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

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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 » Sun Sep 04, 2016 5:46 pm

Diesels do like to be loaded. But, overloading till you start to lug it is more damaging that under loading.

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

Postby North » Mon Sep 05, 2016 3:24 am

Understood re: not overloading. On small correction Ken - yard mechanic said to aim for 2700 a finished boat. Ie after adding weight, not before.
Don't have have pic handy but I believe current prop has little to no 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 » Mon Sep 05, 2016 5:29 am

If Cummins says aim for 2800, then…………

I ran some numbers with calculator, and something isn't right here. Something horribly wrong.
If you turning 3200 with a 15x14, then theoretical speed is 42mph, given a modest slip of 9% that yields 38mph.
If you turning 3200 and getting 23mph, then your prop is acting like a 15x9.
If you turning 3200 with a 15x14 getting 23mph, then your slip is 54%.
I think this prop is grossly cavitating, or tach is off, or prop marked wrong,

Im running a 15x14 moderate cup with a slip of almost zero.
Gordon is running a 16x16 4blade, cupped and slip of almost zero (with full deadwood keel)

My first thought is you need more blade area.
Try a bigger prop maybe as much as a 17dia 4 blade, if you can find one to run.

As a note, I can run a 14x17, or 15x14cup, or 16x16, or a 17x14 4blade. I can turn rpm with all of these.

If your 150hp expects to run the boat about 29mph, then with 2800rpm, your theoretical pitch is 11", that is, if it is 'hooked up' with minimal slip.
Perhaps a 16x12 cupped, if you can get that to 'hook up'.
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 Sep 05, 2016 9:54 am

Kens. I see what you mean, but here is what I know:
- prop bought from a real prop shop, via ebay so I assume they may have measured / confirmed numbers
- tach would not be off More than 100rpms as idle is correct and high idle about 100 from book specs.
- I am not overly concerned at this point as boat performs well in the 17 to 20 kt range, where I will usually run. But, of course I would rather do 20 kts at 2200rpm at a better fuel efficiency or even 2500rpm if possible, rather than at a higher rpm and fuel burn rate. I hate to pay to repitch the current one, in case it doesn't improve performance. I would rather look for a decent deal on ebay or locally and try another diameter and pitch combo. Then, once I get the results, and have a better idea where to go next I can decide which of the two props to perhaps have altered.

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

Postby kens » Mon Sep 05, 2016 11:45 am

Here is what I did:
I paid Frank&Jimmies prop shop, Ft. Lauderdale, Fl. for a used prop, under the agreement that they will exchange/rework it to suit my installation until we got it 'right'. I agreed to pay shipping back to them, and they would send me a different or reworked wheel. They would not offer to rework a brand new prop, but they did agree to rework or exchange used wheels.
We went thru 6 or more props before I accepted the final one. I only paid 1 price for a used wheel but paid shipping 6 or 7 times.

I noticed that as pitch went up, slip went up with it, to no gain in speed, but pitch went up/rpm went down.
I noticed cupping had the single biggest effect of any other rework done.
I tried blade area from a 14.5" 3 blade on up to 17" 4blade. The 17" 4blade did not cut down slip, but it did have tremendous hole shot & power.
I noticed that a cupped prop acted like one with bigger pitch, as far as engine loading. Cupping does not give you more speed, but it does lower rpm.
Example:
A 15x15 3blade light cup ran OK, but the same wheel reworked to 15x14 with cup massaged all the way out to the tips yielded a negative slip number, and fuel efficiency JUMPED !!

My final wheel is a 15x14 medium cup to the tips, and it runs like a 15x16. Im not getting the fastest top end of all my props, this one at cruise is 'hooked up' to a 0% slip number. This is where my fuel economy literally JUMPED.

Ask the guy you bought this prop from if he will exchange with you.
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 » Tue Sep 06, 2016 12:34 pm

Hi Ken - and others... \i can get this brand new - old stick Michigan \wheel 15 x 17 for $350 plus shipping... thoughts?

http://www.ebay.com/itm/252474371737?_t ... EBIDX%3AIT

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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 4:46 am

No, you don't have enough prop information yet to spend money on a brand new one.
Besides, if you running 54% slip with a 14pitch, your slip will go higher with a 17.

I think the cheapest alternative is to take the wheel you got and get it cupped.
Did you determine how much cup is in your current wheel? very little? none?
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 5:05 am

I think very little or none. I think I will do what you say and have my current one massaged a bit over the winter. Boat comes out in a month or so anyway.

Shipping would kill me - time and cost and waiting at borders if I were to use a big prop shop in the USA.

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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 5:31 am

Contact Acme marine or Michigan Wheel, they have prop sizing forms you can fill out online to get a size information better than either of us. doesn't cost anything.
A couple guys on this forum have gone to Acme with good results.

your slip number is so high right now that pitch alone isn't the answer. Im thinking you need more blade area first, then more cup.
My guess is a 16x12 4blade cupped
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 6:40 am

I have not read any detailed books on props, so please excuse my ignorance... I have read definitions / explanations of slip on line, and understand some of the basic, extreme examples, explanations, but don't have a good appreciation for it still.

I understand that increasing the blade area, and or adding cupping may help with slip. But, I would have thought that part of the reason I am (in your calculations) getting so much slip is that the lower pitched prop can't bite into enough water as my rpms increase alot. I am likely wrong.. but I though t increasing the pitch, loading up the engine more and bringing down the rpms would also reduce slip- as I would perhaps top out at 2800rpms with a speed of say 25 kts, versus now getting only 23 kts while spinning / slipping alot at 3200rpms...

Feel free to dumb it down for me, or correct me...
One (large) local shop wants about $200 to re-pitch - may be able to find somewhere more reasonably priced... hence the thought about buying another one (preferable cheap and used) so I would have a spare and I could haul boat and change in a few minutes vs taking mine off and waiting a week to get it back and on again..
I would try more area - 4 blade - or one with mod to heavy cupping if one were availabler used or new at a decent price...

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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 7:43 am

It is true that as you increase pitch your rpm goes down, but, that doesn't necessarily make for efficient prop. In this case you would be expending engine power to drive a big pitch, and not getting much thrust out of it.

The online prop sizing programs are free from Acme and Michigan wheel, try it.

How much does your boat weigh? I re-read your article and noticed you had said you increase the hull plating from 1/8" up to 3/16".
Is this correct?

That guy that said $200 to re-pitch your wheel, I'm proposing DO NOT re-pitch it, Im saying only to add cup to it. leave the pitch alone. !
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 8:03 am

i'll ask a shop about adding cupping.
just finished your complete thread on props / bakers dozen. interesting read.

don't know boat weight but off the top of my head, plans say hull at 1200 lbs, but that was for 1/8". so my 3/16" hull would be about 1800 lbs, if it were stock 23ft. I stretched to 25 so let's say 2000 lbs give or take plus 900 lbs for the engine and gear, so 2900lbs plus a bit more for steering gear, brackets etc -
I think 3000lbs give or take... I may have it weighed on run back at gov't scales. Then do trailer later and subtract.

I still have not seen video of mine running taken from outside the boat - would like to see how it rides... I know the spray rails on chines shed a lot of water to the side. Water does go aft in the bilge.

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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 8:33 am

you do want the spray rails at chine to throw water off to the sides, this is good.
Not only does it give a drier ride, but the force to throw water to the sides helps lift boat out of water, at least run higher in water.
Gordon first ran without any rails at all on chines, he had a very wet ride and not running great.
Then added the chine rails, he got a dry ride and picked up a couple mph.
Butch Barto (Mahogany Tahoe) also added chine rails later and picked up some speed as well.

I ran some numbers on the Michigan wheel site, figured you being a little heavy, and it told me 15x13cup.
If you add cup to the wheel you got, you may get there.
Ask your guy about cupping all the way to the tips, that is what got my Double Eagle to 'hook up' at cruise with minimal slip.
Minimal slip is all your HP going into thrust.
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 8:54 am

ok - I will ask a couple of shops about the cupping. We are only at cottage / boat on weekend now, so I could pull the boat (have my own slip right next to docks) on Sunday, and have a shop do the cupping through the week - then try it again following weekend.
I would like to try a few things, before winter layup if possible...

by the way, Mine cruises straight as well (of course, full keel) and I walked to the stern while at cruise the other day. I overhung my hull about 3" past transom, in case I needed to use as a bit of trim tab... anyway, while running the transome, and hull bottom / extension is a couple of inches out of the water, or more precisely the water is being pulled around/ under to create the effect that the hull extension tab is out of the water.

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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 9:13 am

can you post a picture of this extension?
I don't understand
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:


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