Need advise on RPMs shooting up like 500RPMs w/ new engine

The center of gravity and the center of lift /bouyancy coincide when a boat is at rest. This is not the case when planing as the forces that exist when moving, aerodynamic drag, skin friction, thrust, etc have a vector that is not vertical.

Consider the case of 2 axis force loading for simplicity.

If there are forces that are not acting only in the Y axis, vertical, and there are forces that have a vector that are acting in the X axis, horizontal, an object at some speed, can only have dynamic stability when a couple is introduced. (dynamic stability being a constant speed with no body rotation)

We had some advice when we started our first 21 foot jet boat from the national sales manager of Berkeley pump back in the 80's, and that was to put the fuel tanks at the back of the boat.
Our standard configuration was the engine, pump and heavy duty intake weighing close to 1000 pounds and up to 50 gallons (350 pounds) of fuel almost at the transom.

I agree that the center of gravity of the boat changes as the fuel is used up but disagree that the center of gravity of the fuel tank with fuel has to be at the center of gravity of the boat

 

Agree 100% @Barry . The force couples acting on a boat on the plane bear no resemblance to the much simpler forces on a displacement boat. Consider a high performance boat at speed where the only contact points are the prop and skeg on the outboard, and a tiny triangle at the transom. Where's the CoG then? Who cares? Certainly not reality.

 

so the CoG is the balance of front an back? not to be confused w/ side to side listing? I need a new hydraulic jack for my engine hoist just ordered. plan to lower engine all I can from where the pod is, which will only be 1 1/2" which will improve the situation of to high mount but certainly won't get my cavatation plate 1 1/2" above the keel.

what I can't understand is after transom rebuild I placed pod in exact same position.now the previous Johnson 20" donor vessel was a 2004 tritain 18' bass boat special edition saltwater version no carpet an some other saltwater only items. anyway I read somewhere that this vessel had a special edition 22" shaft engine not sure if this is true or false.


most likely is false (2004 Johnson 175HP model J175PLSRB) bc when I decode this engine J/Johnson 175hp P/power trim L long shaft which is 20" S special styling white saltwater paint RB is year. I still have this engine in back yard so Intend on mearsuring the shaft will report back.

I thought that both of these OMC BRP Long shafts would be exactly the same lenght.

an observation during the transom rebuild. the knee brace fitting was a bit differnt I wasn't able to reuse the formost mount holes in stringer.
had to drill new holes farrer to bow by about 1" however at exact same height on the stringer, it's like the initial trasom rebuild some 10 years ago was actuely leaning back a tad not sure how that could of happened but that along would lower the rear of pod.thanks all you experts for offering up excellent advise.
I want to sort out the engine mount height w/ minimal effort.

edit notice the water leve on the Johnsons (not sure what the round piece w/ 4 bolts is referred to) associated w/ gears how water is dead on middle.
compared to the ETEC water line is well below this round piece which is about the 1 1/2" I have at adjustment downward.

however the Johnsons is mounted w/ exact same upper mount holes.

 

Brain,

I'm sorry for the thread drift, but this is distantly related to your thread.

Barry,

I better understand your standpoint, thanks for clarifying.
While I don't think it critical, I believe variable masses
( primarily fuel tanks ) should be mounted as close to CoG
as reasonably possible in an effort to keep varying coupling
to a minimum. Allowing for more consistent boat handling
characteristic at any speed.
I welcome any reply but I am going to bow out of this discussion.
I would encourage further discussion on a new thread
if someone felt so inclined to initiate one.

Cheers

 

The center of gravity of an object is a point.
There are three considerations when determining the COG
Longitudinal-front to back
Transverse- side to side
Vertical- top to bottom
If you google many of the images found on Center of gravity of a boat, the diagrams are often misleading as they will show a single vertical line with an arrow pointing down. This line is actually the direction of the force that the CoG creates.

This may help to understand some of the computations, though extremely simple. While the presenter of the video shows calculations involving a very limited number of mass items,
if you were to do a boat, each object (technically) should be located, its center of gravity calculated, and the the resulting force added to the FBD to find the overall CoG of the vessel.
As mentioned before, a change in fuel will cause the a change in CoG as well as a person changing his/her/ LGBTQIA2S ( skip it) sitting/standing position in a boat.

 

Barry all good infor here have you sean my threads on the fuel tank relocating?


first shakedown stuff

prop not 100% a few imprefections no vibrations

images of crummie prop.




transducer above hull which actuely needs to be lowered even w/ hull per Ray Marine intstructions.
I believe the cord needs to be tucked in better to reduce the air bubbles
baeckmo
commented on


live well well below hull will elevate (was low on priority list) I believe created a roster tail sean in video # BC this RT was only on the port side.


I intend to reduce pickup tubes lenght a tad an elevate the whole pump where the screen is above the bottom of hull like the transducer


leaking fuild after 1st shakedown run hoping its just prop shaft seals however the fuild is well above where the prop shaft is. will preasure test gear case an reseal, I already have a new water pump will add seals if needed.

my gearcase is none high outputs types only 2 tiny water pickups I think one is water preasure lower an 2nd is speedometer upper,
don't need speedo I use GPS.

2nd hole

hope to recieve new hydraulic jack for my engine hoist mid week so I can lower engine the current allowable 1 1/2" then go for 2nd sereis of shakedowns yes I'll pump a bit of lower unit fuild in to replenish what was leaked out. weird that ts totaly black.I believe the Merc quick silver LU fuild is black? thanks for advising

 

@the brain ,

Good photos.

While there's a lull in the conversation, can I ask you about your moniker, "the brain".
Where did you get that one from?
Just curious.

 

That propeller is not suitable for ventilated operation as in your case; it seems to have no cup and the blade leading edges are blunt and damaged.

 

a few friends just started referring to me as the brain. I'm cursed w/ way to much intelligence.

 

the new ETEC came w/ a 14.25X 22

cupped type prop.

this Viper prop is almost perfect, minus that smallish ding in one fin on left of the image.


I have read these V6 mid sized ETECs (mines 175HP) prefer the 14.25 diameter IIRC they said a smaller diameter will spin faster even though a larger diameter would fit.
I believe the damaged aluminum 14.24x19 is the desired size prop. maybe I should have it refurbished an cupped?

please recommend a decent cupped aluminum for my vessel. my plan is to find the correct size in aluminum then get same size in Stainless steel an stow the aluminum w/ hardware as a spare just in case prop..
I believe the Viper will create to much RPM an not be a good match for my hull I've been considering having it modified to a 20 pitch.

1st height adjustment still waiting on new hydraulic jack for engine hoist.


for 20" setback desired engine height cav plate 1 3/4" above keel so w/out lowering pod engine will still be a tad high by tad over 1"
mite get away w/ adding weight to stern.
during these shakedown runs was w/out the dive plateform it's weight is like 12LBs.




what do you think about the gear case leaking above the prop.shaft?


thanks
baeckmo for advise.

 

If the Viper is 22, it will not make the engine rev higher, it will bog it down.

 

You're correct I had it backwards. I think the optimum WOT RPM for the ETEC is 5850RPM (unsure if thats aluminum or stainless steel) I have read some conflicting information about RPMs an the older versus the newer ETECs.

the internet bot is a little off when I search The desired engine RPM for the Evinrude E-TEC 175 HP "is typically between 4500 and 5500 RPM, with an optimum range of 5200 to 5300 RPM for best performance. This range helps ensure good fuel efficiency, acceleration, and engine longevity"

5300 at WOT seams a bit low for a high tech 2 stroke.

w/ the 14.25X19 aluminum not cupped (not in perfect condition an engine a bit to high) WOT is at 5500 but wasn't moving as fast as previous engine.
how much would the 22 stainless steel cupped lower the RPMs? to much where it's strains the engine?

keep in mind I'm lower the engine 1 1/2" an adding the dive plateform (about 12/14 LBs) for next shakedown runs).
thanks gonzo for advise

 

Afraid you got the WOT rpm's wrong as well. If you check the power curves of the Etec engines from 150 to 250 you will find that they have an exceptionally flat power characteristics. In fact some have more power at 5000 rpms than at max rpms. If I remember correctly, the 175 is maxing out at 5000 or slightly below. All have more midrange torque than the corresponding fourstrokes. So with that in mind I'd say start with a healthy screw with 19" pitch with a decent cup once you have got the engine height corrected, and aim at WOT rpms about 5200 in low load condition.

 

I used all available downward adjustment lowered 1 1/2".

I lowered the engine w/ all the available downward adjustment.

here's before an after at cav.plate I think another 1 1/2" lower would be spot on. placing the cav plate 1 3/4" above the keel.

thanks
baeckmo

will report back after next shakedown

 

is this a cupped prop.?

Michigan / Evinrude 14 1/2 x 19 Aluminum Outboard Propeller # 011008 new michigan wheel prop.

an are there differnt stages of cupness?