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Maintenance Tip of the Week: Gearbox 12/07/2015

Maintenance Tip of the Week – Gearbox 12/07/2015

Always check the level of fluid in your gearbox according to the manual from the manufacturer. Ensure that the level is correct. Gearbox fluid may be checked either hot or cold.

Selecting a Gearbox

 

In this blog, we have covered topics about powering your vessel ranging from engine selection to maintenance, but we have really never covered how to select a gearbox before.

Often, when we are asked to provide a quote or make calculations, the gearbox is often added as an afterthought by the customer. “Just choose one for me.” However, gearbox selection is critical; choose correctly and you will have a long-lasting propulsion system, but choose incorrectly and problems are the guaranteed result.

As an engine manufacturer, we are required to connect to any brand of gearbox. All major-brand gearbox makers comply with specific quality standards, and they give their products a rating, just as engine manufacturers. However, these ratings can vary by manufacturer and close attention must be paid regarding the rating and the intended use. That is why we are often asked to choose: It is sometimes not very straightforward.

Gearboxes are critical systems. They are the device that transforms the torque produced by the engine into usable energy. They couple to the flywheel and to the shaft. The reduction ratio is the way they accomplish this task. The ratio is dependent on the type of drive. For instance, on stern drives, the ratio is normally near 1:1. On surface drives, that ratio is normally from 1.5:1 to 2.5:1. Each manufacturer lists the available rations with their models. The ratios are changed by the manufacturer, usually simply by changing a specific gear, or adding another gear. That is the only difference, in most cases, between ratios, and that is why each gearbox model usually has two or three available ratios.

There are three major manufacturers of gearboxes in the marine market:

  • ZF
  • Twin Disc / Nico
  • Kanzaki

There are also many smaller brands of gearbox, many of which are regional (example Dong-I),  or highly specialized in nature (example, Weismann, who manufactures high quality, customized gearboxes for racing).

Of the three major brands, we provide ZF or Kanzaki on the majority of our packages. Many of our dealers are also Twin Disc dealers, and they often provide their own brand.

Sometimes, gearbox selection is determined by the drive manufacturer. For instance, Konrad drives come equipped with a Velvet gearbox, set at the correct ratio as determined by Konrad’s naval architects. In the case of surface drives, we are usually told the recommended ratio by the manufacturer.

Things to keep in mind when selecting a gearbox:

  1. Annual use. Extremely important. Never choose a gearbox rated for less than anticipated use.
  2. Gear ratio.
  3. Engine HP. Also extremely important to check if the torque produced by the engine is acceptable.
  4. Engine RPM. Also critical. The function of the gearbox is to reduce.
  5. Where is the gearbox mounted?
  6. Single speed?
  7. Mounting angle.

This can often get a bit confusing, but gearbox manufacturers all specify this information pretty clearly. A good resource is Boatdiesel.com’s excellent gearbox selection tool. This will give you a good starting point in selecting potential brands to suit your application. You can find it Boatdiesel GB, along with a wealth of other useful information.

 

 

 

What is Marinization?

 

 

What is marinization?

A diesel is a diesel, right? It should work just fine whether you use it in a boat or a car.

This question occasionally comes up in inquiries to our parts department.

In theory, at least, any diesel engine should operate on any vehicle, land or sea. However, in the years we have been in the marine business, we have never seen one of these independent project come to a successful fruition. The normal scenario is that someone is either given, or has picked up very cheap, an old diesel automobile or truck engine, and they come to us when they cannot get it working.

Marinization is the engineering and manufacturing of engines specifically for marine use.

What are the differences between marine and land engines? There are many. However, the following systems are nearly always different: Cooling, exhaust, and gearbox.

Marine engines need to be liquid cooled. That means that they use a heat exchanger instead of a radiator. As has been written many times before on this blog, heat is an engine’s worst enemy, and the heat produced by combustion must be dispersed effectively in some manner.

As to exhaust, engines used on land typically use a dry exhaust system. Try this on the water and you will have a very noisy engine, indeed. A big issue with wet exhaust systems is that they typically require modified cylinder heads in order to function properly. That old school bus engine most certainly would require new cylinder heads.

Transmissions, or gearboxes, are another issue. We refer back to the old school bus engine. Though you might save a little on the engine, you will need a marine gearbox with proper ratio in order to use that engine on the water. Gear ratios for marine engines are substantially different than on land.

Electronics and electrical systems are also substantially different on marine engines. Something often overlooked is the need for a new control panel, since all of the gauges will be different. Wiring suitable to the marine environment is also necessary. Add in the fact that if the engine to be marinized is electronic, the ECU programming will be completely different. This problem can become extremely expensive to correct, and normally cannot be done by the average person or mechanic.

This brings us to the environment, in general. Marinized engines use components that are manufactured from non-corrosive metals and alloys. Engines used on land are not.

When all of the above is added up in the decision making process, that bargain engine most likely will not turn out to be as big of a bargain as it may seem. In our experience, nearly every one of these projects has resulted in very expensive failures.

Marinediesel has spent the money in properly engineering the marinization of our engines. We have already made the mistakes. We have engineered a bonafide marine engine, suitable for use on boats.

That is the definition of marinization and a very brief explanation about the steps necessary to accomplish the goal of diesel use on water.

 

 

 

How to Match Engine and Propeller – France Helices

 

 

Today’s article is a document published a while back by Mr. Paul Bezzi of France Helices (WEBSITE), that gives a highly detailed description of how to properly match engines, gearbox, and propellers. This is often a tricky task, and we hope that this explanation helps MarineDiesel customers in obtaining optimum performance on their vessels. If you wish to contact France Helices on a project for a quote, follow the link above or give them a call at +33-4-93-47-69-38.

 

A RATIONAL APPROACH TO ENGINE / PROPELLER MATCHING FOR HIGH SPEED AND FAST BOATS

 

Introduction.

 

I am writing this paper with the aim to help those who want decide which propulsion system to select for their particular application.

The majority of yachts and fast boats are targeting speeds in excess of 30 knots. For that purpose engine manufacturers, boat-yards, propulsion manufacturers are increasing power, behaviour at sea and efficiency .Very shortly problems appears due to the lack of know how, and true reason of failure of performance…

I propose to go step by step in the detail of a typical project.

 

1-The hull

There are several type of hull capable of performance, the best hull will be the one that fulfills the owner’s needs.

In fact , most of the errors I observed during the past years were due to contradictions between what owners want and what boat yards can offer. Most of the disputes could have been avoided just by putting some margins in the project. So,

 

The length of the boat for a specific weight will be an important factor.

Not the length Overall ( LOA) but the length waterline(LWL).

A longer hull is always faster with a better behavior than a shorter one at the same weight. (that weight is also replaced by displacement or Greek symbol Δ)

At the end, the best hull will be the one With a minimum of resistance R and the final speed of the boat will be when the propeller thrust T will be the same as R.

 

A simple method to determine the hull resistance is to use the Satvisky equations.

Or, to proceed to tank tests .

For boats with waterline length of less than 40 meter, and weight less than 120 tons, with hard chine hull and constant deadrise, are an acceptable compromise between speed and sea behaviour if the main parameters such as LWL , total power , gearbox ratio, and propulsion are in harmony. The picture below show a typical 25m LOA which successfully reached 36 knots  with attached the table of resistance at the various speeds. 

 

EXAMPLE OF CALCULATION

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HULL DATA
LOA 25 meters
LWL 22 meters
Bpx 4,85 meters
Draft midship 1,12 meters
draft transom 1,12 meters
Deadrise angle midship 17 degrés
dearise at transom 17 degés
spray size 0,18 meters
propeller position from transom 1 meters
propeller immersion h2 0,9 meters
engine centerline 2,1 meters
side inclination 1,1 %
water density 1,025 t/m3
boat weigth 48 tons
lcg 8 meters
propeller diameter 0,8  m
shaft angle 6 degres
shaft length 4 m
ENGINE DATA
number of engine 2
power hp 1300
max revolutions 2300
gear ratio 1,77

 

 

SPEED 0,00 3,39 6,78 10,17 13,56 16,95 20,34 23,73 27,12 30,51 33,90
resistance 2,06 2,21 2,50 2,91 3,45 4,10 4,82 5,49 6,02 6,38 6,63
resistance with flaps 1,86 2,01 2,28 2,67 3,18 3,80 4,48 5,16 5,74 6,17 6,51

The hump is clearly visible at 33 knots. The resistance of the hull at that point is o maximum it will be necessary to check the thrust of the propulsion at that working point  to make sure to get planning.

We can observe that the trim angle of the hull is also maximum at that point and decreases after reaching a top speed running angle of 4.2 degrees with trim tabs.

The trim angle as well as the resistance can be modified by using trim tabs (flaps) or moving the centre of gravity (LCG) forward. Attention must be paid to the fact that moving the LCG forward could increase the wetted area and, by consequence, add resistance.

A quick speed estimation is given by the following formula

V = (SHP TOTAL ^ .551/Δ^.466)* 2.6

In our example V=32.29knots ( with 5 % losses for gears and shafting)

PROPULSION SELECTION

Once we know the resistance, the second step is to calculate an approximate propulsion thrust .

Because the high difference of efficiency between the different propulsion systems, the propulsion system must be selected as a first step.

TABLE OF  PROPULSION LIMITS

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  • The conventional propeller will limit the performance above 35 Knots because of the unavoidable cavitation phenomena and will generate a lift force equal to the sine angle of the shaft multiplied by the thrust force.
  • The CPP will limit the performance to 30 Knots with an additional drag and weight problem due to the size of the hub and 2 % less efficiency than conventional propeller
  • The waterjet limit is around 60 Knots if the diameter of the impeller is correctly sized

Waterjet limits to be considered are:

-weight of the unit and the water carried by the waterjet itself which must be added to the normal weight.

-The particular need of clean water flow  face of the water-jet inlet

-The risk of cavitation if the impeller gets air in the inlet instead of water almost with high values of deadrise angle (in general above 15°)

 

  •       The Surface drives has no theoretical speed limits, though the limits are mechanical properties of the material, and excessive transom forces if used at high speed in rough seas.

Once the propulsion system is selected, the calculations start by assuming for each system an acceptable efficiency comparing the final thrust with the resistance.

The various efficiencies can be obtained trough the propulsion manufacturers or an estimate can be made by using the table below.

 

PROPULSION TYPE AVERAGE EFFICIENCY
CONVENTIONAL PROPELLER 0.66
CPP 0.62
WATERJET 0.68
SURFACE DRIVE 0.71

Then the thrust can be estimated by using a formula.

T= Power*efficiency/ speed

To simplify the calculation and to use coherent factor the Thrust formula can be expressed

T=(146.2 *P* eff)./ Va

Where

Va = knots

P= power per unit in HP

Eff= see table above

Note:

The boat speed is not the flow speed at the propeller. The flow speed at propeller is slower than the speed of the boat. A correction factor must be applied. This correction factor called wake factor is expressed as w factor, so:

Va = V( 1-w)

For a planning hull the w factor is assumed to be 0.02 for a twin engine installation

Hence  Va = V*.98

For the above mentioned yacht the speed of water at propeller to be considered in the calculation will be:

Va=33*.98= 32.34 knots

Also

P = power in horsepower

The power to considered in the calculation is the net power resulting at the propeller after deduction of all losses such as gearbox , additional PTO, shaft losses due to frictional on the bearings , stuffing boxes etc…

As preliminary calculations the values of losses can be assumed to be:

Gear box = 3%

Shaft line =5 %

Finding the necessary power to reach the targeted performance can be now calculated by reversing the Thrust formula.

 

P=( T * Va) / (146.2 * eff)

 

Note : the value of 146.2 is a constant added to the calculation to multiply values which are not coherent  and must be divided by the number of engines.

 

A table of POWER SELECTION can be established.

 

CONVENTIONAL PROPELLER 1133,72 HP
CPP 1206,86 HP
WATERJET 1100,38 HP
SURFACE DRIVE 1053,88 HP

 

8 %  losses must be added to the calculated power

Supposing that conventional propeller is the fist choice, the total power to be installed will be

P total =1134 *1.08=1224 hp*2 engine=2450 SHP

According to engines available on the market   the selected engines are

X  1300hp@ 2300 rpm

THE PRELIMINARY GEARBOX SELECTION

  

The selection of the correct gearbox ratio is of premium importance. The revolutions at the propellers will determine the propeller diameter and the size of the propulsion to be used. The thrust and by consequence the performance of the boat is a direct function of the gearbox ratio selection.

The gear ratio to select is based on the following parameters.

  • The engine revolutions (rpm)
  • The power/weight ratio (hp/tons)
  • The speed of advance or the Froude number ( V/ LWL0.5)

For V/ LWL0.5 > 1 and SHP > 100  the following formula can be used to start calculation.

  • V= Boat speed in Knots=33
  • LWL= length waterline in feet= 22/.3048=72.17 ft
  • P = Total power on board = 2 x 1224 =2448 hp
  • W= Weight of the boat full load (tons) =48 tons(metric)
  • RPM = Engine revolutions per min. =2300 rpm

Optimum gear ratio

GEAR RATIO =1.45 e(0.0038*x)

Where

 

  • x = rpm /  ( P / D ) = 45

hence gear ratio = 1.77/1

Checking in the gearbox manufacturer the available gear ratio the acceptable ratio of 1.77 /1 is selected.

So , the propeller revolutions will be,

Prop rpm = 2300 / 1.77=1299 rpm or 21.65 r/ sec

SELECTION OF THE CORRECT PROPELLER DIAMETER

 There are many ways to selected a correct diameter. The first way is to use empiric formula ,the second way is to use the well accepted method of Kt , Kq ,efficiency curves based on cavitation tunnel test where :

Kt = thrust coefficient

Kq =torque coefficient

Eff= propeller efficiency

J= speed of advance coefficient

The relation between these coefficients is given by

Kt = T / ( p*n^2*D^4)

Kq=Q/ (p *n^2*D^5)

J= Va/Nd

Eff =(J/2*Pi)*(Kt/Kq)

where

P= water density ( 1025 kg/ m3)=1025/9.81=104.5

J= speed of advance coefficient

n=propeller revolutions

The optimum Kt being  known as

0.14 for 3 blade propeller

0.17 for 4 blade propeller

0.19 for 5 blade propeller

0.20 for 6 blade propeller

Turning the Kt formula in Diameter formula the diameter will be

D= (T/p*n^2*Kt)^ 0.25

As a first choice we select 4 blade

Hence

D= ( (6.63/2)/.1045*21.65^2*0.17)^.25=0.80m  and J=Va/nD=(33*0.98/1850)/(21.65*0.8)=0.98

At this stage of project several points need to be checked. The first is to check the cavitation limits and determine the Blade Area Ratio B.A.R

To run the calculation the parameters needed for calculation are:

 

Atmospheric pressure                                                                                  =10100 kg/m²

The hydrostatic pressure due to propeller immersion(0.9m)     =900 kg/ m²

The level of the wave above sea level(assumed 0.5m )                   =500 kg//m²

Static pressure at propeller centre                                                          =11500 kg/m²

Radius at 0.7 r= .80 /2*.7                                                           =0.28 m

Circumferential propeller speed at 0.7 r= U          = 38.06 m/sec

U²       =1449

Boat speed in m/sec=33*1852/3600=                v                             =16.97 m/s

v²                           =288.02

V²= U²+v²=1449+288.02                                                                        =1437.02

½ pV² =0.5*104.5*V         =75084.29

Cavitation criteria = static pressure / ½ pv²                              = 0.153

Burrill coef.    tc.=0.1575*Cc+0.0792                                                  =0.1033

Fp=T/(tc*1/2*p*V²)=3315/(0.1033*75084.29)                            =0.427m²

J=as previous calculation                                                                           =0.98

Pitch /Diameter ratio=0.9511*J+0.21                                                  =1.142

Fa=Fp/ (1.067-(0.229*P/D)=0.427/(1.067-(0.229*1.142)=0.530

Disk area= D²*pi/4=.0.8²*3.14/4                                                          =0.5 m²

BAR=Fa/Da=0.530/0.5=1.06

 

The second is to check the propeller efficiency

We now estimate the diameter=0.8

The BAR=1.06

The pitch=0.931

The blade number=4

The speed of advance J=0.98

The Kt=0.17

We need to calculate the torque Q

Q= 716.2*Power/ rpm=716.2*(1300*.95)/(2300/1.77)=680.68 kg/m=6677N

Kq=Q/p*n²*D^5=0.0418

Kt/Kq=.17/.418=4.06

J/2pi=0.98/6.24=0.157

Eff=4.06*0.157=0.63

The third is to check the final thrust

T=146.2*(power *.95)*eff. / Va= 146.2*(1300*.95)*.63/ 32=3554* 2 engine=7109.43 kg=7.1 ton

The hull resistance is 6.63 which need margin 5% the total resistance is 6.91 tons

      

THE PERFORMANCE OF 33 KNOTS CAN BE ACHIEVED

The fourth concern is the circumferential speed of the screw which should not exceed 50 m/ sec

Revs at propeller =2300/1.77/60=21.65 rev/s

Circumference =.8*pi=2.51m

Circumferential speed= 2.51*21.65=54.38m/sec

This speed is 8% above the limit but can be acceptable if the clearance between hull and propeller is at 15 % of the diameter. If not, the gear ratio must be increased to slow down the rpm and reduce the risk of cavitation.

 

THE PROPELLER DESIGN

 

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The propeller design can some time help to increase the efficiency, the manufacturing class, and also help to get optimized performance.

The new technologies CAD and CNC as well as Hydrodynamic programs help to optimize the performance by increasing the efficiency and behaviour of the propulsion , reducing noise and vibrations as well as reducing fuel consumption. The propeller is working with a certain amount of thrust and torque. The design must guaranty the time life and the safety of crew. In many  application the use of classification rules will solve the problem. The safety aspect of the installation but applying those rules the losses in performance can be estimated to be not less than 10%. It’s the shipyard’s responsibility to decide to go for one way or the other. But, if the decision is made to go for performance first, it is impossible to change for safety after work , just because there is no way to increase the diameter of a shaft nor the thickness of a propeller blade.

So , manufacturers should always calculate the application with a safety factor.

To do so, company internal sizing rules should never be less than factor 2 for safety.

This is the design of the propeller previously calculated.

At this stage of the project the Propeller design helps to clarify the boat general arrangement drawing .

Checking the clearance with the hull , sizing the shaft, calculating the weight

DIAMETER 800 mm POWER 1300 CV 956,8  Kw
PITCH 931 mm REVS 2300 tr/mn 1,05 BAR
BLADE 4 3,4,5 GEAR RATIO 1,77 / 1 65,70 Weigth en kg
BLADE RATIO 1 Leading edge 2,5 mm 42.04 PD² air
SHAFT DIAMETER 75   52.55 PD² water
RAKE DEG, 6 Skew% 0,7

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RADIUS  PITCH DISTRIBUTION l section mm rake mm skew mm THICKNESS Pitch mm Pitch angle
0.2R 0,920 244,968 80 18,039 40,000 856,603 59,59558375
0,25R 0,920 255,293 100 22,703 34,400 856,603 53,73994693
0,3R 0,920 270,628 120 26,503 30,200 856,603 48,64575911
0,4R 0,956 313,939 160 32,789 24,000 890,269 41,52703181
0,5R 0,984 366,084 200 36,644 19,200 916,453 36,10290564
0,6R 1,001 417,914 240 37,045 15,200 931,950 31,71683384
0,7R 1,001 462,853 280 32,789 12,000 931,950 27,91151889
0,8R 0,986 489,310 320 20,355 9,200 917,700 24,53321431
0,9R 0,945 450,650 360 -5,505 6,640 879,795 20,12607725
0,95R 0,890 332,434 380 -26,986 5,600 828,923 19,14580542
0,98R 0,863 205,432 392 -46,222 4,800 803,486 18,06743693
R 0,836 0,000 400 -66,436 4,000 778,050 17,20128356

Mass moment of inertia.

The torsional and whirling calculation will require the propeller manufacturer to calculate the mass moment of inertia in air and in water of the shaft line and propeller.

For propellers, the accurate calculation of weight helps to do so with the mass polar moment of inertia defined as the product of the masses and the square of the radius of gyration of the screw.

                                          I mp=m*i²

I mp = The mass polar moment of inertia in kg cmsec²

m = the mass of the screw in kgcm-1sec²

In general the value of PD² for the propeller is commonly used i.e the product of the weight and the square of twice the radius of gyration. The PD² can be expressed in kgm²

PD²= mass *(k*diameter)²= 65.7*(0.5*.8)²=10.5 kgm² in air

It is commonly admitted that a mass of water is dragged by a turning propeller the final PD² in water is:

PD² water=PD²air *1.25= 13.14  Kgm².

Where  k = .47 < k> .53

Propeller frequency

In some application vibrations could result in harmonics coming from resonance due to the same frequencies between hull, engines, and propellers.

To avoid such inconvenience, it’s necessary to check the natural frequency of each component in the various modes.

The propeller frequency can be easily determined by calculation.

RPM at prop =1299

Number of blade = 4

Frprop = 1229* 4/60=81.99 hertz

VIBRATIONS

 The causes of vibration are many.

When vibrations are observed at sea trial, the method to solve the problem consists of  isolating the various items of the shaft line .

Vibrometer tools can also help to  determine the source of the vibrations.

These types of tools will measure the amplitude of the vibration, the acceleration of the vibration, and then determine the frequency of the measured vibration.

The possible cause of vibration are:

Propeller

  •  Unbalanced propeller
  • Angular difference between blade
  • Cavitation

Shaft

  •  Misalignement of the shaft line
  • Wrong machining of the cone and / or bended shaft
  • Shaft diameter too small
  • Shaft bending when turning due to too important distance between bearings
  • Reaction on bearing due to whirling.
  • Engineering failure ( ex ; rigid shafting coupled to flexible mounted engine)

Engine

  • Diesel Injection failure ( injector nozzle )
  • Distribution timing failure
  • Silent block too loose or too tight ( shore hardness error)
  • Bad matching between propeller pulse ( wake factor variation) and axial admitted thrust of the slient- block .
  • Failure of the gearbox gears or input thrust roller bearings.

Hull

  •  Structural problem in the rear part of the boat
  • Natural horizontal and vertical frequency which generate harmonics
  • Insufficient panel sizing at high speed due to slamming forces ( g too high)

 

Conclusion

 This quick note does not pretend to cover all the aspects of engineering the propulsion of fast boats. Many other matters should have been discussed such as surface drives and going deeper in the propeller design. It simply shows that nothing is impossible to solve . So far, all toolings are available, and computers are a great help to manage technical project if people who use them in the marine industry respect the proportion rules that common sense will bring to their mind.

Thanks to France Hélices engineering team for having provided time and efforts to help me with this document.

 

 

Drives: Which to choose?

 

 

Many of MarineDiesel’s engines are sold as vessel performance packages, as a complete unit: Engine, Gearbox, and Drive, ready to install. Of course, you can always purchase an engine from us and install your own equipment, but packaging allows the shipyard to combine installation, commissioning, and after sale service into a package easily service by our trained technicians, often at a far lower cost than buying each component separately (Since we buy far more gearboxes and drives per year than the average shipyard and most of our partners pass warranty expense to us, we can sometimes deliver components at a lower price.)

This brings up the question: What type of propulsion should I use on my vessel?

There is no single answer other than, “It depends on how the vessel will be used.”

Each propulsion method has strengths and weaknesses.

For instance, on single engine installation, surface drives are not a good option due to torsional pull. On these vessels, a stern drive or water jet should be considered.

MarineDiesel has put together packages with all of the major drive types. What should you choose? See below.

Sterndrive: good trim and reverse. Good box size for davit operation. Traditionally not strong but Konrad and MD is. More drag than surface drive. Good steering. Easy to install and small inside build dimension. Cheap propellers. Normally integrated transmission (not Konrad). Trolling function with MD, joystick control, crash stop. Low cost.
Waterjet: Safety aspects, no propeller. Will not work if there is too much debris. Bad efficiency in low and mid range. Crash stop. Very durable. No trim capability. Must in most cases use transmission. Expensive. Good maneuverability.
Surface drive: Highest speed and most durable in most cases. Expensive propellers. Poor slow speed performance. Difficult to handle torque response in single engine installation. Must use transmission. Can be expensive.
Pod system: Easy to install. No trim function. Falls off on impact, expensive to replace. Designed for pleasure use. Good steering and available with joystick. Very expensive.
Shaft setup: Durable. Needs transmission. Medium efficiency. Complex for boat builder to install. Good reverse and medium steering. No trim.