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