Prop Sizing/Tach options for small OB's

[Jamar]

I now have her up and running and want to be sure she is delivering optimum performance. I run up/down TN river. Lots of current. AT WOT avg speed is 25 mph on a 14' 4' wide john with 2 men < 200# each and minimal gear. Have no tach and its a tiller handle model so no console to ass one to. Speed is from GPS unit. <br />Is there a way to rig a tach from Radio shack or somewhere with alligator clips to check motor rpms so i can do some prop sizing?<br />I have seen the $40.00 tiny tachs......not that i am cheap, just figured i might could make one for 10 bucks or so. Anybody got any ideas?<br />Also does that top speed sound right for this rig. I know there are alot of variables but it sounds pretty close to right to me....seems to be pretty fast for the boat.

 

[Jamar]

Re: Prop Sizing/Tach options for small OB's

Whoops sorry.......motor is 93 model 20hp Merc. Finally running like a new one.

 

[Jamar]

Re: Prop Sizing/Tach options for small OB's

^^^anybody?

 

[Dhadley]

Re: Prop Sizing/Tach options for small OB's

A Tiny Tach will work well. Do a search on the net and you will see where you can buy one for under 50 bucks. A neat little deal.<br /><br />Good luck!

 

[FlyBoyMark]

Re: Prop Sizing/Tach options for small OB's

Jamar..speed is in the envelope too..not bad for a barge that big. When you do tach it, it should be about 5000-6000 RPM with your target rpm at about 5500 RPM. The 25 HP has a bigger carb than the 20 HP so spooling up a little more would produce more HP. Target RPM for the 20 is 5000 RPM(4500-5500 RPM) . If your turning a couple hundred less than 5500 RPM, I would consider dropping the pitch 1 inch thus alowing the engine to breath easier and possibly even gain extra rpm and HP over what you may be getting by lugging the engine.