Prop Advice

[Jango]

Sea Zar, Happy to help out.
Be sure to try and stay with a 14 dia. or perhaps 14 1/2 max. A larger dia (15+) will require a smaller pitch.
Let us know how it works out.
Jango

[Sea Zar]

OK
I've decided not to muck around & have decided to purchase a new prop which I will pick up on wednesday. I've decided this because I think that I really need the option of exchange.

I'm going with a 'Power Tech offshore series' Model OFS4R. Any pro's & con's on this prop are very welcome please.

I have given them all the specs & they have decided that the best option in my case (slippage, deep 'v'..etc) would be a 4 blade, 15.25" diameter, 16" pitch model. A moderate high rake & full tip... which they recommend for offshore intentions. The say that I won't beleive the difference. They also recommended I take on this new rubber hub that apparently never needs replacing. Don't know much about that. I'm still trying to find it on their site.

I would have thought a 17p would be more appropriate but we will see when I take it out for a run.

Seazar

[Sea Zar]

Picked up prop Monday & took it for a run. Though the control & handling was superb the pitch was way off. I lost 5 knots all round & engine maxed out in revs.
I exchanged it for an 18" pitch.That brought the revs down at WOT to 4950rpm. Cruise speed is 25knt at 3000rpm. Still not quite what I want. I'm trying to acheive 30knt at 3000rpm & WOT at 4500 to 4600rpm. So they have decided to up the pitch to 21. It's been ordered & should arrive next week from the U.S hopefully.

I would have thought that the equivalent pitch (19") to the 3 blade would have been correct- like we discussed here. I'm very surprised that it is going to manage 21" in pitch?! Any ideas why?

SeaZar

[Jango]

Sea Zar,

If the blade area remains the same, (Dia) you will drop 300 RPM for EACH inch of pitch added.(with a 1.50:1 gear). If it doesn't drop 900 Rpm, 3" x 300 RPM it is because the blade area is smaller.
Quite often Prop manfacturers DECREASE Dia slightly as Pitch increases. Smaller Dia will offset some RPM drop, but with added slip. (decreased effeciency)
Good luck, you're probably getting close.