Evinrude 6hp — not enough oomph!

Hello Friends–

We finally managed to finish our Cocktail Class racer at the very end of last summer and launched it on Lake Joseph in Ontario.

I bought a nice-looking Evinrude 6hp motor and got it all rigged up and running. It definitely pushes the boat but let’s say I was expecting more. A friend who knows old motors pretty well says it’s running as well as can be expected, at 4500 RPM.

My son was probably about 100 pounds and we really expected the boat to go faster.

Does anybody have any advice? Right now I’m considering looking for an 8hp Johnson/Evinrude motor so I can hopefully use the same cable setup. I’m also curious about the Tohatsu motors, but I don’t see a lot of used Tohatsus for sale here in Ontario, and the new ones are really pricey.

Any advice from the community would be a big help. Hoping to get ahead of this before summer. Thanks–

5 replies:

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RE: Evinrude 6hp — not enough oomph!

So the first thing that comes to mind for me is, is the problem with the motor, the boat or the expectations?

Have you measured the speed of the boat so you could compare it with what other boats in the class do? If your performance is typical then you might as well save your money.

If the performance is low for the class, how so? Is it poor acceleration? Or is it low speed?

You say that your friend has confirmed the health of the motor, but has anyone verified that the prop is a correct match for the motor and type of boat? A mis-matched prop could result in a situation similar to driving down the freeway in first gear. While the motor may be happily turning the shaft at 4500 rpm, that has to be efficiently turned into thrust to get you velocity. Your motor power may be rated at 6 hp  but the propeller  power could be considerably less if you have an inefficient prop.

As far as the 8 hp motor, I'm not sure if that will make a noticeable difference except in improved acceleration, especially if there's a prop mismatch. All other things being equal you may see a 10% or so increase in speed.

The other thing to check is your boat's trim. Make sure that it's angled correctly in the water. The wrong trim can greatly reduce a boat's speed. I've had dinghies double their speed when the trim was fixed.

Good luck,







RE: Evinrude 6hp — not enough oomph!

  Depending on how old it is, it could have any number of different props.  I'd get the model and serial numbers and go to Michigan Wheel to see what alternatives they may have for pitch, # of blades, diameter.  Other prop makers also, but Michigan Wheel is I think the biggest supplier.   A lot of those motors are used for pretty slow speeds on jon boats and such, so a prop change may be in order, maybe more pitch.

RE: Evinrude 6hp — not enough oomph!

   Aside from the aforementioned advice, have you pulled the prop, serviced the lower unit, check the castle nut/cotter pin, and made the sure the prop hasn't been spun?  I've hit enough oyster bars in south FL to know the symptoms.  Just another point to check and some worthwhile maintenance. Is it an aluminum prop?  Maybe a stainless prop is available for that motor.  I've seen spun props that can still power the boat, but slippage reduces the torque.  I second the advice on trim, proper trim can add a lot of speed. 

RE: Evinrude 6hp — not enough oomph!

   THanks very much, this is all very helpful advice. I'll post an update--

RE: Evinrude 6hp — not enough oomph!

   As a start on tuning your CCR's trim, the anti-ventilation plate on your engine's lower unit should be nearly parallel to the bottom of your hull near the transom.  If this isn't the case, you need to adjust the angle of your transom's wedge to make it so!  Temporary fine tuning can be done with shims (popsicle sticks or tongue depressors broken in half) between the wedge and your engine's clamp brackets.  The front of the anti-vent plate will be near the transom.

You didn't say what year Evinrude you are using.  Makes a big difference in speed.  About 1980, the government mandated horsepower shall be advertised at the prop, not the engines' output shafts.  15% to 20% increase in evident power occurred.  A pre-1980 engine should get you speeds in the higher teens (mph), a post-1980 engine should get you speeds in the low 20s.  A 100-pound driver will get a higher top speed than a 200-lb driver.

Also, check your hull's bottom for flatness, fore and aft.  A hook (curve) in the bottom will adversly affect it's speed.  Squaring the corners of running surfaces can improve top speed, too.  Check the Cocktail Class Wooden Boat Association's website (ccwbra.com) for details on keel modification to minimize prop cavitation during turns.  Lots of pictures showing proper running attitude at that website.  And plenty of pictures and racing videos at PropTalk's website (proptalk.com, a Chesapeake Bay Area soft- and hard-copy magazine) under their Racing section.  Good luck.

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