Measuring a propellor

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simon barefoot

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Hi everyone, and a happy new year!
I am wondering how you measure a propellor. Obviously there is the diameter of the circle it makes, but what is the other measurement, and how do you get it?
I have a horrible feeling I'm going to feel a little bit thick, but what the hell, you only learn if you ask!!
BTW, once I know how to measure it I'm going to ask how much they cost.
(Princess 37, 1980, twin 145hp).
TIA!
 
With dificulty but there are two ways it's is by measuring the blade angle

The second easier way is looking at the hub either under the nut or along the root of the hub where it should be marked

However some manufacturers cheat and only stamp a serial number!
 
scottie said:
With dificulty but there are two ways it's is by measuring the blade angle

The second easier way is looking at the hub either under the nut or along the root of the hub where it should be marked

However some manufacturers cheat and only stamp a serial number!
Click to expand...

I once had a little 16 foot cabin cruiser with a 50HP 4 cylinder 2 stroke Mercury on the back. One day I took the boat out from Whitby and realised that there was something wrong. Coming back into the harbour an ex Whitby lifeboat (no name no packdrill) was right up my a*se. I think they call it 'bullying', bloody prat. I opened the engine up, and slowed down! EWL honked and whipped past me, blowing angrily. I'm sure that the program that runs this forum would seriously edit my response to that. I had a problem and TFI didn't give a sh*t.

There used to be a yard, very close to the Humber, that had its own lake. I took the boat there to get the problem fixed. It was there that I learned about the metalastic bush that cushioned the drive on my Mercury, the one that failed to transmit the drive to the prop.

The guy there explained to me that each engine has (generally) a set max RPM. I think in the case of my Mercury it was 6000RPM., though it may have been 5. The idea is that flat out the correct size of prop will not allow more than max RPM. Under pitched it'll over rev., over pitched it'll not reach max RPM. Correctly pitched the engine will run, flat out at the engine's design speed.

I suppose I'm really saying, don't guess, find a supplier that knows.... :rolleyes:
 
simon barefoot said:
Hi everyone, and a happy new year!
I am wondering how you measure a propellor. Obviously there is the diameter of the circle it makes, but what is the other measurement, and how do you get it?
I have a horrible feeling I'm going to feel a little bit thick, but what the hell, you only learn if you ask!!
BTW, once I know how to measure it I'm going to ask how much they cost.
(Princess 37, 1980, twin 145hp).
TIA!
Click to expand...
The "other measurement" is the pitch -- i.e. the theoretical distance the propeller would advance in a single revolution if it were screwing its way through a solid medium like a screw going through a block of wood.

Measuring it accurately without special tools is very difficult, but here's a description of it: http://www.propellerpages.com/?c=articles&f=2006-03-27_manual_pitch_measurement
 
Normally there is no reason to measure it yourself, except out of interest. The original size is almost certainly stamped on the boss. If you need to replace the prop then ask your chosen supplier to recommend a size based on the data of your boat. You can get a ball park size yourself by entering the required data into a programme such as Propcalc on www.castlemarine.co.uk If your boat is a production boat and the builder is still in business they will be able to tell you what the original was.

You can get an idea of prices from the T Norris website, but of course there are many more suppliers.
 
Marking should be on the hub of the prop. Pitch is not easy to measure as it is a theoretical distance carved out during 1 revolution. Best to go to a specialist repair place like Steel Developments. They do excellent low cost repair, and can supply new and refurn props. Important that with 2 engines the props are matched, but could be different size/pitch if the gearbox ratios are different. This can happen where one gearbox has a different ratio in reversing the direction.
 
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simon barefoot said:
Hi everyone, and a happy new year!
I am wondering how you measure a propellor. Obviously there is the diameter of the circle it makes, but what is the other measurement, and how do you get it?
I have a horrible feeling I'm going to feel a little bit thick, but what the hell, you only learn if you ask!!
BTW, once I know how to measure it I'm going to ask how much they cost.
(Princess 37, 1980, twin 145hp).
TIA!
Click to expand...

Going back to basics, can you share your problem with us?

Existing props damaged, engines not reaching rated speed, Ford Mermaids?

Please do not be offended, why not simply look for dimension stamp on the hubs?

Unless suffering badly from the effects of electrolysis plenty of prop shops out there, original props were by Teignbridge I think, so nice quality and refurb perfectly practical.
 
To my knowledge mermaid didnt make a 145hp engine, the 6 cylinder enginesin the 37 were.

120 nat aspirated, 165 turbo but no CAC, 180 and 188, 200, 212, the list goes on but 212 was the biggest fitted at the factory.

Whats the problem, bear in mind if you have the Y hull you will get 15knots max anyway.
 
Thanks for all your replies, as requested, some more info.
Engines are Bedford GM466, I have some details that suggest 1977 vintage, so not the original engines. I have just had the boat lifted out and there is some serious electrolysis damage to both props. (Thanks to the homemade steel barge next door that turned out to have the dodgiest wiring this side of frankensteins brain!!) I have yet to remove the props, I'm hoping the makers stamp is still legible.
Both props are identical, instead of having counter rotating props/gearboxes, they are matched and both turn in the same direction. (I bought the boat as a liveaboard when I was about to be homeless through divorce....small things like the props weren't of much importance then!)
I'm looking to replace the props because of the damage, not through any wish to increase performance (but that would be nice).
I'll get in touch with Princess and see if they still hold the details for this boat, out of interest just as much as anything else.
 
As mentioned propeller technology involves some principles and terminology.

To detect a propellers characteristics and/or possible damages at tool like this is needed
brown-pt2.jpg


If you want to know a lot more about props you may get the free book from Quicksilver here:

http://prop_.spidybot.com/Quicksilver_Propellers.pdf


I've seen somewhere a P37 with 20"x22" props, but not knowing hp and gear ratio it's wild guess that something like it would fit your boat.

As suggested get a calculation based on your boat's design, weight, gearboxes and available hp
 
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