Prop Info for Ericson Cruising 31 Bukh DV 20 Engine

[larossa]

Saw the earlier post for the 3 blade prop for the Yanmar. I still have the orginal Bukn DV 20 engine, still runs like a champ, and I was wondering if someone out there might have any info on the proper pitch and diameter for a three blade prop for this engine. Look forward to hearing from you. I suspect it would be fairly close to the prop used for the Yanmar.

Thanks,

Brian
Hull #2
E 31C

 

[Glyn Judson]

Prop information for your engine.

Brian, If you have any data that will tell you the factory specification for the highest RPM under load allowable for your engine, then you can pitch a prop to give you just that. The prop on our boat is the same one as was on her 16 years ago when I bought her. The engine couldn't achieve maximum factory RPM then and in addition, sooted up the hull aft of the exhaust. I consulted a prop shop and they repitched it for me based on their best recommendation. It still wasn't quite up to the maximum speed, so I had them tweak it again and that got it smack dab on. So I guess the lesson here is that trial and error will bring you to the correct pitch for your particular engine. Remember cavitation and keep the blades the proper distance from the hull. As I recall, mine is a bit more than 1". As with most if not all diesel engines, they should operate at 80% of max RPM to avoid overheating. Our Yanmar 3GMF can run wide open at 3,700 RPM for one hour before overheating. The sustained cruising speed we use is between 3,000 and 3,400 RPM with never an overheating issue yet. As I type this, memory returns and one additional reason I changed the pitch twice was to eliminate the inordinate amount of steam issuing from the exhaust and cooling water. When I finally got it right, the steam was reduced to a mere trace. Hope this helps, now go consult your manual for those ever important numbers. Good luck, Glyn, E31 hull #55, Marina del Rey, CA

 

[larossa]

Prop information E 31C for Bukh

Glyn,

Thanks for the info, regarding the steam coming from the exhaust and cooling water, would the pitch be increased say from a 12 to a 13 to reduce the steam or would I decrease the pitch to reduce? You are right on beacuse yes, I do have a small amount of steam coming from the exhaust. What I don't have on the boat is a tach so I don't know what RPM's the engine is turning. Any recommendations on a good tach to get?
As always, Thanks again for help.

Brian
E 31C

 

[Glyn Judson]

Prop information.

Brian, Talk to your local prop shop to get the best answers. Why not talk to the folks at Michigan Wheel in Grand Rapids at 616.452.6941? Don't let the word steam mislead you. There still is steam in my exhaust but just a fraction of before my repitching exercise. As to a tach, maybe someone else much smarter than I on the subject can chime in to help you there. Do you have a manual or any other data that can give you an idea as to what speed the engine should run at? That's what I based all my pitching changes on. If not, this might help http://boatdiesel.com/Engines/Bukh/Bukh-Manuals.cfm?PDFID=3989200553096 Cheers, Glyn

 

[Greg Ross]

Determining prop sizing

Brian,
Are you running a 2 blade or 3 blade at this point?
The vital bits of data necessary to make the base calculation to size a prop;
Hull speed,
HP and rpm, and
Gear ratio
Reducing pitch would obviously reduce the load on the engine.
Because diesels don't have ignition systems to generate the signal for a tach. you need a sender that will generate that signal. I remember from my toy airplane days that there are tachometers available that use an optical input. Hold it in front of a propeller and set for two impulses per revolution, it provides engine rpm. Perfect for tweaking the needle to get that 2-cycle glo engine screaming at 10,000+ rpm.
There's got to be a knowledge base out there somewhere on retrofitting a tach on the Bukh engine.
Engine rpm is pretty vital as the starting point unless one of the Prop Shops has data for your engine/ reduction gear combo.
Regards,

 

[Frank Langer]

I recently bought a hand held laser tach for $22.00 incl. shipping and love it. I simply had to paste a piece of reflective tape (which was supplied) on the main pulley of the engine, and point the tach at it while holding an on button and it gives a digital reading of the engine speed. It's great. It enable me to see that at 80% of the manual's max. rpms, I was able to cruise at 6.2 knots, which is almost exactly the rated hull speed. So my concern about prop size, and also about whether I was running the engine too fast or slow, were easily resolved.

Frank

 

[Greg Ross]

Awesome

Frank,
That's awesome.
For the nominal sum of $18.00, no DIY mechanic should be without one!

Brian,
Looking at Glyns' Ericson 31 Data summary, he had hull 16 as having the BUKH DV 20, and a 2:1 reduction gear swinging a 16 x 11 x 1" 3 blade. I don't quite understand how there's any clearance with the 16" diameter prop, unless the whole installation was angled to enable more prop/swing clearance. Gear reduction is what constitutes a major consideration, for the Yanmar 2GM 20F with 2.63:1 reduction that I removed she swung a 13 x 10 x 1" three blade. Similar HP but an enornous difference in prop dia./ pitch taking into account the reduction/ prop speed. The BUKH could be a slower turning diesel as well, I'm not familiar with it. ie; making it's peak hp/torque at lower rpm.

 

[Carefree Sailor]

Handheld tach

Hi Frank,

Would you mind sharing with us the brand of handheld tach that you bought?

 

[Frank Langer]

Hi,

Yes, I ordered it through Suntekstore.com on the internet. You can find it by doing a search on that site for tachometer, and it will show several. The one I bought is currently listed at $15.94 US with free shipping, is shown with a blue pouch and is called a Digital Non Contact Laser Photo Tachometer, their item #14002037. The model number on the tach itself is DT2234C. The tach is grey in colour, with a brown top end, has two buttons on it, is light and fits nicely into one hand. It came to Canada from Hong Kong within two weeks as they said and has been great, no concerns. There are other tachs on that website, but this one seemed most appropriate for this use.

I was surprised to find one this inexpensive; every other one I had ever seen was alot more expensive, so I was delighted to find this one, and still really like it.

Frank

 

[Tom Metzger]

Possibly the same one is offered by Amazon for $12.33:

http://www.amazon.com/Professional-..._2?s=automotive&ie=UTF8&qid=1326655999&sr=1-2

 

[Frank Langer]

Yup, it's the same one, a really good deal at that price!
Frank

 

[mherrcat]

Hey, Frank, can you post the engine model and the prop specs you are currently using? I'm curious as I have a 1985 30+. I have an M-18 engine, Hurth HBW-50 gearbox and a 3-blade prop; don't have the size and pitch offhand.

 

[Frank Langer]

Hi Mark,

Our 1984 E30+ has the 16 hp Universal 5416 diesel engine, a newer 2:1 ratio Twin Disc transmission (used to have the Hurth 50), and a 2 blade Varifold 13 x 9 folding propeller. The Varifold factory in Europe thinks the prop is slightly under what it should be--they recommend either a 15 x 9 or 14 x 10 for this boat/engine/transmission. However, the current 13" prop when folded only clears the rudder by about an inch, so to accomodate a larger prop I would likely have to shorten the propeller shaft. I also don't have too much clearance to the bottom of the hull, so would have to look at that before I would take any action.

At 80% of max engine rpm (2800 from the Universal manual) my engine should cruise at hull speed at about 2240 rpm. Using my new handheld laser tach, I determined that in calm conditions the boat cruises at 6.2 knots at 2240 rpms, which seems about right for rated hull speed. At that speed the engine would begin to either smoke very slightly or exhaust steam (I did this test in December), where at about 2040 rpm or 5.8 knots it didn't do that. I tried full throttle for a short while and the boat picked up even more speed, rpms went to 2850 and the exhaust showed more smoke/steam--I didn't like this and immediately throttled back. So maybe if I had a slightly larger prop, as the factory tech suggested, the engine would not exceed 2800 rpms and I might have slightly more power, but I think I'm close enough to not bother to make a change. New larger blades for my prop would cost about $585.00, for what I think would be little gain. I do notice that in strong chop and headwind, the boat will not reach full hull speed under engine alone, which can be a bit annoying when I'm in rougher conditions and want to get to my destination, but again for the cost of new blades, I think I'll stay with what I have.

The tech said that if I was underpropped it could damage the transmission. But I think that would only happen if I run at really high rpm and drive the transmission faster than it wants to go; I guess a larger prop would help prevent that, but I think I can avoid that problem by being a bit careful.

All in all, I'm pretty happy with the current set-up, and would prefer to spend any additional money on new sails down the road than on the drive train.

I hope this information is helpful--let me know if you have any questions.

Frank

 

[mherrcat]

I believe my prop (fixed 3-blade) is 13 x 9. Thought I had that written down here at home somewhere but can't find it.

The specs on the M-18 are 3200 RPM max., transmission reduction 2:1

 

[Frank Langer]

It seems to me that a 3 blade 13 x 9 prop would provide significantly more power than my 2 blade 13 x 9 folding propeller, so your prop may be quite fine for your application. Do you have any problems with perceived lack of power, reaching hull speed, engine exhaust smoking, etc.? Maybe one of the experts could chime in...

Frank

 

[mherrcat]

I just ordered that laser tach from Amazon and will use it to check the engine RPMs.

I found a prop calculator here:

http://www.castlemarine.co.uk/pitch.htm

Based on the info I have about the boat and engine it comes up with a 12.43" dia. prop with a 10.08" pitch, resulting in 5.85 kts at 2,880 RPM.

My prop is also about 4" behind the strut, which may just be getting to the point of possibly being an issue; I think the prevailing wisdom is that the prop not be too far from the strut. A smaller prop would allow it to be moved closer to the strut and still have the hull clearance needed.

 

[Greg Ross]

Is that a Palm Tree I see

Mark,
Is that a Palm tree I see behind your prop in the above image?
I cannot imagine....... it's -8 deg C here tonight, Brrrrrrrr
Prop calculators should never be taken as Gospel, that's why they post all the disclaimer statements!
What the calculator will give you is a starting point, and because every boat design will perform differently, it's necessary to to resort to fine tuning if you want to max out. The power is in the tips of the blades, if you can swing 13" you're better off then reducing to 12" diameter with more pitch.

 

[Frank Langer]

So the calculator is showing that I should have a 15 x 11 prop rather than my current 13 x 9, yet the current one does let me get to hull speed at the expected (ie. 80% of max rpm) rpms. How fast do they want me to go?? :0

I wonder how the 15 x 11 would affect the boat ( 1984 E30+, 16 hp diesel, displacement 9000 lbs.), and whether operating it with the 13 x 9 is causing any damage...

Frank

 

[mherrcat]

Good eye! That is a palm tree, but the picture was taken a couple of years ago in February when it's usually a little colder down here.

Yeah, I used the calculator just to see if my prop was at least in the ballpark, which it seems to be. I guess the tach will help me zero in on what's going on. As I mentioned in my earlier post, I'm not even sure my prop is actually 13x9, I have to find where I wrote down that info to confirm.

 

[Glyn Judson]

Correct prop for an E31 with a Yanmar 3

All, Greg makes a good point as to prop calculators being only a good ball park tool. For years now, two E31 owners single hand their respective boats each summer by buddy boating off the coast of Washington and into Canada. Hull #06 is fitted with a Yanmar 3GM30F with three blade prop while hull #19 has a Yanmar 2GM20F with 3-blade prop and it was a great frustration of the owner of hull #06 to have hull #19 consistently walk away from him when under power. I happen to have close to the same set up in my E31 powered by a Yanmar 3GMF with 3-blade prop as does hull #05. Mine is 22 1/2 HP with a 30 amp alternator while his is 27 HP with a 50 or 60 amp alternator, thus the major reason for the difference in power which at the end of the day, is about the same at the prop for his boat and mine. The point of all this is that years ago I went through three iterations of my prop to zero it in at 3,700 RPM max, the factory specified RPM and his couldn't achieve that. After talking together, I gave him my prop measurements and suggested that he consult with one or two prop shops up there, Tacoma Propellor being one and I forget the other. It so happens that mine ended up at 13 X 12 1/2 but that's not important to this discussion. Armed with my numbers, he spoke to the two places. One flat out said that they couldn't or wouldn't bend a prop to 1/2" increments (I don't know enough about this science and art to know why) and the other told him that my numbers were way off and that he'd never zero in on 3,700 RPM. I asked him to trust me on this one and go with my numbers. He did and hit 3,700 smack on. I should say here before going on too much further that our engines are not designed to operate at that speed for more than 60 minutes before overheating (which they will do, don't ask) but that maximum RPM is an indicator of proper pitch and allows the safe operation for sustained periods between 3,000 and 3,400 RPM and no soot on the transom or being under pitched either. Now he can motor cruise with hull #19 and not only stay up with him but pull ahead with the increased performance of his larger engine. The point her isn't to see who can go faster, but to solve the puzzling frustration of being wrongly pitched and correcting it despite being told otherwise by the "pros" that it wouldn't work based on their calculator. As an aside, can anyone explain what's with the one shop refusing to correct in 1/2" increments? Bending is bending and I imagine if it were me, I'd just stop bending at the half way point between one inch and the next. But there must be more that this dullard doesn't get. Oh and for anyone interested, hull #19 is pitched at 13 X 12 and he can tit 3,700 RPM too. Cheers, Glyn Judson, E31 hull #55, Marina del Rey, CA