13.75 x13p sometimes???

[ed72vette]

I have a 18' Sailfish CC [1350lbs]with a 90hp Yamaha 2 stroke [261 lbs] which holds 52 gal of fuel[312lbs]. The boat came equiped wiht a 13.25x17p Yamaha aluminum prop. With me, the boat and motor, and all the gear will weigh about 2300 lbs. With this combination the boat ran; <br />4000 rpm 25 mph [gps]<br />4600 rpm 32 mph [gps]<br />5400 rpm 36 mph [gps]<br />I tried several ss props with equal or greater pitch than the original aluminnum prop. All would not turn more than 4800 rpm, which is obviously over proped. I than purchased a ss Power Tech 13.75x13p. The hole shot is great but the speed and rpm is not consistant with the pitch in relationship to my original al prop.<br />4000rpm 24 mph [gps]<br />4600rpm 30 mph [gps]<br />5500rpm 33 mph [gps]<br />My question is this 13p ss prop that much different than the 17p al prop or do I have a problem with my tach? I am assumeing that the gps is accurate. Or is there somthing else going on here that I cant understand. Thanks for any help.<br />Ed

 

[walleyehed]

Re: 13.75 x13p sometimes???

17-19% slip ratio with the 17P aluminum and about 7-8% slip with the 13 SS.<br />If tach is correct (seems close) it's showing how in-efficient the alum prop is compared to an SS prop.<br />SS will be lower RPM in the same pitch as the aluminum, but not 600RPM lower.<br />List some of the props you've run, and where (on transom or up some) is the engine mounted?

 

[ed72vette]

Re: 13.75 x13p sometimes???

Thanks for the quick reply. The motor is in the second hole. This is the same hole that the boat came rigged from the manufacture with the al prop.<br />The first prop I tried was a Yamaha ss [black painted] 13x19p. Max rpm was 4500 I did not record the speed. The next prop was a Stiletto 13.25x17p. Max rpm was 4500. I did not record the speed. The stiletto was tested again with the engine in the third hole and had an increase of rpm to 4600. I then returned the engine to the second hole. I have not tried the 13.75x13p in the third hole. Hope this gives you some helpful information. <br />Thanks again for the help<br />Ed

 

[walleyehed]

Re: 13.75 x13p sometimes???

The larger diameter of the 13P would allow you to run a higher engine heigth, which I would recommend.<br />At your weight with a 90HP, I would have thought a 15P would have put you in the 5500+ range. The stiletto 17 pulling down 900RPM from the stock alum 17 doesn't sound right..???<br />Are we sure the stock prop was a new one and a true "17P"????

 

[ed72vette]

Re: 13.75 x13p sometimes???

Kenny,Thanks for spending the time with this. The boat was purchased new and the prop has the Yamaha markings on it indicating it is a 13.25x17p. I would like to raise the motor again. I am going to try a 13.5x 14p ss Mercury Vengance prop this weekend if the weather hold up. Will the 13.5 dimension support the higher height?<br />I would have thought that the 13p prop would have turned the rpms up higher than 5500rpm. This is why I was questioning my Teleflex Tach.<br />Ed

 

[walleyehed]

Re: 13.75 x13p sometimes???

We've seen alot of problems lately with Teleflex Tachs...that really opens another can of worms...<br />Yea, the larger diameter works better "higher".

 

[ed72vette]

Re: 13.75 x13p sometimes???

Is there a particular model or year Teleflex tach that is identified as a problem. Mine is a 2003 vintage lido series with a hour meter.<br />Ed

 

[walleyehed]

Re: 13.75 x13p sometimes???

That series is not included in the ones I know are off...The International series had some problems...yours may be OK.