c.2004 160 Dauntless OptiMax 75

Optimizing the performance of Boston Whaler boats
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c.2004 160 Dauntless OptiMax 75

Postby jaz761 » Mon Jul 02, 2018 6:27 pm

Hi. BACKSTORY: I am a new power-boater and have a 2006 (might be a 2004) Dauntless 16 that I bought used and refurbished from a boatyard. It has a new Optimax 75. I am aware that the Dauntless is a pretty heavy boat. I use the boat in the St. Lawrence River (1000-Islands of New York State) and at the edges of Lake Ontario. I was initially looking for a Montauk 17, but was convinced that the Dauntless would cut through chop a little better and be more stable. I frequently ferry some older folks around between islands and thought the slap would be too much for them. It's almost always windy and somewhat choppy, and there are very strong and somewhat unpredictable currents.

THE STORY: The c.2004 DAUNTLESS 16 feels like a very safe and stable boat, and I'm not looking for blinding speed though I would like it to handle a little more crisply. I am fairly new to powerboats. The c.2004 DAUNTLESS 16 seems to be stern-heavy. I feel that the c.2004 DAUNTLESS 16 takes forever to plane. [Engine speed must be] somewhere around 4.000-RPM before [the boat] seems to get up. [Boat speed has] never been more than about 26-MPH SOG with the wind helping me out. The guy at the boatyard more or less says, "they're all like that." The guy at the boatyard swapped in a different propeller which seemed to help very slightly. I can't remember what propeller is on there now.

REQUEST:I would like to hear any experiences of others [with a c.2004 DAUNTLESS 16 with a 75-HP OptiMax outboard].

Thank you.

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Re: c.2004 Dauntless 16 OptiMax 75

Postby Todd » Tue Jul 03, 2018 9:45 am

Jaz--I have a similar boat, a 2003 160 Dauntless. I know Boston Whaler put two engines on that boat, the standard was an Mercury 90 hp and the upgrade was the 115 hp version. The maximum hp recommended is 115. According to information from the manufacturers, here’s how the weights break down for my boat:

• 2003 Boston Whaler 16 Dauntless, boat only – 1500 lbs dry weight
• 2003 Mercury 115 Four Stroke – 386 lbs (your Optimax weighs 375 I believe)
• 45 gallons of gas @ 6.3 lbs per gallon = 284 lbs
• 4 adults average of 175 lbs = 700 lbs
• Cooler, anchors, gear, two marine batteries, etc. = 225 lbs

Fully loaded, my boat performs well with the 115 and a "whale tale" mounted on the motor. Some people prefer trim tabs but on a boat this size I don't know they are necessary. I can trim the motor to change the planing characteristics but typically don't need to. It planes off well and fully loaded tops out at 38 mph on smooth water. Plenty to blow all the hats off in the boat. Lightly loaded, I can reach 43 mph (GPS measured).

In my opinion, the boat is under-powered for your purpose. The Dauntless from that era was built like a tank. Extremely solid and built to last. Being heavy is not a bad thing in my opinion but you definitely need to provide enough power to operate the boat to it's full capabilities.

Not sure what you can do other than ask the boat yard to give you a credit toward a larger engine, or sell that OptiMax 75 privately to upgrade.

I would definitely look to get a a 115-HP engine--I am no expert on which is best. There are people on this forum who can advise you on the topic should you choose that path.

All that said, overall, the Dauntless is really my favorite boat. I've had everything from a kayak to a 38-foot Sea Ray, and I get more fun out this2003 160 DAUNTLESS, along with the ability to work on it myself. My Sea Ray required a computer diagnostician to do ANYTHING on it.

Best of luck with it, I use this forum for all sorts of questions about this boat and boating in general. You'll find it very helpful!

Take care,


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Re: c.2004 160 Dauntless OptiMax 75

Postby jimh » Tue Jul 03, 2018 10:00 am

According to Boston Whaler catalogue listings, a 2004 160 DAUNTLESS needs a minimum of 50-HP to get on plane. Minimum horsepower to plane means "minimum horsepower to plane satisfactorily with light load."

A "light load" means a minimum amount of added weight, fuel, gear, and minimum passengers. A "light load" is not a full fuel tank, four adults, coolers full of ice and food, and additional gear.

A Mercury OptiMax 75 meets those requirements. The OptiMax 75 is a 1.5-liter displacement three-cylinder engine, and it should be able to develop plenty of torque for getting your boat on plane. This same engine block was used for models with 115-HP rating.

You need to provide more information about the performance of your boat. You need to collect the following data:

ENGINE SPEED as measured by tachometer, include details of the tachometer, i.e., analogue or digital
BOAT SPEED as measured by GNSS receiver, include details of the GNSS receiver, i.e., with or without WAAS augmentation
Details of the propeller, i.e., brand, model, construction, number of blades, pitch, diameter, condition
Details of engine mounting height. See http://continuouswave.com/forum/viewtopic.php?f=7&t=739 for how to describe engine mounting height
Engine TRIM position during acceleration and when on-plane

Collect data for boat speeds from minimum planing speed to full-throttle. Present data in a table listing.

It is not unusual for the engine speed on a moderate v-hull planing hull boat to have to reach 4,000-RPM to achieve plane and remain on plane.

It is not unusual for boats that have an engine near the minimum recommended horsepower to be slow to get on plane or struggle to maintain plane at lower speeds when heavily loaded.

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Re: c.2004 160 Dauntless OptiMax 75

Postby Phil T » Tue Jul 03, 2018 10:52 am

If you review the archives, you will see there are many discussions regardling planing performance problems.


We can help you with tuning, rigging and prop.

We need additional information.

At what height is the engine mounted - See article --- http://continuouswave.com/forum/viewtopic.php?f=7&t=739

What is the prop? Make, model and size?

What is the wide open throttle (WOT) speed and rpm's for the current prop running the boat solo with no gear.
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1992 Outrage 17, 1992 Evinrude 115

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Re: c.2004 160 Dauntless OptiMax 75

Postby jaz761 » Tue Jul 03, 2018 1:54 pm

Thanks for the responses. I will do some reading of those links that were graciously provided and see what I can learn. Also will try to provide the data but honestly just use the boat to get around and am really not sure I would be able to measure all of that stuff with much accuracy. I'm heading up to where we keep the boat now so will see what I can figure out when I have it out tmrw.

The engine is mounted at its lowest position. It's a four-blade propeller. I am not sure on make and model but I have to clean out some weeds anyway so will look at it when I get in the water later today.

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Re: c.2004 160 Dauntless OptiMax 75

Postby GoldenDaze » Tue Jul 17, 2018 11:05 pm

I don't mean to sound too negative, but I have serious doubts that you will ever get adequate performance out of a 75 HP engine on that boat. You are right in all respects, the 160 Dauntless is a heavy boat for its size, and it is somewhat stern-heavy. Even with my 115 4-stroke, my 160 Dauntless (2003) can be reluctant to get up on plane with 4 adults and a full tank of fuel. She will, however, hit 41-42 MPH once she gets going. Over the production history of the boat, it was offered with 90 and 115 2-stroke Optimax engines, as well as 90 and 115 4-stroke engines. As far as I know, it was never offered from the factory with a 75.

To get the best you can from that motor, I think there are 3 things you can do:
1) Run a stainless 4-blade prop, for example something like the Trophy Plus. As Phil and others have pointed out, we can't recommend a specific pitch until we know more about your current prop and your engine speed at wide-open throttle.
2) Add a hydrofoil or trim tabs to help get the stern up when planing off.
3) Run with less fuel in the tank. That 45-gallon tank is simply huge for that little boat, and when full that's another 270 lb. in the boat, and that's a lot.

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Re: c.2004 160 Dauntless OptiMax 75

Postby Jefecinco » Thu Jul 19, 2018 10:26 am

Attempting to get a Dauntless 16 with a 75 HP engine to perform at a safe level in open waters is fruitless.

As the Optimax engine is new you should consider a replacement with a 115 HP engine while the Optimax has value. I believe it is unsafe to use a small open boat in open waters known for very rough conditions that can appear suddenly. A safe boat needs to be able to run from conditions it cannot handle.

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Re: c.2004 160 Dauntless OptiMax 75

Postby Lupi » Mon Jul 23, 2018 8:41 am

I have a 1999 160 DAUNTLESS. The only difference may be the notch in the hul. All the rest is similar. My 160 DAUNTLESS is equiped with the stock engine, the Mercury 90-HP two-stroke-power-cycle engine that still runs well.

I had to change the propeller. When I bought the 160 DAUNTLESS its propeller was an aluminium 16-pitch that was underperforming. I had good advice here in the forum, and I went to a stainless steel propeller that improved the performances.

For me, 90-HP is a minimum for this boat. Next engine, I will search for a 115-HP. I cannot imagine this boat running with a 75-HP engine. I think that loaded with more than four people, the 160 DAUNTLESS cannot plane with 75-HP.


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Re: c.2004 160 Dauntless OptiMax 75

Postby jaz761 » Mon Jul 23, 2018 4:20 pm

This is all sort of bad news. I have had the boat out in windy and choppy weather with a very full load of people and a totally full tank of fuel. It was sluggish but didn't seem unsafe. It seemed to plane fine, though felt maxed out at 25-MPH SOG on the GPS. Most commonly, we have about 700-lbs of people in the boat.

If I try to trade up for a larger motor, won't the boat be even heavier in the stern and more difficult to handle?

I'm not looking to set speed records here, but I am concerned about handling and safety.

I can't figure out what propeller I have on there and unfortunately I am five hours away from the boat most of the time.

My cousin has a Montauk 17 with a 90—can't recall if it's a two or four-stroke—and his boat handles much better, in my opinion. We purchased the boat from a trusted marina who has worked with my wife's family for many, many years. I don't feel that he would've sold us an unsafe boat or tried to pull any sort of fast one. Guess I'll give him a call.

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Re: c.2004 160 Dauntless OptiMax 75

Postby GoldenDaze » Mon Jul 23, 2018 10:10 pm

I wouldn't say that the seller "pulled a fast one", but the boat is certainly under-powered. Plugging my data into the speed calculator at http://www.go-fast.com/boat_speed_predictions.htm (2600 pounds, 115 hp, 42 mph, getting a hull constant of 200) and then substituting your engine at 75 hp gives a predicted top speed of 34 mph. That's actually not bad, and in fact I very rarely run my boat faster than that and typically cruise at about 25 mph, as speed at which the hull rides nicely. Given the prediction of 34 mph and your observation that you top out around 25 mph, you might not have enough pitch to your prop. A very low pitch will help you plane off quickly, at the expense of top speed. Increasing the pitch will increase your top speed, at the expense of planing off quickly. It's a delicate balancing act.

Using the prop calculator at http://www.go-fast.com/Prop_Slip_Calculator.htm and putting in the values for your motor (5500 rpm max and 2.33:1 gear ratio) and the predicted top speed of 34 mph and assuming 10% slip, it suggests a 15" prop.

Turning next to the Mercury prop selector at https://www.mercurymarine.com/en/us/propellers/selector and selecting "bay boat" and "acceleration" (you're going to need it) and a 2006 OptiMax 75 and "weight" and entering 1500 lbs enpty plus 800 lbs people and gear plus 45 gallons of fuel (total weight 2556 lbs) causes Mercury to suggest the following props:

- Black Max (aluminum 3-blade) 13.25 x 15
- Black Max (aluminum 3-blade) 13.25 x 16
- Spitfire (aluminum 4-blade) 13 x 17
- Vengeance (stainless 3-blade) 13.125 x 16
- Vengeance (stainless 3-blade) 13.125 x 17

Honestly I am not in love with any of those suggestions, thinking instead that you would benefit most from a stainless 4-blade prop like the Trophy Plus. However, the Trophy Plus (and the 5-bladed Hi-Five) are only available down to 17" pitch, and I think the direct comparison with my 115-powered boat that suggests a 15" pitch is probably closer to the mark than the Mercury prop selector. I've found the Black Max (I have a 13" pitch for skiing) to be pretty unimpressive overall, it ventilates easily and does not like to shake off the air. The Vengeance (I had a 16" pitch before I switched to my 17" Trophy Plus) has a lot of blade cupping and didn't accelerate well for me (though it had great top-end speed), but a 14" or 15" Vengeance might work okay for you. The Spitfire I'm not familiar with at all, but in general you probably want the performance of a stainless prop. At this point I've exhausted the available Mercury props, and while there are other brands, someone else will have to make suggestions on them.

The big difference with 75 hp is that you don't have a lot of excess power available to push the boat up onto plane. Workable? With the right prop, probably. Ideal? Not really. Acceptable? That's a question only you can answer.
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Re: c.2004 160 Dauntless OptiMax 75

Postby GoldenDaze » Fri Jul 27, 2018 1:38 pm

and while there are other brands, someone else will have to make suggestions on them

But since nobody has yet done so, a little web research suggests the following possibilities:

Evinrude/BRP Rogue 13.25 x 15
Yamaha Talon SS4 (can't easily find available sizes online)
PowerTech NRS4 13.25 x 15

One or more of these might be a good fit for you. All of them are expensive.
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