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Any benefit fitting a SS prop to 75HP 2 stroke

L

leachy

Regular Contributor
Hi All,

Just wondering if there is any real advantage fitting a Stainless Steel prop to an old 75HP 2 stroke. I was looking at a couple of random posts and a lot suggested that it wasn't really advantageous below about 115HP as the smaller engines dont really have the power to take advantage of the SS benefits.

just wondering how true this is.

Regards
Leachy
 
It's not the material that matters it's the pitch and diameter that keeps you in the max. rpm. Stainless doesn't flex as much as aluminum so the thrust is better.
 
Quicksilver said:
It's not the material that matters it's the pitch and diameter that keeps you in the max. rpm. Stainless doesn't flex as much as aluminum so the thrust is better.
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But a 75 HP won't benefit from this- not enough power. The only one who benefits is the place that sells it.
 
jimn said:
But a 75 HP won't benefit from this- not enough power. The only one who benefits is the place that sells it.
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That depends upon the boat and load. I had a 75 Rude and changed props, jacking my engine up one notch since the SS had rake and blade top cupping....don't remember the specs. on the props. I guarantee you it did make a difference. I don't remember the mfgr. of the boat but it was FG, 15', lightweight, cork flotation between the outer hull and inner molded drop in "deck" (floor). It had a little V at the bow for about 2' and the rest was flat with a center flat area that extended about 3/4 across the bottom of the hull. Steering was side mounted console, 2 single person seats. I didn't have instruments on that boat but I could certainly tell that things changed. One of the things that changed was that the wetted area of the hull was reduced as the speed increase was sufficient to only have that center pad in contact with the water reducing drag.
 
Assuming you are replacing undamaged aluminum Merc prop with equivalent stainless Merc prop, performance will be similar. But the stainless will withstand minor impacts (primarily mud/sand/wood) without damage. Where the aluminum would possibly be damaged.
 
Droid said:
Assuming you are replacing undamaged aluminum Merc prop with equivalent stainless Merc prop, performance will be similar. But the stainless will withstand minor impacts (primarily mud/sand/wood) without damage. Where the aluminum would possibly be damaged.
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It depends on which Mercury prop!
 
Texasmark said:
That depends upon the boat and load. I had a 75 Rude and changed props, jacking my engine up one notch since the SS had rake and blade top cupping....don't remember the specs. on the props. I guarantee you it did make a difference. I don't remember the mfgr. of the boat but it was FG, 15', lightweight, cork flotation between the outer hull and inner molded drop in "deck" (floor). It had a little V at the bow for about 2' and the rest was flat with a center flat area that extended about 3/4 across the bottom of the hull. Steering was side mounted console, 2 single person seats. I didn't have instruments on that boat but I could certainly tell that things changed. One of the things that changed was that the wetted area of the hull was reduced as the speed increase was sufficient to only have that center pad in contact with the water reducing drag.
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The OP wrote that its an old 75HP, so I'm not sure he's trying to go terribly fast, or have a great hole shot for tournament water skiing. Sure, a super-light boat will get up and go quickly, but there are a lot more 75 HP on medium weight and heavier boats. Once it's on plane and close to maximum speed, it's not going to have a lot of resistance from the water, though.
 
jimn said:
The OP wrote that its an old 75HP, so I'm not sure he's trying to go terribly fast, or have a great hole shot for tournament water skiing. Sure, a super-light boat will get up and go quickly, but there are a lot more 75 HP on medium weight and heavier boats. Once it's on plane and close to maximum speed, it's not going to have a lot of resistance from the water, though.
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Don't hold your breath on the last statement!
 
racerone said:
As boat speed goes up , friction from the water also goes up.-----And by more than you would think.
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That's why racers get the hull out of the water........Check out Faztbullet's Avatar, an avid racer, and look at how much of his hull is in the water.
 
fitz73222 said:
Thats a great question, because they are two completely diifferent engines. My money would be on the looper 3 cylinder 75 than the 4 cylinder cross flow 75 for anything performance related.
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Is the 3 cylinder a newer design over the 4 cylinder. I don't have a lot of experience with this engine or Mercury really all our boats were either Johnson/Evinrude or Yamaha in the later years. This engine dose seem to go through a bit of fuel though, seems almost as much as a 115Hp V4 Enivrude. from the late 90's.
 
A 4 cylinder is a spin off from the first Mercs developed from the original 25 Hp through the 40 HP (which was available in 45 and then 50 hp engines for years) cross flow engine up to the 6 cylinders which were also that old design, just stacking 2 more cylinders on top of the 4. In the 1989 area Merc completely revamped its lineup with loop charging and a reduction of the number of cylinders. Comments on both sides of the isle about the 4 being a smoother runner but being loop charged was more efficient.

On more cylinders are smoother, I had the last year of the "Tower of Power" 6 cylinder (direct charged)....1988 and that was one "sewing machine" smooth running sucker. Direct Charging was a modification using some of both worlds. It had very good mileage.
 
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