Prop Info for Ericson Cruising 31 Bukh DV 20 Engine

[Greg Ross]

Simple answer on 1/2 increments of pitch

Prop shops are braziers and metal bashers, tools of the trade include dies/ castings that are used to fix the pitch of each blade. They're used like an anvil to hammer the blade down against.
These dies/ castings would have been bought at some considerable expense to make the repair/ reworking of propellers very effecient. The one shop I spent some time in, he had only even inches of pitch dies/ no 1/2 increments so I suspect that's fairly common. To get that 1/2 inch of pitch Glyn is referring to would take some jigging/ extra effort, hence the reluctance.

 

[mherrcat]

So let me see if I understand this.

When I get my laser tach I will want to take my boat out on flat water and check the RPM at full throttle and see what I get. The target in my case is the 3,200 RPM that Universal says is the maximum RPM?

If I am above or below that number I need to adjust my prop to get to that number?

Then, once adjusted, I will want to try to keep my average RPMs when motoring at about 2,500 RPM or 80% of the maximum?

Am I understanding this correctly?

 

[Frank Langer]

Mark, that's my understanding. I don't think you want to run at your max rpm for too long as that could stress the engine in my opinion, but long enough to check the rpms and your boat speed, then throttle back. Important that it be calm, no tide current, etc. so you get an accurate reading. Ideally you should be able to cruise at hull speed at about 80% of max rpm.

Frank

 

[larossa]

Prop Info for Ericson 31 Bukh DV 20 Engine

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.

Gentlemen,

Thanks for the wealth of information regarding the correct prop. I was hoping that maybe a current E31C owner still might have a Bukh engine. As I had stated, I am looking at replacing my current 2 blade prop with a 3 blade. The problem I have now is what is my cruising RPM’s. Which at the moment is quite difficult to obtain without a tachespecially in the middle of winter. So I do know this, consulting my manual the maximum RPM’s is 3000 for 20HP. My manual also states the output ratio is 2.5 to 1 so I need to do a little more digging in regards to this data. Because Michigan Prop is questioning this also and suggesting that it should be 2 to 1. Greg you are correct, I could not swing more than a 14 inch prop due to the hull clearance. I spoke with my Bukh dealer and they do make a tach for this engine. But I do like the price on the hand held that was mentioned in this forum. So it looks like I will have to wait until when the boat is in the water to start prop testing. One other note is I have notice some soot on my transom from the engine. So Glyn you are correct I will have to get some more data before I can decide on the correct prop.

Thanks,

Brian
E31 hull #2
Detroit

 

[Glyn Judson]

Correct prop data for the Bukh DV20 (the same engine as the Westerbeke Pilot 20)

Brian, Unfortunately of the seven or so E31's I show still fitted with a Bukh DV20, I only have prop data for yours and hull #16 and you have that info already. Consult your E31 Owners' List to contact the owners of hulls #05, 06, 07, 12, 15 & 17. Maybe they have prop numbers available to share with you. Glyn

 

[mherrcat]

Haven't had a chance to do my engine rpm/speed test yet since I didn't have a second person to drive the boat last weekend. But I did test the laser tach and it works great.

Today I was looking up some specs and calculating what my maximum hull speed should be and found this interesting site; some of you have probably already seen it:

http://www.psychosnail.com/boatspeedcalculator.aspx

According to their calculations my hull speed should be between 6.74kts and 7.48kts. Based on my LWL and displacement, to get to 7.48kts would require 31.4hp. Their chart shows I should get about 6kts at around 12hp, which looks to be a little more than 80% of my M-18's 14hp max.

Hopefully we'll see next weekend...

 

[Frank Langer]

Mark, hull speed is not an exact science, depending on boat shape, actual (vs. rated) displacement with the boat loaded for normal sailing/cruising, etc. As we have the same boat (though yours is one year newer and a different engine), I'll be interested in your findings.

Our normal sailing speed in decent winds is about 6.5 - 7.2 knots, though we have sailed close hauled at 7.8 knots in about 18 knot winds and reached 8.4 on a broad reach once though the waves might have contributed to a bit of surfing. All of those are with mainsail and a 125% headsail.

When motoring, I can run at 6.2 knots when my tach shows 2200 rpms which is about 80% of max. rpms on our 16 hp universal engine. That's with a 13 x 9 two blade folding prop which the manufacturer says is slightly underpropped--they recommend 14 x 10, but at $585 for the new larger blades I don't think the difference is worth it for me.

Please let us know what you find next weekend.

Frank
1984 E30+

ps. a book I'm reading said we lose at least one hp to the alternator, and more hp if you have anything else running off the engine, so we aren't likely getting the full hp from our engine going to the prop.

 

[mherrcat]

Yeah, I did figure it wasn't going to be exact, but it at least gives me a range to target.

I did confirm, at least from old log entries, that the prop is 13x9. I could probably get a 14" on there, but as you say, the difference might be negligible. I am also a little unsure about the amount of shaft exposed between the prop and the strut. It's only about 3" and I don't think there is any flexing going on, but I am curious what others have; I would probably be happier with 2" but again, maybe it's fine as it is.

 

[Frank Langer]

Mark,

I sort of recall that your prop is a three blade, rather than two blade--if so, then the 13 x 9 might be quite ok. You also need to leave enough clearance between the prop and the rudder, and the prop and the hull bottom.

Regarding the distance from prop to strut, others may have the exact knowledge to comment, but I would be monitoring for vibration on the boat (visible if the lifelines and other lines are jumping around, pedestal is shaking a bit, etc.) and wear/slack in the cutless bearing if the prop shaft has too much flex from being a bit too long.

Frank

 

[mherrcat]

Yes, 3-blade prop.

There is no apparent vibration.

Since the boat is in the water and I have no way of physically measuring things, short of diving, I am basing my measurements on the pictures I have posted. The prop shaft is 1" dia. Based on that it appears in the pictures that the distance between the strut and prop is 3". The distance between the prop and the hull is 2".

I believe I recall from somewhere that the distance between the prop and the hull should be 1.5x the prop shaft dia. Or maybe it was 1.5x the prop shaft dia. between the strut and the prop.

A 2-blade prop would probably cause less drag when sailing as I could try to lock it vertically between the keel and the rudder. A larger 3-blade prop would likely just result in more drag.

Right now I guess I'm just talking through my hat since I haven't actually done any testing...

 

[Frank Langer]

Mark, let us know once you have done the tests. I'm guessing your 13 x 9 3 blade prop will be ok. Yes, it will cause a bit more drag, but unless you're racing, probably not enough to really matter in getting to your destination. I think the 1.5 distance you are referring to was between the prop and the strut, so you may have a little more shaft there than needed, but I'm not sure it will matter, though maybe one of the experts can comment. The distance of about 2" between the prop and the hull sounds ok.

Frank