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-   -   Increase Propeller size? (https://www.cruisersforum.com/forums/f114/increase-propeller-size-116585.html)

Downhill 04-12-2013 16:45

Increase Propeller size?
 
I'm looking to replace my fixed 3-blade 16" prop with a VariProp Feathering Propeller on my Morgan 323. The 16" propeller gives me roughly a 2" clearance between the blade tip and the hull. Here's the question:

Is a 1.5" clearance too tight if I would go with a 17" prop and is it even worth it with regards to power/thrust?

Any insight is appreciated.

Cheer!:thumb:

MARINE ENGINNER 04-12-2013 23:45

Re: Increase Propeller size?
 
Quote:

Originally Posted by Downhill (Post 1407205)
I'm looking to replace my fixed 3-blade 16" prop with a VariProp Feathering Propeller on my Morgan 323. The 16" propeller gives me roughly a 2" clearance between the blade tip and the hull. Here's the question:

Is a 1.5" clearance too tight if I would go with a 17" prop and is it even worth it with regards to power/thrust?

Any insight is appreciated.

Cheer!:thumb:


Dear Downhill what is your engine specs? hp , rpm , hp on crankshaft, engine type model , brand
by having a tighter clearance you will feel more vibration by the propeller thrust force

MARINE ENGINNER 04-12-2013 23:48

Re: Increase Propeller size?
 
also transmission type and brand , gear ratio.
thanks

Kokanee 05-12-2013 00:52

Re: Increase Propeller size?
 
For clearance - the general recommendation for a 3 blade prop is 15% of the prop diameter for clearance above prop, and 5% clearance below to avoid vibration. You only have 12.5%. It appears you are slightly over this already.

As a quick check to see if your prop diameter and pitch are correct, run your engine at wide open throttle. If the prop is matched to the engine and boat, a diesel engine should only just reach the maximum rated rpm of the engine under full throttle.

If it is under-propped, your engine will reach maximum rpm too easily. If it is over-propped it your engine will not reach max rpm and you may see black smoke in your exhaust from unburned diesel. Either is not good for the engine.

Downhill 05-12-2013 05:15

Re: Increase Propeller size?
 
Good info. Thank you. It confirms my inclination to stay with the 16", even though the sales rep says: "no problem with the 17..."

btw I have a 4-108 Perkins, ~38hp @ 3000 RPM with a Hurth 1.88:1 transmission.

MARINE ENGINNER 05-12-2013 09:23

Re: Increase Propeller size?
 
Quote:

Originally Posted by Downhill (Post 1407605)
Good info. Thank you. It confirms my inclination to stay with the 16", even though the sales rep says: "no problem with the 17..."

btw I have a 4-108 Perkins, ~38hp @ 3000 RPM with a Hurth 1.88:1 transmission.

dear Downhill with the program i have and the specs you gave plus some i found in the workshop manual of perkins , it recommends me a 16"x10"pitch propeller

what is your current prop?when you engage do you hear a slight """gap""" noise or a little bigger ,windows of cabins closed and wood hatches of the engine compartment :-).
What i want to tell you is for example jeanneau to their 49ft yachts with shaft year from 2002 to 2007 they increased the pitch to compete the bavaria speed with the saildrive. so with this one inch of pitch you have more ahead force then one inch less pitch when you engage same idle rpm more thrust torque

MARINE ENGINNER 05-12-2013 09:25

Re: Increase Propeller size?
 
forgot to tell you that Kokanee is 100% correct about what he says test also this to check whether your engine corresponds to your current prop

Cheechako 05-12-2013 09:53

Re: Increase Propeller size?
 
16" is plenty on that boat.

Downhill 05-12-2013 10:05

Re: Increase Propeller size?
 
My current prop is a 16"x15. The max RPM I can get is 1500 and at 1000 RPM I'm at 6kn. That's also a reason why I want to replace it. The PO had it repitched to a 15, but I don't know why. Save fuel??? It looks like I'll stick with the 16" prop and VariProp's have an adjustable pitch. That allows me to dial it in for correct RPM's.

MARINE ENGINNER 05-12-2013 10:28

Re: Increase Propeller size?
 
Quote:

Originally Posted by Downhill (Post 1407807)
My current prop is a 16"x15. The max RPM I can get is 1500 and at 1000 RPM I'm at 6kn. That's also a reason why I want to replace it. The PO had it repitched to a 15, but I don't know why. Save fuel??? It looks like I'll stick with the 16" prop and VariProp's have an adjustable pitch. That allows me to dial it in for correct RPM's.

wowwwww.... this a high pitched prop..when you increase your RPM you should reach the maximum rpm of the engine.
No downhill you don't save fuel, your fuel consumption is same and you destroy your engine like this.it works continuously at 100% of load plus no good air suction from the intake valves so you have build up of excessive carbon in your engine
you whould reach the maximum speed of your boat letys say 7.5 knots with max rpm of your engine not the 50% of the MCR
MCR (Maximum Continuous Rpm)
all the engine have drawing with the reversing gear attached and tested
rpm-hp drawing
do you have black smoke at 1500 rpm?

you should go to a proper propeller workshop and find correct propeller for your boat

MARINE ENGINNER 05-12-2013 10:30

Re: Increase Propeller size?
 
for a so big pitch you should at least have a ratio 3:1 to you reversing gear

Cheechako 05-12-2013 10:51

Re: Increase Propeller size?
 
Wow, you sound way over propped. Good that you are changing. I had a 17" max-prop behind my 4-108 (in a 44 foot cutter) Performance was excellent. Personally I wouldnt pitch it for max rpm on a 4-108, but maybe something like 3000 rpm. 2300-2500 is a real nice sweet spot on those engines and you have plenty of engine for that boat. I wont get into the de-rating of engines for comercial vs pleasure use discussion, but it's fine to prop that engine well below max advertised rpm! Not as low as currently pitched though!

CptJ-GS50 22-01-2014 18:49

Re: Increase Propeller size?
 
Repowering my Gulfstar 50 I called Bay-Prop (Alameda) Getaprop (Anacortes) each of the ran my boat and engine info thru the computers and each came up with the same recommendation.

SkiprJohn 23-01-2014 12:49

Re: Increase Propeller size?
 
As the other folks have said, you really need less pitch and I agree that a 16x10 would be good. Your max rpms are way too low. You are running at just above idle speed at 1000 rpm. This will eventually cause overheating in your engine when trying to push it through strong current or wind speeds. That's a great engine you have there and you'll find it really great once you match your prop to engine and transmission and boat.
Good to have you here.

CptJ-GS50 26-01-2014 21:02

Re: Increase Propeller size?
 
Vicprop - Prop calculator for Displacement and semi-displacement hulls

I used this calculator and came very close to what the Professionals came up with. It might be worth a shot.


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