Wot

I have a 1964 18hp Evinrude,and I am trying to determine which prop to use.
Does anyone know what the WOT for this engine is?
I have a digital/laser Tacho on order to check the RPM.
The standard prop supplied with this engine is a 9x10
I am thinking of using a 9x9.
The present prop on my boat is a-- 9.25" X 11" and although it lifts the bow,it fails to plane.
The boat is a 14ft fibreglass boat with steering and remotes,two seats and a decent planing hull.It is quite light in weight.
My idea is to stay on the plane at lower speeds.
Although my present prop has maintained it's dia and has no dings,it is old and has been painted,and the paint is flaking off.It definately needs to be replaced.
The cavity plate sits one inch below the lowest part of the hull.
I'm not interested in the top speed,just to get there and back on inshore fishing trips.

 

http://www.rubexprops.com/boat-prop...r=E0180A&ver=a&model=18HP 2 Stroke (Pin Drive)

 

Try the 9", if that doesn't work, the motor is probably too worn to be any use. If it is hard to start, it probably is not up to spec compression wise.

 

Hello Mr E,

I decided on the 9" Pitch,there's a company in London that is converting a new 9X10 into a 9x9 for me.
The compresson on the motor is fine,it starts very easily.I turn the fuel off when I have finished a trip and run it out rather than using the cut off button.It starts 2nd or 3rd pull every time from cold.I have put new coils in it and points and plugs,and a new impellor,drained the
gearbox,and replaced the oil.The old oil was pretty clean.
Haha,at 71 it's me that's getting tired.

 

If your boat does not plane with the old 9x10" then it will not plane with a new 9x9".

Old engines were rated differently and your old 18 is like a new 15 hp outboard.
If timing is a bit off, dwell etc. then the old outboard has less power than needed.

Lower pitch will help but if your empty boat does not plane, your outboard will never plane a loaded boat.


A word of caution, two stroke engines must never run lean at wot.
You can run lean at idle but not at wot.

 

This is incorrect.

Say you install a 9 by 15 inch pitch prop on the 18hp.

With this steep of pitch, the prop will cavitate due to the pitch angle and not create much thrust. Additionally, the prop might require more hp than what is available below planning speed.



Then you switch to a 9 x 9 inch prop. The prop will cavitate less, demand less power but, depending on the boat parameters, might be able to get the boat on plane.

Inlet velocity to the prop blade, ie boat speed has a tremendous effect on cavitation.

So at pre planing speed, the op prop might be cavitating and not providing the thrust that he needs to get the boat on step.

Most planing boat hulls will have a high resistance point just before planing, and the engine has to get the boat through this point. "Hump speed" is the commonly used term.

Just to provide a definition to the high resistance point, ventilated transom, bow just beginning to come down

 

The present prop is 9.25 X 11,not a 9 X10.
The 9 X 10 I suspect would be borderline.
I have no doubts the 9 X9 will get it on the plane easily.
the dwell and timing is preset on this engine.
I use a happy medium mixture setting and never run weak.
Boats similar to mine are achieving 20mph to 24mph with this engine.
As for the load,there would be one more person and fishing tackle,(sandwiches and a thermos)
Despite their age,these engines are still very popular.
The idea is for it to plane at a lower speed,purely for economy and safety reasons.
hanks for your interest.