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

Designs for inboard or outboard power

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

Postby North » Wed Sep 07, 2016 9:59 am

Just the bottom hull skin extended about 2 inches aft of the bottom of the transom. It allowed for an easier filet weld on outside stern rather than a outside corner weld if the hull bottom had stopped right at the transom.

May see it in the bottom pic if you expand it. its like a super mini swim platform - runs the entire beam- could just get you toes on it if you had to....I now have an anode bolted on top of it which sits maybe an inch under the water. But when running, it's well out of the water.

speaking of waterline - my paint is way too high. At rest, the chines are just out of the water at the stern.

In pic at dock, you can see the waterline at the stern.
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kens
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Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby kens » Wed Sep 07, 2016 10:29 am

this extra bottom skin you got, is it perfectly straight with the running bottom? No hook nor rocker?

When I first launched mine, the corners of the transom would be barely out of the water at rest.

When you are planeing, the water/wake should come straight off the bottom of the transom.
What is this extension out of the water? Do you mean that the entire transom area is not wet?

Here is a pic of my aft chines when cruising, notice the sides and transom are actually dried out, not wetted.


Image
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 » Wed Sep 07, 2016 11:04 am

I'll have to get a pic of it running. This extension is straight - has not bee bent up or down. But the bottom looks like it's an inch above the stream of water when running. Obviously it's running on the hull but perhaps the stern is lifted a bit and appears out of water.
Question or note on your slip calculation of around 50% at 3200 rpms and 23 knots. Not mph so that will make a slight difference. But, main point is the fact that designer says it is inefficient or impossible to push the full keel version past say 23 or 25 kts tells me this increased drag will look like or show up as slip in your calculations. I can never push that larger wetted surface anywhere close to your theoretical 42 mph or whatever it was. Maybe in you no keel hull but not mine. Would that not impact or show up as slip in error?

I do appreciate you doing the numbers and thinking by the way.

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

Postby kens » Wed Sep 07, 2016 11:24 am

Gordon has bigger keel than you, and he is running less slip, and 25kts on 130hp.
The designers notes stated 27kts* concerning the keel drag.

At any rate, you are underpropped, and cupping is the cheapest & easiest way to load the engine more, and this may allow your 15x14 to run like a 15x12 rather the current 15x9. (or 15x10 if you figure in kts)
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 » Wed Sep 07, 2016 12:03 pm

I know Gordon has a heavy keel, but I am not sure that it is much larger / more wetted area, but I do have a large cutout and he does not.
We are close enough to compare though, and as I mentioned if he tried to hit the theoretical 42 mph in the prop slip calculations he would never achieve near that either. The keel has to reduce potential speed even if the calculator doesn't allow for it I don't you think?

Still, we have the same engine - 130hp at 2500 (and 150hp at 2800 but only intermittent use) and I get 23 kts so I am only 2kts off his numbers top end... But, I want to hit 23 kts or even 25 kts at a lower rpm. ideally 2200 (max torque) or 2500 max continuous rating.

Found a good prop shop, but it's a 3 hour drive, so may wait until the winter layup. He can add mod or more cup for about $100 -125 or cup and a couple inches of pitch for about $200. He said my idle speed will go up though - juts have to live with it and shift some in no wake zones, if I want the performance.. He recommends propping for usage level - overpropping a bit .i.e to reach 2500 or 2600 max. I know that's more than most recommend, but 2600 is only 100 less than local mechanic suggested as well.
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kens
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Re: Double Eagle -need to tweak prop pitch or settle with pretty good

Postby kens » Wed Sep 07, 2016 12:39 pm

But, this is what you opened your post with:

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.

Gordon and I noticed that his speed reads the same as rpm, example: 17kts = 1700rpm
22kts = 2200rpm,
25kts = 2500 and so on,

your 14kts @ 2200 is 8kts off Gordon results
your 17kts @ 2500 is 8kts off Gordons

If Gordon is hitting 25kts @ 2500 then you should also, and, you should hit 28kts @ 2800.
You saying you getting 23kts @ 3200, if you and Gordon got similar keel, then you should approximate his numbers, agree?? That would be 23kts @ 2300.
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 » Wed Sep 07, 2016 1:09 pm

Agreed. I need to keep the speed around 20 to 23 kts but hit it hopefully at a much more efficient rpm range.

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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 » Wed Sep 07, 2016 1:22 pm

I'm running a 125 metric HP Yanmar diesel. That is 115 HP here. I have a 1.23:1 reduction with a 13x16 prop. I get about 1 mph per 100 rpm. WOT is 3900rpm. I usually run about 2400 rpm. Continuous rating is 3700 rpm.

This is just as reference point.

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 » Wed Sep 07, 2016 2:07 pm

North wrote:Agreed. I need to keep the speed around 20 to 23 kts but hit it hopefully at a much more efficient rpm range.


Keep in mind also that @ your 2500rpm you are not making 130hp. As long as your prop is hitting your high idle number of 3200, you are not making full rated power, at any rpm.

I have also noticed from my own prop experiments where prop shop gurus say that cupping = 1" of pitch. I say balogny. I have personally seen where cupping loads the engine as much as a FULL 4" of pitch or more.
Also cupping does not increase your idle speed as much as does pitch. cupping shows itself as you get up and running, no so much at idle.
Keeping all this in mind, I think you will end up with something like a 15x10 heavy cup. the 10 pitch would allow your low speed idle, and cupping will come on strong in cruise.

Right now the cheapest way from here is to cup the one you got, what if you ship your prop to that guy that is 3 hours away?
How much is shipping for that? Is shipping cheaper than your gas for that much driving?
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 » Wed Sep 07, 2016 3:35 pm

Yes shipping would be cheaper than driving. And with 2 small kids - can't really get away just to drive the prop up and back. I may wait and try and find any used prop within 14 to 16 inch diameter with a reasonable pitch so I then have a spare. More importantly, I could then always put one back on if and when I take the other off for massaging. I hate to send my only prop away unless it during winter layup. On the other hand, I'd love to be able to try another combination now.

I know cupping may help load the engine better but I can't imagine I would be able to drop pitch down to a 15 x 10 unless it were perhaps a 4 blade.

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

Postby kens » Wed Sep 07, 2016 4:24 pm

I just ran some numbers on a calculator, and a 15x12 cupped seems about right.

From my own experience, I can say that my 15x14 runs with 0% slip. It does have cupping, so I am assuming the benefit of cup makes the slip appear to be zero. Since it is a 14 pitch and it runs like a 15 pitch, it must be cupping that makes it appear to be a bigger prop (0% slip)

I think you could use a 15x12cupped. At your target of 2800rom, a 12 pitch puts you at 28kts. add cupping to negate slip and the numbers are believable.

Why you have such high slip numbers right now, and why your engine isn't loaded, I just don't know. Sometimes a prop is just a dud, I have had a couple of them.

Gordon is running very low slip numbers also.

If I may ask, where did you set your CB when you did your stretch? Is the engine nearly exactly between frame 3 & 4?
I just looked at mine, and my engine is closer to frame 3 than, midway of 3 & 4.
If you added your 3" swim platform, then your 'running bottom' is effectively 3" longer aft of the engine (making the engine seem forward of CB) ??
This was just a chair & cooler moment for me awhile ago. thinking out loud……...
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

North
Posts: 265
Joined: Mon Jul 15, 2013 6:29 pm
Location: Nova Scotia

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

Postby North » Wed Sep 07, 2016 6:25 pm

It's been a while since I did the cb figuring... I have it on a full size piece of paper, but that's at the shop an hour away - wom't ne up there for a few weeks likely. I did some guesswork, but did move the cb based on the stretch - assuming that I was adding bouyancy aft, so I believe I moved the CB aft about 6 inches of so on the drawing (wild guess - can;t remember). Then plotted my engine's cg a few inches aft of that.
here's a pic or two of the engine placement. Looks like its about centered between frames 3 and 4 to me.
I do plan on putting my fuel tank - perhaps 50 gallon in the void aft of the engine.
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Last edited by North on Wed Sep 07, 2016 6:32 pm, edited 2 times in total.

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

Postby kens » Wed Sep 07, 2016 6:31 pm

I found a pic of your build, then went out and looked at my boat. Both of our VelvetDrive tranny mounts are in about the same place to frame 3.
Never mind, we are both about the same on that thought. I moved my engine aft some also with the stretch.
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 » Wed Sep 07, 2016 6:51 pm

Check this out, look at my wake at the amount of foam & froth & air bubbles thrown in the prop's center wake. Notice it seems the prop wake subsides shorter than the side's wakes.

Then look at your second video you posted, for the similar view, and stop it for a freeze frame good look. I think your prop is throwing a lot longer wake of froth & foam. your prop is churning air it seems.

Image

When I am trolling top water lures, there is a speed barely off-idle, that my wake is clear. no foam, froth, nor churned air in the prop wake. Can you do that? This is mostly the speed I troll at, I can see the lures better. I keep saying that my prop is hooked up, it sure is off idle, no froth nor foam to hide the lures.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

North
Posts: 265
Joined: Mon Jul 15, 2013 6:29 pm
Location: Nova Scotia

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

Postby North » Sat Sep 10, 2016 2:45 pm

Just verified #s today - top speed was off by a Knot - was a bit windy and I am getting some good growth n the bottom edges of hull, and perhaps under hull....been in the water a couple of months now with epoxy primer only - no bottom paint... will pressure was off when I take it out in a month or so, if not sooner.

Also, i'll correct a previous statement - the hull bottom / hull extension is of course riding on the water on plane - top is dry but water is coming off the bottom, straight back so it is NOT "out of the water" as I thought previously.


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