P(r)op art · TECHNOLOGY & EQUIPMENT Maximum achievable bollard pull This chart shows the value of maximum achievable bollard pull in Newtons. The two bars per propeller represent

11 Streamlined propellers under test

P(r)op artA fixed blade propeller is not the first choice when one wants to extract the last ounce

of speed from a yacht. The flow resistance in comparison to a folding or a reversible

propeller is considerably higher. How large the difference actually is and what the flow

optimised blade can achieve during harbour trials “Segeln” has found out for you at the

Naval Architecture Test Centre in Potsdam.

Text: Gerald Sinschek

Article translated from German Segeln Magazine 2008

sailing boat is asailing boat, twosailing boats are aregatta.

And if the "Opposition",despite having a similaryacht, pulls slowly furtherahead the reason may lieunder the water. Hangingunder the faster yacht isperhaps a streamlinedfolding propeller whichmakes a small but significantdifference.

Those sailing with a freerotating fixed pitch propellerunder the hull could also betowing along a large bucket.The drag is almost the same.Those who should brake thepropeller according to enginemanufacturer's instructions,may just as well throw asecond bucket overboard andtow it behind which spoilsgreatly the pleasure ofsailing.

At least we have made dragmeasurements in the famousPotsdam Naval Arch. TestCentre (NATC Potsdam)towing tank). There we triedto get to the bottom of amuch discussed topic: Howmuch less is the resistance ofa two bladed folding orreversible propeller incomparison to its fixedcounterpart? How does acomparison look between amodern folding and rotatingblade propeller for cruisingor harbour manoeuvres,when forward and reversethrust is rapidly available inorder not to bump the edge ofthe pontoon?

Test configurationand propeller typesFive standard foldingpropellers, a variable pitchpropeller and three reversiblescrews were in the testprogramme. As a comparisonwe tested a fixed two bladedVolvo propeller, "Racingfolding propeller", which- >-

should have especially lowdrag, and a three bladedvariable pitch screwpropeller. Diameter and pitchfor our defined yachtconfigurations weredetermined by the propellermanufacturers. The task wasto select for a Bavaria 34Cruiser with Volvo engineDl-20 and Saildrive 130S asuitable blade, consideringthe given values for engine,gearbox and Saildrive toachieve as far as possible

optimum performance. Thisshould be about 7.4 kn at fullspeed ahead at maximum3200 rev/min (the maximumengine revs.)

In addition we measuredthe static thrust (forward andreverse) at a given speed.These so called bollard pulltests allowed conclusions tobe made as to themanoeuvrability of thepropeller. The more thrust apropeller produces the fastera yacht accelerates or

comes to a halt. For harbourtrials this is an importantcriterion.

The main purpose of thetest runs in the 280 m longNTCA test basin washowever the towing dragmeasurements. After all thismechanical work of art wasonly built for one purpose;that of making sailing boatsgo faster after the engine hasbeen shut down. The bladesof a folding propeller open upunder engine drive from theresulting

How we tested

A video camera was installed next to the propeller,which showed its behaviour under water

The giant towing carriage travelled at up to eightknots above the test tank

For two days we travelled constantlyup and down the NA Research CentrePotsdam test tank with the enormoustowing carriage to determinemeasured values. (Towing drag,bollard pull forward/astern and powerat cruise and full speed).The towing drag of the folding andreversible propeller in the sailingposition was determined at a speed ofsix knots. With the drive shaft of themeasuring instruments stationary(Dynamometer H39) static frictionoccurred, which influenced themeasured results. During resistancemeasurements the shaft rotated at 0.51/s. Tests have shown the influence ofthis very small rotational speed on thedrag affected the measured accuracy.For the measurements we assumedthat the total engine power wasavailable for driving and that nopower was taken to drive thealternator.The evaluation for the referenceyacht was converted for the densityof Atlantic sea water at 18° C.

The propeller was mounted on a shaftas a dummy based on the Saildrive ofthe Volvo 130 S. Mounted in the bluesection, front right, is a video cameraand underwater lighting.

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TECHNOLOGY & EQUIPMENT

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Centrifugal force and torque.Because the blades are gearedtogether they open and closesynchronously. When theDiesel is shut down both bladesfold in due to longitudinalwater flow stream thusreducing the drag considerably.With feathering propellers theblades are not turned bycentrifugal force but through anintegrated gear drive active inthe prevailing situation forforward or astern. If the enginestops the blades are turned tothe horizontal position by thewater flow where they offer theleast resistance. A special formis the "Autoprop". The twohighly


In the large bar chart the differences in resistanc e between the folding and feathering propeller and the Volvo (upper bars: braked, lower bars:free rotation) are easily seen. The small differenc es can be seen better in the detailed chart. Here t he Autoprop gives a drag value of just over 20Newton, the Gori Racing on the other hand is so wel l shaped that it produces no drag.

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Propeller pitchA ship's propeller is characterised by twodimensions (in inches) : Pitch and diameter. Thepitch describes the theoretical travel in the water ofthe propeller for one revolution. To illustrate this onemay think of a ship's propeller as a wood screw,which with a turn of the screwdriver travels a certaindistance in the solid material. A propeller with asmall pitch similarly does not go as far through thewater for a single turn as a propeller with a largepitch. The diameter is calculated by the experts anddepends on the space under the hull and therequired blade area. These determine first of all theamount of water that is accelerated astern toachieve the required speed. With the featheringpropellers under test the pitch could be varied toachieve optimum conditions.

5" is theoretically the distance in incheswhich a propeller travels for one revolution

Towing Resistance at 6knTowing Resistance at 6knTowing Resistance at 6knTowing Resistance at 6kn

Provisional Results


Maximum achievable bollard pull

This chart shows the value of maximum achievable bollard pull in Newtons. The two bars per propeller represent the forward thrust (violet) andreverse thrust (beige). The small "Maxprop" value can apparently be optimised by careful variation of the pitch, without overloading the engine.Especially high values are shown for the "Autoprop" whose free rotating blades are set in the optimum position depending on rotational speed andtorque.

profiled blades are mounted ona transverse axis in the boss.Depending on engine rotationalspeed and torque they arrangethemselves independently inthe ideal position. When thereis no drive, the

blades turn through 1800 aboutthe transverse axis to the newposition.

Test resultsand special featuresThe situation described at thestart for towing drag one cansee in naked

values: The Volvo held fixedblade propeller decelerates -theoretically - the yacht fromsix knots under sail with over200 Newton. To the measuredtotal drag (approx. 550N) ofthe yacht at this speed, one

must then still add 40%."Braking contribution". Withthe propeller free to rotate thedrag is reduced by at least 50%to 100 Newton. However thisvalue still corresponds to about20 >-

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Left the "forward position" for the "Autoprop", in middle are the free rotating blades in the position "astern" and the right hand picture shows theblades in the sailing position. For this model they offer the least drag in the flow.


Maximum achievable revs/revs for maximum bollard pull

As the propeller blade profile is optimised for good forward travel performance, not only are the majority of propellers considerably better for"forwards" than "astern" thrust values (compare with diagram "Bollard pull" p.?). For travel astern, in addition much higher revs are required toachieve maximum thrust. The "Autoprop" is again in a special position: Not only are the thrust values almost identical, but also the necessaryrevs required.

Provisional result

Percent of total ship's drag.As the drag curve for a yachtincreases sharply with speedand

is not approx. linear, thisrelationship becomes evenworse at low speeds. Thebollard pull values for fixedblades was 1000

N forwards and a good 800 Nreverse, cruising speed was at2000 rev/min, for 7.4 kN theengine ran at 3000 rev/min.

With a robust bronze alloymanufactured folding orfeathering propeller the dragrelationship of the yachtpropeller changeddramatically. Thehydrodynamically polishedpropeller develops justbetween 1 and 20 Newtondrag, which corresponds to aproportion of between 0.2%and a maximum of 4.2% ofthe total drag. Thehydrodynamically optimisedscrews also display theirstrengths at low speeds.

As the most streamlinedpropeller from the standardtest programme was theAllpa folding type. The dragis only a single Newton sosmall

The majority of propellers tested were fitted with a zinc anode. The "Maxprop" (l) gives in addition a morestreamlined transition between boss and Sail drive. Allpa (r) without anode

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Engine speed

Provisional result

In the yacht configuration (see text) the Volvo-Penta "D1-20" delivers maximum power at 3200 rev/min. The propellers represented by the fourviolet bars that are above the line, would have only attained the maximum speed of 7.4 knots with considerably higher revs. They are too "light",the chosen pitch is too small. The Volvo folding prop was the most unsuitable for our boat, with the "Variprofile" and "Variprop" one had to adjustthe pitch bolts

that it can almost be neglected.The Italian foundry ElicheRadice have done a good jobwhich goes unseen. At thesame time the prop gave forthe bollard test measurements1100 N forwards and 800 Nreverse thrust, which weregood results. For thiscomparatively higher revs wererequired. However, Allpa hadincorrectly calculated thedimensions. The 15 inchdiameter was given to allmanufacturers, and the pitchwas to be calculatedindividually based on the ship'sspecification. For the cruiseand full ahead test runs it wasshown that the Allpa prop therequired

Propeller installation

maximum speed with themaximum available revs of3200 rev/min (see above) couldnot be achieved. Theoretically

this propeller would haveneeded 4000 rev/min to achieve7.4 kN. The propeller was "toolight" as the pitch for the nineinch from

Allpa was dimension too small.Similarly streamlined with a

drag value of only two Newtonwas the feathered pro- >-

Research Ldr. Dipl.-Ing Dr. StefanHelma (r) assembling prop.

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Installing a propeller is comprehensible if one adheres strictlyto the instructions. Necessary special tools such as a hookspanner and others are generally supplied. With both foldingpropellers one cannot do too much wrong, the number ofparts to assemble is manageable. Allpa and Flexofold could,however, have made the operating instructions morecomprehensive and also in German.Somewhat complicated and requiring more than two handsis the installation of the "Maxprop". The correct choice ofpitch is somewhat complicated and requires concentration.All safety screws must definitely be secured with a threadlocker e.g. "Loctite" , which is supplied with all propellers.

Model Allpa Flex-o-Fold Gori Varifold Volvo Autoprop AManufacturer/Distrib Eliche Radice/ Allpa, Flexofold, Tel. Gori/Bukh-Bremen, Tel. Bruntons/SPW, Tel. Volvo, Tel. 0431/399 Bruntons/ SPW, Tel.

Tel. 0031-24/377 77 0461/481 56 10, 0421/535070, 0471/77047, 40, www.volvo- 0471/77047,

73, www.allpa.nl www.flexofold.com www.bukh-bremen.de www.spw-gmbh.de penta.de www.bruntons-propellers.com

Price 588.35 € 1044 € 1082.90 € 1008 € 1328€ 1875 €

Guarantee 2 years 3 years 2 years 1 year 2 years

Weight 5.6 kg 6.9 kg ??? 6.7 kg

Type Folding prop Folding prop Folding prop Folding prop Folding prop Automatic Variable pitchpropeller

Diam/Pitch. 15 x 9 15 x 12 15 x 12 15 x 9 15 x 8 15 x variable

Anode available no no yes yes yes yes

Installation/ Very easy install Very easy install Very easy install Very easy install Very easy install Very easy install goodinstructions -ation; few instructions -ation; few instructions -ation, good instructions -ation, good instructions -ation, instructions in instructions in German

only in English only in English many pictures German German

Tests

Towing drag 1 3.1 2.2 3.1 9.7 20/NewtonBollard pull/Newton

forward 1106 1118 948 1172 1368 1486astern 786 681 857 771 766 1499

Engine speed f/2430 f/1880 f/1750 f/2060 f/2840 f/2850max. Bollard pull r/2870 r/1880 r/2100 r/2540 r/3000 r/2810

Engine speedCruise 6.4 kn 2650 1950 1990 2100 2860 1670

Full speed 7.6 kn 3960 2990 3060 3120 4210 2880

Sail load at/kN 1.4 1.9 2.3 1.7 1.9 No measurementavailable

Comments For the cost conscious This sail prop Gori delivered the The sail prop In comparison with The strengths of theAuto-regatta sailor is the excels due to small second best drag- distinguishes itself with fixed pitch propeller the Autoprop lie definitely

"Allpa the very best drag. The bollard value. With the with small towing drag value is small With its performance: As a sail prop pull value for travel good bollard pull values resistance. In other folding and Under power. Theit scored with the astern is however especially for harbour the performance blade props have Motorprop gives almostlowest drag modest, one must travel astern, make could be better, better values. The 1500 Newton thrustin the regular test be careful in the Gori a very the reverse thrust is reverse thrust could fwds and rev. Howeverseries, the bollard pull- narrow harbours good compromise for not spectacular be better, all in all the drag value is highvalues are reasonable racing sailors, who also the prop is an acceptable and propeller isthe price is considerably want relaxed manoeuvres compromise Expensive tooless than the competition. in harbour

I Assessment 00000 00000 00000 00000 00000 I8 I www.segelnmagazin.de/2008


peller from "Maxprop". Thestatic thrust measurements forit were 653N forwards, thelowest value, and also 767 Nfor astern travel are notspectacular values. For tightharbour manoeuvres it can benoticeable. Because with the"Maxprop'' the pitch can bevaried, the thought comes tomind simply to increase thepitch, in order to achieve bettervalues. The required torque forthis cannot be provided by theengine for our testconfiguration. With themanufacturer selected pitch theengine is already

The change over from boss to sail drive occurs very differently. With "Flex-Q-Fold" (l) the region is verystreamlined, with "Variprop" (r) yawning unfavourable gap

at full speed on its power limit.In third resistance place

revolving with only 2.2 Newtontowing resistance

is the Gori folding propeller.The bollard pull values indeedoccupy the lower third of thetest results, but with 857 Nastern and barely 1000 N

forwards the harbourperformance should beacceptable. The 12 inch pitchselected by the manufacturer iscorrect: The speeds

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are available in a healthy speedrange and the engine is notover loaded.

Each with 3.1 Newton dragthe two folding props"Varifold" and "Flex-O-Fold"lie together. They also havesimilar bollard pull values, the"Varifold" can both in forwardand reverse travel producemore power. The "Flex-O-Fold" produces for reversetravel the worst test values:With only 681 Newton thevirtual ship went astern. Onecan approach the edge of thelanding stage very rapidly. Therevs for both

are close to optimum, in otherwords: The pitch is correct, themanufacturers have theknowledge.

A shade more drag isproduced in the water by therotating blade propeller withvariable "Variprofile", and theword "braking" is seen asrelative: measured with thetotal drag for the yacht the"Variprofile" would have hadonly a partial resistance of0.7 %. At maximum revs the expected top speed wasnot attained with deliveredsetting, the prop is too "light".The owner of the virtual testBavaria yacht must once againcrane

his ship from the water andadjust the pitch screws to abetter setting. Here one shouldproceed with care to reach theperfect pitch setting and notstraight away by turning theadjusting screw a completeturn. Even if it could mean anadditional appointment with thecrane.

The "Variprop" with threeblades stands out considerablyin the line up of streamlinedpropellers. The dynamometermeasured seven Newtons,which amongst other things isdue to the ungainlyconstruction and the large gapbetween the propeller boss and

Saildrive leg. It does notdepend on an especiallyeffective blade profile thatwould stick out like a lump inthe flow, apparently not if oneexamines the bollard pullvalues. With 752 Newton undertest it got off the mark, it pulledwith 877 N. As a result thestipulated maximum speed wasnot attained and here also thepitch should be carefullyreadjusted.

Curiously enough the Volvofolding propeller does not goso well with the Volvo engineand Volvo Saildrive. For thetowing resistance - >-

Model Maxprop Variprofile Variprop Gori Racing Fixed Prop Gori 3-bl GManufacturers/ Maxprop/Boattec, Tel. Tel. 0471/770 Tel. 0471/770 Gori/Bukh-Bremen, Tel. Volvo, Tel. 0431/399 Gori/Bukh-Bremen, Tel.

Suppier 0431/3990060, 47, www.spw-gmbh.de 47, www.spw-gmbh.de 0421/535070, 40, www.volvo- 0421/535070,www.boattec.de www.bukh-bremen.de penta.de www.bukh-bremen.de

Price 1688 € 1457€ 1675 € 1558.90 € 292€ 2344.30€

Guarantee 2 years 2 years 2 years

Weight

Type Rotating blade propeller Rotating blade propeller Rotating blade propeller Folding prop Fixed pitch propeller Folding prop 3-fl.

Diam/Pitch. 15 x 12 15 x 11 15 x 12 15 x 12 16 x 11 15x 12x3

Anode available yes yes yes no no yes

Installation- somewhat complicated,good

somewhat complicated,good

somewhat complicated,good

simple simple somewhat complicated,good,

instructions instructions in German instructions in German instructions in German Instructions only inEnglish

Tests

Towing drag 2.0 3.3 7.0 0 99.9 held 1.4/Newton 198.7 free rotatingBollard pull/Newton

forward 653 851 752 1086 990 869astern 767 926 877 665 833 1128

Engine speed f/1380 f/1770 f/1650 f/1950 f/1660 f/1600max. Bollard pull r/1610 r/1950 r/1930 r/2560 r/1810 r/2280Engine speed

Cruise 6.4 kn 2040 2240 2130 2000 2010 2090Full speed 7.6 kn 2950 3440 3310 3090 2990 3090

Sail load at/kn 3.5 2.7 3.1 2.7 3.3

Comments The Maxprop The manufacturer's pitch With somewhat more An immeasurable The fixed pitch propeller Despite the largeOffers a very was selected somewhat pitch the bollard pull drag distinguishes is the "sailing brake" jagged areas thelow value of drag. too low. values would apparently this perfect sail prop. the bollard pull three bladed propellerThe performances An adjustment of have been better. Those wanting to equip values for this gave a very lowunder power is the sail propeller Together with the their yacht for regatta propeller are drag valuemodest and would definitely improve comparatively high Sailing should acceptable performanceand gives lowest the bollard pull results towing drag the consider the price and very goodforward thrust and perhaps lead to Variprop makes a Of wiining and reverse thrust. Thisvalue which cannot be a good compromise good compromise" buy this prop. comparision propcorrected by increasing . . The bollard pull values is howeverpitch are variable. very expensive

I Assessment 00000 00000 00000 00000 00000 I

A folding prop in the engine drive position (l) and folded in to a morestreamlined sailing position (r)

The rotating blade propeller, due to flow, takes up the lowest dragposition (r)

measurements the propeller (9.7

N) is in last but one place whichcan be explained by theextremely long extended bladetips which protrude a long wayinto the flow. The engineersselected the unusual form forthe benefit of good performancewhich the folding propellerproves by the impressive bollardpull measurements: In "reverse"with 768 N it is placed in thelower middle range, and"forwards" with 1368 N gave thesecond best result. The pitch isonly 8 inch which is too muchand in no way compatible with

yacht and engine: The hullspeed cannot be achieved withthis combination. The yachtwould not reach 7.6 knotsthrough the water without 4210rev/min. As the engine only revsup to 3200 rev/min, one mustaccept a reduction in speed.

Here Volvo would do better toextend the range of this or otherpropellers.

With 20 Newton drag the"Autoprop" with its free rotatingblades is still a long way off the

The recommended Volvo fixed blade propeller for our yachtconfiguration delivered exciting comparative values


fixed blade brake" (100 N to 200

N), is by far the worst in thetowing test. In return the bollardpull values forwards as reverseare solely top class (currentlyover 1400 N), the revs despitethis remain comparatively lowand the engine does not have tomake available its last ounce oftorque.

For the best possiblecomparison we have also towedan especially flow optimisedGori racing folding propellerand a three bladed rotating bladepropeller (Gori) in the towingtank. Looking at the giventowing resistance values for the"Racing" we had to rub our eyesa number at times: Thehydrodynamically styled masterpiece offered 0 (in words:zero) Newton drag. And inaddition with good bollard pullvalues and optimum enginespeed range at cruise and fullspeed ahead. However the pitchis a shade too large and underfull load the engine mayproduce clouds of black smoke,because more torque is requiredthan is available and therotational speed is not achieved.The fuel injector balances thisrequirement by injecting morefuel. The additional fuel leadshowever to an over rich mixturein the cylinders and the exhaustclouds become black and sooty.

It is the same picture for thefolding propeller with threeblades: pitch somewhat toolarge, but very good bollard pullvalues and despite the crudeshape when folded a very lowdrag value, with only 1.4

Newtons, within a field of 12

propellers, lay in third place. Forthe advantages of quiet engine

running and outstandingstopping characteristics onemust reckon on a two to fourtimes increase in price incomparison with the other screwunder test.

Folded/ OpenThe propeller blades fold orrevolve very quickly to theoptimum position. At best 530

rev/min was required, to turn the"Maxprop" to the engine driveposition. The blades of thesimple folding Gori propellerhad already attained its idealposition at120 rev/min. Theother way looked entirelydifferent however: Somepropellers required under test acomparatively high flowvelocity in order to fold orrevolve to the ideal position.The "Maxprop" blades onlyrevolved to the ideal sailingposition at 3.5 kn and also the"Variprop" and the three bladedGori only made themselves thinat more than three knots. Theseobservations oppose the actualpurpose of the propellerespecially at low velocity toproduce little drag. It is possiblethat the frictional resistance inthe rotating blade propellermechanism or the geared foldingblade will reduce after longeroperation and will work easier.The measured values for therotating blade propellers canpossibly be reduced byfollowing the describedoperating tips. "Variprop" evenproduce a single sided sheet onhow to get the propeller in to thesailing position. Here is anextract: -sail at three to fourknots forward speed. -Selectneutral, switch the engine offand >-


The two part zincanode is screwedto the mid-sectionand provides inaddition astreamlinedtransition toSaildrive

A housing half with propeller in place,mid-section with bevel gear and zincanode. The other half housing isattached with six bolts

The central bevel gear in mid-section, with which pitchadjusted. The position of the punch marks is read fromtable with ref. to letter.

"Max-Prop"

Centre piece with bevel gearin half housing. Both propellerblades are fitted in the sleevebearing normal to the bevelgear

Mid-boss with rotatable blade bevel gear.Here the bevel gear engages for pitchadjustment for forward and astern travel.

Both halves of housing (right andleft), between them the mid-sectionwith central bevel gear that is bolteddirectly to the drive shaft.

Due to gear connection thepropeller blades movesynchronously when folding inand out

Viewed from behind the innergearing can be seen in theboss, which engages with theSaildrive shaft gear

Shaft nut safety screwand grub screw, whichadditional secure theshaft nut

Almost completelyassembled propeller. Inthe left half the insertedblade bolts and the safetyscrew can be seen

Rotating bladepropeller

Two blades with bolts safetybolts, which secure the bolts inposition

Folding propellerboss. It is initiallybolted to the shaft

The blades are placed in the bossand secured with bolts


now put the indicated enginestate to reverse for a shortperiod so as to stop the shaft.Now select neutral again.Whether or not during thismanoeuvre the sail should behoisted, is not in theinstructions.

(Almost) all theoryis greyThese values and observationscannot be transferred 1:1 toevery other yacht-engine-gearbox combination. Theindividual factors have suchdifferent influences on boatperformance. If the towingresistance is a comparativelymore stable value which cannotbe altered positively ornegatively, it is seen differently

The Gori "Racing-Propeller": closed. The non-existent dragcan be visualised

in the bollard pull test: Hereone can read a definite trend,which is given by the bladegeometry and construction,variations can arise due toengines with different torquecurves of other drivedifferences.

ResultHow great the actual speedimprovement in the water witha hydrodynamically optimisedpropeller is, one cannot saywith scientific certainty if theexact drag curves for the actualyacht are not available. Withestimated values based onexperience the screws candepending on sailing speedachieve at least another halfknot. And that can bring adecisive advantage not only inregattas, but also for sailingtrips the greater range altercruise planning considerably.

Those putting maximum valueon sailing performance, cannotbypass these screws.

even if one, in comparison tofixed propellers, one mustinvest more heavily. Finally thepleasure in sailing is increasedat least quadratically with everysmall increase in speed. Wehave attempted to put screws into three categories on the basisof their characteristics:1) The sail prop: It should firstlyhave low drag and manoeuvrabilityis secondary.2) The engine prop: It should givegood performance under power, bothforwards and astern and at least 800

N should be developed.3) The compromise: It ischaracterised by good engineperformance with low drag. X

The correct propellerf the boat does not attain the specifiedmaximum speed at the specified engine revsor the engine under full load

starts to smoke and soot the incorrect propeller couldbe fitted. An incorrectly specified propeller can alsoconsiderably increase the fuel consumption if one hasto operate permanently at higher revs to achieve agiven speed. In such cases in order to calculate thecorrect screw the propeller manufacturer must amass amultitude of data on ships. In an ideal case one canfind propeller sizing data for popular boat and enginetypes in a data base. For lesser known boat types thecalculation is more difficult. In this case the experienceof the propeller expert is important.

As well as type of ship and boat character (roundhull,) ship's data such as overall length, length andbeam at the water line and above all for a yacht, thedisplacement is important.

For boats with shaft, the position of the propellerrelativeaddition several other distances must be known:The blade tip distance to the underside of the hull isjust as important as the distance to the rudder or skeg.The propeller must be able to fit in to such a tightspace.

To calculate the correct propeller masses ofdata must be obtained.

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TECHNOLOGY &EQUIPMENT

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Documents

P(r)op art · TECHNOLOGY & EQUIPMENT Maximum achievable bollard pull This chart shows the value of maximum achievable bollard pull in Newtons. The two bars per propeller represent