What size prop for a new motor

[Ron570]

I've got a 1984 17 ft aluminum Blue Fin open bow boat. I just rebuilt a 1978 85 hp Johnson for it. The motor came without a prop. According to the shop manual for the motor WOT is at 5000 rpm. Need an idea what size prop to put on this motor? I went onto a calculator on some website and it recommended a 13DX21P. A guy on Ebay selling props said I shouldn't go above 17P with my set up. Which sounds right 21 or 17 pitch or something else? Also reading through the iboat prop posting, some people said to prevent carbon build up on the rings I should go higher than the rpm the manual shows, is this true and how high?

 

[walleyehed]

Re: What size prop for a new motor

That engine should be set-up for around 5800RPM...the range was 4500-5500, and the HP was taken at 5000.<br />Do you have a break-in plan???If you just O/H'd it, give us some info as to what you have done and we can give you some advice on break-in, which the method can change depending on the pistons used, bore-size, etc...<br />The reason we shoot for the higher RPM is because of the fuel available today...it ain't near as good as it was when that engine was designed. You'll see less problems, better over-all performance, better fuel economy and less maintenance if you set it up right at the higher RPM figure.<br />Now I think a 17P is going to be real close based on the fact my buddy has an alum 17ft deep-V with an older 85 Johnny, and we run a 13-3/4X15 on it to get to 5850RPM with a normal load, but we're at 3600ft, so a 17P should be a good starting point.

 

[Ron570]

Re: What size prop for a new motor

For break-in I was just going to use the procedure in the overhaul manual.<br />1. Operate at 1/2 throttle for 2 hrs<br />2. Run at any speed but not sustained WOT for the next 3 hrs.<br /><br />O/H consisted of 4 new pistons Wiseco 0.030 over.I believe they were forged. Also needed a new upper main bearing. Original had a cracked outer race. Also replaced lower main because I got a new one for free. Replaced all powerhead gaskets, seals, new water pump and seals in the gearcase. <br /><br />All I've done so far is started it and let it idle for about 5-10 minutes in my driveway with earmuffs on the water inlet. Water flowed through it real nice, nothing leaked and it idled very smooth. Just to make sure it would start before I brought it to the lake. <br /><br />Thanks for the advise on the prop. Have another prop question. I was going to buy it off this website. They have MW Vortex, Solas Amita and Hustler Turning Point 13 1/4X17 all for around $90. Is there any difference between these? Which is better? <br /><br />Thanks for the help.

 

[walleyehed]

Re: What size prop for a new motor

Honestly, each one of those props will be well suited to different boats, but I'd likely go with the turning point on your style of boat, and it should provide good, over-all performance.<br />As for the break-in, it's best not to run on the muffs for awhile, and vary the RPM every 3-5 minutes-don't stay at one setting very long.<br />Remember to double-oil!!!

 

[Ron570]

Re: What size prop for a new motor

Kenny, thanks for all the great adise. The muffs won't go on again until I've run it for a while. If it keeps running well when I have it on the water I won't be using them at all until probably next year. I just put them on this time to make sure it would start.<br /><br />For the break-in should I stay away from WOT for a while or does it matter as long as I only stay there for 3-5 minutes? How long (many hours) should I operate like this before I start running it at WOT for extended periods?<br /><br />Thanks Ron

 

[Dhadley]

Re: What size prop for a new motor

When you rebuilt the motor and put in the new water pump kit did you service the t-stats too?

 

[Ron570]

Re: What size prop for a new motor

I did put new t-stats in. The machine shop I had it bored out at said the engine had been overheated. Witnessed by the condition of the rubber water diverters in the block. Replaced those also. Also someone told me the cracked bearing race was also caused by overheating. To be honest I thought about reusing them as they opened when I dropped them in hot water but new ones were cheap.<br /><br />Thanks Ron

 

[walleyehed]

Re: What size prop for a new motor

Depending on the quality of work your shop done in boring, I would stay around the speeds that are on plane, and vary from 3000 to maybe 3500, then back to 3000 or so, then 4500, etc...after 3-4 hours, you can go to wide open throtle for short periods of time, then back off to 3000-3500 for a bit. Try and avoid that mid-range area between maybe 3700-4000 as that is about the leanest RPM range, and not a good range to run very long in during break-in, just accelerate right through it. After about 10hours of double oil and following the break-in suggested, you should be ok to run at WOT for longer periods of time. It's still best to pull back once in awhile in that first 25 hours and cruise more than run WOT.

 

[Ron570]

Re: What size prop for a new motor

Kenny, thanks for all the great advise.I ordered a prop yesterday,13.25X17 turning point. As soon as I get it, it'll be on and I'll be heading to the lake.<br /><br />You're the best.<br /><br />Thanks Ron