Mercury 60hp over revving

[alex2293]

Hey guys, Today tried my tiny tach on my boat, a bayliner 1600 (14 ft, 770lb hull) with a 60 hp mercury 2 stroke outboard (210 lbs). With my dad and I, the top speed is 28 mph at 6700 RPM. It is over revving. The prop is stainless steel mercury quicksilver with a 15inch pitch. The blades are pitted and chipped, it has also been repaired once by a welder (not a prop specialist). The WOT should be 5000-5500 RPM. According to a mercury chart with this weight and outboard I could go to 16 or 17 pinch ? What do you think? Is my prop slipping really bad because it is damaged? Thanks, Alex

 

[steelespike]

Based on my calculations I think your tach is way off 28 mph 15",1.83 ratio 13% slip = 4150 rpm.
If we use a 1.61 ratio rpm would be lower. If we use 6700 indicate slip of about 46%You could probably push a house boat with less slip
even with your bad prop.
If 6700 was accurate you would have to go to a 9" prop.Thats pontoon size.
Check the connections on your tach,note the position of the selector switch on the back, operate it a few times to clean the contacts.
You may actually have a little too much prop.Get an accurate tach we can estimate better if you really need to reduce the pitch.
If your motor was turning 6700 it would sound like it was about to fly apart.

 

[alex2293]

The ratio is 1.64 and the tach is a small engine induction tachometer. It is currently set at one spark every revolution. Could this engine use a wasted spark system.

 

[steelespike]

Don't know if it's wasted spark or not but induction tachs sometime pick up stray pulses.
I never did one but it seems that if you use a lower plug and get the wire outside the cowl
there will be less interference.Seems like wasted spark would double the rpm.

 

[alex2293]

Well, good news, I've tried what you said and the rpm makes sense now. The idle is at 790 rpm and the WOT is 4520 RPM at 28 mph. With a ratio of 1.64 and these number the slip is 28 percent. Should I try a 14 or 13 pitch. It does not have the power trim and the height is not adjustable. Thanks a lot for your help.

 

[steelespike]

How is the hole shot now how about water sports power?
reaction to added weight?
The slip seems really high some of that could be a somewhat low rpm,perhaps a little from the prop.
At any rate it appears you need about 500 rpm to get to 5000.
With a little luck a 13" might get you there if we can get to 5000 and about 13% speed could be about 32.
About 25% may get about 28 mph with better hole shot etc.

 

[alex2293]

I can pull a 220 men on two skis easily and have enough speed to drop a ski and slalom a little bit. When pulling someone on skis there is also two 200 pounds men in the boat and 5 gallons of fuel. Hole shot is good, but i can't compare it is my first boat, except a 12 ft with a 6 hp. I, m also able to pull a 220 pounds man on a tube and bring him and the boat to plane and swing him a bit around. I thinkn the high slip is caused by too much pitch and a repaired/pitted/dinged propeller.