TOPIC: tach info needed

Hello everyone, my 58 Evinrude 50hp enhancements have taken some twists and turns. What I have ended up doing is transplanting the entire 61 Johnson 75hp powerhead, outer exhaust tube, and lower unit into the 58 exterior. Since I also switched from battery / coil to magneto and no longer have an alternator I was wondering what tach I could install that would accuratly read the V4? This is on a 65 SeaRay 500 (15 foot runabout, hullweight 650lbs, using a 10-11p aluminium prop). Thanks.

ps. any prop suggestions based on above criteria would also be welcome.

I have been collecting mathmatical formulas to calculate the performance of a outboard v hull. Still making adjustments and tweeking the model. If you would give me some specifications I would run it through my model and get some figures. Then when you do find the right combonation of hardware, it would help in my adjustments to the model. And it might give you a ballpark figure of where to start. The figures I need are Prop size and pitch, gear ratio, WOT Rpm's, Gross operating weight, and HP. THX

Gross hul weight is 650 boat, 244 motor, 100 lbs fuel, and 350 occupants. The HP is 75 @ 5000 rpm, the ratio is 20:23, and the prop is an aluminum 10" 11p. Of course I could really use a tach to help know where I am at with whatever prop. Hope this helps with your tables. Thanks.

Ratio 20:23?

i think that would be a 1.15 gearbox ratio. 23 divided by 20. The 20-23 number is right out of the owners manual. I know that it is a very small gearcase and spins the prop relativly fast...almost 1 to 1. Max motor rpm is 5000rpm and that will spin the prop at 4350 rpm. hope this makes sense.

According to my formulas; @ 5000 RPM (assuming a 10% slip factor)should have a top end of 40.77 MPH Seems high to me. I did my little 720# gross weight with a 40 HP 12 pitch and it came out at 26.29 MPH Let me know your running RPM's when you get a tach. THX