Double Eagle -need to tweak prop pitch or settle with pretty good

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North
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Double Eagle -need to tweak prop pitch or settle with pretty good

Postby North » Sat Aug 27, 2016 1:22 pm

Hi folks,
As most of you know I have sea trialed the new aluminum 25ft double eagle a bunch of times now and am very happy with results.
But, I would like to know if things could still be improved or especially made more efficient at cruise speeds of say 18 to 20kts.
With the Cummins 4bt 150hp engine, velvet drive 1:1 gearbox, 3 blade 15 x 14 prop I am getting the following numbers.
I cleaned and tightened tach pickup on bellhousing as it was going wonky at around 2200 rpms - it would drop to zero at higher rpms.
Now in neutral I am geeting about 3200rpms before it goes wonky and there is a bit more left in her.
High idle for the engine is supposed to be 3100rpms so tach may be off a bit, but idle is 800 so that seems correct.

In gear, I get:
4.5 kts at idle 800 rpms
14.5 kts at 2200 which is where it hits peak torque
About 17 kts at 2500rpms which is max rated rpm for continuous use
About 19 or 20 kts at around 2800 rpms which is max rated rpm intermittent use.
23 kts full out which is I think around 3100 or 3200 rpms - have not been out again since cleaning sensor.

Looking for input but a local marine diesel mechanic said they usually load them to hit about 2700 rpms in gear full out. Following that I could likely add some pitch, but would then increase my idle speed from around 5 kts....that's already on the high side for no wake zones...
Thoughts?
here's a link to the tach readings in neutral to find high idle..

https://www.youtube.com/watch?v=vS2q5x1X488

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Bill Edmundson
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Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby Bill Edmundson » Sat Aug 27, 2016 2:23 pm

North

It sounds pretty good to me for right now. I'd wait until you finish the boat and have an average load in it. Then maybe I'd play with pitch.

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

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kens
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Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby kens » Sat Aug 27, 2016 3:07 pm

you are most definitely under-propped.
The propeller load is what holds the engine to 2800rpm when up and running.
You are now under propped and the high idle (injector pump) is holding the engine top rpm, not the prop loading.
In the video I saw you hit 3300rpm, and then revved it up some more. Either your tach is off, or your injector pump needs adjustment.
You should be able to go completely wide open throttle, and the prop loading maintains 2800. You already mentioned you can get above 2800 when up and running. You need more prop to get best performance.

You mentioned you want the slowest idle, trolling, I have not noticed much difference in my idle speed with different props. I've run a bakers dozen props, and not noticed much difference at idle, except for prop walk when docking.
Look at your prop blade tips, is there much cupping?
Cupping is the tips curved aft. this has the effect of adding in more prop loading, but it doesn't really come on until mid-range on up.
My boat likes a 16x16 plain blade prop, that has best top rpm, top speed, medium prop walk.
Im now running a 15x15 light cup, minimal prop walk.

When you look at prop sizing numbers such as 15x14, it doesn't tell you much until you confirm how much cupping is massaged in the tips. the basic number size can be measured with a computer scan to the .0000nth degree, HOWEVER, the cupping has no means of calibrated measurement. cupping is the voodoo in prop sizing. I have one prop put aside as my 'control specimen' prop. I know that it is a 16x16, no cupping, plain blade, computer measured, it turns up 4200rpm and 28.5mph. I only run it as a base line engine performance calibration tool, I'm afraid to bang it up. it's put away.
Any other prop I run is a comparison to my base line prop.

I have found that cupping does in fact add prop loading, but I am not sold on the fact that is actually acts the same as adding pitch. It is a compromise.
I can run a 15x17, and get my 4200rpm, 28.5mph, but I also get a big prop walk when docking.
Big pitch = big prop walk.
Small pitch = small prop walk
I can compromise and run a 15x15 light cup and hold my rpm, and give away a lot of prop walk.
The fastest props I have run were plain blade (no cupping) like a 14.5x17.
Blade diameter doesn't load the engine as much as pitch does. Cupping loads the engine more than pitch does.
I think you need about 400rpm more prop loading to get correct. Most shops will say an inch of pitch = 300rpm. As a Double Eagle owner I say horse hockey. On my boat an inch equals about 125rpm. Computer scanned and measured.

Different prop shops measure cupping different ways, Bubba's prop shop may measure cupping on a scale of 1-5. Hero's prop shop might measure cupping on scale 1-10. So, you are running a prop (cupping loads engine more than pitch, remember) with a #4 cup. Now you can have 2 totally different props, both of which are 16x15 #4 cup. Was it a #4 cup on a Bubba scale or, a Hero scale? Cupping is VooDoo, remember that!!

That is why I have a base-line prop which has a pure plain blade with no cupping.

If you want a slowest trolling speed, lowest prop walk, size the lightest pitch
If you want best fuel economy, size the larger blade area, least cupping,
If you want fastest speed, size the smallest diameter, to biggest pitch

See if you can get a prop Hero or Bubba to measure how much cupping you got now. This has everything to do with the kind of performance you gonna get.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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kens
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Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby kens » Sat Aug 27, 2016 3:26 pm

here is a link to my baker's dozen props,
viewtopic.php?f=20&t=4425&start=0
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

North
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Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby North » Sat Aug 27, 2016 4:27 pm

Bill - that is a good point about finishing boat / any heavy additions first, or at least leaving room for them in calculations.
The only substantial thing I need to add is a main / larger fuel tank, likely about 30 - 40 gallons.

Kens-. I did not have time to read the other thread, yet, but your explanations above make sense.

I Don't believe the current prop had any cupping. I am not in a hurray, but may chat with a local shop about adding pitch and possibly cupping to load engine more.

May keep eye out for good deals on ebay as well say with 2 - 3 inches of pitch more than current one or combo of pitch and cupping.

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kens
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Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby kens » Sat Aug 27, 2016 6:10 pm

North,
you quoted above:
"About 17 kts at 2500rpms which is max rated rpm for continuous use"

Gordon's Double Eagle has the full sized deadwood skeg, per the letter of the plans, barn door rudder and all, running the 4BT-130hp @ 2500rpm.
He is getting 25kts @ 2500rpm. His 2500rpm is 130hp, continuous duty rating.

You should also be getting 130hp @ 2500. therefore I believe you also should get the same 25kts that Gordon is getting @ 2500rpm.
You are wayyyy under propped.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

North
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Location: Nova Scotia

Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby North » Sun Aug 28, 2016 3:45 am

I do have the same engine/ specs as Gordon (130hp @ 2500rpms) but ,I have the 1:1 gear, so of course won't spin as aggressive a prop as he...but will spin is faster.

Well, As I have no spare prop now - just the one... I will start looking on ebay for a good deal on something with 2-3 more inches of pitch, like a 15 x 17 or so...
Do you think this is in the right range?
When I welded on my skeg bar / lower rudder support, I did not leave quite enough clearance under the prop so I hesitate on using / buying a 16" prop.....

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kens
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Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby kens » Sun Aug 28, 2016 5:33 am

I thought you said you got the 4BT-150hp @ 2800 ??
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

North
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Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby North » Sun Aug 28, 2016 3:56 pm

Correct but 2800 rpm is max intermittent and it get 130hp at 2500rpm which is max continuous rating.

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kens
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Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby kens » Sun Aug 28, 2016 4:37 pm

A 15x17 or so might work, if, you know how much cupping is in your current prop, and know about the newer one.
Another option is the possibly of a 14" dia 4blade. 14" 4blade is almost equal blade area as a 16" 3blade.
These type props are mostly from the ski boat, wake board type boats, most have heavy cupping, but if you see one for cheap, try it.
16x16 is a good option too. Is your tip clearance really very tight?
How much clearance you got with your 15" ?
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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kens
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Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby kens » Sun Aug 28, 2016 6:23 pm

Here is the thread where I found my 'cruise' prop.
viewtopic.php?f=20&t=12109&hilit=rake

Double Eagle has a large forefoot in the bow, and as such has a lot of wetted surface on plane.
I used cupping to emulate rake, and arrived at my best cruise prop.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

North
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Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby North » Mon Aug 29, 2016 5:42 am

Interesting read about the rake. I will keep this in mind as I had been only looking at diam, pitch and cupping as well as number of blades of course.
Forgot to mention. Don't care much about prop walk. Not much call for tight manouvering here. More concerned with loading. If I get a lot of walk, will use it / work with it when needed.
Also, ideally I would find / use a SS prop, due to galvanic corrosion concerns with th aluminum hull. Don't want to sacrifice my hull as an "anode" to a nice bronze prop... but I am not at a marina nor on shore power where that issue is greatly increased due to sharing common ground and creating "circuits" between other boats. Also have my shaft and prop isolated from engine and boat with a drivesaver coupler insert. That reminds me to make sure I took off that strap across the bolts....
May find a SS prop in 4 blade as seems more common with skiboats.
Will keep my eye out.
What kind of pitch would you recommend if I look for 4 blade 14 " or 15 " diameter?

North
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Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby North » Sun Sep 04, 2016 4:04 pm

Update - I removed, cleaned and re-installed the tightened a bit further the tach pickup sensor, on the flywheel bellhousing and it now reads smoothly and doesn't act up and go to zero now, as revs climb..
I guess it could be out, but a local mechanic told me they are normally accurate, as opposed to what a lot of people say about the ones driven off the alternator.
I would not think it would be out too much, as idle is 800 rpms and high idle (in neutral is about 3200rpm - book says it should be 3100)

To Ken's point though, I had boat out today, and can now confirm that I do still reach that high idle- 3200rpms - in gear as well.
So, even though I will eventually add 200 -300 lbs (mostly from larger fuel tank) I do still think I am quite a bit underpropped.
I would think that I need to bring down the rpms to around 2800 - the max rated intermiitent rpms for this engine, or as the local mechanic advised - try for about 2700rpms. So, even after I add more gear and weight, I am still looking to decrease rpms by 400 or so.
normally, I think they say that would equal adding about an inch of pitch, but according to Ken, it doesn't affect it nearly that much, so I would require 2-3 inches more pitch, or a combo of pitch and cupping.

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psychobilly
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Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby psychobilly » Sun Sep 04, 2016 5:13 pm

Thanks for your updates North. All this info is invaluable for the next person that sets up with this combo.

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kens
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Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby kens » Sun Sep 04, 2016 5:26 pm

Well, that says a lot.
Yes, you are a whole lot under propped. You have not yet reached WOT.
Your injection pump is de-fuelling even tho the throttle lever is all way up, you are actually de-fuelling to prevent rpms going farther than 3200.
You don't know where WOT is until you get prop load under 3200.
We don't know how much more prop you need to get under 3200.
By the time you get there, you might have a 35mph boat !!

Look around at some other boat props and notice how much cupping is in the blades, then look at yours.
Either pay a prop shop to measure, or you can estimate how much cupping you got, slight, medium, heavy amount.
Look at a ski boat, Mastercraft, SkiNatique, those guys generally use heavy cupping, maybe you can see it.
The big outboards with a SS prop mostly use heavy cupping. How does your compare?

the prop you running now, does it have no cupping at all?? is it flat blades?

I respectfully disagree with the guy that says prop to 2700, then you add weight to your boat.
You should prop your diesel to 2800 or higher.
In your case prop it to 2900, then as you add weight your rpm will fall towards 2800 target.

Your boat yard guy saying to prop to 2700, then add weight, your rpm will fall towards 2600; an overloaded condition, don't do that.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:


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