2 blade prop for big merc?

[100333624]

Hi guys, I ended up picking up a 16' boat that came with a 1978 Mercury 800, the 4 cylinder model. The motor has the original 3 blade prop on it. The boat ran exceptionally fast and I was impressed, it did about 40mph with me and the previous owner on board, not to mention the excess water weight it had at the time (have removed about 100 pounds of it so far)!! Anyways, the boat picks up speed very fast and planes in only a matter of seconds... This is my first fiberglass boat so things are a bit new to me so bare with me. I spoke to someone about the performance of this boat and they mentioned to me that it would be wise to get a tachometer to know how much RPM I'm doing at WOT. He also said that either my boat is light for its class size, or the motor runs very strong for its age (I think both). He also said that he BELIEVES there were 2 blade speed props for these motors, and that possibly getting one could gain me quite a few mph top speed. Now my question is do they make 2 blade props for motors of this size? Are they safe running or do I risk blowing my motor? What are the pros and cons to going with a 2 bladed prop? I appreciate any insight into this, thanks guys!

 

[jestor68]

Re: 2 blade prop for big merc?

In the early days, Mercs were optionally equipped with 2 blade bronze props that really didn't work as well as a 3 blade prop.

They are now passe, because they vibrated and performed poorly compared to a modern 3 blade prop.

I wouldn't recommend them.

 

[H20Rat]

Re: 2 blade prop for big merc?

Yep, you absolutely need a tach before touching the prop. Its MANDATORY, at least if you value your engine at all. 40 mph on that combo is pretty much right where it should be, although you can never trust a pressure based boat speedo. Sometimes they are dead on with a gps, sometimes they are a LONG ways off. (usually on the optimistic side, although the one in my current boat reads about 5 mph low at top speed.)

 

[Frank Acampora]

Re: 2 blade prop for big merc?



Third on the tach--"You can not get where you are going unless you know where you are starting."

However, for your interest, I have attached a couple of photos of BRONZE props in both two and three blade.

Yes, they are passe' but nothing looks quite as pretty as a newly polished bronze prop. Specifically, the prop in the third and fourth photos was a Mercury racing prop. Yes, you can still find 2 blade bronze Mercury props--both racing and thru-hub exhaust on ebay. They do run well, with a little more vibration, but no, they will not "hurt" your engine.

It used to be that on a light hull, a two blade would deliver more speed. Now, with modern prop design, a three blade delivers better performance than the old two blade design. In fact for serious racing, like offshore, even more blades (4 or 5) are used.

Note however the last photo of a stainless steel three blade. You would be much better off to buy a correct stainless prop for your engine.

 

[Faztbullet]

Re: 2 blade prop for big merc?

What are the pros and cons to going with a 2 bladed prop? I appreciate any insight into this, thanks guys!

As posted in earlier posts get a tach before doing anything. The 2 blade aluminum props where usually a tad bigger in diameter and are 1-3 MPH faster than a 3 blade prop. They are a tad slower on hole shot as they are more efficient, transfer a lot of torque back thru steering and only good on light boats and for just playing around, its not that good at pulling skiers but can if skier is experienced.

 

[100333624]

Re: 2 blade prop for big merc?

So from what I take it, yes they do make these 2 blade props but they are old school, and old design.. So I'm better off going with a modern stainless prop perhaps.. But not until I get a tach first... Oh and by the way, the speed was recorded on gps not speedo! This boat is mainly used for fishin as the lakes I fish are quite big and wide and I like to get there faster, not to mention that I'd need to get through chop because our great lakes here in Ontario can develop some big swells fast! Maybe I'll be looking at a 3 blade with higher pitch? I'll be back to bug you guys about choices but after I get a tach! I guess I'll just wait till next season then... Thanks guys I appreciate the info!!

 

[Bondo]

Re: 2 blade prop for big merc?

So from what I take it, yes they do make these 2 blade props but they are old school, and old design.. So I'm better off going with a modern stainless prop perhaps.. But not until I get a tach first... Oh and by the way, the speed was recorded on gps not speedo! This boat is mainly used for fishin as the lakes I fish are quite big and wide and I like to get there faster, not to mention that I'd need to get through chop because our great lakes here in Ontario can develop some big swells fast! Maybe I'll be looking at a 3 blade with higher pitch? I'll be back to bug you guys about choices but after I get a tach! I guess I'll just wait till next season then... Thanks guys I appreciate the info!!

Ayuh,.... I haven't seen a New 2 blade prop for a Mercury mid-sized outboard, since the current mid-sized outboards were the Big Boys on the transom...

'n that was sometime in the mid/ late 60s...

 

[100333624]

Re: 2 blade prop for big merc?

Oh and I forgot to ask, is there anything else that can be done to get a little more performance out of an outboard? I'm not looking at racing I just want this motor to perform GOOD, the previous owner really just sort of neglected this boat and it just sat outside in one of the most "tropical" regions of Ontario... Like could putting better sparks and high octane gas make a difference? Or premium mercury oil? What about polishing the lower unit? I'm just pulling random facts out of my @$$... Lol thanks guys!

 

[beaverliner]

Re: 2 blade prop for big merc?

Oh and I forgot to ask, is there anything else that can be done to get a little more performance out of an outboard? I'm not looking at racing I just want this motor to perform GOOD, the previous owner really just sort of neglected this boat and it just sat outside in one of the most "tropical" regions of Ontario... Like could putting better sparks and high octane gas make a difference? Or premium mercury oil? What about polishing the lower unit? I'm just pulling random facts out of my @$$... Lol thanks guys!

hahaha!!! Tropical regions of Ontario... you made me laugh!! flurries in the forecast for tomorrow!!

 

[Bondo]

Re: 2 blade prop for big merc?

Oh and I forgot to ask, is there anything else that can be done to get a little more performance out of an outboard? I'm not looking at racing I just want this motor to perform GOOD, the previous owner really just sort of neglected this boat and it just sat outside in one of the most "tropical" regions of Ontario... Like could putting better sparks and high octane gas make a difference? Or premium mercury oil? What about polishing the lower unit? I'm just pulling random facts out of my @$$... Lol thanks guys!

Nope,...

Nope,...

Nope,...

Nope,....

 

[Frank Acampora]

Re: 2 blade prop for big merc?

Summer is the nicest week you have up there!

You can for zero dollars experiment with the engine height on the transom. The cavitation plate can sometimes be set about one inch above the bottom of the hull. Less drag and better performance--maybe! There is a fine line after which you cross, the engine will ventilate and performance will be reduced.

Also, search the topic porp venting. With a vented prop, a higher pitch can be used (if the engine can turn it adequately.) while still retaining hole shot performance.

Post a couple of photos of the back of the boat on the trailer with the engine trimmed to its normal running position in the water. That would help some.

Then, add the weight of the boat, the normal load it is expected to carry, its top speed now and the current prop (look for numbers inside the hub or between the blades. Do this after buying a tachometer and include currwent RPM at wide open throttle. Then, the members on this forum will be able to help you

I don't think any two blade props are made for larger engines anymore BUT, you can still find used on ebay.

 

[100333624]

Re: 2 blade prop for big merc?

That sounds good Frank, I'll look into that for sure! I've pulled the motor off and it's on a stand for the winter... The bottom of the boat is evenly leveled with the cavitation plate.. The boat hauls @$$ and gets on plane instantly, also gets to top speed very fast... My boat is empty right now and I'm guessing is around 250 pounds? I can kinda lift it up on one side off the trailer while its on it... Mind you the floor has been removed so add another 50 pounds to that weight.. I'm gonna wait till spring where I can get it in the water again and test it with a tach. Thanks Frank!