This program is designed to give a quick estimate of propeller

 The logic of the program is simple. After entering hull and engine

 The more complex Bp (power coefficient) method of propeller

 For both types of calculation, engine power is derated 5% to account for

 Of course you may not want the most efficient propeller for a sailboat

 This is a bare bones program written in elementary Basic. It should run

Here is a program, written in elementary BASIC, for calculating the proper propeller for displacement boats. I posted it to the TWL a few years ago but I keep getting requests for it from time to time. It will get you in the ballpark and should provide a good starting point for selecting the right propeller after repowering. In addition to specifying propeller characteristics, the program performs a horsepower calculation necessary to achieve hull speed and indicates the efficiency of various propeller pitches, diameters, and rpms. This program works better than the one used by Michigan Wheel or Federal Propeller. It will run on ANY computer which supports a Basic interpreter. Try it and see. BASIC PROPELLER CALCULATION FOR DISPLACEMENT HULLS - L. R. Zeitlin This program is designed to give a quick estimate of propeller specifications for typical displacement hulls such as sailboats and trawlers. I must caution that propeller specification is an arcane art full of compromises and approximations. It is rare good fortune if you get it right the first time. This program will get you in the ballpark, about + or - 5% of the right value. >From there on it is experimental trial and error. From a theoretical point of view pure displacement hulls work best with large, slow turning props. Generally the diameter is determined by hull clearances. The pitch is the main variable under your control. Fortunately the pitch of most bronze propellers can be adjusted a couple of inches either way at relatively low cost. While the program is designed to give you the most efficient prop for a given engine speed, reduction ratio and hull speed, most of us can live with, and even enjoy boats that donbt squeeze the last mile from a tank of fuel. The logic of the program is simple. After entering hull and engine characteristics, hull speed is calculated using the standard formula KT = 1.34 x b WL. An estimate of power required to reach hull speed is obtained using Keithbs formula (line 90). This usually works out to about 1 HP per 500 lb displacement. A slip estimate is made using Gerrbs formula for displacement hulls (line 110). Given power, slip and shaft RPM, propeller pitch and diameter are calculated using Crouchbs method. The more complex Bp (power coefficient) method of propeller specification is used as a check on the simpler approach. Va (velocity of advance) is estimated (lines 347, 348) and Bp is calculated (line 350). Lines 350 through 410 provide estimates of Delta (advance coefficient), P/D (pitch/diameter) ratio and efficiency based on the Taylor Bp charts. The calculation is made to provide optimum efficiency at a given Bp. It is not necessary to know exactly what these terms mean. Elaborate tables have been plotted delineating the relationship between power coefficient, advance coefficient and efficiency. The program uses the variables which you input to specify the propeller which will bring you closest to the point of maximum efficiency. For both types of calculation, engine power is derated 5% to account for gearbox and shaft losses. Small modifications in propeller specification (up to 15%) are usually acceptable if pitch is increased 2b for every 1b decrease in diameter. Since the equation constants are based on actual in water and tank tests, any inputs which would give a result out of the measured range is rejected. For example, if you specify a 6" diameter propeller to push a 40' trawler, you will be asked to try again. Of course you may not want the most efficient propeller for a sailboat or motorsailer. Efficient propellers tend to be big and have a lot of blade area. This translates into unacceptable drag when sailing. In that case, play around with the prop RPM and the diameter until you find one that suits. This is a bare bones program written in elementary Basic. It should run on any computer which supports a Basic interpreter, even palmtops. All PCs can run it from the TRS 80 or Commodore Pet to the latest Dell or iMac. If your computer does not have a simple Basic interpreter, you can download several freeware versions from the internet. Try (www.chipbasic.com/). I use the program on an old HP100 LX palmtop running a copy of Radio Shack Basic for the PC. No provision is made for printing. I assume that you have a pencil and pad handy to record the results. If you have a Basic interpreter, simply copy the text file starting at line 10. Most Basic programs are able to interpret text files. If you have comments or questions, contact me at: LRZeitlin@aol.com 10 PRINT "Propeller calculation program" 20 PRINT "Copyright 1997: L. Zeitlin" 27 FOR x = 1 TO 30000!: NEXT x 29 PRINT: PRINT "CROUCH'S METHOD FOR OPTIMUM HULL SPEED PROPELLER CALCULATION." 30 INPUT "Waterline length in ft."; L 40 INPUT "Displacement in lbs.";D 50 INPUT "Engine rated HP"; HP 52 HP=HP*.95 'assume 5% HP loss in power line. 55 INPUT "Engine RPM at rated HP"; RPM 57 INPUT "Percent RPM used for calculations";PRPM 58 DRPM=PRPM*RPM/100 'desired RPM 59 HPREV=HP/RPM 'horsepower per revolution. 60 INPUT "Gearbox reduction ratio"; RG 70 HS= L^.5 * 1.34 'hull speed calculation. 80 PRINT "Hull speed ="; INT(HS *100)/100;"KT" 90 RHP = D * ((HS/(11.963 * L^.5))^3) ' Keith's formula, required HP for hull speed. 100 PRINT "Required HP ="; INT(RHP *100)/100 105 IF RHP > HPREV*DRPM THEN GOTO 900 110 SLIP = 1.4/(HS^.57) 'estimate of slip, Gerr's formula 120 HSRPM = DRPM/RG 'desired shaft RPM at hull speed. 130 PITCH = (HS*1215.6/(1-SLIP))/HSRPM 140 DIAMETER = (632.7 * ((HPREV*DRPM)^.2))/HSRPM^.6 150 PRINT:PRINT "Three bladed propeller:" 160 PRINT "Diameter ="; INT(DIAMETER * 10)/10; "inches." 170 PRINT "Pitch =" INT(PITCH * 10)/10; "inches." 180 PRINT:PRINT "Two bladed propeller:" 190 PRINT "Diameter ="; INT(DIAMETER * 10.5)/10; "inches." 200 PRINT "Pitch =" INT(PITCH * 10.1)/10; "inches." 205 PRINT "Slip estimate=";INT(SLIP*100)/100;"percent. 210 Thrust = 62.72 *((HP*PRPM/100*DIAMETER/12)^.67) 220 PRINT: PRINT "Static thrust =";INT(Thrust * 10)/10;"pounds." 225 PRINT:INPUT "Would you like to try another calculation using Crouch's method (Yes=1, No=2)";Q 226 CLS 227 IF Q=1 THEN GOTO 30 230 PRINT:INPUT "Would you like to try the Bp method of optimum propeller calculation (Yes=1, No=2)"; Q 240 IF Q=2 THEN GOTO 500 250 CLS 260 PRINT "Bp METHOD OF OPTIMUM PROPELLER CALCULATION." 263 INPUT "Choose method: (1) Calculate block coefficient. (2) Use average block coefficient."; Q 264 IF Q=2 THEN GOTO 300 270 INPUT "Waterline beam in ft. =";WLB 280 INPUT "Hull draft, excluding keel or skeg, in ft. ="; DFT 290 Cb = D/(L*WLB*DFT*64) 295 PRINT "Block coefficient ="; INT(Cb * 100)/100 296 GOTO 303 300 Cb = .53 303 INPUT "Desired engine RPM"; DRPM: DHP = DRPM * HPREV 306 PRINT "Available shaft HP at desired RPM ="; INT(DHP *10)/10 310 INPUT "Desired speed in Kt."; HS 313 RHP = D * ((HS/(11.916 * L^.5))^3) 'required HP for desired speed. 316 IF RHP > HPREV*DRPM THEN GOTO 1000 318 PRINT "Required HP for desired speed ="; INT(RHP *10)/10 320 SRPM = DRPM /RG 'shaft RPM. 330 SHP = DHP 'available HP at desired RPM 335 DX=0 347 Wf = 1.11 -(.6*Cb) 348 Va = HS * Wf 349 PRINT "Va="; INT(Va*100)/100 350 BP = (SRPM * SHP^.5)/Va^2.5 355 PRINT "BP"; INT(BP *10)/10 360 DELTA = 103.143 + (4.73 * BP) -(.034 * BP^2) + ((1.57/10000) * BP^3) - ((2.964/10^7)* BP^4) 361 IF DELTA <0 THEN PRINT "Entered values out of range. Try again." 362 IF DELTA <0 GOTO GOTO 303 365 DIAFT = (Va*DELTA)/SRPM 367 PRATIO = 1.014 - (.014 * BP) + ((1.72/10000) * BP^2) - ((9.873/10^7) * BP^3) + ((2.047/10^9) * BP^4) 370 PRINT "DELTA"; INT(DELTA *10)/10 375 PRINT "DIAFT"; INT(DIAFT *100)/100 380 DIAIN = DIAFT * 12 395 PRINT"P/D RATIO"; INT(PRATIO *100)/100 397 PITCH = PRATIO*DIAFT 398 SLIP = 1-(HS*101.3/(PITCH*SRPM)) 399 If SLIP <0 THEN PRINT "Entered values out of range. Try again." 400 IF SLIP <0 GOTO 303 410 EFF = .742 - (.006 * BP) + ((5.086/10^5) * BP^2) - ((2.209/10^7) * BP^3) + ((3.835/10^10) * BP^4) 420 PRINT "Three blade prop diameter ="; INT(DIAIN *100)/100;"inches." 425 PRINT "Two blade prop diameter =" INT(DIAIN * 105)/100;"inches." 430 PRINT "Pitch ="; INT(PRATIO * DIAIN*100)/100;"inches." 440 PRINT "Slip ="; INT(SLIP*100)/100;"percent." 450 PRINT "Efficiency ="; INT((EFF-(DX/2))*100)/100; "percent." 451 Thrust = 62.72 *((RHP*DIAFT)^.67) 452 PRINT "Static thrust =";INT(Thrust * 10)/10;"pounds." 455 PRINT: INPUT "Do you wish to modify engine and speed variables (Yes=1, No=2)";Q 456 IF Q=1 THEN GOTO 303 457 INPUT "Do you wish to change recommended propeller diameter (Yes=1, No=2)";Q 458 IF Q=1 THEN GOSUB 2000 460 PRINT:INPUT "Another propeller calculation? (Yes=1, No=2)"; Q 465 IF Q=1 THEN CLS: GOTO 29 470 'IF Q=1 THEN GOTO 30 500 PRINT "GOODBYE" 505 FOR x = 1 TO 10000: NEXT x 510 END 900 PRINT "The engine does not have enough power to reach indicated speed." 902 INPUT "Do you wish to modify initial variables (Yes=1, N=2)";Q 903 IF Q=1 THEN GOTO 30 904 IF Q=2 THEN GOTO 500 905 RETURN 1000 PRINT "The engine does not have enough power to reach indicated speed." 1002 INPUT "Do you wish to modify initial variables (Yes=1, N=2)";Q 1003 IF Q=1 THEN GOTO 303 1004 IF Q=2 THEN GOTO 500 1005 RETURN 2000 INPUT "Desired diameter in inches"; DDIAIN 2005 DX = 1-(DDIAIN/DIAIN) 2010 DIAFT =DDIAIN/12 2020 DELTA = (SRPM * DIAFT)/Va 2025 PRATIO = (3.28) -(.03*DELTA)+((1.231/10000)*DELTA^2)-((1.719/10000000#)*DELTA^3) 2027 2030 GOTO 370 2040 RETURN ************** It's Tax Time! Get tips, forms, and advice on AOL Money &amp; Finance. (http://money.aol.com/tax?NCID=aolprf00030000000001)