Prop Question for 1977 20hp

[Ginder75]

I am looking at new props for a 1977 20hp serial number 4580916. I have found several suppliers but they carry similar sizes.<br /><br />My question is this: I am finding either 2 blade props in an Eleven Pitch, or Three Blade Props in a Nine Pitch. Not to mention the various materials they can be made from.<br /><br />This motor is on a 15' Duracraft fishing boat that may top the scales at 1000 pounds fully loaded, gear and all. I have a 2 blade prop in Eleven currently, but I was looking for a back up and a little more speed if possible. I do not have a tach, but the engine does not seem to bog down at full throttle with this load. It is slow to plane, but then all is fine.<br /><br />If anyone can help me make sense of the 2 verses 3 blade configurations I would greatly appreciate it. And feel free to offer your opinions on the different prop materials out there.<br /><br />Great Website and Great Information<br /><br />Thanks,<br />Ginder75

 

[Motor Boater Bill]

Re: Prop Question for 1977 20hp

According to the Seloc manual, which is the only direct reference I have found on this, the two blade 11 pitch should give more speed than the three blade 9 pitch, which is recommended for heavier boats and heavier loads, such as water skiing. They include some caution about the potential to over-rev with the three blade prop if lightly loaded. I have a three-blade on my boat and took it for its first full scale test on the river, and I think I need to switch to the two-blade. It seems to need a bit more traction at wide open throttle.

 

[Ginder75]

Re: Prop Question for 1977 20hp

The other comment I have heard on the two blade verse three blade was that the three blade provided smoother operation. My motor acts like a ticking time bomb at idle, but at full throttle it purrs like a kitten. I have been through the electrical system, I have worked on the Carb, and adjusted this motor as well as I can adjust it. My only complaint about the motor is its idle. I know I can not compare it to any of the newer models on the water, but I feel it should preform better than it does.<br /><br />I am running down rumors now!<br /><br />Thanks for information.<br /><br />Any one else want to take a stab at this one?

 

[ShafferNY]

Re: Prop Question for 1977 20hp

This is interesting. <br /><br />I was given a 1978, 20hp merc that has the three blade prop on it and given an extra two blade prop.<br /><br />I just automatically assumed that the three blade prop probably pushed the boat better, so that's why it was on there verses the two blade.<br /><br />I haven't put it on my boat yet, but now I'm curious.

 

[Ginder75]

Re: Prop Question for 1977 20hp

I keep asking and still no solid answers. Some say the two blade is a racing prop, and the three blade is a work horse. I am happy with the performance of my 2 blade in 11 pitch, but thought a 3 blade maybe better since all newer motors have three blade props.<br /><br />My real question was is this - Is a nine pitch 2 blade the same as a 9 pitch 3 blade? Will they preform similar? What can be expected from a three blade over a two blade?

 

[Dhadley]

Re: Prop Question for 1977 20hp

Thats an excelelnt question. And very hard to answer. Heres why -- to compare any two props, any two, they have to be the same material, brand and style. Any variance and youre no longer comparing apples to apples.<br /><br />Some props of the same pitch will run a higher X dimension than others. Some have bow lift, some stern lift. Some have overall lift. <br /><br />I would expect that a good 3 blade prop will work better overall than a 2 blade. It has to do with blade area. <br /><br />To properly set up a rig, a tach is critical. However it can be done. Lets try something. Look at your 3 blade prop and count the splines. Also, what diameter is it?

 

[Ginder75]

Re: Prop Question for 1977 20hp

I know that is part of the problem. Any deviation and it is a different test. Heck you can have two props from the same manufacture in the same style, size, and pitch and they will act a little differently.<br /><br />I have a 9 7/8 dia, 2 blade prop in 11 pitch. I do not know the number of splines offhand, but I can either look it up or post tomorrow.

 

[Motor Boater Bill]

Re: Prop Question for 1977 20hp

Pitch describes the distance a propeller would move forward in one revolution, assuming a theoretical "no slip" situation. So, regardless of the number of blades, in theory an 11 pitch prop will go faster than a 9 pitch at the same RPM's. In practice, a three blade prop may slip less than a two blade, at least during accelleration. <br /><br />Ultimately, it is about matching the boat, motor and prop so that you don't lug the motor and don't overrev. Most of the experts on this board will say you really need a tach to fine tune this. There won't be one right prop for that motor, but there may be a best choice for that motor on your boat, with whatever load and use you normally have.

 

[Motor Boater Bill]

Re: Prop Question for 1977 20hp

Acording to the prop chart in the Mercury factory service manual (C-90-74041), the 48-3348A1 prop is an 11 inch pitch, two blade, and for boats up to 1000 pounds will run 22-33 mph at maximum RPM. <br /><br />The chart also shows a prop #48-33482A1, two blade (I wonder if this is a typo and it is really a three blade?) with a 9" pitch, for boats weighing 850-1600 pounds, making 13-24 mph at maximum rpm.<br /><br />If anybody has one of those 48-33482A1 props, let us know if it is two or three blades. My three blade nine inch pitch is a Michigan Wheel, and it is supposed to be the replacement for the Merc 48-33482A1.

 

[Motor Boater Bill]

Re: Prop Question for 1977 20hp

Correction, the two blade 11 inch pitch is #48-33480A1.