Overrevving/Underrevving & Tach question 85hp merc

argonia1021 · 08-31-2009, 10:44 AM

I had to sell my big 18 foot tri hull to a friend so i could make some auto repairs. my buddy is also a carpentar so my mom and dad is having his do some home repairs and in return gave him their much smaller 14 foot long trihull. the 18 footer had a 85 HP merc on it the smaller boat rated for 80ho had a 75hp chrysler on it. Yesterday he decided to swap engines because the 18 footer had almost no towing cabilities because of the sheer size of the boat. So now he has a 14 footer with a 85hp mercury on it with a 19 pitch prop. Okay cool we took her out for a spin and she ran great instant hole shot top speed of about 35mph. However this 85hp merc on the smaller boat sounds louder like its getting more revs. neither boat has a tach on it and i know we need to to get one on. However until we do what is your guys opinion, was this motor getting underrevved on the bigger boat at WOT or is it getting overrevved on the smaller boat at WOT? The engine does seem warmer to touch but is not hot to touch. quick specs on the big boat and small boat. big boat was 18 feet long and a little over 6 feet wide trihull smaller boat is about 14 feet long and about 5 feet wide trihull.
Also do you have to use a tach specifically made for boats (like from bass pro) or can you just go to the auto parts store and buy one from there and use it.

bradh · 08-31-2009, 11:08 AM

You have to use a tach designed for a 2 stroke marine engine. Most likely you need more pitch and you are over revving the engine. If your hull is rated for 80hp and you have an 85 hp engine on it you should probably put the 75hp chrysler back on.

argonia1021 · 08-31-2009, 11:16 AM

i told him we are probably over revving the poor thing that the easiest way to fix it is to just go to a 21 pitch prop. However if it was getting underrevved on the big boat and its proper revs are what its attaining on the small boat then i dont want to underev the motor anymore it did always have a hard time getting the big boat up on a plane even with a hydrofoil. And my apologies i meant to say small boat is rated for 90HP. shouldve proofread my post. but without a tach (we have every intention on getting one eventually) what is your guys opinion on if im over revving now or under revving before?

xtreme76 · 08-31-2009, 01:32 PM

There is no way anyone can tell you if your rpms are to high or low based on information provided. However if you know the gear ratio of the motor and your gps speed you could use a prop pitch calculator, guess the slip, and that would give you a GUESS as to rpms. I would suggest getting a tach and taking it easy until you do.
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Propeller Calculator / Prop Calculator

08-31-2009, 10:44 AM	#1 (permalink)
argonia1021 argonia1021 is Wishin' I was Fishin' and Wishin' the fish would bite!! Member Join Date: Jun 2007 Posts: 171 Thanks: 4 Thanked 23 Times in 9 Posts	Overrevving/Underrevving & Tach question 85hp merc I had to sell my big 18 foot tri hull to a friend so i could make some auto repairs. my buddy is also a carpentar so my mom and dad is having his do some home repairs and in return gave him their much smaller 14 foot long trihull. the 18 footer had a 85 HP merc on it the smaller boat rated for 80ho had a 75hp chrysler on it. Yesterday he decided to swap engines because the 18 footer had almost no towing cabilities because of the sheer size of the boat. So now he has a 14 footer with a 85hp mercury on it with a 19 pitch prop. Okay cool we took her out for a spin and she ran great instant hole shot top speed of about 35mph. However this 85hp merc on the smaller boat sounds louder like its getting more revs. neither boat has a tach on it and i know we need to to get one on. However until we do what is your guys opinion, was this motor getting underrevved on the bigger boat at WOT or is it getting overrevved on the smaller boat at WOT? The engine does seem warmer to touch but is not hot to touch. quick specs on the big boat and small boat. big boat was 18 feet long and a little over 6 feet wide trihull smaller boat is about 14 feet long and about 5 feet wide trihull. Also do you have to use a tach specifically made for boats (like from bass pro) or can you just go to the auto parts store and buy one from there and use it. __________________ Fish ON!!

08-31-2009, 11:16 AM	#3 (permalink)
argonia1021 argonia1021 is Wishin' I was Fishin' and Wishin' the fish would bite!! Member Join Date: Jun 2007 Posts: 171 Thanks: 4 Thanked 23 Times in 9 Posts	i told him we are probably over revving the poor thing that the easiest way to fix it is to just go to a 21 pitch prop. However if it was getting underrevved on the big boat and its proper revs are what its attaining on the small boat then i dont want to underev the motor anymore it did always have a hard time getting the big boat up on a plane even with a hydrofoil. And my apologies i meant to say small boat is rated for 90HP. shouldve proofread my post. but without a tach (we have every intention on getting one eventually) what is your guys opinion on if im over revving now or under revving before? __________________ Fish ON!!

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08-31-2009, 11:08 AM	#2 (permalink)
bradh bradh is driving a sweet '72 C10 Member Join Date: Apr 2008 Location: Columbus Posts: 443 Thanks: 245 Thanked 204 Times in 101 Posts	You have to use a tach designed for a 2 stroke marine engine. Most likely you need more pitch and you are over revving the engine. If your hull is rated for 80hp and you have an 85 hp engine on it you should probably put the 75hp chrysler back on.

08-31-2009, 01:32 PM	#4 (permalink)
xtreme76 xtreme76 is bummed open water is wrapping up and trying to get excited about hunting. Member Join Date: Jun 2008 Location: Central Ne Volunteer Hunter Education Instructor Posts: 129 Thanks: 68 Thanked 32 Times in 26 Posts	There is no way anyone can tell you if your rpms are to high or low based on information provided. However if you know the gear ratio of the motor and your gps speed you could use a prop pitch calculator, guess the slip, and that would give you a GUESS as to rpms. I would suggest getting a tach and taking it easy until you do. - Propeller Calculator / Prop Calculator