Caculating boat speed

[nordy]

Hello,<br /><br />I have some questions caculating engine speed of a rib with two engines:<br /><br />1.) E-Tec-50hp: i=2,67, prop=3 x 14 x 17<br /> measured max speed = 59.9km/h / 6000 rpm<br /><br />2.)Honda 50hp: i=2,09, prop=3 x 11.5 x 14.25<br /> measured max speed = 59.4 km/h / 6000 rpm<br /><br />I must calculate the E-tec with a far less slippage of only 4 % due to the 14 " diameter and an additional 1 '' of pitch due to cupping to calculate 6000, the measured value.<br /><br />The Honda on the other hand needs a more common slippage of 8 % and a less pitch increase of 0.5 '' due to cupping to meet the measured 6000.<br /><br />Do you think that all is realistic.<br /><br />I know increasing prop diameter decreases slippage, but will it make 4 % ?<br /><br />And are the assumptions for cupping realistic ?<br /><br />The are derived from:<br /><br /> http://www.mercurymarine.com/chapter_4_-_propeller_technology <br /><br />Greetings from Germany<br /><br />Nordy

 

[markvan]

Re: Caculating boat speed

I don't think the 14" prop will give you that much of a decrease on the slip%, what type of boat is it, what does the hull look like

 

[f_inscreenname]

Re: Caculating boat speed

nordy; It looks like you are into numbers. You should have fun with this thing http://www.boatramp.com/prop_applet/PropAnalyzerAppletG.html

 

[Bondo]

Re: Caculating boat speed

I think your Slippage %s are Way Low.........<br /><br />I Don't Think you ever even approach those kinds of #s in the Real World.......

 

[nordy]

Re: Caculating boat speed

Hello,<br /><br />the boat is a rigid inflatable boat, it's a quite light boat.<br />It's dimmensions are (in meters) 4,70 x 2,10 and it's weight is around 450 pounds netto (without engine, equipment and passengers).<br />With two persons on board the weight is 1100 pounds.<br /><br />Due to the moderate weight I think the slippage will be on a low level.<br /><br />Nordy

 

[Bondo]

Re: Caculating boat speed

You Might be Right,............<br /><br />But,.. I Doubt you'll get the Slippage Below 8/10%............<br /><br />Racing Boats rarely get below 10%.......

 

[nordy]

Re: Caculating boat speed

Hello Bondo,<br /><br />remember the E-Tec's prop has a diameter of 14'', it's 2.5 greater the that of the Honda.<br /><br />Increasing diameter will decrease slippage.<br />The 2.67 gear ratio will give the shaft enough torque to handle the higher load.<br /><br />This also means a decrease in shaft revolution.<br /><br />This all will lower the Bp (Power coefficient)for an optimized prop efficiency of around 80 %, I suppose.<br /><br />It's a sophisticated design by Evinrude.<br /><br />Nordy

 

[markvan]

Re: Caculating boat speed

I agree with Bondo, I doubt that your slip will be below 8-10%, I was able to find some hull designs that are considered high performance racing hulls, and when they are in top shape some report slippage in the 5-6% range.........