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I put my new-to-me Dauntless 13, with its 1997 Mercury 40 HP two stroke, in the water for the first time yesterday. I was playing around with the trim to cut down on porpoising and noticed that at WOT it was running about 6000 RPM. This was basically the upper limit of the gauge on the console so it seemed a bit high. What is the normal range for revs at full throttle?

This is my first boat, so obviously I don't have much experience. Thanks for your help!

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I am not sure if this correct but if you look at this web sight, I hope this helps.

http://www.piranha.com/enginelist.php...ubmit.y=11

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The 1997 Mercury 40 HP two stroke outboard has a recommended WOT engine speed range of 5000-5500 RPM and uses a 2:1 gear ratio.

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If you are hitting 6000 RPM you are clearly underpropped.

Porpoising is often caused by an outboard mounted too low and propellers that do not grip well.

Exactly what propeller is on your 1997 Mercury 40?

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Thanks for the replies all!

Tom, I will have to check later today and report back.

Could high RPMs be an effect of trim not being set exactly right (too high or too low)?

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By itself, no.

When you're checking the propeller, see how high on the transom the motor is mounted. My suspicion is that it is set all the way down.

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The propeller has the following numbers: 48 816704 A40 13P

The motor mount bracket that straddles the transom has a bolt through the second highest hole (so the motor is one hole higher than the lowest setting).

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OK, that propeller is the 10-1/2" x 13" Mercury Black Max aluminum ("Large Blade Area") three blade propeller.

The motor is mounted "One Hole Up", not a bad place to be.

To relieve the porpoising and bring your engine RPM to within the recommended specifications I recommend the motor be raise one more set of bolt holes and a good stainless steel propeller be fitted with increased effective pitch.

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Thanks for the info. Looks like a ~200 rpm reduction for every increase in pitch by 1"...is that the rule of thumb?

Could I go up slightly in diameter also (which I think would lower rpms at WOT)?

I found the Powertech ILC3 in a 11.25 x 15...is this a good choice? What are some other recommendations for SS in the ultra low cost range?

Alternatively, could I just choose not to run the engine at WOT? If I stick to a max of 5500 rpm is that causing any undue stress or working the engine?

Again, thanks for your help Tom!

P.S. DKroger, you live very close to me. Your boat looks fantastic!

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Be careful of rule-of-thumb's and how you interpret them, they often break down. In your case it does not hold true at all. A one inch change in pitch will result in an inverse change in engine speed of about 400 RPM. Furthermore, that only holds true if you are changing the pitch of the specific model of propeller you already have, in this case the Large Blade Area Black Max aluminum. Switching to a different model of propeller can result in very different RPM for a given pitch.

What is your definition of "ultra low cost range?"

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Ideally, less than $250. The cheaper the better. We are just looking to get out on the water on a nice day and cruise around. No skiing, no high performance needed. May troll for salmon and drop some crab pots, but mainly just out for a Sunday afternoon stroll...

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I recommend the 10-1/2" x 13" the Stiletto Triad. It can be had for $200, delivered with hub kit.

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Tom, Would you recommend the Triad 3.5 or the Triad 1 or 2?
What are the major differences between them ( I see that the 3.5 is a two piece)...?

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Triad I and Triad II are the old model names. Triad 3.0 and Triad 3.5 are the new model names. They went to the new names to make it more clear which prop you need.

3.0 (I) means the propeller has a 3 inch diameter hub and fits motors with 3 inch diameter gearcases (usually 9.9 to 30 HP)

3.5 (II) means the propeller has a 3-1/2 inch diameter hub and fits motors with 3-1/2 inch diameter gearcases (usually 35 to 70 HP)

Your Mercury 40 uses a 3-1/2 inch gearcase so you want the Triad 3.5.

If you find a vendor selling the Triad II, it may be old stock from as long ago as four years, or they may still simply one using the old name; even the factory does this sometimes and it makes things very confusing.

The old props used a pressed-in rubber hub, the new ones use the field-replaceable Guardian hub kits so you can use one propeller on any brand of outboard.

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Hello again Pickles1107, just an F.Y.I.. The Dauntless 15's have a tendency to porpoise, the hulls (running surface) design is very sensitive to excessive outboard (positive) trim. It took a few boat outings for me to figure out which O/B trim setting worked best.

I suspect that the Dauntless 13's may also suffer from this situation. An additional factor to consider is surf conditions (light to moderate chop, boating into or with the wind) and finally weight distribution. Allocating to much weight to the stern area will cause more bow rise which in turn will prompt the porpoising condition and only become more exaggerated by O/B trim and settings (motor mounted to low or high on transom).

Your top side fuel tank is already at the stern (roughly 7 lbs per gal. of gas) your battery may also be located at a corner in the stern area (roughly 15 lbs or more depending on size) plus your weight and a passenger seated on your helm bench. Unless you are using the console for what ever storage you can, I would suggest relocating the battery to that location if possible for the sake of moving some weight forward.

As Tom previously described, improper prop and outboard height setting will also be a contributing factor for porpoising and boats lack of good performance.

Food for thought...see ya.

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Hi Miami,
Thanks for the notes. The battery is currently located in the console. I found that trimming down eliminated the porpoising effect.

Tom,
The seller also included an additional prop. After checking it out, it is the 10.5" x 15" Black Max prop which I suspect is the original.

Would the Stiletto Triad II in a 13" pitch have roughly the same WOT throttle rpms as the Black Max 15" pitch?

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I wouldn't assume so but try the 15" BlackMax and let us know how it does. It might be a useful data point.