prop for a 70 johnson

[cesandroid]

Im running a 1985 70 Johnson on a StarCraft st150 trihull with the OEM 131\4 x 17. Good hole shot and 35m/hr turning at 5300rpm. Would be nice to go a bit faster? Any better props out there that would perform better? Thanks

 

[JimS123]

I had one of those oldies back in the day. I'd say your performance is pretty good now. I did swap for an OMC SST prop and it very slightly improved the gas mileage and added a very slight bit to the top end, but it wasn't worth the cost.

Unless you boat in shallow waters at times and want the added durability of stainless, I'd keep what you have.

 

[Sea Rider]

The wot rpm range for said motor is 4500-5000, at 5300 as usually loaded it's near the max rpm, so very little to improve. Going down 1/2 pitch to 16.5 from current 17 prop will gain slight more top end speed but sacrfying a bit the current hole shot..

Happy Boating

 

[WesNewell]

Hm... The chart I use show a wot range of 4500-5500 rpm and a GR of 2.42. What's the dry weight of your boat. I find from 745# to over4 1100# depending on source. If you want more speed, the first thing you should check is the engine mounting height. Mounted too low can cost you up to 20% of top end speed. That's the very first thing to check.

 

[JimS123]

Hm... The chart I use show a wot range of 4500-5500 rpm and a GR of 2.42. What's the dry weight of your boat. I find from 745# to over4 1100# depending on source. If you want more speed, the first thing you should check is the engine mounting height. Mounted too low can cost you up to 20% of top end speed. That's the very first thing to check.

5500 would be the correct WOT rpm for that motor. The upgraded "Stinger" was 75 HP and rated for 5800 rpm.

After trying each hole and doing a performance curve in each, I ran mine in the 3rd hole up.

I must say that the OMC 70 was probably the best 2-stroke motor I ever had.

 

[cesandroid]

Good points here. I forgot to mentioned I did set the height of the motor as well as I do trim it up for best results. If I go to a lower pitch wouldn't that increase rpm and speed maybe decrease? I think dry weight is around 800 lbs

 

[WesNewell]

Lowering the pitch should increase rpm's, but the increase in rpm's should keep the sped about the same. If your boat is only 800#, you should see more than 35 mph with a 70 hp engine with no extra people.

 

[Sea Rider]

How many souls were on board when the motor achieved 5300 wot rpm from max 5500 rpm factory stated, would you say that's the normal load you'll have on board frequently ?

Happy Boating

 

[JimS123]

Lowering the pitch should increase rpm's, but the increase in rpm's should keep the sped about the same. If your boat is only 800#, you should see more than 35 mph with a 70 hp engine with no extra people.

It's a Tri-Hull. How accurate is the speedo?

 

[cesandroid]

The speedometer tends to peak at 31. 2 adults in the boat for most cases. GPS gives me 35.

 

[JimS123]

Your data isn't far off. You are doing just fine.

 

[Sea Rider]

Yep only 200 OFF from max 5500 wot rpm range, personally if boating with 2 up on constant basis would stay with current 17 prop. Will still handle 3 up nicely with good water performance sacrificing current wot rpm and top speed reduction hard to determine, just for the record test with 3 up and report current wot rpm run and top speed...

Happy Boating