Revs

[timw]

New prop on now, and maximum revs I get at wide open throttle is 5200rpm.


Anyone know if this is about right for a 225 4 stroke?

Can't see the speed as the speedometer is playing up.

Thanks

[Richard B]

According to Yamaha's web site http://www.yamaha-motor.co.uk/produc...?view=techinfo the WOT range for your engine is 5000-6000 rpm. However, looking at your earlier post, you were pulling 5500 with the old prop?

You need to know waht speed you're doing with the new prop. If the speedo's playing up, have you got a GPS to take a speed reading from?

If your speed is lower, then sounds like you've had poor advice with the choice of prop.

[timw]

Hi Richard,

I think it was 5500 before, but can't be sure.

I'm off to but a handheld GPS to see what I'm really getting.

By the way, any thoughts on why the Yamaha speed guage likes to read between 15mph and 20 mph at any speed and even while standing still!!!?

[Richard B]

Don't know... but... has the fault occurred since you damaged the prop? If so, could be damage to the pitot tube ion the outboard leg.

[Alex-341]

Originally Posted by timw

Hi Richard,

I think it was 5500 before, but can't be sure.

I'm off to but a handheld GPS to see what I'm really getting.

By the way, any thoughts on why the Yamaha speed guage likes to read between 15mph and 20 mph at any speed and even while standing still!!!?

The guage starts at 10mph (presumeably becuse of the following), you must remember that they arnt acurate in anything that flat waters, and even then not that great but they work. Also remember becuse its measured on the leg so any tide will be added to your speed.

Best is to use a GPS when compareing props but the speedo is nice for a quick glance.

200-300rpm per inch of prop is rough guide.

[jyasaki]

The speedo works using water pressure. There's a tiny (1/8" or less) hole in the leading edge of the power unit. This allows water to pressurize it as the boat moves forward. There is a plastic tube that runs from the foot to your speed gauge. The gauge reads by reading how much air pressure is being generated (the air is trapped in there by the water.

Often, you'll get a bit of sand or mud or kelp or some other garbage that plugs the pitot hole in the lower unit. Result is that any pressure that was present at the time of the plugging is trapped there, resulting in that speed being displayed forever.

Fix is to take something like a small drill bit or a paper clip, and clear the hole. It is usually located about in the middle third of the leading edge of the lower unit.

One other thing to note is that the zip tie or whatever secures the hose to the gauge should be inspected every so often. A hose that comes off will spray seawater all over the inside of your console.

Hope this helps;

jky

[Roy Smith]

If you are still not getting the rpm, you could look at how your engine is set up if you have got to much leg in the water, the drag could hold you back,
from memory and i know iam going to get corrected but i think
The cavatation plate should be about 2" above the v of the hull
Roy

[timw]

great info...many thanks. will be checking all these things.

(Does 5200rpm at WOT sound about right?)

[Richard B]

Originally Posted by zodiac rover

The guage starts at 10mph

Err, no it doesn't! The Yammie speedo is fairly accurate at single figure speeds. I know this from many hours trolling along at leagal speeds in harbours!

Originally Posted by zodiac rover

they arnt acurate in anything that flat waters

Um, not strictly true, but they certainly are dependant on sea state, although they can lose the reading on flat water just as easily as on a rough sea. I suspect this depends on the hull profile and engine height - the behaviour will be different on every setup.

Originally Posted by zodiac rover

Also remember becuse its measured on the leg so any tide will be added to your speed.

Depends whether you're talking about speed over the ground or speed through the water! Speed through the water is the one to measure for prop testing, which is what the engine pitot guage tells you. Speed over the ground (ie GPS speed) is the one to give you an accurate ETA for passage planning.

Originally Posted by zodiac rover

Best is to use a GPS when compareing props but the speedo is nice for a quick glance.

No! The GPS gives speed over the ground. This can be very misleading. If you use GPS speed for prop testing, your figures will only be accurate if you subtract the tidal speed (assuming a following tide, add it for a "foul tide" - that's tide against you, non-yotties ) or if you do a run each way, average the two speeds.

Originally Posted by zodiac rover

200-300rpm per inch of prop is rough guide.

Yep, not a bad rule of thumb I reckon

[Alex-341]

i can see your point about the GPS, BUT the guage we have starts at 10mph and certainly isnt very accurate perticually in rough sea.

[Andy Moore]

What do you reckon the max revs for an Optimax 135 will be? The prop is a 19" Laser, I think.