Props

fasterfitz

Anyone here got any knowledge of Boat propellors ?

i have bought a boat but we damaged the prop on it. Now I want to get a better prop for it but as far as i can see I have a choice between a trade off in take off power and top speed. Basically can we not get a prop that does both?

Any help and comments appreciated....

elmariachi

hi fasterfitz,
I think Iknow what you're getting at here so i'll do found this for you.
From working and driving rescue ribs for the past 4-5 ribs I can say that choosing a stainless ,preferably heat treated, prop will provide you with many years of good durability that seems to evade the weaken aluminium cast propellers.
Here's a link to some advice from a proof read site which is top notch
good luck
http://www.theoutboardwizard.bizhosting.com/how_to_choose_the_correct_propeller.html

Spinnaker

a yacht or rib ?

we replaced a prop on a sigma33 last year. expensive and a a marine engineering job!

if its a yacht:

- gori props (italian) are well recommended
- be sure to protect your new prop with a shaft or other anode
- i can provide with the marine engineer who changed ours

JohnCleary

Firstly, there are 2 factors when deciding the size of your prop.
Diameter and pitch. Now the diameter will be set, you want to get the largest diameter as possible without hitting the cavitation plate or going below the skeg. On my 75HP Yamaha the diameter is 13 3/4"

Now the factor that effects acceleration & speed is the PITCH of the prop. The pitch is how far, in a perfect world, the boat would move with 1 rotation of the propellor.

If your pitch is too high, the propellor will try and cut too much water when moving, thus putting more of a load on the engine and slowing down your acceleration and top end speed.

If your pitch is too low, you will have great acceleration but a low top-end speed, and poor fuel economy due to your engine over revving.

In regards to prop-repair, Montega (google it) Engineering in Dublin will do a full repair of any aluminium prop for 70euro and they do a good job too!

This is just a general guideline off the top of my head. Ive been engrosed in boats/boat mechanics for about 12 years now and bought my first RIB in 2002, though I had been driving them since the age of 12-13. I have been volounteering with the RNLI for over a year now and captain my inshore lifeboat since completing the ISA Powerboat Level 3 course. If you want more help youll have to be more specific, ie. what type of boat, what engine (make and model)

Powerboat.ie

I would endorse John's opinion on Montega. If you cannot find them on Google give Phil Knowd a ring in our office Lo Call 1890 70 40 90 or email him at courses@powerboat.org and he'll send you contact details.

If you drive a prop with even a small chunk out of it the prop will be unbalanced and this can cause gearbox and egine wear up along the drive train.

One other factor on pitch.. Too low a pitch means not enough load on the motor which can also lead to over revving.

As John said , if you can tell us what type and size are your boat and engine we should be able to give more accurate advice.

Kind regards,

Stuart

JohnCleary

Stuart - I must contact you RE ISA PowerBoat Level 2/3 courses. I rang you last year and you rang me back but I had so many things going on at the time it just wasnt possible!

Rumour tells me things are going very well for you at Hudson Bay, hope all is well!

-EDIT-

Montega Engineering Ltd.
Jamestown Road
Finglas
Dublin 11
Tel: +353 1 8362315
Steel fabricators. Specialists in marine engineering and
repair of bronze, aluminium and stainless steel propellers.