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

[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

[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'

[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