Re-pitching prop

[Ken13559]

After servicing on my Yanmar 2GM20F and polishing up the Campbell Sailor 3 blade prop, I can do 3650rpm where previously the max I got was 3350rpm. Now I thinking of re-pitching my prop from 6"Pitch to 14"D x 7"P to get more drive and to take advantage of the available margin in rpm.
The additional 1" probably drop my max to 3450rpm.
Have I got it right ? Should I or shouldn't I re-pitch ?

 

[petersea]

My opinion for what it is worth is leave it alone. Here is why....Are you 100 % positive your tack is 100 % accurate ?? Yanmar tachs are not noted for accuracy so get it checked before you do anything ! second reason, I am assuming you just relaunched boat recently because you polished your prop so you have a "like new" prop with no marine growth and presumable your boat bottom is also clean as a whistle so when you get some growth your figures will change slightly. Also cost of re pitching and cost to change out prop unless you are diving and doing all this yourself don't appear to give you very much more. At what rpm do you reach hull speed would be some good info to put into this equation also. 3650 is a little fast in gear how many rpm do you reach in neutral ? What service work was done to engine ? just some things to think about before doing anything.

 

[Ken13559]

Pete,
I dive under to polish the prop & shaft using a Hooka. The boat was launched 6 months ago. New oil and new exhaust mixing elbow (old one was choked up), adjusted tappet/valves and repaired raw cooling water pump (was leaking). Last I checked, my Yanmar tach reads 50rpm more than handheld tach. So the 3650 could actually be 3600rpm. My hull speed is supposed to be 7.09kts but I never got near it. At 3650rpm, I'm doing about 6.1kts. I'm two minds here; I'm comfortable with the engine running smooth and prop is responsive if I gun at throttle and yet I feel the engine rpm has some margin which if I re-pitch the prop I could get more power to move against tide/wave. So I'm not sure if decision to re-pitch is right or should I just leave it. I'm pissed off evert time I motor alongside my friends at same speed and when I asked what rpm they were doing and was told 1500rpm or about 1800rpm when I'm doing 2300rpm or more. One has same engine as mine and the other about same horsepower although both are 28footer and mine is 32. My boat displacement is just about 2500lbs more than the others.

 

[petersea]

Ken, another suggestion is to get together with a good prop shop and run the numbers for your boat. Check in archives there are some threads about Campbell sailor props and a place in California that has a good reputation. You will need engine and transmissions spec as well as boat info including weight. See what they say about the prop you have now, and ask their thoughts. Did you install the original Campbell prop ? or PO ? I'm am not a big fan of re pitching props IF they were set up original and worked correctly, maybe yours was not ! One degree greater prop pitch will generally reduce rpm by 100 rpm. Get set up with a prop shop and lets us know the outcome, you should be able to reach hull speed at less then full throttle. Something does not sound right with your set up.

 

[Ken13559]

Pete, I installed this prop. Think I'll talk to West by North (or is it North by West) and see what they say. Thanks.

 

[Don12364]

Ken,
Your 6" pitch seems incredibly fine. Whilst we don't know whether your gearbox ratio is 2.2 or 2.6 or 3.0:1, a more normal pitch would be from 10" up to about 14". (Gearbox ratios on www.marine-power.co.uk/2gm20f specs.htm).
The ratio should be on a label on the gearbox or in the makers pamphlet about the engine.

Once you know this I suggest you visit www.castlemarine.co.uk and click the blue word 'PropCalc'. Scroll down and download and then run 'Propinst.exe'.
This will give a suitable prop for your boat/engine combination though minor refinements could be made from then on - and you can impress your friends too!
You can input your own prop diameter but I guessed some details for your boat and it calculated 15.8" diameter by 12" pitch for a 2 blade and 14.7" x 12" for 3 blades.

 

[Ken13559]

Donalex, thanks. I'll check out the site. My gear raito is 2.62:1.

 

[petersea]

Ken, you may want to check out the boat info section here on Hunter Owners, spec listed says hull speed is 6.0 for your boat. Maybe you are trying to get an impossible speed ? ? something more to check......

 

[Ken13559]

Ken, you may want to check out the boat info section here on Hunter Owners, spec listed says hull speed is 6.0 for your boat. Maybe you are trying to get an impossible speed ? ? something more to check......

oh..., I uses a web program to calculate hull speed. Ok I'm not expecting to reach 7kts. Yes, more to check. Thanks.

 

[petersea]

OK maybe be bad info entered for the 326 hull speed I check some others same size and your figure is closer. I also did a hull speed calculation based on 28.4 feet water line length and speed should be closer to 7.1 not 6.0. next time I will do my own math rather then rely on pre entered info. The prop shop still looks like your best bet. When you purchased the Campbell sailor how did you or they determine sizing ???

 

[Ken13559]

They gave me 14"D x 8"P at first. Good power but max at 2800rpm with black smoke.
Return to WbyN for check and it was re-pitch to 14"D x 6"P. No more problem with smoke, works fine but just doesn't seems to have much power (unlike the first pitch) to tackle tide/wave though. Now after I think my engine is ready to take on alittle more pitch.

 

[Shaun]

Prop Pitch

Hey, What did you find out on our prop pitch?

I have a 94 model 33.5 with a Cambell sailor 3 blade prop. My max rpms are about 3,100 instead of 3,600 while in gear. I'm thinking I should be able to get more rpms out but the rear end of the boat drops down about 6 inches while motoring at these sort of speeds, so it may be trying to outrun the bow wave. My engine also runs warm at over 2800 rpms.

So..

I've just replaced my exhaust elbow this week, (my first time, it was 9 years old) and found it to be about 75% clogged on the exhaust side. Im thinking that I may get better rpms on my next time out on the boat due to the cleaner exhaust flow, less backpressure.

I suspect Im a little over-propped but I don't get any black smoke when pushing it.

 

[Ken13559]

well after re-pitching, my rpm max out at 2900-3000. I expect 3200-3400rpm. anyway, its got better bite and seems faster. i haven't time to do proper warmup and try max rpm again, in anycase i'll live with what i have .........for now.