Prop speak

[Romany 162]

I am up for a new prop unhappily.

Currently have a 16 x 12 Briski 2 bladed non geared folder which has given years of service - but alas no more. It drove a Townson 36', and was driven by a Yanmar 3GM30.

 

Aside from the obvious solution of buying a new one of the same spec (around $1000 new), what other options should I consider? Have heard good and bad things about Kiwi Props but little else about anything else.

 

I understand 16 to refer prop diameter, 12 refers to pitch (in degrees) - or am I totally off the mark?

 

What level of performance would I expect to achieve if I were to fit a 15 x 9, 2 bladed non geared folder. A single word response would be OK but don't let me stop you explaining the full range of factors I should take into acccount before I pay my hard earned dosh.

cheers.

[Fish 0]

We have a Kiwi prop and are very happy with it. Its on a Birdsall 37 with a 37hp Izuzu, so fairly similar in general size and weight to a Townson 36.

I can't remember how long we have had it, but in the order of 10 years. They do need regularly maintenance, i.e. if you dry out every 6 months or so they need a once over for barnacles (the same with raw water intakes and stuff) A bush thing did split and the prop wouldn't feather, but that was after about 10 years I think, was replaced without cost for the item.

 

Performance wise, great. Have a vague idea ours is 17. They are very very good at going in reverse, as the blades flip around the right way and are a propler facing backwards. Very good for parking in tight spots. Forward speed is excellent, 6.5 knts normal, 7 knts in flat water, plenty of 'torque' for pushing into a chop.

 

I don't actually know, but think a Kiwi is a wee bit more than $1,000 though.

[Romany 162]

$110/inch. I checked.

 

And thanks for the feedback.

[Bogan 8]

If you switch to the 15x9 you will find that your engine must rev higher for a given speed . Ignoring slippage, your boat will progress 9 inches per prop revolution instead of 12 for your present prop.

 

If your engine doesn't get close to its max revs with the present prop this may be good - your engine will rev more freely and be more powerful when heading into a strong wind or big sea (by being able to develop more power at a lower speed).

 

On the other hand, if your engine easily gets to max revs with your present prop, you don't need the lower pitch, you will lose alot of top speed, and your fuel comsumption per mile will probably increase.

[wheels 543]

Bogan is pretty much on to it.

Prop diameter is kind of akin to wider tyres on your car. The more rubber on the road, the less slip. Slip is inefficiency. But diameter is limited by the distance between the tip of the Blade to the bottom of the Hull. You take the Diameter of the prop, times that by 10%, and that distance must be no less than the 10% sum. If it is too close, then you get a terrible hammering affect.

Pitch is the angle of the blades. As Bogan said, the 9" means that for one revolution, your prop should move forward 9", less the inefficiency, or slip. At low revs, the slip is big, but reduces to much lower figures at speed. A really good prop may get to 10% efficiency. A two blader folding is not so good. The number of blades is also all about getting that "grip" in the water. But more blades means more surface area in the water and that takes more power to turn it.

Have you seen this one

http://www.trademe.co.nz/motors/boats-m ... 381589.htm

[Island Time 1,232]

I have a 17" kiwiprop you can have for $1000 if you want it. I took it off Island Time when I repowered with a larger engine, as it was too small. Im very good condition, would not have 200hrs on it. Three blades are HEAPS better, fwd and reverse, and being pitch adjustble, will suit you perfectly. You may have to change the spline - this one is for a 120s Volvo saildrive, not sure if it's the same or not. Kiwiprops will rebush with the right spline if requred. PM me if you are interested.

[Romany 162]

Thanks to all for replies.

 

So Wheels let me tell you how I hear your comment - and converting these inches things to metric just because all my tapes are that. Please correct me if I have misunderstood.

I need to determine exactly where on the shaft (or the line thereof)the prop will be located, and then measure from that point to the hull at 90 degrees to the line of the shaft centre. In the case of say a 406.4mm prop (16 inch x 25.4mm/inch = 406.4mm), the tip of the blade should pass no closer than 40.4mm to the hull?

 

I presume that prop measurements state the distance between points of the blade which are furtherest from the centre of the shaft. In the case of a square tipped blade this could well be the leading corner of blade 1 to leading corner of blade 2 ? Or is the difference so small as to be irrelevant?

[wheels 543]

Yep youv'e sussed it. And nah it's not something you have to be dead perfect with. The bigger that gap, the quieter the prop noise into the Hull. That 10% minimum is the standard industry rule of thumb minimum, or the noise becomes too much and the hammering too hard on things.

The trademe prop I assume has come off a shaft. I am not sure what you require, so ensure you have the correct shaft mounting before commiting to any prop.

What exactly is wrong with your old prop? Have you had it checked by and engineer for rebuild?? It should be easy enough to rebuild a Bronze Prop.

[Romany 162]

Biggest issue is that approx 50% of it is located somewhere off the Naval Base

 

Its coming down to finance as these things always do...

 

Thanks for your advice.

[wheels 543]

So you have just lost a blade? or has it broken something else. The cheaper unit I pasted the link to, could provide you with a blade. Then you simply get the blade set to the required pitch by a propeller outfit. Or contact the various propeller places and ask if they have a blade either new or second hand.

[Chewing Gum 17]

We have 3 blade feathering autostream about 10years old and it has never had any problems. Three blade is definitely the way to go. They are worth every cent. Two blade folders are rubbish by comparison for drive especially in a chop and in reverse.