Outdrive Props Mod?

[boatone]

I have made contact with Steel Developments (previous post - helpful arent they!) and in discussion the subject of modifying props for lowspeed performance arose. Apparently they dont charge any extra for doing this during a repair/refurb.
The idea is to alter the pitch of the props to improve thrust at low speeds and to enable the engines to run at somewhat higher revs for a given speed. At the moment around 1200 revs on both engines (Volvo AQD32's) gives max Thames permitted speed. As I will be mainly Thames based and the age of the boat (mid 70's) precludes flat out high speed performance anyway I'm tempted to do this. Could I also expect more economic performance dieselwise?
Anyone had prior experience or care to comment?

TonyR
boatone@boatsontheweb.com

 

[oldgit]

If you are talking about cupping! I had this done and it certainly did not make matters any worse.Difficult to say it was better due to props being totally knackered before refurb.

 

[adarcy]

Dear Tony,

If you are going at the same speed through the water at higher revs, the boat drag should be the same but there will be a very slightly higher internal loss from the engine (friction etc). After all we don't drive along the motroway in 3rd or 4th when there is a 5th gear. Against this, there may be a minimal advantage at running the engine closer to its optimal therma and mechanical efficiency (bmep and lowest specific fuel consumption).
However, I would have thought it would be better for the engine's longevity running in its normal working rev range rather than just above idling but I doubt if you will notice much fuel consumption difference. One would expect to notice a change if a prop is too "tall" and a boat never reaches its max engine power. If a prop is not "tall" enough, I would only expect a limited top speed which may not concern you on the river.
If you decide to get the prop changed and it makes a big difference, I'd be v interested to hear.

Anthony

 

[boatone]

Thanks for your views Anthony
As I only bought the boat at the latter end of the summer and have not used it in anger before having it lifted for the winter, my problem is that whatever I have done I am unlikely to 'notice a difference'. I am really only concerned with what prudent steps to take whilst its possible to do these things. As the props need attention anyway (one has a bent blade) it seems sensible to address the issue of size and pitch relevant to intended use. I agree that the engines should be better off running at closer to design revs rather than close to idle speed - (might also be able to get the throttle levers to stay put if they're not right on the edge of idle!).
Somewhat surprised at the paucity of replies to this one.....not usually a problem on this forum!

TonyR
boatone@boatsontheweb.com

 

[BarryH]

I had my props repitched when steel developments refurbed mine. I had the pitch increased as the boat was under prop'd anyway, if I remember rightly they said they can take the pitch eitherway by an inch, but in saying that an inch in difference will only give about 300rpm difference. Dont quote me on that tho, SD are the chappies to work all that out for you

 

[Col]

One of the main reasons for increasing engine rpm is to prevent " bore glaze "
This is a condition where your cylinder bores dont reach full operating heat and load, so surface becomes glazed, piston rings cant seal properly and causes increased oil consumption and smoky exhaust.
Increased rpm will give you a better charge rate for your batteries.

 

[scottie]

Dont worry about glazing as this normally only happens in initial use of engine

The aqd32 engine is most likely original and with 270 drives were early ones at that

They never liked too much hard work and best used in larger sailboats when they were governed down to 3000 rpm.

The idea of increasing the size of the props is based on the premis that you have excess power for river cruising which is sound but remember the age of these engines. The dia is limited by the o drive but pitch can be increased.
Ask sd to try running the figures though there calcular based on non planing for river use .
ps the props hafts may not be tapped for centre bolt fastening so you will have to stick to the tab washers these are made on a vernier princeple so you will only get one or two trabs to fit.

 

[boatone]

Deleivered tjhe props to Steel Developments on Friday for (hopefully) a full refurb.
I say hopefully cos the guru there immediately drew my attention to pitting caused by previous owner antifouling prop and which may be bad enough to defeat successful repairs. I await their call with baited breath but Be warned - DONT antifoul props!
In the process of sorting this lot have become all too familiar with the deficiency/inaccuracy of manufacturers documentation. Twin outdrives so props are left handed and right handed...fortunately I sussed this out for myself but nowhere in the maintenance manuals does it even mention this, let alone draw attention to the importance of refitting props on the right outdrives .....oh what fun....throttles ahead....hey, why the hell are we going backwards etc etc!!!!!
Another usefull advantage of the digital camera here... took a quick pic before removing props so can now see which goes where when I put 'em back.


TonyR
boatone@boatsontheweb.com