The Proper PROP

Dec 28, 2016
33
Hunter 34 MiddleR
I have and greatly enjoy a 1987 Hunter 34. It has a Yanmar 3GM30F, turning a 15" two-blade prop. While under power, I can reach about 5 mph. I am
considering replacing with a three-blade prop for more speed. Can anyone recommend the proper size and make, three-blade prop ?
I have owned this boat since 2016, and have benefited from your answers and assistance. I really appreciate this, and reading this Forum , and I wish
you all -All the best and smooth sailing in the New Year !
 
May 17, 2004
5,032
Beneteau Oceanis 37 Havre de Grace
5 mph (4.4 knots) sounds quite slow for a 34’ boat. What RPM’s are you turning at that speed? You might want to make sure there’s not some other underlying problem causing that slowness before you invest in a new prop. I’d expect a top speed closer to 6.5 knots.


Are you interested purely in motoring performance or also sailing speed? That and your budget will guide you to a fixed, folding, or feathering prop.

If sailing performance is a consideration I’d recommend a flex-o-fold. The drag under sail is minimal, motoring performance is better than a fixed prop (except in reverse, but it’s still adequate there), and maintenance requirements are minimal. If you call them with your engine HP, transmission gear ratio, and shaft size they’ll recommend a well matched prop size.
 
Oct 22, 2014
20,995
CAL 35 Cruiser #21 moored EVERETT WA
The change from a 2 blade to a 3 blade is not likely to change your boat speed. It could smooth out vibration.

Boat speed is achieved by a clean smooth bottom, good prop design, and a properly sized/pitched prop matched to your engine and boat.

Talk with a prop shop for a best match. Owner at WestbyNorth would be my suggestion.
 
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Jan 4, 2006
6,444
Hunter 310 West Vancouver, B.C.
As per @Davidasailor26 the #1 item here is the RPM, and have you confirmed this with an optical tach (< $30). No need to break the bank with the tach.

ALSO ........................
1640710055979.png


....................... are you operating at 80% max. RPM ?
 
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Nov 6, 2006
9,885
Hunter 34 Mandeville Louisiana
Same here as Mark. My prop is a fixed two blade Michigan is
15 by 15 inch and drives the boat easily at 6.5kts. Something is out of whack, speed measurement or rpm or fouling on prop and or bottom .
 
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Jan 22, 2008
1,654
Hunter 34 Alameda CA
Out 1984 H34 came stock with a 15 x 15 prop that moves us at least 6.5 knots. That is at 3000 rpm. The engine has always liked that rpm. It keeps the mixing elbow nice and carbon free.
 
Dec 28, 2016
33
Hunter 34 MiddleR
My bottom remains clear of marine growth throughout the season, but I did see a lot of barnacles on my prop, when the boat was pulled in November. There has not been a problem with the engine, nor with vibration. During the winter months when most boats are on the hard, I have noticed that most have three-blade props. Very few with two-blade. Also, I have read that speed does increase and handling, particularly when backing-up, is much more responsive. Part of the reason that I'm only getting 5 mph is that I'm afraid to run the engine
over 2,200-2,500 rpm's. It does run fine, but it's old (34 years), and I don't want to push it too hard. I'm thinking that a three-blade prop will move the boat faster at say, 2,200 rpm's than my two-blade. Seems that I'm motoring a lot since the wind is usually not strong enough.
I know the folding props work well and reduce drag, but their high cost outweighs the drag of a fixed prop.

Thanks for your response and opinion.
 
Oct 22, 2014
20,995
CAL 35 Cruiser #21 moored EVERETT WA
It does run fine, but it's old (34 years), and I don't want to push it too hard.
I can understand this thinking....
Coddling a diesel engine can cause more problems than running the engine at 80-85% WOT (wide open throttle).

Diesel engines are aged by "Hours of Operation" rather than the chronological years of existence.
Life expectancy of yanmar Diesel engine
 

Sailm8

.
Feb 21, 2008
1,746
Hunter 29.5 Punta Gorda
You should be cruising at a higher RPM like around 2800. But first you might want to purchase a cheapo laser Tach and check the accuracy of your tach. Diesels love to work not be babied.
 

dmax

.
Jul 29, 2018
971
O'Day 35 Buzzards Bay
"but I did see a lot of barnacles on my prop"
This will definitely inhibit your speed, the prop needs to be clean.
 
Jan 4, 2006
6,444
Hunter 310 West Vancouver, B.C.
Found this one spring (but never again) and it almost stopped me from making the 1,000 yards to the lift for the annual haul out:

100_3259.jpg


Barnacles will do that.

There is a problem with your boat. Keep digging and you'll find it.
 
Dec 28, 2016
33
Hunter 34 MiddleR
My thanks again all, for your responses and comments. Based on what you've said, I going to stay with the two-blade prop this
year and try cruising at 2,800 rpms, which I've never done. Also, since moving to the Eastern Shore of Maryland, I've experienced
considerably more barnacles on my prop and shaft than I used to, on the Western Shore side. Don't know why that is, but the zink oxide
paint that was effective, doesn't seem to work well over here. I'll have to switch to another paint, and clean my prop and shaft on a regular
basis.
Happy and Healthy New Year to you all !

Barney
 

Sailm8

.
Feb 21, 2008
1,746
Hunter 29.5 Punta Gorda
In my experience, there are so many variables with barnacles such as boats location, water temp, salinity. that making comparisons of products erc. is almost impossible. One thing that works for sure is a diver cleaning the bottom and prop on a regular basis.
 

DArcy

.
Feb 11, 2017
1,691
Islander Freeport 36 Ottawa
Choosing the wrong prop can be bad for your engine. When I bought my boat the previous owner had the same thinking as you and switched from a 2 to a 3 bladed prop because he figured it would "have more bite". When I first launched the boat it had a 16x13 3 blade and I was only able to run at around 2,000 rpm and just over 5 knots. I knew this wasn't right, the engine (Pathfinder 50) is rated to run at 4,000 rpm and other Freeport 36 owners say they can get up to 7 knots under power. After running it hard for a while I was able to get to about 5.5 knots and just over 2,200 rpm. I suspect the PO was babying the engine which allowed some carbon deposits causing reduced performance. At the end of the season I switched out to to the 2 blade 17x8 and was able to run up to 3,800 rpm at 7 knots. Over propping your boat (two big and/or too much pitch) can be bad for your engine and will limit it's life.

Another thing I noticed when switching from the over-propped 3 blade to the proper sized 2 blade was a reduction in prop walk. In reverse the prop walk with three blade was so bad I was not able to turn the boat to starboard while in gear. After switching to the 2 blade I have some prop walk but not nearly as much.

You may want to check the manual for your engine for recommended cruising rpm. My manual says "Top speed of the engine is 4000 RPM and recommended sustained cruising speed is 2800-3600 RPM." Check the manual for your engine, the Pathfinder is a high revving diesel.
 
Jan 2, 2022
1
Hunter 310 Worton Creek
FYI - should you decide to change to a different three blade prop, I have one I just removed from my Hunter 310 that is a 15-12 in very good condition. I have a friend who had the same engine on his Catalina 320 and ran a 15-11 before going to a folding prop. Just food for thought. If interested you can contact me at plind19713@verizon.net
 
Dec 28, 2016
33
Hunter 34 MiddleR
Choosing the wrong prop can be bad for your engine. When I bought my boat the previous owner had the same thinking as you and switched from a 2 to a 3 bladed prop because he figured it would "have more bite". When I first launched the boat it had a 16x13 3 blade and I was only able to run at around 2,000 rpm and just over 5 knots. I knew this wasn't right, the engine (Pathfinder 50) is rated to run at 4,000 rpm and other Freeport 36 owners say they can get up to 7 knots under power. After running it hard for a while I was able to get to about 5.5 knots and just over 2,200 rpm. I suspect the PO was babying the engine which allowed some carbon deposits causing reduced performance. At the end of the season I switched out to to the 2 blade 17x8 and was able to run up to 3,800 rpm at 7 knots. Over propping your boat (two big and/or too much pitch) can be bad for your engine and will limit it's life.

Another thing I noticed when switching from the over-propped 3 blade to the proper sized 2 blade was a reduction in prop walk. In reverse the prop walk with three blade was so bad I was not able to turn the boat to starboard while in gear. After switching to the 2 blade I have some prop walk but not nearly as much.

You may want to check the manual for your engine for recommended cruising rpm. My manual says "Top speed of the engine is 4000 RPM and recommended sustained cruising speed is 2800-3600 RPM." Check the manual for your engine, the Pathfinder is a high revving diesel.
Thanks for your advice, DArcy ! Based on what I've seen here, I'm going to stay with my current two-blade prop and cruise at 2,800rpm's, which is 80% of my WOT.
 
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