Underwater Prop Change

I've always had a suspicion my prop was undersized for the boat and the type of waterways we do and intend to use it on. The engine never seemed to be "working" which is not ideal for a diesel and is only really used at the lower rev range.

I did some research into the very black art of prop calculation, at first online with various web based calculators then with about half a dozen UK based prop manufacturers.  In all cases the maximum diameter I can swing is a 21" so this was a definite as too was the max RPM, HP, and gear ratios and the hull size and weight.

All online and all manufacturers calculations came up with the same pitch within an inch or 2,  with most pitching at the mid range of 21.  So it was decided a 21 x 21 and still a 4 blade. Up from 21 x 16.

Old and new props side by side

The initial plan was get a lift out and the back the same day at a local crane day, but the more I looked at it and them more advice I sought I decided it would be worth at least trying to do it under water. If this didn't work or something serious happened the crane was still an option.

I'm fortunate that the weed hatch on the boat is quite large and access good and Garth at the boat yard had the necessary prop puller on hand.

Weed hatch

The prop nut on a Vetus shaft is a flange nut so there would be no problem with either lining up a nut and split pin hole or having to drill a new one.  The only down side was two 2mm thick SS flanges had to be hack sawn off and hack saw movement was limited.  This part of the job took a quite painful hour or so, but once done the nut was easily removed.

Prop nut showing the flats where the flanges lock

Old prop, nut removed

Then getting the prop puller on was a bit of a fiddle but about half an hour of adjustment had it ready to start winding on the pressure.

Lots of lines attached so as not to lose anything

After a bit more tightening that sweet moment the taper broke.  The prop was off. :-)

This lot was removed and the new prop offered on the the shaft and measurements taken to establish the key wouldn't be too proud for the new prop.  I was undecided that the prop would sit entirely on the taper and not the key.  In the end I decided it was probably fine but removing .5mm would do no harm so I did this gently and ended up, once it was flat and square again, at .7mm removed.

Then it was time to fit the new prop.

All complete with shaft anode refitted

In all it took around 6 hours, and of course the only part dropped was the non ferrous prop nut which required the donning of  the wet suit and snorkel and a short dive to the river bed about 4' down to retrieve it.

All in all a good days work and a worthwhile experience. 

The purpose was to load the engine a bit more at as the 16 was not making the engine work and would easily reach max RPM of 2500 which is never needed or used. Also I wanted the fast cruise RPM to be at the top of the torque curve at 1500rpm.

RPM             Speed MPH

                   Previously 16".          Now 21".
  800            2.6                          4
1000            3.5                          4.8
1200            4.5                          5.5
1500            4.8                          6.5    Turbo now heard working
1800            6.0                          7.8    And speed building

I suspect given all the right conditions 8.5mph would be achievable.

The conditions were wind on against 2 mph (ish) flow.

Observations so far are noticeably less rudder judder throughout the range, more rudder authority, less rudder input to keep on line, turbo heard working, acceleration much better (not really required) stopping massively improved prop walk slightly more pronounced, engine ran 5°c hotter on prolonged 1800+ test but this returned to normal 85°c very quickly at 1500 and maximum rpm now 2200 so slightly over propped as expected. No black smoke at any point on the test.

In conclusion I think this has been worthwhile. I can now cruise very nicely at 1000 and fast cruise at 1500 and gallop at 1800.

These are only preliminary tests on deep water so like all things boat there are infinite variables.

The predicted all round performance/economy improvement was 20% which I think is probably about right.  Someone else can do the calculations for me if they like.

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