Why won't my Yanmar reach redline?

[mckenzie.keith]

So just to recap.

1) Bottom and prop were still fairly clean, according to the divers I hired to clean the bottom and prop. They cleaned it again anyway.

2) Exhaust elbow was supposedly cleaned by mechanics in San Diego which is too recently for the elbow to be clogged already. I am assuming it is good.

3) No signs of black smoke or black junk in exhaust water at 2900 RPM.

4) Just confirmed throttle (governor whatever) lever on the engine can be driven all the way to the full-throttle stop by the cockpit "throttle" control. So it is not a lack of travel problem.

5) Tach is reasonably accurate. Less than 50 RPM off as far as I can tell. This was verified with a handheld digital tach on the crankshaft pulley or harmonic balancer or whatever that big wheel is connected to the crankshaft on the non-drive end.

6) The current pitch (13 inches) is much higher than what flexofold personnel are recommending. Prop is also too small in their opinion (17 inch diameter is too small).

7) As a place to swim, Ventura harbor is not terribly inviting. But I was only in the water for like a minute to measure for the prop clearance.

So, I think it is now clear that the boat is over-propped and that is the only really plausible explanation left for why I cannot achieve 3800 RPM.

There is slightly over 12 inches from center of prop shaft to bottom of hull, so there is room for an 18 inch prop. It will be a lot cheaper for me to buy two new 18 inch blades from flexofold than to buy a 3 blade prop. I am not for sure yet on that decision but I think I will probably go with the two-blade 18 to save money, unless the flexofold guys are fairly adamant that I should go with three blades. I can also keep the old blades as spares.

I am most grateful to you all for sharing your knowledge, wisdom and experience, humor etc.

 

[El Borracho]

^^^ What maximum RPM did you attain in neutral with the control at max?

 

[mckenzie.keith]

^^^ What maximum RPM did you attain in neutral with the control at max?

I did not try to go beyond 3800 RPM in neutral. I adjusted slowly up to 3800 and then stopped. However, I would say that it was near the max mechanical travel. I'm still on the boat. I can do that if you think it could help us to know. I just thought that since the motor says 3800 rpm max, maybe I shouldn't go beyond that.

 

[accnick]

Usually, they are governed so that they won't exceed maximum rpm.

 

[DDW]

I'd advance slowly to the stop just in case - but the governor should not let you overrev. Might as well find that out now. Get it a little warm first which may require forward gear in the slip.

 

[DDW]

This is a Veem brand. The little blue stripe is an interceptor strip of hard plastic. By changing the depth of the little lip you can change the pitch cheaply and fast (within limits). This is clever.

That is interesting. The strip is on the trailing edge?

There are newer strange props like the Sharrow. And the Volvo folding. It is just possible that there is not a sharp peak in efficiency around the optimum planform, so anything that looks like a prop works.

 

[Zonker]

Yes the trailing edge. Used them on some 24 and 32m alum cat ferries and they certainly could adjust the pitch (maybe +/- 1 or 2"). So yes it seemed to work.

I remain to be convinced by the Sharrow. Certainly less noise from some videos of drones with 3D printed copies of them with similar styling. Yes you get rid of tip losses, but you're still dragging that useless curvy tip through the water. The tests on their site seem like the competing fixed prop was deliberately chosen to be a bit off.

Lots of prop shaped things work but I think efficiency can vary quite a bit with blade shape (well +/- 5% easily). Just being off of the optimimum diamter/pitch for a particular boat matters as much or more.

 

[mckenzie.keith]

^^^ What maximum RPM did you attain in neutral with the control at max?

OK I just did that. 4050 per tachometer. I motored at low speed in reverse at the slip for about 15 minutes. Then I put it in neutral and slowly pushed the lever to the max. It hit 4050 on the tachometer, but as noted previously the tach is reasonably close. I did not note any explosions or flying shrapnel. But I felt some hairs on the back of my neck start to raise up, so I reved back down to idle for a bit and then shut off the engine.

 

[mckenzie.keith]

OK, so here is another aspect.

Max engine speed is 3800 RPM. Reduction ratio is 2.23 giving a max shaft speed of 1704 RPM. It seems that Flexofold does not recommend any 18 inch prop for this high of a shaft speed. They are saying that the saildrive ratio of 2.49:1 would have yielded a lower shaft speed and been more optimal for a sailboat. I guess this is limited by tip speed?

The SD60 is available in both ratios, 2.23 and 2.49. The Yanmar shop who recently replaced the old saildrive with the SD60 never brought this to my attention or discussed it with me and seemingly, they made the wrong choice. At one point I did ask him whether the drive ratio was the same and if not which way it was changing. But he didn't look into it and I didn't follow-up beyond the initial question either.

The shop knew that the boat was likely already over-propped because they had a copy of the engine survey report where that was mentioned. Probably they should be aware that 1704 RPM is a little too fast for a sailboat prop shaft in general. Or at least on the fast side.

Tip speed for an 18 inch prop at 1704 rpm is about 41 m/s (134 ft/s). This is just based on speed * radius. Ignoring the fact that the blade is spinning in a stream of flowing water.

Changing the reduction ratio to 2.49 would bring that down to 36.5 m/s or 120 ft/s. If I had the 2.49 ratio, I could use an 18 inch prop with a lower tip speed than the 17 inch prop on there now. I am a bit miffed by all this. Shouldn't the yanmar shop be more pro-active in something like this?

 

[longy]

Pacwest has good mechanics - but they are definitely not marine engineers. At the time lack of max rpm was discovered, the root causes could have been a lot of different items. I'll bet they have no knowledge of prop dynamics, just does the engine spin it.

 

[kinardly]

^^^ Undoubtedly correct but immaterial since OP is now up in Ventura and Pacwest is in San Diego. Does Flexofold have a recommendation that doesn't involve changing out the sail drive, i.e. 17 inch blades at slightly increased pitch? Seems to me the basic parameters can be tweaked to provide the same end result. BWTHDIK

 

[mckenzie.keith]

^^^ Undoubtedly correct but immaterial since OP is now up in Ventura and Pacwest is in San Diego. Does Flexofold have a recommendation that doesn't involve changing out the sail drive, i.e. 17 inch blades at slightly increased pitch? Seems to me the basic parameters can be tweaked to provide the same end result. BWTHDIK

3 blade 17" x 10" is their best recommendation without changing gear ratios. But they note that the slip is still kind of high and it is not optimal. If I stick with 2 blade they suggest 17" x 12". We just moved the boat to Morro Bay Friday and Yesterday.

 

[mckenzie.keith]

<redacted> has good mechanics - but they are definitely not marine engineers. At the time lack of max rpm was discovered, the root causes could have been a lot of different items. I'll bet they have no knowledge of prop dynamics, just does the engine spin it.

Engineers or not, it was, in my opinion, a failure to not look into or at least not mention that there was a drive ratio change. I don't expect them to help me pick a new prop.

But they were replacing an old SD31 with a new SD60, and thereby changing the drive ratio. There were two ways they could have changed it, up or down. They went down. But never even mentioned this to me. I am miffed and venting and will probably call them to just make them aware of it.

 

[DDW]

As a point of reference, my Volvo (ZF) saildrive has a 2.19 ratio, and I am swinging a 19" prop. But the Volvo (Perkins/Cat/Shibaura) engine max rpm is 2800 (which is one big reason I spec'd it over the Yanmar).

 

[mckenzie.keith]

As a point of reference, my Volvo (ZF) saildrive has a 2.19 ratio, and I am swinging a 19" prop. But the Volvo (Perkins/Cat/Shibaura) engine max rpm is 2800 (which is one big reason I spec'd it over the Yanmar).

2800 RPM / 2.19 = 1278 RPM max shaft speed. Tip velocity for 19 inches is 32 m/s (from reference frame fixed to boat). Yes it makes sense. The faster your engine the higher reduction ratio you need. And vice versa.