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Old 05-11-2015, 03:58   #1
jbe
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Is your Yanmar SD20 slow?

We have a 2014 Marlowe Hunter e33 with a 3ym30 engine and 20SD sail drive.
We are in a fleet of boats transiting the ICW and everyone is faster than we are including much older Hunter 31's. Even at 3000 rpm we are barely able to keep up. We have had the RPM's tested and the gauge is correct. Granted it is a sailboat but heck I would like to motor at a decent speed. Others in our group agree we should be able to go faster. We have only 400 hrs on engine and have done multiple oil changes and services. Any thoughts or is this just the way it is? I would like to hear from others with the 20SD.
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Old 05-11-2015, 04:55   #2
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Re: Is your Yanmar SD20 slow?

Check to make sure you have the same prop diameter and pitch as other sister boats.

Are you able to easily get the engine rpm to full limited revs while underway? If so, you may need a larger diameter or pitch prop.

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Old 05-11-2015, 05:34   #3
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Re: Is your Yanmar SD20 slow?

We have the prop provided by Yanmar. The recommended props are a 16.5 fixed or a 16" folding. Don't have any idea about pitch.
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Old 05-11-2015, 07:03   #4
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Re: Is your Yanmar SD20 slow?

Have you had the bottom cleaned lately?
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Old 05-11-2015, 07:25   #5
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Re: Is your Yanmar SD20 slow?

At 3000 rpm you're below the cruising rpm for this engine. At WOT you should reach 3700 rpm's. At cruise you should be at 33-3400 rpm's. This should give you in the neighborhood of 7.5 knots of cruise speed. What kind of speeds are you expecting? Are you trying to keep up with the trawlers in the group? Does the older Hunter have a 28hp engine also?
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Old 05-11-2015, 07:59   #6
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Re: Is your Yanmar SD20 slow?

We just had the bottom cleaned and painted, all oils changed, belts done etc.
We are only sailboats and the 31's have smaller engines. Today at 2950 we are doing 6 mph with little current. The prop is a 16 x 11. Just checked with dealer and that is what all e33's are equipped with. If we were dragging something wouldn't we feel it? Also, I have not heard our cruising RPM's should be over 2750. Where can I find that confirmation?
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Old 05-11-2015, 10:24   #7
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Re: Is your Yanmar SD20 slow?

Quote:
Originally Posted by jbe View Post
We just had the bottom cleaned and painted, all oils changed, belts done etc.
We are only sailboats and the 31's have smaller engines. Today at 2950 we are doing 6 mph with little current. The prop is a 16 x 11. Just checked with dealer and that is what all e33's are equipped with. If we were dragging something wouldn't we feel it? Also, I have not heard our cruising RPM's should be over 2750. Where can I find that confirmation?

Check out your owners manual, all the info is there including proper breakin. That's where I found all the numbers I quoted. 6 mph @ 2950rpm sounds about right to me..If the sellers didn't give you the manual you can find it online..
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Old 05-11-2015, 11:14   #8
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Re: Is your Yanmar SD20 slow?

Quote:
Originally Posted by jbe View Post
We just had the bottom cleaned and painted, all oils changed, belts done etc.
We are only sailboats and the 31's have smaller engines. Today at 2950 we are doing 6 mph with little current. The prop is a 16 x 11. Just checked with dealer and that is what all e33's are equipped with. If we were dragging something wouldn't we feel it? Also, I have not heard our cruising RPM's should be over 2750. Where can I find that confirmation?
I have a 2GM20 and it goes well. Your prop sounds about right. I used to have a folding prop of the recommended pitch / diameter. I think it was finally 16"X10. I did have it adjusted an inch or two finer than original to achieve 3400 rpm flat out. Now I have a Kiwi Feathering prop which is still 16" but the pitch is not relevant as they are in degrees and adjustable. That didn't change the flat out speed, but is better sailing (in my opinion) and adjustable pre set and reverse is much better.


You should theoretically be able to achieve full revs 3600 at full throttle in calm conditions. I've chosen 3400 myself. The prop shop suggested 1" of pitch is a change of 100 rpm maximum rpm as an approximation.


The only problem I've had that sounds similar to your problem was the rubber fairing under the hull around the sail drive leg coming loose and forming an effective water brake. It's possible that's happened to you since you were hauled out. Mine is coming loose at the front again. I'm going to make a fibreglass replacement with a small gap screwed and Sikaflexed on next haul out.


The other thing that greatly affects motoring efficiency is barnacles on the prop. They can appear fairly quickly. I use Prop-Speed which it seems can only be applied by a licensed agent. It does work very well. Herreshoff wrote that a prop is driving a boat forward with the same force that the hull is holding it back. He said that because the prop is 1/1000 the area of the hull underwater, 1 barnacle on a prop is like 1000 barnacles on the hull ( or similar).


Flat out, we motor faster than most similar sized yachts but I usually cruise around 2900 / 3000 at just over 6 knots. We can get up to 7.2 knots @ 3400 with ideal conditions. That is correctly measured at slack tide no wind and running 2 ways. Our waterline is 27.5' / 28'. ( you are quoting MPH which isn't knots)


Another thing that can greatly affect engine efficiency is your exhaust system. It should be the same internal diameter as the exhaust bend all the way including into and out of your water lock. I think that is 1,1/2". Often people stick a plastic hose extension in a cast skin fitting at the transom outlet and that reduces it to around 1", making a huge restriction, as it also carries the cooling water. I once found that my water lock connections were apparently 1,1/2" from the outside but inside, the thick wall made it only 1". I replaced that.


If you can only achieve 2750 rpm and your hull is clean and clear you could also check that your throttle cable is correctly adjusted. You can see that just by the injector pump. Disconnect the cable to make sure the lever on the motor is traveling the whole way to its stop inside the engine when the cable pulls it. The full cable travel should be limited by the stop at that engine lever, not back at the controls.


I hope that all helps
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Old 05-11-2015, 14:48   #9
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Re: Is your Yanmar SD20 slow?

First are you able to achieve full revs in neutral? If so that eliminates mechanical elements like linkage and pump and with a clean bottom that only leaves the prop and sail drive.
My guess would be that's where your problem is. Possibly faulty from new or more likely it's taken a hit somewhere and thrown one of the blades out of pitch or bent the shaft. These may only show up with machine testing unfortunately.
Try this. Pick a calm day with little current and try standing as close over the prop as you can then get someone to slowly increase speed till you hit WOT. Feel for changes to the vibration levels. If there's damage you'll find the vibration will increase significantly at certain revs. Normally between 1800 - 2400 but not exclusive to that range.
Agree with Graham HO on Prop-Speed. Expensive but worth every $$$. We do the props and the sail drives. I'm currently on my 3rd year since they were done and expect I'll get one more season before redoing the Prop-Speed. She comes out on the 14th Dec so will find out then. I've found it much more effective on the sail drives than soft antifoul.
You didn't mention your location? Here in Oz we have real growth issues and typically slip and anti foul every 12 mths.
Remember also this is a diesel and unlike a petrol engine you won't hurt it running at full revs all day. Indeed you'll do more damage running it at lower revs for extended periods. We typically run both engines at WOT for at least 30 min every time we move.
Getting a bit long winded sorry. Best of luck with it.
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Old 05-11-2015, 15:17   #10
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Re: Is your Yanmar SD20 slow?

Is that 6 mph, or 6 kts?

6 kts doesn't sound too bad.
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Old 05-11-2015, 15:33   #11
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Re: Is your Yanmar SD20 slow?

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Is that 6 mph, or 6 kts?

6 kts doesn't sound too bad.
6 knots at 2750 revs sounds fine to me as long as they are accurate measurements.

Another check that is easy and worth doing is a static "thrust" test. If your boat is well secured fore and aft in a berth, warm the engine in gear and then try full throttle forward while still in gear. My boat will achieve around 2700 rpm and give out quite some black smoke. You only need a few seconds. That is eliminating any hull problem.
I think my prop pitch and motor are in fine condition, though as I've been traveling and the boat has been sitting, there may be some small prop growth.

You shouldn't achieve full revs while moored in gear or the prop pitch is too fine. If you can only achieve 2000 / 2500 the pitch may be too course or some other problem.
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