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Author Topic:   Mercury 60 vs 90

kswoods posted 08-18-2007 10:00 PM ET (US)
I have a 2005 Bennington 20' pontoon boat with 25 inch pontoons. It has a 2005 Merc 60 HP Bigfoot 4 stroke engine/
Typical cruising performance is fine. I can tube lighter people with only 1-2 onboard. I’d like to get a bit better performance to tube adults. I am interested in upgrading this to a Merc 90HP motor.
I’d like to know if people think I will see an increase in performance sufficient to make the move. I am getting quotes of about $4,600 with trade in, which doesn’t seem too bad.
Thanks
Kevin

Moe posted 08-19-2007 07:21 AM ET (US)
What size prop do you have on the 60HP (diameter and pitch)? What is the maximum rpm shown on the tach when pulling a tube now?
myakka posted 08-19-2007 07:38 AM ET (US)
Kevin
Use the $4600 to buy a good used 15' Whaler with max hp. Keep the pontoon boat as the movable floating party dock. Tow the 15 with the pontoon boat to the center of your lake drop anchor and let the fun begin.
Mike
jimh posted 08-19-2007 07:50 PM ET (US)
Use Crouch's Calculator to predict the improvement in performance.
Speed increases with horsepower exponentially to the 0.5 power. Therefore:
(90/60)^0.5 = 1.22
Speed will increase by a factor of 1.22.
lordswork2007 posted 08-19-2007 10:41 PM ET (US)
I would be surprised if the exponent were really .5, if the boat is planing cleanly. Once well above the transition to plane, frictional resistance predominates. Frictional resistance increases as the square of the speed. Power is force times speed, so power increases as the cube of the speed. In practice, boats may decrease wetted area with increasing speed, to a point, but I bet there are few boats that actually increase their speed by the power to the .5. I would bet its more like .4 or .38. And that is NOT considering the increased weight of the larger powerplant.
I saw recently that aero drag at 50 mph is nearly 1/3 the total. Aero drag is clearly a square term, and power to overcome it is a cube with speed. So...
I wonder if there is good data on this.
Tohsgib posted 08-20-2007 11:21 AM ET (US)
Save your money...adding 30hp will do nothing for you. Find someone or dealer to give you a test ride in that combo you seek, I bet all it does is rev higher and maybe goes a couple mph faster...they are not performance boats. Friend has an 18 with a 30, we dropped a 70 on it and it did not do much except cavitate more and push the bow into the water.

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Author	Topic: Mercury 60 vs 90
kswoods	posted 08-18-2007 10:00 PM ET (US) I have a 2005 Bennington 20' pontoon boat with 25 inch pontoons. It has a 2005 Merc 60 HP Bigfoot 4 stroke engine/ Typical cruising performance is fine. I can tube lighter people with only 1-2 onboard. I’d like to get a bit better performance to tube adults. I am interested in upgrading this to a Merc 90HP motor. I’d like to know if people think I will see an increase in performance sufficient to make the move. I am getting quotes of about $4,600 with trade in, which doesn’t seem too bad. Thanks Kevin
Moe	posted 08-19-2007 07:21 AM ET (US) What size prop do you have on the 60HP (diameter and pitch)? What is the maximum rpm shown on the tach when pulling a tube now?
myakka	posted 08-19-2007 07:38 AM ET (US) Kevin Use the $4600 to buy a good used 15' Whaler with max hp. Keep the pontoon boat as the movable floating party dock. Tow the 15 with the pontoon boat to the center of your lake drop anchor and let the fun begin. Mike
jimh	posted 08-19-2007 07:50 PM ET (US) Use Crouch's Calculator to predict the improvement in performance. Speed increases with horsepower exponentially to the 0.5 power. Therefore: (90/60)^0.5 = 1.22 Speed will increase by a factor of 1.22.
lordswork2007	posted 08-19-2007 10:41 PM ET (US) I would be surprised if the exponent were really .5, if the boat is planing cleanly. Once well above the transition to plane, frictional resistance predominates. Frictional resistance increases as the square of the speed. Power is force times speed, so power increases as the cube of the speed. In practice, boats may decrease wetted area with increasing speed, to a point, but I bet there are few boats that actually increase their speed by the power to the .5. I would bet its more like .4 or .38. And that is NOT considering the increased weight of the larger powerplant. I saw recently that aero drag at 50 mph is nearly 1/3 the total. Aero drag is clearly a square term, and power to overcome it is a cube with speed. So... I wonder if there is good data on this.
Tohsgib	posted 08-20-2007 11:21 AM ET (US) Save your money...adding 30hp will do nothing for you. Find someone or dealer to give you a test ride in that combo you seek, I bet all it does is rev higher and maybe goes a couple mph faster...they are not performance boats. Friend has an 18 with a 30, we dropped a 70 on it and it did not do much except cavitate more and push the bow into the water.