Which prop is best ?

[MARNEN]

Not unrelated to my recent post re prop shaft projection.
I have two props. The one that was on the shaft which has 16x9 SL 4403 stamped on the boss, and a spare that came with the boat which has 16x8 1188 7250A TM stamped on it.
Any body in the know able to advise me which would be best to use.
The boat is 9tonn ,steel, 32 feet o/a length with a full keel and transom hung rudder.
Thanks as always for any help .

 

[macd]

There are various prop calculators on-line to help with that decision, and many earlier threads on much the same subject.
You'll need engine power and waterline length, as would anyone helping making the choice for you.

 

[Tranona]

Impossible to say without doing the calculations. As macd says, put your boat data into one of the on line calculators such as Propcalc on www.castlemarine.co.uk

However the two are very close, just 1" different in pitch. The lower pitch one will allow you to achieve approx 200-250 engine rpm more. You should aim to use the one that achieves closest to maximum rpm in flat water. The sizes sound roughly right assuming you have a 30hp or so engine and a 2:1 gearbox.

 

[Trundlebug]

Contact a prop manufacturer or refurbishing company like
http://aewoodward.co.uk/

or

http://clementsengineering.co.uk/products/marine-stern-gear/propellers/

Tell them the relevant info about your boat, including engine and gearbox info as well as the above info in your OP above. They have professional design software that will calculate the right prop for your boat.

Although there's still nothing quite like practical experience with a displacement boat, especially if it has more than adequate power to achieve hull speed. In those circumstances overpropping is common and can be of considerable benefit.

 

[Stemar]

If there's somewhere you can dry out easily to swap, try 'em both. Assuming they're both 3-bladed, as Tranona says, without doing the sums, neither sounds far wrong.

The finer pitch should give you a bit more drive into a sea at lower speed, the coarser one should give slightly more speed at lower revs in benign conditions, but the difference is unlikely to be huge.

Before you do, give each of them a welt with a bit of wood. If one rings and the other thuds, use the one that rings and throw away to the one that thuds - it's losing its zinc.

 

[lw395]

Presumably the previous owner changed to the one fitted, because the other one was not ideal?
If these are fixed three blade props, either will be equally effective at slowing your boat under sail.

 

[Tranona]

I see from your other thread that you have an 18hp 2GM. If indeed your boat does displace 9 tonnes (on the very heavy side for a 32', even if it is steel) there is no way you can achieve hull speed. I suspect you have got a 2.6:1 reduction to reduce the shaft speed to be able to swing a 16" prop and that the 8" pitch one would probably be better as it might allow you to reach full rpm to use the maximum of the meagre power you have from the engine.

You really need 30hp minimum to shift that amount of weight if you are going to rely on the engine to maintain cruising speed in adverse conditions. otherwise it is is an "auxiliary" in the traditional sense rather than an alternative source of power to the wind!