82 90 hp merc is not 90 hp

[Lone Duck]

I have a 90hp merc ser# 7158587 I was told it was built in canada in 1981 and is called a 1982. apparently they measured 90hp at flywheel. At the prop you lose 15% which makes it 76.5 hp I have a jet on it which loses another 15% which makes it 65 hp in the water. Is this correct? If so is there anything I can do to the engine to give me another 20 hp? The boat is a flat bottom wooden 23 footer I built from Glen-L plans. It floats at rest in 6" of water and I have not touched bottom yet in 4" of water at 3/4 throttle. The boat weighs about 1200 lb loaded ready to go. I love sneaking up the shallow back channels of the river, but would like a little more speed . I am burning 5 gal phr. at 3/4 throttle. I am a retired small engine mech. but have had little time on outboard engines. Some advice would be appreciated. Thanks

 

[Chris1956]

Re: 82 90 hp merc is not 90 hp

You are essentially correct about how the HP is measured and the losses. The powerhead HP measurement should be within +- 10%. Merc had the reputation to derate their motors years ago.

You can make up for the jet losses if you switch it with a prop lower unit. BTW - A Jet version of my 150HP Johnny is called a 105 jet. Your losses due to the jet drive may be greated than you think.

 

[Lone Duck]

Re: 82 90 hp merc is not 90 hp

You are essentially correct about how the HP is measured and the losses. The powerhead HP measurement should be within +- 10%. Merc had the reputation to derate their motors years ago.

You can make up for the jet losses if you switch it with a prop lower unit. BTW - A Jet version of my 150HP Johnny is called a 105 jet. Your losses due to the jet drive may be greated than you think.

Thanks for your input but a prop won't run in 4" of water. Guess it's a bigger hp motor or forget about more speed.

 

[steelespike]

Re: 82 90 hp merc is not 90 hp

I think you are over estimating the losses on hp rating its probably closer to 5% Not hardly enough to tell without a speedo. As far as measurable differance its just the pump percentage.You may want to be sure the pump is in excellent shape if your running in 4 inches of water it may need a rebuild.Also be sure the motor is sound and in good tune.Check the compression.Should at least 100psi.
5 gallons per hour is not bad for a 90 at 3/4 speed.wot is about 9 gph.

 

[Lone Duck]

Re: 82 90 hp merc is not 90 hp

I think you are over estimating the losses on hp rating its probably closer to 5% Not hardly enough to tell without a speedo. As far as measurable differance its just the pump percentage.You may want to be sure the pump is in excellent shape if your running in 4 inches of water it may need a rebuild.Also be sure the motor is sound and in good tune.Check the compression.Should at least 100psi.
5 gallons per hour is not bad for a 90 at 3/4 speed.wot is about 9 gph.

Jet pump is in excellent shape I have all the data on how to maintain them. Come the end of April I plan on doing all the checks on the engine,just aquired a manual . Thanks all for your help.