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Author Topic:   c.1985 Mercury Thunderbolt 40-HP

mitch13 posted 01-22-2007 09:57 PM ET (US)
My son recently acquired a c.1985 Mercury "Thunderbolt" 40-HP. What is the maximum rated engine speeed in RPM? My son can only get 4,200-RPM from the motor. He just installed a Doel-fin on it, and, although it gets on plane much quicker, the WOT engine speed dropped to 4,000-RPM.
What propeller [should be used with a c.1985 Mercury Thunderbolt 40-HP motor on a 16-foot aluminum boat]?
Thanks,
mitch
jimh posted 01-22-2007 10:10 PM ET (US)
What is the total weight of the boat, the motor, and all the gear and people?
What is the gear ratio of the c.1985 Mercury Thunderbolt 40-HP motor?
bms1939 posted 01-22-2007 10:34 PM ET (US)
2.0 gear ratio 4500-5500, 1.83 ratio 5000-5500. If you dont have power trim I would move the pin up a notch or two, if the motor is trimmed too far down it will not reach maximum rpm. The doel fin lifts the stern wich is probably pushing the bow down into the water even more than it was without it. To reach max rpm you usually need to be fully trimed up, if you dont have power trim you wont be able to run the boat fully trimmed. You will have to find a happy medium to run the boat without sacrificing the boats ability to plane and to reach top speed. With power trim you can have the best of both worlds.
mitch13 posted 01-23-2007 12:46 AM ET (US)
Thanks, bms, exactly what I was looking for. I'll pass it along to my son.
Tom W Clark posted 01-23-2007 01:01 AM ET (US)
If the fin is affecting top speed at all, it is mounted too low. Raise the motor so it is ABOVE the surface of the water when the boat is on plane.

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Author	Topic: c.1985 Mercury Thunderbolt 40-HP
mitch13	posted 01-22-2007 09:57 PM ET (US) My son recently acquired a c.1985 Mercury "Thunderbolt" 40-HP. What is the maximum rated engine speeed in RPM? My son can only get 4,200-RPM from the motor. He just installed a Doel-fin on it, and, although it gets on plane much quicker, the WOT engine speed dropped to 4,000-RPM. What propeller [should be used with a c.1985 Mercury Thunderbolt 40-HP motor on a 16-foot aluminum boat]? Thanks, mitch
jimh	posted 01-22-2007 10:10 PM ET (US) What is the total weight of the boat, the motor, and all the gear and people? What is the gear ratio of the c.1985 Mercury Thunderbolt 40-HP motor?
bms1939	posted 01-22-2007 10:34 PM ET (US) 2.0 gear ratio 4500-5500, 1.83 ratio 5000-5500. If you dont have power trim I would move the pin up a notch or two, if the motor is trimmed too far down it will not reach maximum rpm. The doel fin lifts the stern wich is probably pushing the bow down into the water even more than it was without it. To reach max rpm you usually need to be fully trimed up, if you dont have power trim you wont be able to run the boat fully trimmed. You will have to find a happy medium to run the boat without sacrificing the boats ability to plane and to reach top speed. With power trim you can have the best of both worlds.
mitch13	posted 01-23-2007 12:46 AM ET (US) Thanks, bms, exactly what I was looking for. I'll pass it along to my son.
Tom W Clark	posted 01-23-2007 01:01 AM ET (US) If the fin is affecting top speed at all, it is mounted too low. Raise the motor so it is ABOVE the surface of the water when the boat is on plane.