????hwsii?????

[junior1113]

just bought a 07' mercury 8hp 4 stroke 2.08:1 ratio 5000-6000 rpm range. put it on my 1969 feather craft 10'8" alluminum johnboat . i get 15.8 mph wot with a 7.5 pitch. 550lbs total weight. my question is what is the estimated rpm with 10% slip. or what speed would be achieved at 6000 rpm with no slip. trying to figure out if im within limits without buying and hooking up a tach. thanks for for time, PAUL

 

[steelespike]

Re: ????hwsii?????

Your kind of going about this wrong way around.I can tell you your speed is
pretty close to what an 8 would do with a setup like that.And it isn't over reving.Go ahead and get a tiny tach;about $40 and very easy install it will work on any 4 stroke with the same nimber of cylinders.With that and your gps you can tell how good your setup is and get rigged for the best speed/rpm and motor health.You can probably raise the motor on the transom just add 1/4" sticks to set the height while you tighten the clamps.Raise it to a point just before venting(cavitation) becomes a problem.Be sure it pumps water and clamps have a good grip.

 

[hwsiii]

Re: ????hwsii?????

Paul, I ran the numbers for you using the different suppositons for RPM and prop slip. But I do agree with Spike, you can buy a digital tachometer and hour meter here: http://cgi.ebay.com/ebaymotors/Hour-meter-tachometer-dirt-bike-atv-motorcycle-jet-ski_W0QQcmdZViewItemQQhashZitem2a00fc400eQQitemZ180405157902QQptZMotorcyclesQ5fPartsQ5fAccessories for only $ 20 with shipping.

Prop Slip

I would try raising the motor as well, and I would start with about 1" in height above the keel and you should be able to gain a couple more MPH out of it, from my analysis.



H