Need confirmation on 90hp actual hp rating

[FBPirate95]

I have a 1985 90hp Johnson. Does anyone know if this hp rating is at the flywheel or at the prop? The reason I ask is I'm repowering a 17' boat. Whe I looked at the max rating it says its rated to 150hp. This got me a little worried that I would be under powered. However at lunch today I was in a local boat shop and the owner said not to worry. Due to my boat being a '79 the hp rating listed is for hp at the flywheel, and my outboard is rated at the prop. He said taking into account the hp loss from fly wheel to the prop is about 1/3, the 150 rating is more like 100, and my 90hp should be just fine. Can anyone confirm if he's right???

 

[bob johnson]

Re: Need confirmation on 90hp actual hp rating

1985 seems kinda early for the HP rating at the PROP... I would have thaought that prop rating change occured closer or into the 90's

bob

 

[tashasdaddy]

Re: Need confirmation on 90hp actual hp rating

i think it was 1986 the change was made. the hp loss is only about 10%, not 1/3. jet lower units loose 1/3 in their interchange.

 

[1946Zephyr]

Re: Need confirmation on 90hp actual hp rating

Isn't it something like 82.1369375876096847 hp?

Just kidding.

 

[bob johnson]

Re: Need confirmation on 90hp actual hp rating

i think it was 1986 the change was made. the hp loss is only about 10%, not 1/3. jet lower units loose 1/3 in their interchange.

my old codger mechanic, has told me that some motors are over rated and some are under rated, as well as they are allowed to be within a certain % and still meet the requirement of labeling the motor the HP they call it.

for example he had told me that the 1988 70 hp motor i had picked up really was barely putting out 64 HP!!! in top form!!

I was also told that many times a larger cubic inch motor really has more power than a less cubic inch motor of the same HP label

might be felt torque though, not actually peak HP.....it would seem

bob

 

[FBPirate95]

Re: Need confirmation on 90hp actual hp rating

Isn't it something like 82.1369375876096847 hp?

Just kidding.

If you just rounded that last didget to a seven your calculation is worthless to me

 

[JB]

Re: Need confirmation on 90hp actual hp rating

It doesn't matter. Either way that 90 is not enough outboard for that boat. Your friend has greatly exaggerated the difference in HP, and the difference in torque in the real world operating range is even less. He is using creative math to try to justify the 90. Forget it.

The rating of your boat has to do with several factors other than actual hp of the outboard. . . things like weight and low end torque. A JohnnyRude big V6 150 before and after the rating change has the same weight and very close low end torque. Consider the rating good for modern, as well as older, 150HP engines.

Using the rule of thumb that at least 70% of rated max HP is needed to get satisfactory performance from a planing hull you need at least 105HP.

I wouldn't use anything less than a 115HP on that boat.

 

[bob johnson]

Re: Need confirmation on 90hp actual hp rating

It doesn't matter. Either way that 90 is not enough outboard for that boat.

Using the rule of thumb that at least 70% of rated max HP is needed to get satisfactory performance from a planing hull you need at least 105HP.

I wouldn't use anything less than a 115HP on that boat.

that CANT apply to bass boats!!!!!

they run 300 hp... and I have a hard time believing those boats wont plane just fine with 115 hp motors OR LESS!!



bob

 

[FBPirate95]

Re: Need confirmation on 90hp actual hp rating

It doesn't matter. Either way that 90 is not enough outboard for that boat. Your friend has greatly exaggerated the difference in HP, and the difference in torque in the real world operating range is even less. He is using creative math to try to justify the 90. Forget it.

The rating of your boat has to do with several factors other than actual hp of the outboard. . . things like weight and low end torque. A JohnnyRude big V6 150 before and after the rating change has the same weight and very close low end torque. Consider the rating good for modern, as well as older, 150HP engines.

Using the rule of thumb that at least 70% of rated max HP is needed to get satisfactory performance from a planing hull you need at least 105HP.

I wouldn't use anything less than a 115HP on that boat.

Thanks for the info. Even though it may not be enough....that's what its going to get. For now anyway. All I need to do this summer is push me around the river some so I can lose some bait.

 

[surrender]

Re: Need confirmation on 90hp actual hp rating

Dont worry. Put the 90 on and enjoy. Just prop it right you'll be fine. My 1987 Mckee craft is rated for 160 hp. I run a 1996 88 special. Runs 33 mph and i havn't finished tweeking it. With only 35 gallons of gas i wouldn't want a 160 hp on this boat. Could only run for a few hours and would need to re fuel.

 

[Dhadley]

Re: Need confirmation on 90hp actual hp rating

The 1985 90 is rated at the prop. It's basically identical to a 1984 115.

How well it will or will not work depends largely on hull design and set up.

 

[FBPirate95]

Re: Need confirmation on 90hp actual hp rating

The 1985 90 is rated at the prop. It's basically identical to a 1984 115.

How well it will or will not work depends largely on hull design and set up.

Thanks for the info. Is there anywhere this type of info is documented?

 

[tashasdaddy]

Re: Need confirmation on 90hp actual hp rating

the 90 will push that boat just fine. trust me.

 

[FBPirate95]

Re: Need confirmation on 90hp actual hp rating

the 90 will push that boat just fine. trust me.

I'm sure it can, I just like learning. I'm not looking to go high speeds or be skiing on it, so it will be perfect. Especially since its the only one I have...lol.

 

[rolmops]

Re: Need confirmation on 90hp actual hp rating

I did read a few years ago that the change was made in 1982.
The maximum allowable horse power on a boat is not what it needs to move very well.Other concerns like the difference between fiberglass and aluminum and flat bottom or deep V were never really addressed when these maximum hp rules were made.
To give an example:
My 1984 19 foot Sylvan is rated for 145 HP maximum,but the factory supplied them with 75 horse Mercuries and going reverse even at very slow speeds would make the water slosh over the stern of the boat due to the weight of the engine.However,this 75 horse merc makes that 19 footer move at 38 to 40 mph(GPS) so there is no real need for this boat to carry anything larger than a 75 horse.
Maximum allowable horse power has a lot to do with beam ,stern width and height and overall length but not so much with what the boat actually needs to move very well according to its needs.

 

[Dhadley]

Re: Need confirmation on 90hp actual hp rating

OMC began the changeover in 1985 with some models. The V4 crossflows were one of those re-rated in 1985.