Johnson 3HP 1965 propellor options?

[TFO]

Now My 1963 Johnson has done a 12 hour continous drive pushing my sailing boat from our summer harbour in the swedish archipelago into the calm waters of lake maelaren to soon to rest on shore until the summer season starts on the 1;th of may.<br /><br />It was the first drive with a 1965 straight gear and the 3-blade propellor. The shaft down to the gear has been welded togheter from two shafts to fith the length of the sailboat extension and the odd combination of vintages.<br /><br />I feel the thrust is excellent at low speeds but the RPM a bit to high for and old engine. <br /><br />Does anyone know what propellor options are available for the straight gear. Did about 3.8 knots and that is about the same it did with the original tilted gear and the two blade prop.

 

[Paul Moir]

Re: Johnson 3HP 1965 propellor options?

The speed is going to be completely depend on the boat you have it attached to. What length is your sailboat? How much does it weigh (appx)?<br />With two strokes, bringing the revs down by giving the engine with a higher load is extremely harmeful. They are much happier when they can rev freely as it'll run much cooler and the rings won't coke up as fast. Ideally, your 3hp should wind up to 4500RPM.<br /><br />Likely, the propeller you have is the best one for your sailboat. Take it off and examine it for a number (usually 6 digits). The ones I've seen had the number cast on the inside of the hub of the prop.

 

[TFO]

Re: Johnson 3HP 1965 propellor options?

Expected the speed of the boat to increase with the switch to 3-blade and horizontal mount. The earlier, tilted, propeller mount from 1961 by design, looked like it was wasting energy making a lot of vertical thrust. Imagine a completely vertical mount - It would not do any work at all.<br /><br />According to the laws of physics, a small diameter propeller on a slow spead boat (4 knots) need to eject water at higher speed than a big prop to obtain the same thrust.<br /><br />Higher speed of the water ejected means more power lost compared to more volume and lower speed.<br /><br />(In theory of course) <br /><br />Will check the propeller marking.

 

[TFO]

Re: Johnson 3HP 1965 propellor options?

Have checked the 3-blade prop on the 1965 horizontal gear mounted on the 1961 3HP Johnson. It's marked 6x6. <br /><br />Given that the boat will do between 4 to 4.5 knots (4.6 to 5.2 MPH) at full speed - Is the 6x6 the right prop to use?

 

[Paul Moir]

Re: Johnson 3HP 1965 propellor options?

Sorry I havn't gotten right back to you, I was just doing a little research before posting back.<br />The right-angle drive was built with sailboats in mind. Not just is it more efficient than the weedless (110° angled) drive, it has a higher gear ratio (12:25 vs 17:28) so it can swing a larger, 8" diameter prop (larger props are more efficient, and better "pushers"). The props available for it were 4.5" pitch and 5.5" pitch. I think the one you have on there now is really for a later 4hp (or something else) weedless drive unit, as they all took 6" diameter props.<br /><br />Does it look like you've got a lot of room between the tip of the propeller blade and the anti-ventilation plate?<br /><br />9.3

 

[TFO]

Re: Johnson 3HP 1965 propellor options?

Ok, got it! The 6x6 prop and the right-angle drive might come from different engines.<br /><br />I higher gear ratio was what i was looking foor. Slower rotation speed and bigger prop is perfect. The prop may be wrong. Will check how much room there is for the prop.<br /><br />Thanks for the research.

 

[TFO]

Re: Johnson 3HP 1965 propellor options?

I was wrong about the marking of the prop. It's marked (8 x 4 1/2).<br /><br />Time to dwell in theory..<br /><br />Did some calculations based on the gear ratio figures, kindly supplied by Paul above, and found that the engine need to deliver only 0.77 HP's at 4 knot boat speed at 4500 RPM. Another inch added to the pitch and the engine need to deliver 1,96 HP at the same speed. What this means is the thrust at 4 knots will be 44% higher with the higher pitch prop. <br /><br />Compared to the current situation where the boat uses 0.77 HP aplying 1,96 HP will cause an increase of speed to a point where the boat consumes the exact amount of power that the engine can deliver at that new speed. <br /><br />It has to do with the fact that the low pitch prop turns through the water at 8 knots and the high pitch prop at 9.78 knots. <br /><br />If the boat speed would reach 8 knots with the low pitch prop the engine, at 4500 RPM, will run without load and will make zero thrust whilst the higher pitch prop will deliver thrust up to 9.78 knots before being pulled through the water.<br /><br />On the other hand, at 1 knot boat speed the low pitch prop will demand 2.35 HP from the engine whilst the high pitch one will need 4,52 (which sure is not available so in reality the high pitch prop will force the engine RPM to stay below 4500 at 1 knot speed)<br /><br />So for my 4 knot boat to go beyond 4 knots it seems the high pitch prop is a must.<br /><br />Where can I buy one? <br /><br /><br />P.S<br />In reality the water from the prop will rotate and there are friction losses and turbulence arround the prop. But the basic facts about speed, pitch, RPM and power gives a good hint on what could be expected.

 

[Paul Moir]

Re: Johnson 3HP 1965 propellor options?

The difference between the speed of the propeller wash (theoretical speed) and the speed of the craft is known here as "slip", and is measured as a percentage.<br />So with the 4.5" prop at 4500 engine RPM and a 12:25 gear ratio, you're going 8 knots theoretically and 4 knots actually. That means you have a slip of 50%, which is pretty typical for a sailboat.<br />*IF* the engine will spin at 4500 rpms with a 5.5" prop, then the theoretical speed will go from 8 to 10 knots, and if the slip remains the same 50%, then the boat will go 5 knots?<br /><br />That's what you're saying, right? But there are some serious limits on that. If you're anywhere close to hull speed, you can't expect the slip to remain the same. The closer you are, the more it will increase. You haven't told me yet how long the boat is (water line length if you have it), so I have no idea if you're close enough to matter. The weight would give me a fair idea as well, as we can generally say to use a 5.5" prop on lighter boats.<br />The 4500 RPMS is an assumption as well, since you haven't used a tachometer on it.<br /><br />That said, the 5.5" pitch, 8" diameter prop is still available from your local Johnson/Evinrude dealer for about $35USD. Part Number is 315858.

 

[TFO]

Re: Johnson 3HP 1965 propellor options?

The calculation I think does not show the actual power of the engine but is a way to compare two sets of props at a certain boat speed and RPM.<br /><br />I think my calc indicate that assuming we keep the engine att 4500 RPM and a big hand is moving the boat at 4 knots the low pitch configuration will only ask for 0,8 HP from the engine and the higher pitch prop 1,96HP. It would mean that if the big hand released the boat it would leave the operating point more rapidly and accelerate to a higher speed with the higher pitch prop.<br /><br />Another way to view it is, keeping the 4500 RPM and consuming 3HP from the engine will happen at a boat speed of 0.1 knots with lower pitch and at 2,6 knots with the higher pitch.<br /><br />It looks like the low pitch is for towing elks over a lake at hunting season. <br /><br />The length of the boat is 7.68 m and the weight is 2000kg (2.2m wide). In sailing mode it will rarely go beyond 7 knots and then the power from the sails are way beyond what the Seahorse can deliver.<br /><br />Thank you for the article number. I will ask at our local delaer if they have the prop in store.

 

[JB]

Re: Johnson 3HP 1965 propellor options?

Prop topic.

 

[TFO]

Re: Johnson 3HP 1965 propellor options?

The black 8x5.5" propeller has now been tested. The speed of the boat increased and the max RPM lowered as wanted.<br /><br />

<br /><br />Used an ironless DC motor (PortEscap 23L21-213E) as RPM tachometer by just holding the shaft in the cavity at the end of the crankshaft and got steady readings on a DVM. The speed was read from a GPS and there was zero wind. <br /><br />A way to verify that the RPM/volt from the DC motor datasheet is right is to make a mark on the shaft and view it in the light of a normal incandescent lamp. It works like an accurate stroboscope. In a 50Hz grid the mark will be steady at 6000 RPM and in a 60Hz at 7200 RPM. (Incandecent lamps flickers at twice the grid frequencey) An andjustable dc voltage source and a volt meter is needed. The same calibration/check can be done against a TV-screen with a tape strip on the shaft.<br /> <br /><br /><br />

<br /><br />Now, with data at hand, the max RPM with the old low pitch prop was not as high as I guessed. The 3HP's of the engine I think is specified at 4000 RPM. The engine might need some tuning? It has new plugs, points, ignition cables and adjustable fuel nozzle from a later year carburetor.<br /><br />With the new prop the RPM got no higher than 3000. <br /><br />Wonder what experts can read from the diagram?