PROPELLERS

[Clyde_Wanderer]

Hi All.
Further to my question a few wks ago reg the slowness of my boat under power.
The boat is now out of the water, and I measured the prop this eve, aprox 13.5" dia, but looks as if someone has squared off the tips of the two blades, or has anyone ever seen blades like this before? ( not radiused tips, but sort of flat) am wondering if it was done to prevent blades hitting hull or skeg.
Anyway does anyone know of a prop supplier who sells sail boat props, (not just out board props) and in Britian, not America.
And can anyone tell me how I can tell the pitch of the prop as there is no writing on it, and what other spec do I need to get a replacement to fit the shaft?
I am thinking of going for a three bladed prop to see if it will give better speed from my 12hp engine, any thoughts?
And while I am on can anyone advice where I can get Monel revits at a reasonable price for small quantities?
Any advice appreciated, Eamonn.

 

[oldharry]

Square end blades are not all that uncommon on smaller props - but yours may have been cut back to reduce the diameter. Props can be had from Lancing Marine.

You won't get any better speed froma 3 blade prop except in adverse conditions when the extra blade area helps to give more thrust, than you woud with a properly matched 2 blader. The 2 blader will also allow you to sail a bit faster, but this varies from boat to boat.

 

[srp]

Lancing marine - www.lancingmarine.com/
Sillette - www.sillette.co.uk
ASAP - www.asap-supplies.co.uk/

The prop size should be stamped on the boss - you might have to clean it up with some wet n dry to uncover it.
The number of blades doesn't necessarily have a bearing on boat speed - diameter, pitch and blade area are the main factors. Most companies selling props can work out the correct size for you over the phone if you give them the boat details, or you can post them here (but you'll probably get 20 different answers, every one of them swearing they're right and everyone else is wrong). It sounds as though your prop has been reduced in diameter - quite a common practice. You should have a bit of clearance between the prop tip and the surrounding hull/skeg/rudder, about an inch on a 12" prop is ok, but a bit more is better.
Can't help you with monel rivets (do you mean monel pop rivets?) - unless you want to drive down to Cowes to pick them up.

 

[Richard10002]

Someone posted this a while ago... looks cheap to me

http://www.seascrew.com/acatalog/Sea_Screw_Catalogue_Monel_Rivets_378.html

 

[nordic_ranger]

Have a look at this site http://www.castlemarine.co.uk/pitch.htm . It allows you to do propeller pitch calculations. Try eBay for propellors and monel rivets some bargains to be had.

 

[PeterGibbs]

One tweak to what you have been advised by others: a prop delivers only what the engine will support so do get a proper prop sizing for your hull, engine output, gearing etc. Output is improved, if the engine will support it, by the degree of "disc" presented by the prop. Therefore a three bladed prop is likely to offer a fuller disc than a two bladed. This confers greater thrust but perhaps a little more drag when sailing.

I permit my 3 bladed prop to freewheel when sailing - no noticeable drag produced. But when I need the power to get a wiggle on, the 3 blades deliver it! I have had 2 bladed props in the past on earlier yachts - good for churning the water, less satisfactory for making way into a choppy sea!

PWG

 

[Pye_End]

Agreed. Choice of props is not just about full hull speed in a flat sea. It is about grip whilst manoevering and boat speed whilsty motoring into wind and waves. This is where surface area is important (this reduces cavitation). Also choice of pitch is also important - do you want the power to be transmitted whilst the boat is motoring at top speed or when the boat is going slower, but you want it to be going quicker.

Prop calculators use hull speed, but in my view that is your starting point not your ending point.

Pitch calculation - there is a post on this a few months back if it is not stamped. It is slightly more complex than you first think.

This might be helpful

There was another with a better description of how to measure pitch but cannot find it at the moment. If you want more then let me know.

 

[PCUK]

Pitch calculation:
Measure angle between maximum leading edge and maximum trailing edge of prop.
For example: 40 degrees.
Divide 360 by answer, in this case 40 = 9
Sit prop' upright on trailing edge of boss and measure distance from surface to maximum height of leading and trailing edges.
For example: leading edge 3 inches
Trailng edge 1 inch
Deduct smaller from larger = 2 inches
Mulitply 2 inches by 9 (from earlier calculation) = 18
PItch is therefore 18 inch.

 

[Aja]

[ QUOTE ]
Hi All.
...but looks as if someone has squared off the tips of the two blades, or has anyone ever seen blades like this before? ( not radiused tips, but sort of flat) am wondering if it was done to prevent blades hitting hull or skeg.[/QUOTE]

I have seen this before. It is done after manufacture to reduce 'singing' in the propellor at certain revs

Usually caused by cavitation at the blade tips. The process to try to elimenate this is to square off the tips - as you have.

Donald

 

[Clyde_Wanderer]

Thanks all,for your replies, but as you can understand I am still as much in the dark.
My boat is 30ft, fin keel, weighs aprox 3.25 ton, or is that same as displacement, has a Perkins parama 12hp engine, and a ZF5 gearbox.
What I really need is a prop that will give me a little more speed against tide etc, without having to rev the engine to its full revs.
Would a three bladed prop with a higher pitch not give me that?.At the moment my boat is exceptionally slow,even taking the dirt on the hull into consideration, and i know the engine is bordering on the minimum size for the boat size.
I would have imagined that a three bladed prop would give a little more speed than a two blade, but some say yes and some say no, so who do I believe?
Again Thanks for your replies, Cheers, Eamonn.

 

[Pye_End]

Do you achieve full revs currently? You really want to aim on matching a prop that will achieve, (or pretty well), full r.p.m. To put a prop on which is delibertately overpowered will mean that you will miss out on some h.p. from your engine. If you are happily reaching full revs now it is possible that you are under-propped or correctly propped.

What is the gearbox gear ratio? You will need this to work out shaft speed, and therefore work out correct prop.

Generally a 3 bladed will improve engine performance, one way or another, but top speed is governed ultimately by the hull - what is your hull speed and what are you currently getting?

Raising the pitch alone may or may not improve ultimate boat speed. If you find that you are getting cavitation it may be the wrong thing to do - raising the blade area (eg going to 3 blades you may find puts extra load on the engine and does some of what you are aiming to do. It may or may not raise your top speed in a calm (depending on where you are with hull speed), but it will certainly improve your top speed motoring into more sloppy conditions.

Think you need to put your particulars into a prop program (or get a prop supplier eg Bruntons to do it for you) and start with that.

You will need to supply them with the taper as well to match with your shaft - just measure the holes at either end, and the distance between them will be fine, or calulate the angle if you want.

One other point - slow motoring speed - make sure your prop is clean - in some instance more important than the state of the bottom.

 

[ostra4]

purchased some monnel rivets from foxs marina ipswich , ordered 6 rivets , delivered in two days , they had assorted sizes , havnt got phone handy but check their web site and you need to speak with the rigging dept .
regards
andrew