Success! Problems with getting on a plane.

[RandelS]

You guys have been great in the past and have solved several of the boat problems I have presented on this forum over the last several months. Thanks guys.

I finally got the boat we have been working on to the lake this weekend to test everything out. To remind everyone, this is a 1989 Skeeter Bass boat with a Johnson 150GT VRO with a 8? engine ?jack? plate (none operable). I purchased the boat a year ago and have been reconditioning several years of neglect. The engine has been pulled off the boat and repainted and the lower end completely rebuilt ? water pump, seals etc. in addition the carburetors have been rebuilt and spark plugs, spark wires replaced and a new fuel/vro pump installed. The motor starts right up, idles and revs like a champ. The boat floats nearly flat in the water; however, the bow rises upon acceleration, which is expected; however, it is difficult for the motor to get the boat up on a plane - taking nearly a minute to get there. Once on a plane the boat screams. During acceleration and running the motor sounds great - no gasping or sputtering ? just solid roar. Trimming the motor inward helps tremendously during acceleration and trimming the motor outward (a little) once a plane is reached makes the boat fly (spooky fast). Again the problem, even with inward trim, is getting on a plane ? how should I approach getting a better ?out of the hole response.? How important is engine height on the transom? This could have been changed slightly during the reinstall. Any ideas, comments, suggestion are greatly appreciated.

 

[Daviet]

Re: Success! Problems with getting on a plane.

Is the cavation plate even with the bottom of the boat? What RPM are you turning when on plane. Could be over proped, Do you know what size prop you are running.

 

[jriegle04]

Re: Success! Problems with getting on a plane.

i bought a whale tail hydrofoil and it helped. it went from feeling like i was preparing for take off to a nice smooth plane

 

[RandelS]

Re: Success! Problems with getting on a plane.

Let me check these things out and report back.

 

[mrcj001]

Re: Success! Problems with getting on a plane.

Trim tabs will solve this problem ........

http://www.nauticusinc.com/

 

[RandelS]

Re: Success! Problems with getting on a plane.

Daviet / jriegle04

Additional findings - the Cavatation (AV) plate is about 3 1/2" above the bottom of the boat. Could this cause the delayed planning? The prop is a stainless steel "Shooter" with 4 blades and approximately 13" in diameter. To get on plane requires about 3000 rpm and once on plane requires about 2000 rpm before it rapidly falls of the plane. I am confused about the dynamics of trim (in or out) versus Cavatation plate heigth versus prop pitch. Do you know of any resource materials that cover these topics in some detail? Prior to putting on a hydrofoil I would like the boat to perform the best it can.

Is there a better forum for these type of questions?

Thanks for everyones thougts any further suggestions are greatly appreciated.


R

 

[Pain'n'da'boat]

Re: Success! Problems with getting on a plane.

rpms too low...sounds like your over propped
need smaller pitched prop

and you def need a hydrafoil

 

[Daviet]

Re: Success! Problems with getting on a plane.

You say it takes 3000 RPM to get on plane, it that the max RPM at wide open throttle? If so i would think you are way over propped. I don't have specs but I would think that you should be turning in the neighbor hood of 5500 RPM.

 

[jonesg]

Re: Success! Problems with getting on a plane.

Prop might be too big if all you get is 3K assuming the engine is set up correctly but its too high by 3+ inches.
I'm surprised the prop is wet. The whale tail if mounted on the cav plate should be even with the bottom .

 

[RandelS]

Re: Success! Problems with getting on a plane.

I do have the factory service manual, but can't seem to find the proper transome height adjustment. Actually that was one of the best suggestions I got when I started first using this forum - I will continue to browse through it. In general should the AV plate be even with the bottom of the boat (is this ideal)? if so what is the advantage of a operable "Jack Plate" ? Also if the AV plate was at the height of the bottom boat the prop would still be about 18" below the water line does this sound correct?
thanks in advance

 

[kencat]

Re: Success! Problems with getting on a plane.

Randel, good place for info on high perf boat/engine setup is here - http://forums.screamandfly.com/forums/forumdisplay.php?f=21

Now this is just my opinion here, but I would do everything under the sun before putting on one of those whale fin things.....seems like a bandaid for something that is not right.

That jackplate you mentioned could be causing you grief. Can you elaborate more on what is not working with it? and provide some info on the jackplate dimensions?

 

[kencat]

Re: Success! Problems with getting on a plane.

if so what is the advantage of a operable "Jack Plate" ?

I love reading about high perf boats, so what i'm saying is not from experience but from my own research.

The jack plate can do two things; provide setback (move the motor further away from the transom) and allow the prop height to be moved up and down. Ideally the jack plate is hydraulic and can be adjusted by the pilot to give the performance he wants for the conditions he's in.

Setback changes the balance of the boat. The further back the more leverage the motor has to lift the bow without requiring any trim. The ideal for maximum speed is for the hull to be flying at its best designed attitude while the propshaft is parallel to the water surface i.e. no positive or negative trim. With setback the prop can be in cleaner water and the motor can be raised higher.
See - http://forums.screamandfly.com/forums/showthread.php?p=1362960

Raising the motor higher reduces drag from the lower unit giving higher top speeds. Also, in high performance applications using surface piercing props you want the motor high so the prop is partly out of the water. There are modifications out there to put extended nose cones and low water pickups so the motors can be raised beyond where a regular motor loses the water pickup. Caution is needed when raising the motor so that water pressure is maintained for cooling. Raising the motor is also supposed to help planing and increase economy.
See - http://www.trailerboats.com/output.cfm?id=2087029

Try googling some of the key terms and you should find a wealth of information out there.

 

[RandelS]

Re: Success! Problems with getting on a plane.

Kencat/Others

Kencat thanks for the link. I have reviewed.

I have attached photos that shows the notched transom(12" deep), the depth of the jack plate is 5" and the distance to the lower unit to the transom (20" where the speedometer pickup is), the AV plate height is 3" above the bottom of the boat is 3", however, the AV plate is nearly 4" below the notched transom. What is the correct AV plate height when considering a notched transom and 5" jackplate? Any suggestion are greatly appreciated.

 

[HybridMX6]

Re: Success! Problems with getting on a plane.

I'm not too technical on this sort of thing, but that angle in the last pic makes it look like the notched transom combined with the location of the CAV plate could be causing a lot of turbulence for the prop. Does the jack plate work? I'm not positive of what you are talking about, as when I think of jack plate I think of something used to raise or lift the motor vertically (not like trim and tilt). If you can adjust it, I'd try to lower the motor approx. 2". I have a feeling your are cavitating a bit, and the prop is having a hard time getting a solid bite in the water.

 

[RandelS]

Re: Success! Problems with getting on a plane.

Unfortunately, no this particular jack plate is not operable.

My reading has lead me to believe that the AV plate can be moved up 1" from the bottom of the keel for every 12" that the lower unit is from the notched transome and can be moved up an additional 1" for every 10mph over 40mph (not to exceed 5").

I think, since the lower unit is approximately 20" from the transom and the boat will do somewhere in 50's the height of the AV plate should be about 3-4" above the bottom of the keel of the boat. After remeasuring, it is currently at 2" above the boat, meaning it is not that far off. But I wanted to get opinions all you guys, before chaning the motor height. Any suggestion are greatly appreciated.

 

[kencat]

Re: Success! Problems with getting on a plane.

Randel,

Your theory is probably all correct, but obviously something is not working. You are going to have to experiment to see what happens.

You must tell the folks here what your WOT (wide open throttle trimmed out for max speed) rpm is so they get a feel for whether the prop pitch is ok.

Also, the jack plate is putting more setback to the motor over the original setback of the notched transom. Maybe there is too much setback now. Why not take that inoperable jack plate off and set the motor on the boat transom at the cav plate at bottom level position and try it? After giving WOT rpm as it all may simply be a prop issue. If you need a jack plate get one that works?

Have you seen this - http://www.marinepartsman.com/installing-an-outboard-on-your-boat.html

What hp motor is the boat rated for? Possible combo problem if motor is overweight with too much setback.

 

[jonesg]

Re: Success! Problems with getting on a plane.

Randel,

You must tell the folks here what your WOT (wide open throttle trimmed out for max speed) rpm is so they get a feel for whether the prop pitch is ok.
.

Right, impossible to help due to lack of numbers.