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Proper Propellor

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.
I realize that a three-blade will produce additional drag while sailing.
Thanks for your help
"Barn-dog"
 
May 17, 2004
3,469
Beneteau Oceanis 37 LE Havre de Grace
How slow is very slow, and what speed does the engine turn at? Just want to make sure the prop is the issue before you invest in replacing it.
 
Jan 22, 2008
1,591
Hunter 34 Alameda CA
We still have the original 15 x 15 two bladed prop. We always motor at 2,800-3,000 rpm and move along at an acceptable 6.5 knots with everything clean. Fuel burn rate is a bit over 0.6 gallons per hour. Ours is a 1" shaft.
 
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Jan 11, 2014
7,827
Sabre 362 113 Fair Haven, NY
If you decide the best course of action is a new prop, then it will be best to rely on the prop manufacturer's recommendations. There are too many variables that affect prop performance to get meaningful recommendations from a forum.

Some basic information will help identify if it is the prop or something else. @Davidasailor26's question is a good start. What speed are you getting in what conditions at what engine speed?

@Allan12210's report sounds about right for a 34' boat in relatively calm conditions. Is this close to what you are getting?
 
Jan 22, 2008
8,050
Beneteau 323 Annapolis MD
If you decide the best course of action is a new prop, then it will be best to rely on the prop manufacturer's recommendations. There are too many variables that affect prop performance to get meaningful recommendations from a forum.

Some basic information will help identify if it is the prop or something else. @Davidasailor26's question is a good start. What speed are you getting in what conditions at what engine speed?

@Allan12210's report sounds about right for a 34' boat in relatively calm conditions. Is this close to what you are getting?
:plus:
 
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SFS

.
Aug 18, 2015
1,972
West Marine Kayak Tampa Bay
dlochner is correct, if you decide you want a new prop, you need to contact your local prop shop for the correct recommendation. Come prepared with the following information:
1) Engine max horsepower (see engine manual)
2) Engine max RPM (see engine manual)
3) Transmission ratio (see engine manual, or placard on side of transmission case)
4) Size of prop shaft (which you can measure at the aft end of the transmission)
5) Max clearance between hull and tip of prop shaft (the prop won't be this large, as there are "rules" regarding clearance)
6) Which way the shaft turns in forward gear (see engine manual)
7) Length of the boat at the water line (see owners manual, or sailboatdata.com)

The prop shop will also be able to intelligently discuss the advantages and disadvantages of two versus three blade props, and the advantages of specific blade shapes. You can have this discussion with a phone call, so aside from collecting the above info, your time investment is low.

If you decide a prop change is needed, used props are much less expensive than new ones, and can be found on eBay, local prop shops, and marine salvage yards.

Having said all that, I agree with the above posters that you should at least evaluate a few other things before spending any money on a prop. More info from you will help us help you more accurately.

Edit: And perhaps the most obvious question: is the hull fouled, or do you have it cleaned regularly?
 
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Jan 11, 2014
7,827
Sabre 362 113 Fair Haven, NY
5) Max clearance between hull and tip of prop shaft (the prop won't be this large, as there are "rules" regarding clearance)
This distance will depend on the prop size, better to have the measurement from the center of the prop shaft to the hull. This will determine the maximum prop diameter.

Edit: And perhaps the most obvious question: is the hull fouled, or do you have it cleaned regularly?
And, is the prop fouled?
 
Jan 22, 2008
8,050
Beneteau 323 Annapolis MD
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.
I realize that a three-blade will produce additional drag while sailing.
Thanks for your help
"Barn-dog"
Baconsails.com has a number of props in their back (sail loft) room.
 
Dec 14, 2003
1,319
Hunter 34 Lake of Two Mountains, QC, Can
We still have the original 15 x 15 two bladed prop. We always motor at 2,800-3,000 rpm and move along at an acceptable 6.5 knots with everything clean. Fuel burn rate is a bit over 0.6 gallons per hour. Ours is a 1" shaft.
I have a 3-blade 15 X 11 prop and get pretty much the same results as Allan. As others have said, you need to make sure that everything is in order before you blame the prop for lack of speed. Are all underwater surfaces clean, without barnacles ? If that is a yes, next thing is to make sure engine is reaching proper rpm under load. If not, then the problem is not with the prop ! At that point, it could be caused by several things, i.e. exhaust elbow restriction, fuel restriction due to dirty filter, etc. Plenty of posts discussing these issues. Good luck
 
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Jul 7, 2004
8,006
Hunter 30T Cheney, KS
Not enough to go on from the post, but as others have mentioned. you are not going to go faster than hull speed. So if you are used to a powerboat it will feel very slow. How slow is slow?
 
Dec 28, 2016
29
Hunter 34 MiddleR
Thanks all, for your replies. There certainly is a lot to consider. 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.
 
Jan 11, 2014
7,827
Sabre 362 113 Fair Haven, NY
Thanks all, for your replies. There certainly is a lot to consider. 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.
Diesels run best and longest when they are run hard. Typical cruising RPMs are 80% of wide open throttle (WOT). Extensive running at slow speeds or idle is bad for the motor. Doing so allows carbon and unburned fuel to build up and allows blow by in the cylinders because it does not reach proper operating temperature. The proper prop for your boat and motor will allow the motor to run at its optimal speed.

Before purchasing a new prop, find out what the WOT RPMs are and then run the motor at about 80% of that number.
 

SFS

.
Aug 18, 2015
1,972
West Marine Kayak Tampa Bay
Hull speed with your waterline is 7.17 mph. The 3GM-30has a 3850rpm redline, 3600rpm one hour rating, and 3400rpm continuous. Run it harder than you are, and every hour, take it up to 3600 for 10 minutes to blow all the garbage out.

Before making any decisions about buying another prop, see if making the above adjustments get you what you want in terms of speed. Also, when measuring speed by GPS, remember that is speed over the ground, and may be helped or hurt by currents (and wind). Good luck.

Edit to add (and make more stark): According to the power curves, at 2000 rpm the engine is producing 4 (four!) horsepower at the prop. At 2500 rpm, about 8.5 hp. If you have been getting 5 mph out of 8.5 hp, you've been doing well with a 12000 pound boat. Imagine what you would get if you nearly tripled the power at the prop to 22 hp by running at 3250 rpm.

You may want to become more familiar with your engine manual.
 
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