Johnson 30 hp: Changing gear wheels

nordy

Petty Officer 2nd Class
Joined
Aug 13, 2003
Messages
117
Hello,<br /><br />is it possible to the change the serial 13:28 wheels for the 12:29 wheels from the 15hp-Johnson ?<br /><br />I need a 0,414 or near by gear ratio.<br /><br />Best greetings<br /><br />Nordy
 

muskrat

Chief Petty Officer
Joined
Apr 13, 2004
Messages
445
Re: Johnson 30 hp: Changing gear wheels

what the model number and are you just trying prop size?????
 

nordy

Petty Officer 2nd Class
Joined
Aug 13, 2003
Messages
117
Re: Johnson 30 hp: Changing gear wheels

It's the J30RSSR, model 2000.<br /><br />I'm just using a 10 x 15 with too low engine speed but with ideal prop efficiency.<br /><br />To increase engine speed on the desired level (5800 rpm) I had to take a 3 x 10 x 12, but the efficiency is worse and therefor there's not enough increase in boat speed.<br />My engine has it's power maximum at 6000 rpm due to several power increasing measures.<br /><br />Greetings <br /><br />Nordy
 

G DANE

Commander
Joined
Nov 24, 2001
Messages
2,476
Re: Johnson 30 hp: Changing gear wheels

The 15 gears are different. I had exactly same experience on my 35 HP. Running 10x15 gave me less slip that the 10x13, both OMC propellers. I think you would be dissapointed with the gear change, the high effeciency probably comes from less prop rpm, turning prop faster probably makes slip grow. I would look for a better 10x13. I have an old slightly damaged 10x13 SST propeller that could be rehubbed and worked. Interested ??
 

nordy

Petty Officer 2nd Class
Joined
Aug 13, 2003
Messages
117
Re: Johnson 30 hp: Changing gear wheels

Hello Dane,<br /><br />I have a program, that enables me to calculate all performance parameters on the basis of the Wageningen kt and kq-cofficient polynoms.<br />Those coefficients depending on prop diameter, pitch/diameter-ratio,expanded area-ratio and number of blades usually have to be harmonized with the measured v max and n max, because of differences between open water and hull conditions and due to differences between the used and the Wageningen propeller type.<br /><br />Here are the results of the analysis, which are in some case contrary to your statement:<br /><br />1.) 3 x 10 x 15 - Ae/Ao = 0,33, z = 3<br /><br />v max = 48 km/h / 5101 rpm - measured+calculated<br />slip = 15,8 %<br />Bp = 4,66<br />Advance ratio = 1,263<br />Eta = 86,3 %<br />engine power = 35 hp / 5100 rpm<br /><br />Gear-ratio 0,408 instead of 0,464:<br /><br />49,8 km/h / 6015 rpm<br />engine power = 39 hp /6000 rpm<br />all other dito<br /><br />2.) 3 x 10 x 13, Ae/Ao = 0,33, z = 3<br /><br />v max = 48,2 km/h / 5605 rpm<br />slip = 11,2 %<br />Bp = 5,21<br />Advance ratio = 1,154<br />Eta = 82,7 %<br />engine power = 37 hp / 5600 rpm<br /><br />3.) 3 x 10 x 17, Ae/Ao = 0,33, z = 3<br /><br />v max = 48,3 / 4752<br />slip = 19,7 %<br />Bp = 4,15<br />Advance ratio = 1,365<br />Eta = 93,2 % !<br />engine power = 33 hp / 4800 rpm<br /><br />The 13 " has the worst and the 17" the best calculated propeller efficiency, due to a high pitch / diameter-ratio and a low engine speed.<br />The v max values differ only a bit !<br /><br />Slip doesn't affect prop efficiency directly.<br />The prop with the best efficiency has the greatest slip, due to a heavier load.<br /><br />A smaller gear ratio would help most, v max would rise about 2 km/h.<br /><br />A very useful prop efficiency parameter is Bp (Taylor coefficient).<br />Prop effenciency is best when Bp is lowest and near 4 and P/D is > 1,3<br /><br />The formula is:<br /><br />Bp = i gearcase*(Pe /(v boat/1,852*(1-w))^5)^0,5<br /><br />Pe in hp<br />v boat in km/h<br />w = Wake in 0,0x<br /><br />Conclusion:<br />It seems a 10 x 13 doesn't make sense.<br /><br />A lower gear ratio would be best choice, because it doen't deteriorate prop efficiency.<br /><br />Greeting to Danmark<br /><br />Nordy
 

G DANE

Commander
Joined
Nov 24, 2001
Messages
2,476
Re: Johnson 30 hp: Changing gear wheels

Interesting, but a lot of theory, beyond my skills. Where does blade cup fit into the formula, and blade thickness SST compared to Alu ?? Holding theory up against practics, would tell mee that having your 13 pitch worked a bit, adding cup to trailing edge and tip of blades should add the few percents to effeciency. But what do I know. I dont understand you can make a program that so exact compares different props. Is rake in the formula, besides from the increased blade area, bow lift and decrerased hull drag ?. Did you have your motor dynoed to determine exact horsepower at different rmps. Adding cup would be a cheap way to make it run at right rpms, with the most effecient prop. Now I dont know what prop you have, make, ALU or SST. I'm just a trial and error guy :D Besides, I'm not sure it actually is contrary to my statement, maybe it confirms it. Im sure DHadley know a lot more of this.
 

nordy

Petty Officer 2nd Class
Joined
Aug 13, 2003
Messages
117
Re: Johnson 30 hp: Changing gear wheels

Hello Dane,<br /><br />I can understand your doubts, but did you ever hear about the Wageningen B-series propellers, the most extensive model propeller series in the world,tested between 1937 and 1964 ?<br /><br />Here are the basic characteristics of B-series:<br /><br />- wide tips, airfoil section from blade root <br /> to 0.7 radius, and circular back from 0.8 <br /> radius to tip.<br />- 250mm diameter with hubradius/R = 0,17-0,18<br />- constant radial pitch distribution at outer <br /> radii R<br />- 15° backward rake angle<br />- a blade contour with fairly wide tips<br />- segmental tip blade sections and aerofoil <br /> sections at inner radii<br />- no consideration of cavitation<br />- 0,045-0,055*D max blade at propeller axis<br /><br />The used OMC 10 x 15 prop differs from that, no doubt.<br />But that doesn't matter.<br />Because the thrust and torque equations are met with the two measured performance boundaries:<br /><br />V max = 48km/h / 5100 rpm<br /><br />kt influences mostly v and kq n.<br /><br />The values for the coefficients are varied around the J-v max point (J= v prop/n prop*D) so both equations will fulfill the boundaries.<br />You may add more perfection on this with a third or fouth boundary maybe v max/ n max with full load.<br />Now you have matched kt and kq, respecting your special prop geometry and hull to open water differences (the B-series' coefficients are gained in open water condition, there's no hull).<br /><br />You can introduce now different prop sizes' kt's and kq's to the equations by superposing their 3-rd order polynoms and linearizing them around J- v max, which is between 1,0 and 1,4.<br /><br />I believe this theoretical approach is quite exact.<br />Before all relative differences are calculated well.<br />And that's what matters when comparing two props.<br />You mustn't know quite exact engine power for this.<br />What depends is an approximate correct relative torque curve in the n max-area.<br /><br />The 39 hp is roughly estimated by me but the percentage power increase and torque decrease is near correct.<br /><br />The reason why gear ratio change is the best choice is it doesn't influence the propeller's performance at all.<br />With same boat speed there is only a higher engine speed and a higher power output but same prop speed.<br /><br />Greetings<br />Nordy
 
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