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Author Topic:   2000, Dauntless 18', Merc 125 Classic Saltwater - Prop question
TomT posted 05-04-2004 09:53 AM ET (US)   Profile for TomT   Send Email to TomT  
The engine operating range is 4750-5250 RPM with a redline at 5500 RPM and peak horsepower at 4900 rpm. My boating is in small inland lakes. My interest is in economic cruising speed (4300 rpm) and top speed as I am a fisherman with little joy from pulling skiers. I just had my tach calibrated by a Mercury authorized mechanic.

This boat came with a 13x17 black max and would only hit 27 mph @ 5400+ rpm. I tried a Michigan Wheel Ballistic XL 22, Rapture 21 and Ballistic 21. At 2900 lbs loaded I could only get 4400-4600 RPM on the 21 pitched prop and less on the 22. The Rapture 21 and the XL22 allow the boat to hit 40+ on the GPS while the Ballistic 21 would hit 37 mph. The rapture with 3 open vent holes has a very good holeshot even in the 21 pitch however I am lugging at only 4400 rpm WOT.

According to the Mercury guide I need a 19 or 18 and wondered if anyone has any recommendations. Does anyone have experience with Rapture or Ballistic props (I may be limited to Michigan Wheel products as they have my money already on the props). The Rapture has the advantage of durability, replaceable hub and vent holes and easier repairs. The ballistic I am not sure about getting any extra speed.

Thanks for any input.

PS – I also rigged a trolling motor to the transom and linked it to the boat’s steering and it works well and keeps the bow clear for fly fishing. I can post pictures of the setup if anyone likes.

Moe posted 05-04-2004 11:49 AM ET (US)     Profile for Moe  Send Email to Moe     
Something is wrong with your Black Max numbers...

27 mph at 5400 with 2.07:1 gearing and a 17 inch prop is 36% slip!

37 mph at 5400 with 2.07:1 gearing and a 17 inch prop is a much more likely, but still high, 12% slip.

I'd guess an 18 inch cupped stainless prop, which should perform about like a 19 inch, would put your max rpm at about 5000 rpm at 40+ mph with 8% or less slip.

You aren't "lugging" the engine at 4400 rpm. Lugging occurs at speeds under about 2-2.5 times idle speed, when the flywheel doesn't have sufficient inertia, for the given load, to damp speed-up/slow-down between power pulses. It results in snatching in the drivetrain between power pulses, even though you may not feel it in drivetrains with tighter gear lash.


TomT posted 05-04-2004 12:39 PM ET (US)     Profile for TomT  Send Email to TomT     
Thanks Moe,
Since I am dealing with available props that Michigan Wheel makes they have a 19 rapture. You mentioned and 18 cupped would be like a 19 but they make 17 and 19. Would you feel the 19 is the way to go?
Richard Quinlivan posted 05-04-2004 12:48 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     

I run the same engine on a 1975 Dauntless 17. It is a dual console like your ventura but is smaller and lighter. I have my Merc 125 mounted as high as it will go on the transom and have a merc High5 and a Laser 2. Both with 20 inch pitch. I get about 43 mph with the High 5 and 45 or so with the Laser 2. Since your boat is somewhat heavier I would think that a 18 inch pitch with a cupped SS prop would be about right.

TomT posted 05-04-2004 12:59 PM ET (US)     Profile for TomT  Send Email to TomT     
Thanks Richard,

What are the WOT rpms with those two props?

Moe posted 05-04-2004 01:18 PM ET (US)     Profile for Moe  Send Email to Moe     
I'd guess the 19" Rapture would put you at 4800 rpm max and the 17" would put you at 5200 rpm max, both within your operating range. Top speeds should be close with the 19" probably a little better, but not by much.


Wet Foot posted 05-10-2004 09:10 PM ET (US)     Profile for Wet Foot  Send Email to Wet Foot     
Hi Tom and Richard,

I'm trying to learn more about the Merc 125HP. Has it been a dependable engine? Does it idle roughly? Would you purchase it again? Does the shifting from 2 cylinder to 4 bother you? Have they made the engine for a long time? Any comments are appreciated.

Richard Quinlivan posted 05-10-2004 09:36 PM ET (US)     Profile for Richard Quinlivan  Send Email to Richard Quinlivan     
Wet Foot

My 125 was purchased new in 1995. That was not the first year it was available. There has been a lot of favorable dialog in this list re the 115 2 stroke that was tested on the new Nantucket model. Thd a 115 and the 125 are essentially identical. The 2 cylinder to 4 cylinder transition has been a non-issue for me. I go through it as I come onto plane and don't even notice it. I have replaced plugs and a waterpump and a starter in the 8 years I have owned it. It always works well.


Sal DiMercurio posted 05-10-2004 09:46 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Your not lugging that engine at 4400 rpms @ wot, your destroying it very, very, very, VERY fast.
If I'm not mistaken, that engine is rated to either 55 or 5600 rpms & 4400 rpms is a full 1,000 rpms below where it needs to be.
1,000 rpms equals 5 inches of pitch no matter how you cut the cake.
If your only getting 4,400 rpms with the 21 pitch prop, you need to drop down a full 5 inches pitch to either a 15 or 17p prop.
Who in the world suggest to you to try those high pitch props ?
Certainly don't know what the hell their talking about.
Forget whaqt maximum speed you can get & look at your tach, it's telling you that you are "WAY" over propped.
It dosen't work like ....I want alot of speed so I'll twist a 21p prop.
The prop dictates to the engine which prop can be twisted, not the engine saying, hell, I want a 21p.
You really should listen to what I'm saying or your going to be looking at a new engine to twist that 21p because that 125 is going to the happy hunting grounds real quick like if you continue running her with that much prop.
You don't just over prop at wot, it starts as soon as you put it in gear all the way through the rpm range, same as driving your truck in 4th gear all the time, up hills & starting out at the stop sign.
That engine is to small for anything over a cupped 17p.

TomT posted 05-13-2004 07:15 PM ET (US)     Profile for TomT  Send Email to TomT     
The merc dealer recalibrated my tach before the tests and said it was reading 200rpm too high and adjusted the fine adjust. Then I conducted the tests again and the results seemed like it is 600 rpm lower at wot than before. I took it back and they retested it and said it was within 100 rpm of the flywheel tach at 1000, 2000 and 3000 rpm and felt no need to further test the accuracy at 4000 and 5000.

Last weekend I put the 17p oem aluminum back on and maxed out at 4800-4900 rpm at 39-41 mph. According to Whaler they set that engine up originally with a 21p. The dealer said I should be running a 19p which I have a 19p cupped rapture to try next weekend.

The math seems to tell me that if I can truely (with an accurate tach) only turn 4850 on a 17p and want to be at 5250 (top of cruising range) with redline at 5500, then I would need a 14 or 15. This seems a far streatch from the factory set up of a 21.


Any other dauntless owners with a 125 out there?

TomT posted 05-13-2004 07:23 PM ET (US)     Profile for TomT  Send Email to TomT     
Regarding the shifting from 2-4 cylinder..

It idles smoothly and quietly and I am told the other two cylinders are normally set up to come on at about 1700 rpm but it is adjustable. There is no perceptible feeling when that happens because as you know that boat is far plowing a lot of water in that range. It is not a range you would ever cruise at being it is well below plane unless you were just trying to create wake for jet ski yahoos to launch their brainless bodies off of.

It has been a great engine. Next time I will look for a boat that all else being equal will have the max rated HP engine on it. It is a good runner, doesn't smoke and you can troll at idle if you troll fast. I installed an electric trolling motor on a gaerling kicker mount for fishing.

I have only replaced the impeller with 4 years and I think the water pump with over 400 miles on it.

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