View Full Version : Raised motor another hole but have a question
O.K... raised the motor one more hole off the transom (now it is in the third hole up) When considering the height of the cavitation plate above the lowest part of the transom, should I look at this with the motor trimmed to it's optimum angle or in it's lowest trimmed position? Since the displacement of water will start back up right after the transom, and it would be at an angle would that make sense?
Reason I ask is that when trimmed (in garage after many test runs so I know where it is) I can take a straight edge and place it under the cavitation plate to see where it would line up with the transom (about 3/4 to 1 inch above).
Will not be testing again until Wednesday, and will have a pressure guage installed before then as well...don't want to burn up an engine at this time of year...too much fishing to do all summer.
Thanks
Steve
Tom P
04-09-2002, 03:34 AM
I measure mine with the boat level and the motors main drive shaft 90 degs from the floor. The motor is not tucked in and not trimed out.
Same here. Motor shaft parallel to the transom. 1" above may not perform well on some boats. Making sure you have a water pressure gauge hooked up first is definitely a good idea. Especially when trimming up.
Thanks for the responses..now..another question: What kind of pressure should I be seeing on the gauge? I have a 1999 50 hp Johnson 2 cylinder. It will be in it's third year of running without an impeller change, so that might get done later this year. Full throttle is 5400 RPM I cannot seem to find that in my owner's manual, so I am guessing that I will have to find out while under way..unless someone knows it.
Thanks
Steve
You should contact Bomb directly on that one. I wouldn't think it's any less than 15 PSI but I'm pulling that out of my a$$.
Your motor should have a warning if temps get too high but that's not something I'd want to wait to occur.
A safe bet is looking back to verify your cavitation plate is still at least touching the water if not slightly submerged while at WOT. Then trim out in increments and look again each time.
But this opinion is from a guy whose cav plate is even with the bottom of the hull and no PTT. I'd seek input from someone who knows better.
I wouldn't be suprised, if you might be able to raise your motor even one more hole higher.
Because of the "hole" that boats leave in the water - it is not unusual to have the cavitation plate - 2-4 inches higher than the bottom of the transom - with the motor leg - parallel to the transom; at optimum height for best running.
As a boat moves forward - the water fills the hole - in which the boat was sitting - and thus fills the hole with water to supply water to the elevated propellor.
Basically keep raising the motor - until, with the motor trimmed for optimum running, you are getting excessive cavitation in waves and or moderate turns.
When you get to the spot of excess cavitation - drop the motor down one hole, and you will be fine.
To really optimize the motor height position - it is not a bad idea to try a few different props at the same time. Often by running a stainless prop - of a high rake design, will allow you to run a higher motor height, because the prop will "grab" much better.
Take care
REW
REW,
If I am not cavitating, even when the motor is raised yet another hole, is it possible that I could lose water pressure to the outboard with the inlet too high?
I have yet to install a pressure guage, but after doing some searching of archived threads, I have noticed that you have not mentioned a pressure guage...So...even though it is a wise investment to potentially save the engine and help to indicate a faulty/worn impeller, would the motor still be considered safe? What would you recommend? (your opinion is highly valued and I appreciate you taking the time to respond)
With my motor being quite limited on props, I did do some work with what I could afford and settled on a Solas Alcup 3. Pressure cast aluminum, progressive pitch and cupped design that performed really well so far (motor one hole up off transom), and cornering was much better than my standard aluminum.
Thanks
Steve
I will use this term -- "in general" if you mount the motor - within the limitations of the motor mounting holes" you will be safe.
i.e. - if you have not installed an after market lift or jack plate - you "normally" will have no watter pressure issues, with the motor in its highest - bolt to the transom position. This is a normal maximum lift of about 4 inches.
If you installa a jack plate, so that you can lift the motor 5-8 inches, it is likely that you will need a low water pickup.
As a fairly - crude - check of water pressure, note the angle and direction of water coming from the hole on the side of the motor during operation. Even at the highest position at the highest speed - you should be seeing a strong water flow from the side of the engine.
Take care
REW
GR8WTHUNTER
04-09-2002, 08:49 PM
I am not an expert or a mechanic, btu if I was going through the process to find the "best" holes for the best run, here is what I would do.
First, get the pressure gauge installed. Then drop the motor to the original setting and run it. Take notes as to the pressure at various rpms. Then bump the motor up one hole at a time and continue to take notes. At the first sign of losing pressure, drop back down one hole. I also would discus what I was doing with the mechanic at your dealer for his input.
Good luck
VMS
What kind of boat and motor are you running and also what diameter prop?
It's an Alumacraft Lunker V16LTD console, Johnson 50 2 cyl, 2 stroke with a Solas Alcup 3 in 11-3/4 x 17 pitch. Boat is not a deep vee by any means...more along the line of the modified V hull. Currently pulling 5400 RPM with motor one bolt hole up. So...Just seeing if I can tweek just a bit more out of the rig, albeit I cannot get too much more than what I have.
Steve
VMS,
Expect an increase of 100 to 150 RPM with each bolt hole raise of the motor. Be careful you don't exceed the max WOT allowed for that motor. Do you know what the max allowable is?
I agree with REW that a consistant stream of water out of the Pee Hole is a reliable indication that the motor is getting enough water. I'm not sure a consistant stream of water out of the hole means the engine is getting enough water pressure. And I don't know how one effects the other. However I believe you can not have too many gauges monitoring what the engine is doing every second it's running. It's just like on a car. I hate relying on idiot lights to tell me there's a problem AFTER it's happened.
I think you can safely test the motor to the max height without doing permanent damage. I've never gone higher than the third hole on my boat but I think I'll try it this weekend.
Thanks to all who replied. I definitely have a nice stream of water that comes out...more like a garden hose than a pee hole..lots of water flow, so I will hope to not have problems. speaking of installing a pressure guage, I will have an in-dash compass up for sale (possibly)...would anyone be interested?
Motor max WOT is 5500...I am getting real close and may have to have prop worked on to increase pitch or maybe add a little more cup to the trailing edge to get more bite. Seems my bow lift is fine right now so tip cupping would be out...just trailig edge.
Steve
CharlieB
04-10-2002, 04:22 PM
Level boat on trailer and put a straight edge on pad of boat, then measure from the floor up. Level motor and measure to the center of prop shaft. Divide prop diameter in half and add 1" and this should equal the same number you had to the pad. This is with a stainless prop and will allow your blade to tick 1" out of the water at the top of it's swing. Be sure to watch water pressure for no less than 12#.
Charlie -
Is this a valid measurement of boating operation??
I understand your theory, and also your measuring - points and technique.
I am wondering if you are taking into account the "hole" effect of the boat moving out of its hole.
When the boat is on plane - and moving forward at its top speed - the boat will be exiting its hole in the water, and the water will be quickly filling the void left behind the boat.
Assuming that the prop is at least 6 inches behind the transom - the water level will be some height above the planing pad of the boat, by the time the prop gets to the water.
You are indicating a 1 inch tick of the prop above the water level - I tend to agree with your measurement, but I also tend to believe that the water level will have risen that 1-2 inches, so that there really won't be any of the prop out of the water, by the time that the prop gets to that section of water.???????/
Take care
REW
Charlie,
I do have another question on your measurement.
You indicate to "level the boat on the trailer".
While, I do agree that this is a perfect way to get boat to boat consistency, in setting the hull prop relationship, but is it realistic?
The thing that is going to happen with any boat, when it is on plane - is that the boat will NOT be level -- any planing boat - will have a bow up attitude to some degree, some a little, some more.
Again a question - for this particular boat -- If a person knows the degree of normal up tilt on the bow -- could easily be determined by a protractor and a scale, while running the boat at maximum speed -- shouldn't your measurements be taken - with the trailer tilted to this same angle - for the overall optimum performance??
Thanks very much
REW
Hawgeye
04-10-2002, 09:00 PM
Great comment REW. I am going to expiroment with this idea this spring by using a level in my boat and propping one end up with something measuring the "lift" with a ruler, and the other end will be on the floor to get that angle back while checking set up. At that point, I wll also check the optimum trim angle by my trim guage. I realize that the trim guage is not extremely acurate but it should get me close. I then can take that same level and determine with the motor trimmed at the WOT level, where the prop is. This may allow me to determine whether I have room to move it up another hole or not.
Then again, i suppose I could just move it but it seems like an easy expiroment.
If you raise the motor up, will it also get your boat higher out of the water removing some of the hole left from the boat, or will it cause the boat to want to go more bow up and dig the back down more? Just a thought.
Hawgeye
I think the most interesting thing about this whole topic is this guy only has a 50hp motor on a 16 foot aluminum boat. Were not talking about a 225 Opti on a Triton bass boat that you are looking to hit 70mph. That boat and motor is only going to go so fast no matter how high you mount the motor because it's only a 50 hp motor. I think he posted something like this before.
PJM.
You are correct in that I have posted before about my rig. I finally made the changes, but, as you stated, I am not going to gain much at all..but just enjoy working with the rig. Figure I might as well make the most of what I have. Fishing season is not quite in full swing so at least I get to tinker a bit.
Steve
CharlieB
04-11-2002, 02:39 PM
You are right when you talk about coming out of the hole you do make a large void in the water. I always started out with the motor tucked in tight and trimmed as the boat was comming up, this increased my RPM'S all the way through the hole shot. I am used to running high performance Bass boats and have always used this as a starting point to get the height somewhat correct. When you are running down the lake and your motor is trimmed correctly you should not have water blowing into the air above the motor cowling. Most boats you see have their motors way over timmed and so you are correct that the angle has changed. A properly trimmed high performance boat will not only have bow lift but it will also lift in the transom area. This is how you get more boat out of the water and unwetted which gives you more speed. Here we are talking about a fifty horse motor and an aluminum boat which might run 30MPH.(Not picking on anyones boat) I feel the key here would be to set the motor in the overall best position for hole shot, boat handling and speed. If your not running a stainless prop with cupped blades this whole method is a moot point. If someone really wants to get their walleye rigs going with the big motors, get ahold of Rich Boger and have him tune your prop to your boat. You can't beleive what a difference that will make. You give him your RPM'S, speed (GPS or radar) brand of motor, HP and make of boat and for a couple of hundred bucks he'll have your boat blowing right past another boat just like yours. I now drive a 680T with a 60HP tiller and just putt along at 35MPH. Used to love that riding on the edge when I was young. Can still feel that boat lifting on the pad as the air gets underneath it and you feel the whole boat lifting and it feels like you only have about a foot of boat in the water. Whoooooooooooooo!
Hawgeye
04-11-2002, 02:42 PM
I'm with ya! I too like to tinker with my boat...probably way too much. It cuts into my fishing time! That's what us engineers go through with anything we buy. I drive my wife crazy every time I over analyze something! :)
As long as we're having fun, right? Oh well, REW always provides really good info for us...makes you think.
As you said -- we engineers -- are always tinkering and trying someting different. Part of the enjoyment of this wonderful hobby we call boating and fishing.
take care
REW
I have a 16' Lund Rebel with a 30 HP motor. Things I've learned on this site resulted in an increase of 5 MPH (a 20% improvement) and decreased fuel consumption. At 31+ MPH, I'm moving as fast or faster than similar boats with 40's and 50's. It doesn't matter if you have a performance boat with a 225 or a small aluminum with a 25 to 50. If you can keep improving speed, handling, and efficiency, why not do it. I also love the tweeking. What I apply here can also be applied to my future big rig.
KP..could not agree with you more. The only funny part, is I felt I was going big when I went to a 50!!! Heck..I doubled hp and tripled the boat weight...not to mention my gut getting larger as well. :-)