prop selection

[jvb]

what prop should i use for a 1973 chrysler 70HP outboard, 15'6" fiberglass ski boat

 

[walleyehed]

Re: prop selection

The prop that puts you near the top of the recommended RPM range....

 

[1981Baja]

Re: prop selection

I agree Kenny. JVB didn't give us enough information to work with.<br />boat weight/loading, top speed at wot, wot RPM's with current prop, pitch of current prop, etc

 

[jvb]

Re: prop selection

i dont have a tach, so i am not sure where it is at now. the motor is stamped with 'full power speed range: 4400 to 5100 rpm' i believe there is a 13", 3 blade, 15 degree prop on there now. this doesnt seem to give it much top end, but it gets on plane in about 2 seconds. i also have a 13", 17 degree prop that i could use. the boat loading is around 900-1000 lbs

 

[moderator1]

Re: prop selection

Tach is most important instrument you could have....It may even save your engine not knowing the RPM now...<br />15 and 17 are in inches not degrees-FYI.<br />Diameter and pitch are both measured in inches..

 

[jvb]

Re: prop selection

i switched to the 17" pitch prop, didnt see any speed gains but it did take a second or two longer to plane. shouldnt the 17" pitch give a faster top speed than a 15"?

 

[walleyehed]

Re: prop selection

Only if the engine has the power to spin that prop at the top of the RPM range...other-wise, you may be lugging the engine.<br />I won't make a suggestion of what prop to run without any RPM readings.

 

[jvb]

Re: prop selection

i am still looking for an rpm gauge, trying to find a smaller one since there isnt much room left on the dash. most online prop selectors are saying to use a 15" or 17" for this boat and motor, so i think i am in the ballpark at least...

 

[walleyehed]

Re: prop selection

Without a tach, all we can say is "maybe" and "I think"....we can't say "we know"....that would kill me, not knowing...

 

[jvb]

Re: prop selection

yeah, the tach is a main priority right now. with the fishfinder, speedo, ignition, and switched, there is no room left for the standard 3.5" size tach. i am trying to find a smaller one, like 2" or so should do it. anybody have any ideas?

 

[walleyehed]

Re: prop selection

If you'd asked me, I'd a said take the speedo out...makes a good hole for the instrument that tells ya what your engine is actually doing under the hood...

 

[Boilermaker]

Re: prop selection

That makes a lot of sense Ken. Those Speedos are so inaccurate its just a joke of an "instrument". If he still wants to keep his speedo>>>how about that "Tiny-tach"???

 

[walleyehed]

Re: prop selection

Tiny-tach is good, and seems to be very accurate on the small engine apps I've used it on. I may have to use one on my own boat, as I think I'm having a tach issue myself.

 

[jvb]

Re: prop selection

the speedo is definitly not worth the space. it doesnt even kick in until you are above 25mph, and the boat is currently maxing out at 31, which seems a bit on the slow side to me... it seems to be really sluggish response wise. i spoke to a tech at teleflex and he pointed me in the right direction. so, i ordered a 7k model today - i should have it installed by saturday!<br /><br />i checked out the tiny tach site, pretty cool. which model would work for 6 pulses?