Proper Prop / YSB-8

[Desiderata]

We are enjoying our '78 E-27 thoroughly, but we sometimes think she's a bit underpowered. She is equipped witha single cylinder Yanmar diesel YSB-8. Our understanding is that the YSB-8 generates about 7-8 H.P. While fine under most circumstances and EXTREMELY frugal in fuel consumption, we occassionally have trouble in headwinds and are required to play the tides thru passes here in FL. The warm waters here, require me to swim the prop and rudder at least 2-3 times monthly in the summer or the prop will foul with (damn) barnicles. Even then, we wonder if the 3 blade prop currently occupying our shaft is the right one. Anybody know how to identify said prop and perhaps know what prop would offer improved performance?
Many Thanks for any advice,
Mike & Cheri

 

[Glyn Judson]

Determining the right prop pitch.

Mike, Two things need to be consulted in order to determine the proper pitch of your propellor. You'll need a tachometer and I don't know if the YSB-8 has one and the other thing is having access to the operators manual. Here's the trick. Find out what the maximum operating speed of the engine is under way from the manual or your regional Yanmar distributor and monitor the tach to see if you can hit that number. Choose a time when the water is flat and there is no wind like early in the morning and perform your test. If the RPM's are below the maximum factory stated speed, there's a good chance that you're over propped. Severe over propping can also result in sooting of the hull near the exhaust fitting too. Don't discount also the possibility of carbon restriction in the exhaust elbow from over propping also. If you can get the engine to run faster than the spec, you're most likely under propped. Shortly after getting our boat I had to go through this same exercise and without ever changing the fixed three-blade prop, went through three pitch iterations before dialing in on that magic RPM. In doing so the tell tale soot disappeared along with an excessive amount of steam exiting with the exhaust. Try this as a starting point and see what the result is. To be clear, I had to remove the prop, take it to a local prop shop and have the pitch tweaked twice. As I recall, I also reduced the diameter a bit too based on the need to have the blades at least 1" from the hull to prevent cavitation. This may not be the final solution for your prop but it's a way of clearing away the confusion of prop pitch science, actually magic as much as science. The reason I say that is that my results conflicted with what two prop houses told another owner of the same boat and engine as ours. I assured him that having used my boat as the test bed, he needed to trust my results and not theirs. He went with the same numbers I have and got the same satisfying result with his engine/prop/hull speed as I have. Good luck, Glyn Judson, E31 hull #55, Yanmar 3GMF, Marina del Rey, CA

 

[Desiderata]

Thanks Glyn, for a great description of how to handle this circumstance. I've been wanting to add a tachometer and now have a good reason to go ahead with that project. Thanks again.
Mike

 

[Glyn Judson]

Adding a tach.

Mike, Fitting a tach is all well and good but bear in mind that if the manual or the factory rep technical person can't give you the maximum and sustained cruising RPM's (two different numbers), having a tach won't help in determining the correct prop pitch for your boat. To clarify, the two different RPM's, given my engine as an example, are that mine should be able to reach 3,600 RPM maximum but can only be run at that speed for one hour before overheating the engine whereas the sustained cruising RPM is 3,400 RPM. Now that's my engine, a totally different design from yours so please don't infer from the above numbers that your numbers will be the same. I was merely clarifying the reason for two different RPM's stated for my 3GMF. So I guess if it were me, I'd want to see if those numbers can be gotten before going to the expense of buying and fitting a tach. Come to think of it, my tach gets pulsed information from a wire attached to a magnet in the bell housing that responds to another magnet whirling around on the edge of the flywheel. You might be taking on waaay more than you want to by adding a tach unless you can see evidence of a magnet or dummy plug in the bell housing that would suggest a tach for your engine as an option. But then what do I know? It could be that fitting a tach to your engine is a totally different set up. Check with that rep for an answer to that one. Without blaming him for any inaccuracies, I fell it necessary to invoke Loren's 2 cents worth of value to all the above that I've written. But I think you get the flavor of what all I'm trying to share with you. Good luck, Glyn Judson, E31 hull #55, Marina del Rey, CA

 

[Loren Beach]

I would give Glyn's advice at least $.03 in this case.
Some sorta-kinda related news: I did a multi-day delivery in a Yamaha 33, with the YSM12, down the Washington coast. That boat must weigh about 10K#. We could drive it at 6 kts all day long.
That "little" horizontal single-cylinder diesel puts out a lot of torque, imho.

In the smaller E-27 with considerably less waterline, I would suspect that your top motoring speed is more dictated by your DWL....

I imagine that you would want to first use a sub-$100. optical tach to check revs and be sure your surmises match your realities.
Then do some figuring on what the prop should be.


Or, just fly Glyn out to advise you in person... his new guide dog-in-training could use the free ride too!

Best,
Loren

 

[Tom Metzger]

Very interesting...

With all of the warnings about getting the specs, I was wondering why no one just hit Google for the info. Everyone's guide dog can't be out to lunch over four or five time zones. So I did look. Easily done.

http://www.ship468.org/boats/Catalina30/YSM12/index.htm

In chapter 1 the specs are listed. As Glyn so ably pointed out, there are two listings for max RPM: 3000 rpm one hour & 3000 rpm continuous. Yup, 3000 rpm, I guess it is easier when both numbers are the same. The difference is that the one hour rating is for 12 hp, and the continuous rating is for 10 hp.

Obviously, a little more info is needed. I have had too long a day to trust my reasoning now, so I will leave it to others as I go out for dinner.

 

[Loren Beach]

Tom, I found the same site with the same search...
I was puzzled a bit about any differences in the two engines (the 8 and the 12), closely-related as they were in so many ways.

Loren

 

[Tom Metzger]

Loren - The two engines have serious differences; weight, displacement, operating RPMs. Basically, two different, but similar engines.

 

[Loren Beach]

8 mighty Yanmar horses

Loren - The two engines have serious differences; weight, displacement, operating RPMs. Basically, two different, but similar engines.

Ah so.
In that case the cited Catalina 30 site seems to be only about the YSM-12 and not the smaller Yanmar that started this thread.

Another search turns up this combined manual.
http://www.yanmar.com.au/superseded/YSM8YSM12/Binder1.pdf

Hope this helps a bit.

Best,
Loren

 

[eknebel]

tach aftermarket add on

Most aftermarket diesel tachs use pulses from the alternator, so adding one shouldn't be a problem. You do have to set a switch for variations in alternators, but it wasn't hard to figure out. As a added "bonus", the tach becomes a indicator when your crew switches batteries to off with the engine running... the tach goes to zero with the engine running, telling you to make time for diode replacement in the alternator