The Proper Prop

Dec 28, 2016
29
Hunter 34 MiddleR
I have a 1987 Hunter 34, with a 27hp Yanmar 3GM-30F , turning a two-blade prop. The boat is very slow, when under power. Can anyone advise as to whether I can put a three-blade prop on the shaft ? If so, what is the correct pitch and diameter ? The shaft is either 1 1/8" or 1 1/4" in diam. I need to measure it.
The boat motors at 4 mph when running the engine at 2,000rpm's. I don't like to push the engine much harder than that; she's 33 years old. But even at 2,500 rpm, the boat is still only doing about 5 mph. When the wind is good, I can sail her at 6+ mph. I check my hull and prop once a month for any growth and fouling. That has not been an issue over the years. The engine has always run very well and smooth. It's just that I have noticed that other sailboats while under power, are always cruising past me at a higher speed. Also, while on the hard this time of year, I have seen that other sailboats, even the smaller ones, have three blade props. It's rare to see another boat with a two blade prop like mine has. So, I was just wondering if it's worth the trouble to replace it.
 
Nov 6, 2006
9,225
Hunter 34 Mandeville Louisiana
First thing is to check your tachometer with a laser one to see what the real RPM is.. The Yanmar tachometer on our boats is notoriously inaccurate.. I normally run my engine at 2800-2900 rpm (real RPM) The engine is designed to run day in and day out at 3400 rpm, but my engine seems to have a sweet spot at 2800 ish and uses about a half gallon of fuel per hour at that RPM. ..
something like this is sufficient: Amazon.com: AGPtek® Professional Digital Laser Photo Tachometer Non Contact RPM Tach: Automotive
The propeller on my boat is a 15 inch diameter by 15 inch pitch, as supplied originally by Hunter. .. my gearbox ratio is 2.63.. Boat will motor at 6.5 knots or so ..
The theoretical hull is about speed is 7 knots.. which, if the prop is correct for your gearbox ratio, would be reached at a real RPM of 3400-3600 RPM. For this kind of discussion, speed needs to be measured as speed through the water and not GPS speed which would be affected by currents..
in a nutshell, verify gearbox ratio (tag on the gearbox), Verify the propeller diameter and pitch (should be marked on the hub), Verify engine RPM ..
Your two bladed prop has less drag when sailing than a three blade prop.. if you sail more than motor, and it is the correct pitch and diameter, it sounds close.. if you motor more than you sail, then a three blade may be your right choice.. lots of folks are satisfied with Campbell Sailor props and it is reported that their three blade has the least sailing drag of three bladed props.. If you decide to go that way, get a local propeller shop involved for sizing and balancing..
EDIT: I didn't see the other posts from another thread... but I think the answers are all consistent
 
Dec 25, 2000
5,048
Hunter Passage 42 Shelter Bay, WA
Unless someone here has the information you need, I would defer to a prop shop for the correct three blade for your boat. Ours came with a fixed three blade and I really like how it maneuvers the boat. Before making the switch what happens when you push the RPM to say 3,200? That should be about the normal cruising RPM for that engine without doing any harm. Our first boat had a 2GM engine with a two blade fixed and it would deliver about six plus knots at that RPM.
 

SFS

.
Aug 18, 2015
1,972
West Marine Kayak Tampa Bay
Barn-dog, it is usually adequate to start just one thread on a topic. Now that you provided additional specifics in your first thread, the answers have become more specific in addressing your question.
 
Sep 20, 2014
1,282
Rob Legg RL24 Chain O'Lakes
Don't waste your money on a tach:
I would think you should be able to see your exhaust pulse pretty well.
Depending on your phone, there are other apps that will strobe your flash just like a timing light.
 
Dec 28, 2015
1,357
Laser, Hunter H30 Cherubini Tacoma
Don't waste your money on a tach:
I would think you should be able to see your exhaust pulse pretty well.
Depending on your phone, there are other apps that will strobe your flash just like a timing light.
That guy has got some brains huh?
 
Feb 21, 2013
3,776
Hunter 46 Point Richmond, CA
Since your boat is on the hard I would measure the prop's circumference then divide that number by 3.14 to arrive at the diameter. Both my 1987 Hunter 31 and 2004 Hunter 386 came with a two-bladed prop and could not achieve hull speed under motor. After replacing both with a three-bladed fixed prop I was able to achieve close to hull speed under motor. So its your call if you want more speed under motor. I find the three-bladed prop also provides better acceleration, which helps maneuverability, and maintains speed in choppy water. As Terry Cox suggested contact a prop shop for proper prop size and quote, depending on the type of prop (fixed, feathering, folding) you want and how much you want to spend.

For reference you can calculate the hull speed of your boat in knots as follows: square root of the waterline (not overall) length in feet multiplied by 1.34
 
  • Helpful
Likes: jssailem
Sep 20, 2014
1,282
Rob Legg RL24 Chain O'Lakes
That guy has got some brains huh?
It depends what his career is in real life. In my past life I was an audio engineer, so what he is doing makes perfect sense. Now that I think about it, you could probably just use a frequency counter app. Put some sort of noise making flapper on the flywheel and measure the frequency of the noise. I use a frequency counter app made by Keuwlsoft for Android.
 
Oct 22, 2014
16,090
CAL 35 Cruiser moored EVERETT WA
Wow Mike. You are right That guy was smart. And what a cool websight.

I found this discussion.. "The Dark Side of Solar"!

I knew about this but had never seen the information assembled in a rational form. An interesting read as we consider trashing our present systems for the "Clean Wonderful Solar Solution". the-dark-side-of-solar-power
 
Dec 28, 2016
29
Hunter 34 MiddleR
First thing is to check your tachometer with a laser one to see what the real RPM is.. The Yanmar tachometer on our boats is notoriously inaccurate.. I normally run my engine at 2800-2900 rpm (real RPM) The engine is designed to run day in and day out at 3400 rpm, but my engine seems to have a sweet spot at 2800 ish and uses about a half gallon of fuel per hour at that RPM. ..
something like this is sufficient: Amazon.com: AGPtek® Professional Digital Laser Photo Tachometer Non Contact RPM Tach: Automotive
The propeller on my boat is a 15 inch diameter by 15 inch pitch, as supplied originally by Hunter. .. my gearbox ratio is 2.63.. Boat will motor at 6.5 knots or so ..
The theoretical hull is about speed is 7 knots.. which, if the prop is correct for your gearbox ratio, would be reached at a real RPM of 3400-3600 RPM. For this kind of discussion, speed needs to be measured as speed through the water and not GPS speed which would be affected by currents..
in a nutshell, verify gearbox ratio (tag on the gearbox), Verify the propeller diameter and pitch (should be marked on the hub), Verify engine RPM ..
Your two bladed prop has less drag when sailing than a three blade prop.. if you sail more than motor, and it is the correct pitch and diameter, it sounds close.. if you motor more than you sail, then a three blade may be your right choice.. lots of folks are satisfied with Campbell Sailor props and it is reported that their three blade has the least sailing drag of three bladed props.. If you decide to go that way, get a local propeller shop involved for sizing and balancing..
EDIT: I didn't see the other posts from another thread... but I think the answers are all consistent
Thanks kloudie1. The guy I bought our 34 from, raced it. So, I assume that he changed the prop to a two-blade. Unfortunately, we do a fair amount of motoring, due to low winds. So, I would like to try a three-blade prop on her. From what you've said, it looks like you also sail a Hunter 34 ? I will check my gear ratio. I am not comfortable with running the engine at higher than 2500 rpms on a regular basis. I think that might cause problems later on. I'll contact Miller Island Prop Shop and inquire about the Campbell sailor Prop.
 
Dec 28, 2016
29
Hunter 34 MiddleR
Thanks kloudie1. The guy I bought our 34 from, raced it. So, I assume that he changed the prop to a two-blade. Unfortunately, we do a fair amount of motoring, due to low winds. So, I would like to try a three-blade prop on her. From what you've said, it looks like you also sail a Hunter 34 ? I will check my gear ratio. I am not comfortable with running the engine at higher than 2500 rpms on a regular basis. I think that might cause problems later on. I'll contact Miller Island Prop Shop and inquire about the Campbell sailor Prop.
 
Oct 22, 2014
16,090
CAL 35 Cruiser moored EVERETT WA
Barn-dog. It is your diesel and you get to decide how you want to run it.

All the diesel study I have reviewed, suggests that diesels like to run continuously about 80-90 % of wide open throttle. This keeps the engine running at optimum temperatures. Surprisingly it maintains a good fuel energy level.

Gearing the prop to push the boat using that power level is a key factor in getting the best efficiency out of the engine.

When a diesel is run at lower RPM’s you get lower temps and partial fuel burns. This is exhibited as soot in the engine exhaust and clogging in the exhaust elbow. Doesn’t do much good to the piston and rings.

Diesel engines in Trucks last 100’s of thousands of miles when used on open freeways and interstate highways. When they are used in traffic and Urban streets their performance is compromised.

Wish you all the best with your boat.
 
Dec 14, 2003
1,319
Hunter 34 Lake of Two Mountains, QC, Can
As John said, it's your boat hence your decision. A general rule of thumb is to go down in pitch when you add a blade. I have the same boat, same engine, same shaft. My tranny is 2.61/1 Yanmar Kanzaki (plate on tranny housing). You are currently running a 2-blade 15 X 15 RH. FYI, that is what I was running also. I went to a 3-blade 15 X 13 and was getting some lugging, not being able to reach max RPM. Prop shop re-pitched it down to 15 X 11 and bingo, I am now easily reaching top rpm (3400) and cruising at 2700 (80% rpm) I generally reach 6.2 to 6.5 knots (over the water speed). Of course that assumes that bottom, shaft, strut and prop are clean. Shaft is 1 inch.
 
May 25, 2012
3,841
john alden caravelle 42 sturgeon bay, wis
the S.S.Middeltown, a converted T2 tanker, converted to lake bulk usage. had a 19', one piece, bronze prop, 4 blades, turned 119 RPM at the shaft, and hit 28MPH on a nice day.
the chief was very proud of his prop. would never give us more than 98RPM no matter how much the old man bitched. every time the the captain called for more speed, the chief would claim 'she's wide open'
the chief knew to baby that old steam engine to keep the breakage down. WW2 gear.
i did see 24 MPH one day, but only that once
:biggrin:

the QEII cruised at 38 MPH. yeah, cruised. she has 4 props and a very big boilers :yikes:
 
Oct 22, 2014
16,090
CAL 35 Cruiser moored EVERETT WA
I had the opportunity to cross the Atlantic twice aboard the SS United States in the 60’s.

The SS United States, which made its maiden voyage on this day in 1951, remains the fastest liner ever built, having taken just three days, 12 hours and 12 minutes to cross the Atlantic. That's an average speed of 34.51 knots.

38.32 knots (70.97 km/h; 44.10 mph) (trials) 43 knots (80 km/h; 49 mph) (claimed)

5ECE7A5A-6F63-4E01-ADC0-80F885E6F621.jpeg
 
  • Like
Likes: jon hansen