CONTINUOUSWAVE

I am ready to purchase a new outboard for my 1988 Montauk 17, choosing between Yamaha F70 (253-lbs) and F90 ( 353-lbs) engines. For 2022 my performance goal is to be able to pull wakeboarders and water skiers.

An important factor: I'll operating the boat on lakes at 3,000 to 4,000-feet above sea level. The air at those altitudes has less oxygen so the engine power is reduced. Also the fresh water has reduced density compared to saltwater. All aboard will be under 200-lbs body weight. There will be at least three people on the boat.

Q1: Between the Yamaha F70 and F90, which engine do you recommend for my application?

If the F70 had enough power to do this, it'd be an easy decision for me.

I know of one 90-HP in stock. I have not done research on [the availability of] an F70
--

Ken

I also know that changing the pitch of the propeller can make a difference [in performance].

Q2: how does changing propeller pitch affect performance?

kdellaero@yahoo.com wrote:Q2: how does changing propeller pitch affect performance?

A propeller with smaller pitch will be easier for the engine to turn.

A propeller of smaller pitch will generally allow the engine to accelerate faster.

A propeller of smaller pitch will allow the engine to accelerate to a higher full-throttle engine speed.

A propeller of smaller pitch will reduce the boat speed at full-throttle.

An old rule of thumb is that a reduction in propeller pitch of 2-inches will give an increase in engine speed at full throttle of about 400 to 500-RPM.

kdellaero@yahoo.com wrote:An important factor: I'll operating the boat on lakes at 3,000 to 4,000-feet above sea level.

Many modern engines have a barometric pressure sensor that provides input to the engine control module to improve calculation of the fuel-air mixing ratio. Engines with that feature do not tend to have a reduction in power output with higher elevation or the degree of reduction in power output with higher elevation is less than would be expected with an engine that has no compensation built-in for altitude. For moderate increase in altitude there may be very little reduction in power output in a modern engine.

The best advice regarding operating an outboard engine on the specific lakes you plan to be using may be to contact local boaters and get their experience with the particular engines you plan to be buying and operating on those specific lakes.

Having advised many owners at elevation, you will need the 90hp.

At 4,000 feet your 90hp engine will only produce 79.3hp.

For those interested the formula is: Horsepower Loss = Original Engine power x Altitude/1000 * 0.03

Even at sea level you will need 90hp with 3 persons in the boat pulling a tube or skier.

As for propeller, you will need to do some testing with different models and pitches to get one that works for you.

Note that outboard engine inventory is scarce. Engines in stock are being marked up several thousand dollars.

A 90-HP engine from any mfg are going for $14,000 to $16,000 installed. A 70-HP is 12,000 to $14,000 installed.

Warning: if you order an engine, lock the price in at the time of order in writing .Expect to pay a deposit. Many are getting price increases after months of waiting at the time of delivery that are far higher than quoted.