Should I change the Prop

[pckeen]

Hi,

I bought a one year old Yamaha F90, to repower a 1967 Starcraft Holiday. The motor was virtually new - only 28 hours on it. All components on the boat - steering etc. were changed. The motor came with a propeller, which has remained on the boat.

I took it for a test run last night (myself, wife and three kids). The boat & motor ran really well, no signs of trouble. However, when running WOT, it ran at 6100 RPM, at a speed of 38.5 mph. Should I consider changing the prop? The operating range for this boat at full throttle is supposed to be 5000-6000 RPM.

Best regards,

Peter

 

[SkiDad]

Re: Should I change the Prop

Depends on what you want to do. If mostly cruising I would go up 2 pitches and gain a few mph and lower rpm. If skiing you prob have the best prop just watch rpm.

PS. If you do go up 2 pitches you will now have the choice of which you like best and keep the other for spare.

 

[mpsyamaha]

Re: Should I change the Prop

I would definitely try a 17p prop on there, it will likely go faster and improve fuel economy. Although, like already stated, the current prop sounds great for heavy loads/watersports.

 

[steelespike]

Re: Should I change the Prop

I didn't see a prop size mentioned,can't really figure your numbers without a prop size.

 

[pckeen]

Re: Should I change the Prop

Thanks all. I'm not sure of the pitch on the propellor now - I'll take a look at it and figure it out. Mitzyman, you hit the nail on the head. I'm primarily cruising right now. Thanks all.

 

[jestor68]

Re: Should I change the Prop

Since the motor's hp is rated at 5500 rpm, you can definitely afford to increase pitch 2 inches from whatever is on there now. This should reduce your WOT rpm by about 400 and provide more economical operation over all.

Most of us have two props; one for general purpose(cruising) and one for heavy load/skiing. You already have a heavy load prop(6100 rpm). The other will be the general purpose/cruising prop. Running at about 400 rpm less at any given speed; it will save quite a bit on gas, as well as wear and tear on the motor.

 

[mpsyamaha]

Re: Should I change the Prop

I didn't see a prop size mentioned,can't really figure your numbers without a prop size.

good point, looking at it now, I have no idea where I came up with a 17p suggestion! lol

 

[pckeen]

Re: Should I change the Prop

The water was finally warm enough to look at the propellor that is on the boat. There are three sets of stamps on it which read: 6E5 Y-4 17K. It is an aluminum 3 blade prop. What do these stamps mean?

 

[steelespike]

Re: Should I change the Prop

If you look under the prop nut you may find the actual numbers. diameter X pitch. 17K may be the pitch.??

 

[steelespike]

Re: Should I change the Prop

If you look under the prop nut you may find the actual numbers. diameter X pitch. 17K may be the pitch.??
Prop calculator ,guessing 10% slip, using a 2.31 gear ratio comes up with a 17.11 pitch. 17".
Once your sure of the pitch the suggested increase in pitch should deliver improved speed and economy.

 

[pckeen]

Re: Should I change the Prop

Thanks - I'll try that. I found some part numbers on the Yamaha website. 6E5 seems to be the first three digits of the part number for several different pitches of propellors. 17 is one of the pitches for that propeller, so I'm guessing this is a 17 pitch propeller. Trouble is, there are more than one diameters for this part number, and I want to keep the same diameter propeller for the new prop.

 

[pckeen]

Re: Should I change the Prop

Steelspike - you were right. I went back to take a second look at the prop. Stamped into the side were the numbers 13 1/4 X 17-K. So this a yamaha prop listed on the Yamaha website (I think).

I think I will stay with an aluminum 3 blade prop - from what I have read on iboats, the performance improvement with a steel prop for a motor this size is so minimal that it isn't worth the extra expense of steel.

So this raises a new question. If I move up to the 19 pitch Yamaha prop, the Yamaha Website, indicates that the diameters and pitches available are 13 x 19, or 13 1/2 x 19. I have a 1968 Starcraft Holiday, which I believe is a deep V boat - so I assume I should be going with the 13 1/2 x 19.

With the information above, what prop would you reccomend? Should I stick with a Yamaha prop or go to another make? What will happen if I change the diameter of the prop?

 

[jestor68]

Re: Should I change the Prop

The Solas Rubex 13-1/5 X 19 would produce the most predicable results, as the diameters are basically the same. Expect 300-400 rpm change.

FYI: Solas makes Yamaha props.

 

[pckeen]

Re: Should I change the Prop

Interesting development - I started another thread on what I thought was an unrelated issue. On a recent run, the Yamaha began 'bumping' every few seconds at WOT. It has been suggested that I have hit the rev limiter at 6100 rpms, and as a result, the motor is switching off....

After Jestor's comment - I had decided to order the prop he suggested, then was comparison shopping with the local dealer (becuase shipping costs are so expensive from the states). My dealer suggested I shouldn't bother with the new prop.

What do you think about the bumping?

 

[MARV1]

Re: Should I change the Prop

You are overreving the outboard, bump up to a 19" pitch prop and it'll drop rpm's to where it needs to be.

 

[steelespike]

Re: Should I change the Prop

You can run that prop if you watch the rpm stay below 6000.
The 17" prop will be peppier but will require a little more rpm for a given speed
If you feel you have good power with a strong hole shot now a 19" prop will lower the rpm 300 or 400
should produce a little more speed for a given rpm but be slightly less peppy.
As time goes by I think the motor will be happier.And you may use slightly less fuel.