Prop replacement problem, what would you do/advise?

[Norman_E]

For anyone to calculate the correct prop, they need to know the maximum power RPM of the engine, as well as the figures given in your post above. Essentially the faster the prop is turning, the smaller it needs to be. If your engine produces 6 bhp at 2000 RPM, giving prop speed of 1000 RPM it will take a bigger prop than it would if it revved to 3000 RPM for the same horsepower. Horsepower in any engine is a function of torque multiplied by speed.

I am on the boat so don't have access to a program called PropCalc which I have at home. It is a free download.

 

[Quidi Vidi]

Forgot to add that the max revs according to the handbook is 3600rpm, we don't have a rev counter on the engine so I had to use this figure in my calculations. I posted the black soot problem last year and the replies all pointed towards being overpropped which led me to the online calcs and Silette. We checked the exhaust elbow at that time and it was absolutely clean, we had only done about 10 hrs since launch and about the same amount of hrs since. I will check it again at the weekend, is there anything else that I could check or look for whilst I'm there?

 

[ithet]

Forgot to add that the max revs according to the handbook is 3600rpm

In which case the Vicprop calculator would seem to agree with your 10x7 figure.

 

[Tranona]

In which case the Vicprop calculator would seem to agree with your 10x7 figure.

It is not the max revs that is important, it is prop shaft speed, so the gearbox reduction ratio is key. 2:1 is common, but so is 2.4, 2.6 and with Yanmars 3.1. I don't know what the ratio is on the Petter, nor whether there were alternatives.

 

[Quidi Vidi]

It is not the max revs that is important, it is prop shaft speed, so the gearbox reduction ratio is key. 2:1 is common, but so is 2.4, 2.6 and with Yanmars 3.1. I don't know what the ratio is on the Petter, nor whether there were alternatives.

2:1 reduction at the gearbox

 

[Tranona]

Then the 10" is more appropriate, 7 or 8" pitch.

Have you tried getting somebody to repitch your 12"? That would be the easiest way to go. An inch of pitch will change revs by 3-400. The specialist will also measure what you have now, which may not be what is stamped on the boss.

 

[ghostlymoron]

Then the 10" is more appropriate, 7 or 8" pitch.

Have you tried getting somebody to repitch your 12"? That would be the easiest way to go. An inch of pitch will change revs by 3-400. The specialist will also measure what you have now, which may not be what is stamped on the boss.

I'd still be concerned about the reduction in shaft size.

 

[Tranona]

I'd still be concerned about the reduction in shaft size.

The actual shaft size is perfectly adequate, but as others have noted, there may be an issue with the transition between the two diameters. 3/4" was a common size for that engine.

 

[Plevier]

I can't answer all your questions but a new shaft is surprisingly cheap by boating standards.

The last one I had (for a significantly larger boat) was not much over £100

Was that a long time ago? I've just had to get a new 25mm shaft 1300mm long and the typical price in 316 with one taper and one plain end was close to £200. I couldn't even find a source of material for £100.

http://www.lancingmarine.com/pricepdfpages/sterngearprices0212.pdf for prices from our local company Lancing Marine (a bit dearer than some)

 

[vyv_cox]

As usual, going to a marine supplier will cost you a lot more than a general engineering shop. One of the latter in Bangor in N Wales charged £60 for a 25 mm shaft. You may need to provide full details of what is wanted, which may be known by a specialist.

 

[Plevier]

As usual, going to a marine supplier will cost you a lot more than a general engineering shop. One of the latter in Bangor in N Wales charged £60 for a 25 mm shaft. You may need to provide full details of what is wanted, which may be known by a specialist.

Try and find one in Brighton!
Web designers and games writers, as many as you want, but engineering shops, very few. I found about 6 but 5 not interested, other one very busy and not sure he could do it well enough. Yes I provided a full drawing.
As for material I contacted a number of stockholders all over the country and for 25mm ground 316, it was over £100 from all of them for a 1500mm length.

 

[Quidi Vidi]

Try and find one in Brighton!
Web designers and games writers, as many as you want, but engineering shops, very few. I found about 6 but 5 not interested, other one very busy and not sure he could do it well enough. Yes I provided a full drawing.
As for material I contacted a number of stockholders all over the country and for 25mm ground 316, it was over £100 from all of them for a 150mm length.

I've got a stainless fabricator in the sanderson Centre here in Gosport who has done a couple of small jobs for me and is very good, however i'm putting off replacing the shaft until time and finances allow me to sort it out. On top of the shaft i would need a lift out and some time in a yard, breaking through the cockpit sole and removing the skeg and then making good again. All very expensive on top of the price of having the shaft made up. I need some prices and who and where locally i can get the prop repitched and/or resized, I intend to fit a rev counter to the alternator this weekend to see what pitch/size i need to bring the revs up. I am also considering the bore reducer idea but need to find more info on that, they are readily available and seem common practice in the USA but no one over here seems to have experience of them. Colin (colhel of this forum) has PM'd me and kindly offered to help so hopefully by putting our heads together and using his engineering expertise we may come up with a solution soon. In the meantime please feel free to offer any advice that you may have and thanks for all your replies so far.

 

[ianat182]

Possibly mentioned on the old thread but have you the time to visit the Chandlery barge at Bursledon. They have a container full of propellors of all sizes and pitch,and less than a new price usually.
If no luck with them take your propellor to Hamble Propellors at Swanwick.
Their factory is located next the old Brickworks;Turn into Swanwick Lane (oppositer Premier Marina and take the first left down Coal Park Lane.It's at the end over the bridge.

They made me a new shaft and propellor 3 years ago when I fitted a new engine, and are very helpful, so may repitch your existing prop if its dia is OK,or supply a new shaft

ianat182

 

[Tranona]

Try www.hamblepropellers.com in Swanwick. Or First Marine Propulsion in Shamrock Quay

 

[VicS]

Quote:
Originally Posted by Bower15
...wouldn't a blocked exhaust be affected equally under load or out of gear.

No - the volume of combustion products is much greater under load.

Andy

The volume of combustion products under load will be greater but at the same revs surely not that much greater. Will it?

 

[pyrojames]

Combustion product volume will roughly scale with fuel used, so a significant increase over idle.

As far as shaft reduction and replacements go, Darglow suggested turning down the taper on my 35mm shaft to fit their Featherstream, and if done properly there shouldn't be a problem. Remember if you change shaft size you will also need a new stern gland, cutless bearing and shaft coupling at the gearbox. It will all add up.

 

[Tranona]

Combustion product volume will roughly scale with fuel used, so a significant increase over idle.

As far as shaft reduction and replacements go, Darglow suggested turning down the taper on my 35mm shaft to fit their Featherstream, and if done properly there shouldn't be a problem. Remember if you change shaft size you will also need a new stern gland, cutless bearing and shaft coupling at the gearbox. It will all add up.

But he is not changing shaft size, but possibly new shaft that is not reduced at the prop end so that he can use the new prop.

 

[Quidi Vidi]

Photo's added

Thought i would upload some photo's to show the shaft and prop etc. Unfortunately my crew were a bit too keen and our home made prop puller worked too well so i didn't manage to get any pictures of it in action.

 

[Quidi Vidi]

More photo's

As you can see from the previous set of phot's the prop was quite clean and not fouled much so it is its size thats causing the problem. These show it cleaned up and put back on, there was no locking mechanism for the nut so we used loctite and punched the end of the thread, will drill and tap the nut next time and use grub screws to lock it. Once it was back on we painted it with white boot topping, will report back on how well this works at a later date

 

[Tranona]

That looks like a good old Stuart Turner prop - have a dead one of those as a garden ornament! Seems in good nick and will probably take a bit of re-sizing. Take the photos to one of the specialist I suggested and see what they say. When you come to finally lock the nut on, consider drilling and tapping at an angle through the end of the nut into the thread on the shaft and locking with an M3 socket headed grub screw. Bit fiddly, but works well.