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I am the new owner of a 1978 20' outrage fitted with a 1995 150 Yamaha saltwater series that has about 500 hours on it. It runs smoothly throughout the current rpm range. I believe the prop is original to the 1995 motor installation. However at wot it only reaches about 4000 rpms @ 32mph. The motor is rated at 4500-5500 at wot. I was told the prop may be to big. I believe the current prop has a 19 pitch. Any insight or suggestions would be helpful. Thanks

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There are several factors that determine top performance

Engine mounting height
prop style, material and size
Smooth hull.
Weight of gear, load in the boat.

For the motor in question, you want wide open throttle (WOT) to be 5200-5400 rpm's.

Without knowing what the existing prop is, load in boat etc., you are over propped.

You need to pull the prop off and look on the inside of the hub for the part #. This will tell you make/model and pitch.

Prop sizes and performance is NOT universal across brands and materials.

Selecting the correct prop is a bit of science so don't just run out and buy a 17" pitch of whatever.

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Thanks Phil,

Without pulling the prop I can tell you the hull has bottom paint, the engine has a 25" length mounted 1 3/4" from the top of the transom, 62 gallon gas tank (near full) 2-3 adults, 2 batteries. The prop is stainless steel and is 13"

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Still need part #. Stainless or Aluminum?

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Hey Phil,

I pulled the boat the other day and checked the prop size. It is a13 3/4 x 17 m.

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More accurate and more so everyone know what height the engine is mounted, is by using this chart.
http://www.whalercentral.com/articles...cle_id=106

If you are mounted 2 holes up, then you need a little less pitch prop in the same brand.

If you switch brands, then maybe someone else here can tell you what pitch the other brand of prop will suit you best.
http://www.whalercentral.com/articles...cle_id=108

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After some searching I see a prop guru has recommended the Yamaha painted stainless steel prop (not Performance Series) in 14.25 x 17.

For your information your motor has 1.86 to 1 gear ratio with WOT as 5500.

I would guess that you should be able to hit 40 mph at WOT with the motor and prop dialed in.

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In case it's of any help...

I have a 1992 Yamaha 150 engine, which I believe is essentially the same as yours (but mine is not the improved "Saltwater Series" version) with the 25" shaft and gearcase at 1.86. Mine is mounted on a 19 Outrage II, which is a heavier boat than yours. We are using a 4-blade stainless steel Mercury Offshore Series propeller (which I understand is now called the "Vensura" model), with 14 1/2" diameter and 17" pitch.

The engine revs up to 5400 rpm, the boat can reach 43/44 mph with a moderate load, and seems to be a good match for the engine/boat combination particularly in the mid-range speeds.

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Thank you for your input/research. The motor is mounted two holes up.
I'd love to see a little improvement in performance so I will try one of the suggestions.

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Phil,

I am still researching prop choice. Can you tell me how going to a larger diameter prop with the same pitch will increase my WOT?

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The prop you have now does not have the best "grip" and is "slipping". It turns but does produce a lot of thrust.

Propeller design is complicated and I understand the basics. By increasing the total diameter of the prop, you produce more thrust. The pitch, or angle of the blade, is important for creating thrust but also allowing engine to increase RPM.

Think of rowing a boat with:

toothpicks
oars
snow shovels

You are looking to increase speed over water while at the same engine RPM.

Hope this makes sense.

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Thanks Phil,

Still not sure? I would think if my prop is slipping (spinning too fast or not gripping the water effectively) that my WOT would be higher than required with my current prop. My WOT now is only about 4000-4200 rpm's. I'm concerned this is too low.

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Since you already have a prop that is not particularly aggressive (13 3/4" diameter x 17" pitch), it seems to me the issue could be fuel delivery more so than prop size/pitch.

Is your engine able to rev any higher when it is in neutral?

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Thanks JRP. Yes, it will rev in neutral with no problem. I thought maybe the butterflies on the carbs were not opening all the way, but I had a Yamaha dealer look at it and he indicated they were. I am still going to verify this for myself to be sure.

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Another possibility is a flow-restriction in the fuel lines or fuel filters. in neutral (no load) it might not be noticed, but under load the fuel-flow requirements are much greater to achieve full engine rpm. So that's another area to explore besides the carbs.