Propellor dynamics

[ballabunch]

Having spent most of the weekend removing the props from a newly acquired Fairline Fury, I discovered that the starboard prop is 14x16 and the port one is 14x15L. Great, yet another bill to be loaded on the old plastic. But before lashing out I thought I'd see if there were any old salts out there willing to part with a little advice. I'm rebuilding this boat as a fast economical (?!?) cruiser - I'm not looking for quick acceleration nor top speed. The engines are Volvo AQ130Cs driving 270 legs, and have undergone major overhauls. Despite this I don't want to run at full throttle all day - between 2.5 and 3k sounds reasonable given their age.

So, being new to the propellor game, I need to know the effect of pitch on the power and revs required from the engine. Does greater pitch require extra power overall or would it just take longer to get on the plane and achieve cruising speed (with better economy)? Should I go for 15 or 16inch pitch?

PS. Any good ideas to free a prop 'frozen' to the propellor shaft? I've soaked it all in WD40 but it ain't gonna budge.

 

[Chris_Stannard]

I suggest that both of your engines turn in the same direction so one has an extra idler in the gear train to give you outward rotation on both shaft when going ahead. This means that when you go astern on the outboard shaft the stern will kick in towards the jetty. In order to reduce viabration you have to run both engines at the same revs so the pitch difference is intended to make up for the fact that the propeller speeds are different, the one with the greater pitch will be turning more slowly.
If you put propellers with a greater pitch on you will find that the revs drop for a given speed but you will increase the strain on the gear train and the thrust blocks, not something I would wish to do in an older boat. a good diesel engineer I know puts a smaller prop on older boats, they cannot go so fast since the engines usually have a rev limiter, but they last longer.
To get your prop off you need a propellor puller. much like a hub puller for a car, three legs coming up onto a central web with a screw down through the middle. Wind up on the screw slowly and the prop is pulled off. Best idea is to borrow one from the boatyard, or to get a freelanc engine man to come and take it off for you, it will take him about 10 minutes and should cost less than buying your own tool. My advice would also be to get hime to put the props back as they do need to go on absolutely straight if you are to avaoid viabration.

Good luck

Chris Stannard

 

[tr7v8]

Firstly the 270 legs can run in either direction with the same gear ratio, all you do is move a link on the gear linkage to make forward on the controls clockwise or anticlockwise prop rotation. As regards getting a prop off, you need a three legged puller and a blowtorch.
Try with the puller first and tighten it up and whack the centre of the puller with a reasonable size hammer. If this doesn't work then heat the hub of the prop and try the puller again, DON'T overheat the shaft you'll fry all the seals in the leg.
If all else fails then either get professional help (bigger puller and/or hammer) or cut the prop off. It's cheaper than knackering the shaft and gears.
I assume you have a manual, Keypart do one for £ 20.
Steel Developments can possibly mod both props to be a match or replace one and would advise on sizing as well.

Jim
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[ballabunch]

Thanks Jim. I've got a large bearing puller which I've used (together with the hammer), but still no joy. I also heated up the prop hub, again nothing. The splined shaft pressed into the prop hub seems to have movement on what appears to be a rubber mounting (the prop is pulled back half an inch or so but the splined shaft is firmly seized onto the prop shaft) - when I release the puller the prop moves back up the splined shaft. I've now left it in a 'stressed' position since Sunday, and I'll give it another dose of PlusGas (dismantling lubricant) and try again on the weekend. I'd rather not have to resort to cutting the prop off, but this may be my only hope!

Thanks for the advice re Steel Developments.

Nick