Engine Rev accuracy

[Wideocean7]

Hi All. I am about to take ownership of an aluminium yacht which has a 50hp Perkins Prima deisel. She has a s/s Autostream S1 self-feathering prop with adjustable pitch. Based on the rev counter, the pitch has been set for the engines Maximum Rated Speed of 3,000rpm.

The thing I have noticed is that at 3,000rpm (per the guage), the yacht speed through the water is only 5knts. Her displacement is only 8.9 tons and I would have expected a higher speed for this "weight". The prop cavitates at revs higher than 3,000rpm so effectively there is no useable power left.

At the wharf, the engine revs to 5,000rpm wide open throttle and not under load.

I don't know if such high revs (5,000rpm) are normal and am wondering if the rev counter is reading far too high engine revs. My aim is to make sure the prop is adjusted properly for the engines Max Rated Speed of 3,000rpm. But I don't know if the gauges are reading rpm correctly.

Any constructive suggestions appreciated.

[Wotname]

You could get a hand held tachometer and check it for yourself. This is sample but there are dozens of others on eBay etc.

https://www.jaycar.com.au/digital-tachometer/p/QM1448

[Charlie]

Just went through this exercise with my Yanmar to get fit for a new prop. I think it is important to get both the speed at the crankshaft and after the reduction gear. I used a harbor freight tool worked fine.

[Fore and Aft]

Wideocean7 according to the brochure 3000 rpm is where your Perkins max rpm should be at. That link below has a manual you can download.
I see the Perkins M80T has the 4500 RPM? I googled some images of one and compared my survey photos and boy they look similar.
I also found an exploded diagram of your prop. Those two nuts on the left of the Prop assembly are going to become your new best friends. I really cannot see any way you can properly/cheaply adjust the prop without diving on it and adjusting those bolts. The other way is going to be adjusting the prop, get dropped in the water, sea trial then back to the travel lift to get lifted and repeat the process. I know it took about three swims before I was happy with my prop pitch.
Cheers
https://www.martec-props.com/install_Autostream.htm
https://forums.sailboatowners.com/th...22.1249923404/

[RaymondR]

If the rev counter is taking it's signal from the output on the alternator the reading can be affected by a number of factors ither than true engine speed. Belt slippage for instance.

[Wideocean7]

Thanks all. Your comments are appreciated. It sounds as if first, I need to check the revs to see if the gauge is reading correctly and second adjust the prop accordingly. Does anyone have any instructions how to measure crankshaft/prop shaft revs if I buy a tachometer? (Yep, I have Googled it!). Fore and Aft - I will be in the water multiple times amongst the dolphins and sharks.

Thanks again. Andy

[sailormed]

Just glue a piece of the refective tape on the crankshaft and point the laserdot of the rpm gun on it. Easy.

[Fore and Aft]

Andy it will be interesting to see if the gauge is wrong. When do you take possession of your yacht?
Cheers

[Wideocean7]

5hrs ago😀!!

[Snore]

There is an app for that! Check App Store, I found one for my iPhone that uses the phone’s flash as a strobe tachometer. Cost was under $3USD