Composite Props

[jwalker]

John, I'm a bit lost by your comment. Do you imply that a light prop is likely to allow the clutch to chatter?

[Ed the Duck]

Have got a propulse prop on my 40hp engine. Can confirm that you lose abit of top end speed compared to the 3blade alloy prop - but this is prob as much due to the 4blade area as to differences in material.

I run the boat onto and off a sandy beach covered in a thin layer of small stones- metal props always seemed to take quite a few dinks that didn't seem worth repairing (but no doubt reduce the effectiveness). I have ran the propulse prop for a year now and the leading edges seem to shapen themselves rather than deform. (Will try and attach some pictures) Havent had to replace a blade yet.

The ability to change pitch is useful - I can drop it right down on days when the surf is up and can lengthen it on flat days.

Having a SIB, I prefer to carry two spare blades rather than a full spare prop - the theory being that even if I shear off all 4 blades I can replace 2 of them and limp home with a balanced prop. - As I say theory - this prop seems pretty robust so far.

All in all, more than happy with the prop.

[Simon B]

JW,

The dog clutch thing, is this because it is easier to start and stop a lighter prop? which theoretically is better mechanically but being light the prop has no flywheel effect so will slow more quickly when loaded up.

[jwalker]

Originally Posted by Simon B

JW,

The dog clutch thing, is this because it is easier to start and stop a lighter prop? which theoretically is better mechanically but being light the prop has no flywheel effect so will slow more quickly when loaded up.

Yup.

[quinquarimarine]

Originally Posted by jwalker

John, I'm a bit lost by your comment. Do you imply that a light prop is likely to allow the clutch to chatter?

JW. A couple of things came to mind in my previous post as.

1. Yes, there is some chatter but this may be only with the power and loads that we use. The engineering on the plastic props, as you found, is not perfect. For example grab the blades and see how much movement there is between them and the hub.... a few mm as a norm on a pyra.

2. The actual engaging of the dog itself. We all know that on an OB we must make a positive action on the dog to avoid gear wear. The action is created at the control box and transmited to the gear box. However at the far end is the resulting action of the prop (with the integration of engine forces). Thus at the end of the scale is engaging a shaft many times with no prop on it. This will not give a "good dog" movement. Inbetween is a prop with poor bite such as a plastic prop.

What I am putting forward is that many only look at the engine side of the prop shaft .... the prop side must account for forces as well.

Now for the honest bit !

I studied for three years propellor theory as part of N.Arch but it was all for big slow boats. It was a long time ago, cant remember much and it was nothing to do with outboard props. !!!

Notwithstanding the above we are aware from 15 years of practical commercial experience that the combination of propping/boat/operator will define dog life. A prop with slip will cause problems on the dog.

To add complexity you must remember that on our operations we may have 100 gear changes per hour (in and out of caves and the "Bitches" etc).

ps. hows the boat coming along !

Cheers

John
www.quinquari.co.uk

[jwalker]

Originally Posted by quinquarimarine

1. Yes, there is some chatter but this may be only with the power and loads that we use. The engineering on the plastic props, as you found, is not perfect. For example grab the blades and see how much movement there is between them and the hub.... a few mm as a norm on a pyra.

John

On the Piranhas I used, the blade location was very firm. The blades actually needed to be driven the last few millimeters into the hub by the clamping plate. From your experience, it seems as though the quality may be variable.

2. The actual engaging of the dog itself. We all know that on an OB we must make a positive action on the dog to avoid gear wear. The action is created at the control box and transmited to the gear box. However at the far end is the resulting action of the prop (with the integration of engine forces). Thus at the end of the scale is engaging a shaft many times with no prop on it. This will not give a "good dog" movement. Inbetween is a prop with poor bite such as a plastic prop.

What I am putting forward is that many only look at the engine side of the prop shaft .... the prop side must account for forces as well.

I don't understand your reasoning at all. The dog is simply a set of square cornered spigots which slide into location in an opposing set of similar spigots within the drive gear. The reason for requiring a crisp action at the gear lever is to engage the spigots with as little clipping of the corners as possible. As the spigots engage, the prop has to accelerate up to speed instantly. The more inertia the prop has, the more impact there will be as the dogs(spigots) locate.

IMHO, of course.

ps. hows the boat coming along !

It's complete. We are both waiting for the gales to subside!

[ADS]

Just in case anybody with a 50HP-90HP Merc/ Mariner wanted to 'bite the bullet' and give one a try there is this propulse on ebay:

http://cgi.ebay.co.uk/ws/eBayISAPI.d...516734220&rd=1



Alex