HP each!
    How about a set of 4 350HP Yamahas?
     

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  15. Ilan Voyager
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    Ilan Voyager Senior Member

    In France we call that "le complexe de la petite bite", the complex of the small dick that the guy need to compensate with big cars, useless jewels, big watches and these monstrosities...Or the nouveau riche complex, the bling bling society.
    From the point of view of a NE these things are marvels of inefficiency with the diminutive propellers robbing power from each other.
     
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