2 stroke vs 4 stroke prop differences

[ed72vette]

I just had to re-power my 18' Sailfish CC. The 19 year old Yamaha 90 hp 2 stroke bit the dust. The prop on the 2 stroke was a Power Tec 3x13.5x15 which truned 5200 rpm at wot, which was right in the power range. I re-powered it with a Yamaha 90 hp 4 stroke. I used the prop off the 2 stroke and the engine wants to go over 6000rpm which indicates it needs more pitch. Is there a relationship to how much bigger prop is needed going from a 2 stroke to 4 stroke with the same hp on the same boat?
Ed

 

[dingbat]

I just had to re-power my 18' Sailfish CC. The 19 year old Yamaha 90 hp 2 stroke bit the dust. The prop on the 2 stroke was a Power Tec 3x13.5x15 which truned 5200 rpm at wot, which was right in the power range. I re-powered it with a Yamaha 90 hp 4 stroke. I used the prop off the 2 stroke and the engine wants to go over 6000rpm which indicates it needs more pitch. Is there a relationship to how much bigger prop is needed going from a 2 stroke to 4 stroke with the same hp on the same boat?
Ed

You can’t assume they both make the same HP.

Actual output can be plus or minus 10% of the “rated” HP.

Example: The 2 stroke could be 81 hp and the 4 stroke could be 99 hp.

Extreme comparison but possible with the rating system

 

[racerone]

Well----That would depend on the gear ratios.----Are they the same on these motors ?

 

[ed72vette]

You can’t assume they both make the same HP.

Actual output can be plus or minus 10% of the “rated” HP.

Example: The 2 stroke could be 81 hp and the 4 stroke could be 99 hp.

Extreme comparison but possible with the rating system

Same 2.0 gear ratio

 

[QBhoy]

All about the gear ratios. Are you dead sure they are both the same ratio ? Be unusual perhaps.
Apart from that. Are you using the old tacho from the 2 stroke ? Was that perhaps a 3 cylinder and the new engine a 4 ?

 

[jimmbo]

Gear Ratio is only part of it, WOT Rpm range is also a factor, as is Torque, when it comes to what differences in Props

 

[fatlenny]

are you positive you have the motor height set properly and not cavitating? The replacement motor has same length leg? What is your speed when hitting the power range of this new motor?

 

[ed72vette]

are you positive you have the motor height set properly and not cavitating? The replacement motor has same length leg? What is your speed when hitting the power range of this new motor?

Both engines have the same length shaft 20". The yamaha dealer set the motor in the same location as the 2 stroke.

 

[harmanoff]

bet your tach needs adjusted to the proper setting for the new 4 stroke motor

 

[ed72vette]

bet your tach needs adjusted to the proper setting for the new 4 stroke motor

The dealer switched to the 4 stroke setting on the Yamaha tach

 

[QBhoy]

So. Thinking about it. And what I said earlier. I’m almost sure if the new engine is from the last few years…it’s not going to be the same ratio as your old one. Your new engine will be something like a 2.15-1 ratio at least. Explains the situation for you.

 

[dingbat]

What model number are you running?

Engine Comparison | Yamaha Outboards

Not sure which engine is right for your rig and boating environment? The first step is to get a side-by-side comparison of current Yamaha Outboard engine specs, to see how they stack up. Review stats like displacement, alternator output and much more.

yamahaoutboards.com

 

[QBhoy]

What model number are you running?

Engine Comparison | Yamaha Outboards

Not sure which engine is right for your rig and boating environment? The first step is to get a side-by-side comparison of current Yamaha Outboard engine specs, to see how they stack up. Review stats like displacement, alternator output and much more.

yamahaoutboards.com

Good question. If it’s the presumed standard f90…it has a lower gear ratio than the old 2 stroke. Will be 2.15-1. Might explain things

 

[ahicks]

Regardless, it's pretty clear here you're going to need a little more prop. I think your red line is about 6k, but I would look for something turning about 5500. At 6k, the 4 stroke is going to sound pretty busy. 5500 sounds much better and generally does that with little loss of performance.

 

[Scott06]

I just had to re-power my 18' Sailfish CC. The 19 year old Yamaha 90 hp 2 stroke bit the dust. The prop on the 2 stroke was a Power Tec 3x13.5x15 which truned 5200 rpm at wot, which was right in the power range. I re-powered it with a Yamaha 90 hp 4 stroke. I used the prop off the 2 stroke and the engine wants to go over 6000rpm which indicates it needs more pitch. Is there a relationship to how much bigger prop is needed going from a 2 stroke to 4 stroke with the same hp on the same boat?
Ed

As a rough rule of thumb every 2” of pitch up or down will move wot rpm by about 400 rpm. with the different options in blade size , design, cup etc this is not 100% but a decent rule of thumb