Here's a really stupid question!

[Fastjeff57]

How high does a lower unit (prop) have to be above the keel to generate a rooster tail?

I'm running a Mark 55/ Merc 400 lower unit with a three cylinder Merc on it and have never seen a rooster tail. The cavitation plate is 2 inches above the pad, and the prop is a homemade chopper of 17 pitch (that is not cavitating at this height). All I see is two spumes from the leading edge of the lower unit below the cav plate.

Jeff

PS: I'd post some photos but... What's with this deal? Was always able to post them before.

[filthy phill]

well the cav plate is far too low to cause a rooster unless you were trimmed out realy crazy.

lets say the prop is 12 inch diameter for example and a 1 inch space above the prop to cav plate.
1/2 of the 12 is 6 plus the 1 is 7, so realy the cave plate needs to be about 6 inches above the plate to get the rooster going.
to surface properly and get a real good one going the cave plate needs to be 7 inch above the pad atleast.

the rooster is only realy made when some of the prop is out the water, but with its centre line on the surface it gives the full effect. but could be slowing you down ???

sometimes a rooster is just wasting power if a bit too high, ideal with a surface prop would be with top of rooster no higher than top of motor to get best thrust.

lots of theory's about the rooster tail, but unless running on the surface you dont want to see it behind your boat.


yes the forum is playing up big time again, could not make a new thread last night or post any pics.

No such thing as a silly question, they are all valid...
I normally ask Daft questions, and that is a different matter
62845

[filthy phill]

well the cav plate is far too low to cause a rooster unless you were trimmed out realy crazy.

lets say the prop is 12 inch diameter for example and a 1 inch space above the prop to cav plate.
1/2 of the 12 is 6 plus the 1 is 7, so realy the cave plate needs to be about 6 inches above the plate to get the rooster going.
to surface properly and get a real good one going the cave plate needs to be 7 inch above the pad atleast.

the rooster is only realy made when some of the prop is out the water, but with its centre line on the surface it gives the full effect. but could be slowing you down ???

sometimes a rooster is just wasting power if a bit too high, ideal with a surface prop would be with top of rooster no higher than top of motor to get best thrust.

lots of theory's about the rooster tail, but unless running on the surface you dont want to see it behind your boat.


yes the forum is playing up big time again, could not make a new thread last night or post any pics.
and not allowed to post pics today neither.
someone has definitely altered the settings...........

[Fastjeff57]

Thanks, Phil. My prop is 9 inch OD, Gonna measure the tip to cav plate distance.

Jeff

[filthy phill]

most people when running suface type props cleavers and what not, seem to run with centre line of the prop up as near as possible to the surface when running.
I know it sounds crazy, what that is the whole point of the surface type props.
some are ok with just say 1/3 of a blade out the water, some like to be completely out the water, thats where the rooster comes in, unless you have a jet with a diverter.
you also get a rooster with a conventional type propeller, but that normally only happens when over trimmed and running it higher that it is designed for.

put your motor with cav plate about 4 inches above the pad, might gain a lot more than you think, just need to see how it handles in the corners !! that can often be
the case of people running surface props too low , they dont like the corners on surface ...

[Fastjeff57]

Four inches up! Yikes! I've struggled like mad to get my homemade props to grab at half that height.

Forgot to add.... My boating area (just down the road) is fairly rough most of the time. Running a 12 foot Deep Vee with a 10 inch pad can be rather 'thrilling' in a serious chop. I need for the boat to plane right off--now!--if I get knocked off plane by a wake. So far the prop digs right in and goes--something I doubt would happen at 4 inch up. Agreed?

Jeff

PS:: Thanks a bunch for your advise. It's all in my Keeper File.

[filthy phill]

just go up 1 inch at a time until you find the right place for it to be a good all rounder.
no point in having fast top end if it wont go in the rough water that you use..

top speed is not always going to win a race, if it dont come out the corners it aint going to win..
there is a real good race on you tube with a nice 56 motor racing a sst60.. the sst60 is a lot faster on the straights, but the 56 loses it on the turns.
makes for a realy good race and they are equal in amount of time per lap.

[zul8tr]

Jeff If set up right with correct prop on a good designed hull that can stand the engine lifted high at full speed the rooster tail will not be that high since most of the rearward water from the prop is mostly straight back. The higher the tail the more vertical thrust wasted. Ex. on my hydro at full speed with 6 degree rake 2 blade prop the rooster is maybe 18" high. I use a 7" diameter prop and run the shaft center 3/4" below bottom thus 2-3/4" of blade out of water. Also with a very efficient boat set up correct and engine high with a surface piercing prop the water going backward off the prop is a little bit greater speed than the boat speed going forward it can never be equal since slip is present. Pete

[filthy phill]

sounds about right

[Master Oil Racing Team]

When you are racing, and I am talking closed course such as 1 mile or a little larger or smaller, you are going to set up for your best time around the course. Filthy phill is right about winning. Most checkered flags are between halfway on the front straight to about 3/4ths down the front straight. You have to be in front after exiting from the final pin rather than being the fastest and ahead coming up to the next turn bouy. With this in mind, the testing and setups on outboards with surfacing props the result is a roostertail. In order to get competitive, the motor has to be lifted to reduce drag. The higher the motor is raised, the less the drag is, but there is a point where you will lose water pickup. Then you will fry your motor. Before that happens, there is a point where you will lose control as Phil mentioned. You could jack the motor so high you could swap ends in a turn. We always ran the center of the lower unit about and inch to 1 1/4 below the bottom of the boat. We ran 6 1/2 to 7 1/4 diameter props, and there wasn't much of a cavitation plate. It was bolted on to the lower unit and was about three inches wide and five or six inches long.

So far I have given kind of an explanation of how we set up and raced our engines. Now I will say that we never tried to make a roostertail per se. All the boat racers did their setups to win, and made adjustments according to the size or the course, the wind, etc. Roostertails were just a result of our setups. I have seen on our lake a number of jet boats that raise their nozzle just to make a roostertail. The can look impressive, but lose speed because the jet of water is not pushing the boat forward, but burying the transom. Jet skis do the same. Some people with jet skis tap into the water discharge to send a jet of spray into the air. All of that is cool if that's what they want to do, but just to make a roostertail doesn't mean you are going fast.

Can't give any advice on your boat though. You need to jack the motor up for sure to get a roostertail, but with a V bottom and small pad, you are going to have to do some serious testing, and make sure you always are hooked up to your kill switch.

[Jimboat]

How high does a lower unit (prop) have to be above the keel to generate a rooster tail?
here is an article on 'Rooster Tails (http://www.aeromarineresearch.com/publications/PB&R_Apr2012.html)'

[Fastjeff57]

First of all, thanks very much for taking the time to reply to my question.

I thought that, since I've never seen a rooster tail on my boat, that might indicate something that needs to be done about it. I now see that a rooster tail is not needed, especially since I boat in really rough water for a 12 footer. (Was out yesterday and got clobbered by large wakes of boaters coming back in late in the day. Mornings for me from now on!)

Okay, so I'm up to 42 mph at 5,800 rpm with the latest modification to my homemade prop. The cav plate is 2 inches above the bottom (deep Vee. 20 degree deadrise hull with a 10 inch pad. Love to post some photos but...) The 9 by 19 inch chopper bites well in all cases, planes off swell, holds in tight turns, etc. And it holds up to about 10 degrees trimmed out past vertical. I have another 3/4 inch left in the homemade jack plate, so I'll be trying that next, along with re-installing my 'hot rod' motor; an older Merc triple with external reeds and three carbs. Its power band runs strong up to 6,500 rpm, where the stock triple (I'm running now) 'falls off a cliff' above 5,500 rpm. (Merc spec say it has a 4,800 to 5.300 rpm operating range.) Should be interesting!

Again, thanks for all the help.

Jeff

[filthy phill]

that extras 3/4 inch might get you another 1mph maybe more, worth a try.
when in the rough stuff the rooster ona small boat would be a waste if its a big rooster, and when running in the rough I always keep motor down.

ideal thing to do is get a real cheapo endoscope camera and attach that to the back of the boat point toward the cav plate. these are fully water proof and around $15 for a cheap one.
they are brilliant for looking inside the motors and up exhausts etc and great for watching the cave plate on the phone.
you can keep the videos they take and watch em back later.
This is great to use as you can then see if the plate is riding on the top of the water at full throttle while your in your normal driving position .
The closer the prop to the surface will be best for top speed with your prop, and being as you made it , you know how to change any part of it too.

would like to see this boat run, as 42mph is not hanging around at all !!!!

[Fastjeff57]

I DO have a camera and been taking video forward and aft all along. (I post it here but...) At 1 3/4 inch up the cav plate is clearly above the stream coming back, and I'm now at 2 inch and will shoot for 2 3/4 inch (max possible without modifications).

Dying to try the 'hot rod' motor as it makes at least 15 more hp at a thousand plus more revs (which should be perfect with the present prop).

Jeff

[Fastjeff57]

Update: Built several props this summer, and got the my dink boat up to 42 mph. (I know: big deal, but that's 6 mph faster than before.) Raising the cav plate from a 1/2 inch above the keel to 2 inches up was primarily responsible. By the end of summer I got my homemade props to hold with the cav plate just under 4 inches above the keel. The temp gage went up so fast I actually saw it move, so more work needs to be done on my water inlet system.

Jeff

PS: My respect for prop gurus like Ron Hill knows no boundaries!

[Ron Hill]

PS: My respect for prop gurus like Ron Hill knows no boundaries!


Thanks, Jeff but, I basically call myself a BLACKSMITH. Over the years many of my best props were props we just beat on and and twisted and beat on them some more.

Today, my son, Chad, has some interesting equipment to measure propellers and reproduce them like "POPCORN".

Test, test and more testing. You mentioned water temperature. Over the years, I would hook up a long water hose from the "PEE HOLE" and have the water run on my hand, usually my throttle hand in kneel down boats. When the water started getting hot, I'd let off the throttle and slow down....

PS

I loved O.F. Christner, I always thought of him as more than a welder.

[Fastjeff57]

..."Test, test and more testing."

That's surely what I have been doing, Ron; and relearning what guys like you already know. I too am basically a blacksmith, and damn proud of it! In the old days, those guys could make practically anything the towns people needed.

Jeff

[filthy phill]

Blacksmiths were and are very clever people who did and do wonderful work. and I salute the blacksmith's of the world.
To be a blacksmith you have been gifted from birth to work with metal, you can only tech those who can take it in, and taking it in is not as easy as some might think.

But you must not forget about us who have mastered the art of bodging up stuff, I call myself a bodger as I can bodge anything to get me home.
In the fine art of bodgeneering I would definitely come under a master bodge artist, from now on I will call myself a Bodgesmith just to make it sound a bit
more up market.
We in bodgecraft use a close tolerance of around 1/4inch for our very intricate stuff and about 1/2inch for normal everyday work.
Anything under 1/8th tolerance is a master craftsman to us and all the talk of 1,000ths we think is pure mythology as our rulers dont even have marks
smaller than 1/8th so we know it is just fantasy for normal people to work like this in reality :confused:
Those who claim to realy work in the 1,000ths are obviously wizards, witches, or beings from a distant galaxy and should not even talk to the like of us in the damp
garden sheds of the world.
These people do not understand that the oily , greasy pile of scrap and rubbish they see in our corners of the sheds is actually our master pieces that took blood, sweat
and lots of bandages and cups of tea to get them looking like this.. :cool:
When I see props made by the likes of ron and chad hill I drool at just how good they look, to me they look more like a prize trophy rather than something to get wet
and mucky.
But these master pieces only work when put under water so its obviously something to do with black magic or sorcery of some kind, maybe even Voodoo :eek:
Obviously he is one of the above three mentioned and a Wizard.

Now to fastjeff57
His home made prop work is great, he has made his own and done some great work and got good results, this has to be applauded by us normal mortals.
Not only has he "had a go" at making a prop, its running very good and he is now getting great results with fine tuning his set up.
I take my hat off to him ( all us bodgers wear beanie hats as we are not yet up to peak cap standards). and I love to see home done stuff working good.

And putting a powerhead off one motor onto another totally different mid -section and then making a custom prop to fit this is just incredible so he also must
be a top ranking understudy to a wizard or sorcerer !!!

This talk of testing is just to fool us into thinking he is not sure what he is doing, we are not fooled, we know that you know what your doing.
you have already done what to us is the impossible task of making scrap metal into a piece of working marine art, you sir are a genius just fine tuning a great
piece of work.

All of us at Bodge school salute you, we cannot take our hats off again as its freezing out here in the cold damp sheds of doom, danger and high voltage.
We just keep looking for your next results of " testing " and we are not disappointed in what we are seeing so far.

Keep up the good work.

[Fastjeff57]

Thanks a bunch for your most considerate comments.

Read your post above on your own prop travails and am at a loss as to what I'D do next in your place.


I'm sure somebody here will give you good advice.

Jeff

PS: In my "machine shop", the most sophisticated tool is a cheapo Harbor Freight drill press.

[filthy phill]

A harbour freight drill press is 100% better than no drill press.

with your boat and motor you will get to speed you desire and then some, I think its brilliant that your doing so well with it.
and no way I could ever make my own props, I could make one and make it look good, but no way would I want to trust my welding ha ha .

props are weird in that they dont look much different to each other and yet perform so differently.
I have seen props that are crap and yet a tiny little bit taken off in a certain place and they work great.
recently I tried a brand new prop I got off ebay cheap, its just an alloy 12 3/5 x 23 alloy prop nothing special, tried it and total disaster, was churning up terrible at slow speed
and when running on plane was corkscrewing the boat.
I took it off and looked at it and noticed the leading edge was extremely thick, so I sharpened ut like a razor ( not realy good for alloy props) but it transformed it into a
decent usable prop that handles ok. atleast your home made prop is working very good.
I like to see home made stuff going as planned.

[smittythewelder]

Wonder if any other old guy recalls seeing anything like this.

Back in my era (Pleistocene), well maybe 1962 or thereabouts, and growing up watching the Unlimiteds throwing their monster roostertails at the races on Lake Washington, I saw something outrageously hokey on a family trip east. Crossing a lake in the Midwest was a big heavy slow pleasure boat, something like a 20' Thompson, droning along at maybe 28-30mph . . . and throwing a high arcing roostertail!! I should say, "roostertail of sorts"; it looked more like the tight solid stream of water you'd get out of a garden hose with no nozzle and a lot of pressure behind it. I recall my uncle, who had a little marina on that lake, saying that some little boating hardware company had invented an attachment you could screw on the bottom of your transom with the sole purpose of scooping up water and throwing it up in the air behind your boat to simulate a roostertail and make your tub look FAST!!



(FWIW, as props got more efficient, the newer Unlimiteds threw roostertails that, disappointingly, were lower and less impressive than they were in the old days.)

[filthy phill]

there is a company who actually make these today, I will try and find it for you, it looks like a little anchor type thing and gives impression of a roostertail.

[Fastjeff57]

I remember those. Some were nothing more that a hunk of tube aiming upwards on an angle that bolted to the cav plate. Impressive!

Jeff

[filthy phill]

well nowdays people are too lazy to make stuff, they go and BUY a rig to do this.
here is the site that sells the product and even shows videos of it attached and working.
and yes people are paying good money for these things ha ha

http://roosterboost.com/

[Ketzer]

Humans are funny.

[Fastjeff57]

Year end report: Made some serious progress this year (though the boat is still a slug--hull dynamics problems). Was able to build two homemade props that allowed running the prop shaft about 1 1/2 inches below the padl without slippage. Whoopee! Motor overheated instantly at that height, but the prop held. Made some runs at lower heights (where the water inlet still functioned) and gained over 5 mph from where the cav plate was just above the pad.

So, to answer my original question--Does a prop slip more just as it is raised, then bite when its raised somewhere? No. If that sucker won't grab when it's raise a little, it'll slip even worse as it's raised some more.

Next year I'll be running my water inlet via a hose and fitting mounted on the boat, so she'll stay nice and cool. Got that idea here (see photo) from a really smart fellow (whom I don't know, but thanks!)

Jeff

[hupiveneilija]

It defenetly slips more and more when you go up. Idea go higher is to get gearcase drag lower,
but that you know, by using more pitch on prop to take care of that higher slip and gain speed.

If need to correct, please do so.

-J-