Need advice if current prop is OK

[browntdb]

When I purchased our 2000 CD22 in January, it came with updated twin 2005 Yamaha High Torque 50 HP EFI engines. The props that came on the boat are Yamaha 13.5 x 15K. I had our CD 22 out yesterday for the first major run. I found that in relatively calm water, I could get about 25 mph out of the engines, but the peak RPM I could get full throttle was 4800 rpm. One mechanic told me recently that I should look to change props if I could not get 6000 rpm from them.

I don't plan on running full speed very often, but I don't want to be using props that cause unneeded fuel consumption or make the engines labor too much.

Any suggestions?

Thanks,

Terry

[Chester]

The prop has way too much pitch, better to buzz the engine than to lug it. Better yet to have it propped correctly. You need to get at least another 1000 rpm out of the motor.

[Sunbeam]

I don't know specifically what prop you should try, but can say that the mechanic has the right idea. Basically, you want the engine(s) to run up to their full, recommended WOT (wide-open throttle) RPM, in order to not lug, etc. the engines (even if you don't plan to travel at WOT, it's still the same recommendation). This is not specific to outboards, as inboard diesels are propped the same way (testing for proper RPM at WOT).

6,000 RPM is probably about right, but you can look in your manual. The manual for my Yamaha 80 gives the WOT spec (Yamaha manuals are downloadable from their site if you don't have one).

Sunbeam

[NORO LIM]

I think you are considerably over-propped and are lugging those engines. I'd try something like an 11 3/8 X 12, 3-blade for starters. ( Yamaha's 3-blade aluminum, # 69W-45952-00-EL, for example.)

Maybe somebody else has the same engines on the same boat and can chime in from experience. (I have Yamaha F50's on a slightly different hull.)

I think you should be getting more speed out of the boat, and definitely higher RPM at WOT. Load and altitude changes sometimes make a couple sets of props advisable. If, for example, you were getting those numbers with a very heavily loaded boat running at high altitude, then maybe those props would be OK with a light boat at sea level.

[SEA3PO]

I have twin 40 Yamaha's and the best I can spin is 5200
I am using the factory props that came on the motors.

Joel
SEA3PO

[thataway]

There are several issues: Trim of the boat, weight of the boat, any drag such as trim tabs (none seen on photo) or how the engine is trimmed.

Did you trim the engines to get max RPM for that throttle setting. That is to put the throttle almost to full (Lets say 4500) and then trim the motors up and down to be sure that you are at maximum speed for that RPM. Be sure that the bow is not trimmed down too much.

The WOT for the T 50 is 5000 to 6000 RPM--I would shoot for the middle of this.

Remember that the T 50 is a high thrust motor, and has a different (lower) gear ratio than the F 50! (T 50=2.33:1, F50 1.85:1). Also the props for the T series go from 13 1/4" to 14" in diameter, larger than the F or standard motors. I suspect that Noro Lim was looking at a prop for the standard 50 hp rather than the high torque motors. The larger blades and lower gear ratio will match better for the T series. The F series use the lower diameter props.

You should be getting 30 mph out of that boat. Go out again, and play with the trim, get the engines at max speed for the throttle setting, then push the throttles all of the way down for a minute or so--and you should be seeing 5500 RPM. If not, then drop down to a 13" pitch.

See if you can find a dealer near you who will allow you to borrow props if this does not work out. I would drop 2" to start with. I suspect that trim may allow faster speed and higher RPM.

If there is no dealer who will loan you the props, Ken at Prop Gods is probably the best source of props in the US--He has a program where you can send $30 and pay shipping, for props to try.

Let us know what happens, make a graph of speed vs RPM and trim angle--send it to me if you want.