aqua lark not planing

[modman]

hello im brian im new here i recentley got an aqua lark it came with a 25 hp jhonson the guy i bought it from had a 40 hp evenrude with a 19 pitch prop it did 52 i put a jack plate and a yamaha 3cyl 40 hp with a 16 and i wont get out of the hole or plane motor seems like its running fine

[ProHydroRacer]

Go faster!

[Yellowjacket]

With the jack plate you are moving the motor back and on a boat that short it makes a difference. You could try kicking the motor in more, or try getting your weight out over the bow to get it planed off, but I'd try it without the jack plate first and see if that helps.

[modman]

i thought the jack plate would of helped plane better no its about 6 inches back u think it would be better without it

[Yellowjacket]

i thought the jack plate would of helped plane better no its about 6 inches back u think it would be better without it

aahhhh no the jackplate is making it worse, moving the motor back really hurts you getting up on a plane....

That thing is so short that even if you got it planed off, with the motor that far back it would probably porpoise like crazy anyway...

Lose the jack plate for a start and then get some more weight forward and it'll be fine.

[modman]

i measured it today its 3 inches back i have to keep the engine up because its a 15 inch transome and the motor is a 20 cavatation plate is even with botton of boat, and yes i can get it to plane leaning over front and it does porpoise would it help if i drop the motor down a inch or two

[Yellowjacket]

The first problem is that the motor is too far aft. The boat was really made for a short shaft and you need a short shaft motor for it to work right. Moving the motor 3 inches doesn't sound like much, but these newer motors are heavier than the older generation motors too, so you have more weight and it's further aft.

The second problem is that your motor is too deep. This puts the prop lower and this makes the bow want to lift. With the cavitation plate even with the bottom of the boat the prop is a lot lower than it needs to be and this is also is going to make it hard to get on a plane as well as porpoise once you get up there.

You might think about a set of planing flaps on the back of the boat. It's a bandaid, but it will give you more lift when you are trying to plane, as well as let you adjust them to get the nose down when you are up on a plane.

As I said, your basic problem is that the CG is too far aft, and if you add some more lift at the back you might be able to fix it. I'd think about a pair of flaps that are about 8 inche wide across most of the transom. You probably don't need sexy electric flaps, simple turnbuckles will work to adjust them. You need lift all across the range, from getting up on a plane to full speed and this will probably do it.

[modman]

how far up should i be with cavatation plate from bottom of boat sorry its 1-2 inches below bottom i can raise up about 4 more inches not sure where it should be u guys are helping alot thanks so far i want to get this thing in high 50s doing mods to motor too head reeds lightened flywheel want a ron hill 11/20 cleaver too need to smoke the other minies on the pond lol

[Fastjeff57]

You'll need nitrous to go that fast with a 25 John-Rude.

Jeff

[Yellowjacket]

The limit as to how far you can raise it is that it stops pumping water. Maybe somebody with experience with your motor can offer some advice, but higher is better.

[modman]

thanks guys raised the engine boat comes out of the hole and it screws just need to float the rear up a little now adding tach si will know whats going on then time for all the mods

[phillnjack]

First of all the boat is very short and it will never ever get out of the hole fast with anything bigger than a 15hp.
i had the same problem with a 30hp on a apollo 9ft 6in boat.
A15hp being so much lighter lets the stern get up quicker.

Now go get a set of doel fins and put them on the engine and see the difference.
Trim it in a touch and the bow will try to plough,trim it out and you see the sky ha ha
these little boats need to be just right.

you will never get 50 with a 25hp johnson engine on that boat unless you spend thousands, and i doubt very much a 40hp would do it neither.
so many people think they do 50mph, when infact most are barely touching 40 !!!!!!!!
the boat was not designed for such high speed.
the best i got from a suzuki 30hp on the apollo was 34mph and that seemed like 100mph in that little boat.
once it got over the hump it flew like a little bat out of hell.
with 2 people it took ages to get on plane.
I used mine on the river and at sea in some hairy waves, it was a blast but very hard work
keeping it in a straight line due to chine walk.

The engine being a long shaft is a big disadvantage as it makes the boat top heavy at the aft end and can cause a torque problem on such a small boat.
Look around for short shaft, it realy will make a big difference to the handling.

you will have massive amount of fun with the boat though, i enjoyed scaring myself silly with the little apollo ha ha


these were just my observations with using tiny micro speedboats and bigger outboards than they were meant for.

phill

[modman]

ran the boat 42 mph on gps with 16 pitch stock aluminium prop its a 40 hp yamaha 3 cyl doing reeds head lightweight flywheel and exhaust and a ron hill 12 /20 cleaver next and then see what happens i will get it up to 50

[phillnjack]

The gear ratio of your engine is 2.00 - 1 ( presuming this is not a special ratio you have on the engine)

Max revs of the engine is around 5,500 (unless this has been hotted up)
that gives you 2,750 rpms at the prop @2.1 reduction .
the prop has a 16 pitch ,
so in theory without slip this would travel 16 inches on each revolution of the prop.

Ok lets see what this realy would be in the imaginary world of no slip props.


Its very simple mathmatics to calculate theoretical speed with no slip at all.
The rpms are 5500 at the flywheel,
the gear ratio 2.00-1 so its half of engine revs =2750
so we have a pretty easy starting point.

2750 rpms per minute, x 16 inches per minute. = a speed of 44000 inches per minute
divide this by 36 to give yards and you get .= 1222.2 yards per minute.(36 inches = 1 yard)

Now 1,222.2 yards per minute x 60 to give you how far in 1 hour = 73,332 yards per hour. There is 1,760 yards in a mile
so just divide 73,332 by 1,760 to give miles per hour.= 41.66 mph.

This is only theoretical with NO SLIP AT ALL and no drag from the boat ?

Maybe you have a special race engine :cool:





phill..:)

[fs5]

the 3cyl 40 has a 1.85/1 gear ratio mate.

[phillnjack]

1.85 --1. gear ok

theoretical maximum speed would be 45.7 mph. with engine running at WOT 5,500 RPM
that is without any drag, any slip , mirror glass water !!!!!!!!!!!



phill

[modman]

ok so ur telling me gps is wrong

[capnzee]

The GPS only tells you the speed you are doing over the earth, ground etc. If you are running downstream with a current of 10 mph, your GPS will include the speed of the current plus the speed that the boat is running. eg. boat speed 32 mph, speed of current 10 mph resultant speed 42 mph. A GPS reading is only accurate over "dead" water as in a small lake with no movement. Rod

[capnzee]

see my answer below****
The gear ratio of your engine is 2.00 - 1 ( presuming this is not a special ratio you have on the engine)

Max revs of the engine is around 5,500 (unless this has been hotted up)
that gives you 2,750 rpms at the prop @2.1 reduction .
the prop has a 16 pitch ,
so in theory without slip this would travel 16 inches on each revolution of the prop.

Ok lets see what this realy would be in the imaginary world of no slip props.


Its very simple mathmatics to calculate theoretical speed with no slip at all.
The rpms are 5500 at the flywheel,
the gear ratio 2.00-1 so its half of engine revs =2750
so we have a pretty easy starting point.

2750 rpms per minute, x 16 inches per minute. = a speed of 44000 inches per minute
divide this by 36 to give yards and you get .= 1222.2 yards per minute.(36 inches = 1 yard)

Now 1,222.2 yards per minute x 60 to give you how far in 1 hour = 73,332 yards per hour. There is 1,760 yards in a mile
so just divide 73,332 by 1,760 to give miles per hour.= 41.66 mph.

This is only theoretical with NO SLIP AT ALL and no drag from the boat ?

Maybe you have a special race engine :cool:

****If you want a real easy way of figuring your speed at 100 per cent efficiency (you will come within a mile or two of the way you figure it)::::subtract 1 inch from the pitch of the prop, in this case it is a 16 inch prop so use the number 15 and multiply it by the rpm of the prop 2992 (assuming engine is running 5500 with a 1.85 gear ratio) result will be 44.59 mph. Not too far off when you are in a hurry(within 1 to 2 percent) and not figuring the drag. Most modern props may be called 16" but in reality, this is an average pitch measured through a specific area of the prop. It may be more than that near the hub and it may be less than that at the tips, a little "cup" also changes the resultant pitch--in other words all of this scientific **** is not worth much except to start arguements. Results are what counts. Go for that 52 mph, you may or may not get it but will have fun trying. Rod



phill..:)

[fs5]

42mph is about right for a 16p alloy prop on the limiter.thats about 12% slip ,wich seems about right for a alloy prop .
modman to run a cleaver mate ,you going to have to jack your motor as high as it will go and still suck water.if you can swing the cleaver and hold your boat on the water you will get way over 50mph

[fs5]

how far up should i be with cavatation plate from bottom of boat sorry its 1-2 inches below bottom i can raise up about 4 more inches not sure where it should be
start off with the centre of your "prop shaft" 2 1/2 below the hull .keep an eye on the pisser/telltale and keep going up untill it stops sucking water.then lower it a 1/2 inch.
it might pay to fit a new impellor kit ,if the one thats in there is pretty old

[Fastjeff57]

..."First of all the boat is very short and it will never ever get out of the hole fast with anything bigger than a 15hp. i had the same problem with a 30hp on a apollo 9ft 6in boat."

Too true! My 9 foot dink boat crawls onto plane with me aboard, and it needs "help" (push the bow down) with a passenger aboard--and that's with 70 hp!

Short, relatively heavy boats plane off at a steep angle. If my boat was twice as long it would plane easier!

Jeff

[capnzee]

a couple of additions such as "whales tales" bolted to the cav plate will assist the boat getting on plane. They are used extensively on small/short "blow" (rubber dinghies) boats. Once on plane, these additions to the cavitation plate will usually be out of the water reducing the drag they would cause if the engine was mounted lower. They will also dampen out a tendency for your boat to "porpoise". These "plates" are often used in outboard drag racing set-ups. They will help in acceleration by getting the boat on plane quicker. Rod

[Fastjeff57]

Agreed (and thanks--got trim tabs on mine and they DO help).

jeff

[Yellowjacket]

a couple of additions such as "whales tales" bolted to the cav plate will assist the boat getting on plane. They are used extensively on small/short "blow" (rubber dinghies) boats. Once on plane, these additions to the cavitation plate will usually be out of the water reducing the drag they would cause if the engine was mounted lower. They will also dampen out a tendency for your boat to "porpoise". These "plates" are often used in outboard drag racing set-ups. They will help in acceleration by getting the boat on plane quicker. Rod

The problem with a whale tail is that only provides lift when he is getting up on a plane. As the OP noted earlier, once he does get it up on a plane the boat is porpoising. I know if he goes really fast the porposing will go away, but I don't think he is going to get that much speed out of it. If the whale tail is out of the water (since he should jack the motor up to the point where it isn't even close to the water when underway), it isn't going to do much to address porposing. The only thing it is doing is redirecting some spray, and that isn't going to create much lift back there.

His real problem is that the CG is too far aft relative to the planing surface. For that reason he'd probably be better off with fat trim tabs that not only provide lift to get him up on a plane, but also would get the CG further forward relative to the back edge of the planing surface and prevent the boat from porposing. The drag from the trim tabs isn't any higher since he doesn't have to angle them down, he can just set the flat to the bottom of the boat. Most likely he will gain speed with the trim tabs.

JMHO

[Fastjeff57]

Agreed. I've installed "automatic" trim tabs on mine to cure that irritating porpoising actin at low speed. Haven't had a chance to try it yet (long story), but it looks like it should work. Trimming the LU in pushes down on the...you know!

Jeff

[capnzee]

The trim tabs are probably the way to go. What about mounting them just an inch above the bottom, in that manner they would only touch if the bow tries to come up? By the way, that is a good looking Mark 50 or is it a 55? The method you have devised to control the trim tabs is certainly unique, hope it works! Rod

[Fastjeff57]

Thanks, Rod. Dying to find out how the tabs work, but the wife is pretty sick right now so I have to stay close.

The motor is actually a 70 hp triple mounted on a mark 55 type tower. Built three of them to date and like them (unlike most Merc guys!) Weighs only 150 pounds with electric start and a FGS.

Jeff

[capnzee]

Those are great looking units! Great paint jobs. Hope they work out well. Did the 3 cyl blocks "bolt" right up to the Mk 55 mids? Rod

[Fastjeff57]

Thanks! It's a fairly easy conversion, and the power output is far more than my old 44 cube 50 hp. I also built external reeds for it over the winter, and that's worked out really good; a noticable power improvment over the stock triple (that has rather crappy reeds). Makes nearly 1,000 rpms more with the same prop!

Jeff

[phillnjack]

quote "I also built external reeds for it over the winter"


What are external reeds . .:confused:
its impossible to have external reeds (as far as i know )

unless we have an American v English translation mix up going on. :o


and agree those engines do look preety good, they look like blown up pics of good little model engines. :)




phill. . .:)

[Fastjeff57]

Translations can get you!

Instead of the usual Merc reed valves inside the main bearing pieces, I added external Yamaha pyramid reeds to the outside of the crankcase cover. Made for a shocking increase in power!

I did a 44 cube four and a 30 cube twin Merc earlier, but never got to fully develop them.

Jeff

PS: Here's a shot of the set up with the humongous Chrysler carbs (that bogged like crazy).

[modman]

hey the motoe is up as much as i can go without sucking air im taching 6000 rpm boat is screwing but u can here prop loose bite on the water and see it in rpms too what would i go with for diameter cleaver and pitch

[capnzee]

If you are turning 6000 rrrr's now, (with a stock aluminum prop), I think I would go with a SS cleaver of the same pitch and diameter. You have to start somewhere and after trying it, you may find some trades or you may have to buy another prop as you close in on exactly what you need. No one can tell you what to do at this point--it takes a lot of testing. Rod

[modman]

42mph is about right for a 16p alloy prop on the limiter.thats about 12% slip ,wich seems about right for a alloy prop .
modman to run a cleaver mate ,you going to have to jack your motor as high as it will go and still suck water.if you can swing the cleaver and hold your boat on the water you will get way over 50mph

well motor is up and boat comes out of hole good now. boat with motor trimed tack is like 6500 with one person want a ron hill cleaver but not sure what diameter would be best and pitch and 3 or 4 blade ,its pretty hairy with 16 but have to beat other guys lol im just tailing them now boat chine walkes like hell love it . but rpms are up to hi what can i get away with for a prop and still be ok out of the hole love my 40 hp yamaha

[capnzee]

6500 rrr's, no slip, go up 2 inches, same diameter--S/S Ron Hill cleaver. 3blade--stop the chinewalk by simply "tweaking" the wheel towards the side that is lifting, most likely the chine walk starts with the left side lifting first. just stop it by a litle tweak against the lift.