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View Full Version : Porposing Boat Question??????



Cane Pole
02-12-2005, 05:54 PM
Ok. I am sure most of us has experienced this annoyance. My Jon (17 fft aluminum Spectrum) will porpise with this slightest up trim at full throttle. Since I am in the market for a new engine, I went to some local dealers today. Of course, I got different answers from different people I talked with. The Merc dealer made the most sence of it. He suggested the Merc Big Foot as it had a larger cavitation plate. I am not into hydro-dynamics, so is this just a sales pitch? Any experiences here anyone? I am buying something at the boat show in Jackson, Tn. this next weekend. No doubt about that. Just need inputs here.

crap-king
02-12-2005, 05:59 PM
CP -
Most of the time all you have to do is adjust the trim and it will work its way to smooth on out - I did put a hydra -foil on the foot of my engine the whale tale type - and it seems to plane out quicker and takes away most of the porposing effect
good luck on your purchase

Moose1am
02-12-2005, 06:33 PM
If you are trying to adjust the electric trim as HIGH SPEED this should happen. I would find the sweet spot at a lower speed and not make any trim adjustments while traveling at high speeds. Not sure if the cavitation plate is going to help much at high speed. But then again I am not an engineer and didn't take the Statitics class or hydrodynamics either. Fluid flow.

My cousin would know all about that as he got his degree in Mechanical Engineering and was in the Navy working on the Big Subs for many years. He's more of a nuclear engineer but I am sure that he has some background in fluid dynamics too.

Bottom line is get the trim right and it won't porpoise up and down. You don't need to be going too fast anyway. Take your time and enjoy the lake and don't fall out of the boat.

Find a good dealer that you can trust and work with. Ask them for references and talk to others that have bought boats from the dealer and get their input. If the dealer won't give your a list of people that have done business with them then walk away. They will still be there later on if you can't find somone else. There are bound to be some good guys out there TOM.







Ok. I am sure most of us has experienced this annoyance. My Jon (17 fft aluminum Spectrum) will porpise with this slightest up trim at full throttle. Since I am in the market for a new engine, I went to some local dealers today. Of course, I got different answers from different people I talked with. The Merc dealer made the most sence of it. He suggested the Merc Big Foot as it had a larger cavitation plate. I am not into hydro-dynamics, so is this just a sales pitch? Any experiences here anyone? I am buying something at the boat show in Jackson, Tn. this next weekend. No doubt about that. Just need inputs here.

Barnacle Bill
02-12-2005, 07:01 PM
Ok. I am sure most of us has experienced this annoyance. My Jon (17 fft aluminum Spectrum) will porpise with this slightest up trim at full throttle. Since I am in the market for a new engine, I went to some local dealers today. Of course, I got different answers from different people I talked with. The Merc dealer made the most sence of it. He suggested the Merc Big Foot as it had a larger cavitation plate. I am not into hydro-dynamics, so is this just a sales pitch? Any experiences here anyone? I am buying something at the boat show in Jackson, Tn. this next weekend. No doubt about that. Just need inputs here.

Is your motor mounted properly on the boat? With the motor vertical, look at the cavitation plate on it. It should be even with or just slightly higher (1") than the bottom of the keel.

TAE73
02-12-2005, 07:06 PM
Trimming at high speeds is a must. You cant set a motor to run all speeds, and have it do good at top end or holeshot. You have to have the motor trimmed all the way down to take off or it will lift the front up to the sky. When you start with it down low, the gradually add trim to lift the bow as you increase throttle. But once you get to much bow lift you will start to porpose or loose speed. Once there bump the trim down just a tick. This is a learning curve for all boat and motor componations.

His problem is this, the boat is loosing bow lift and then gaining it back. Causing it to porpuse, my lowe did this with a 115hp on it. Only way to solve the problem is like Crap-king said or by putting a jackplate or offset. Offsets work as leverage and will gain more weight on the back and it will keep it from porpusing. I have to play with the trim on my boat to find the sweet spot, by watching the tach and mph. Once I get it opened all the way up.

Cane Pole
02-12-2005, 07:24 PM
Is your motor mounted properly on the boat? With the motor vertical, look at the cavitation plate on it. It should be even with or just slightly higher (1") than the bottom of the keel.

Bought it new in 1996. Dealer installed it. Doesn't porpise until i try trim it out.
Trim down, it is ok. Just wondering if there is a weight ratio problem here (front to rear). Tanks, batteries, my fat butt all towards back of boat. I don't know.

fatboy
02-12-2005, 07:35 PM
Don't forget to include my fat butt

Barnacle Bill
02-12-2005, 07:41 PM
Bought it new in 1996. Dealer installed it. Doesn't porpise until i try trim it out.
Trim down, it is ok. Just wondering if there is a weight ratio problem here (front to rear). Tanks, batteries, my fat butt all towards back of boat. I don't know.

All that weight aft will certainly contribute to the problem. Can you move any of that gear forward? If not, a doel type fin would help, but they make trimming very sensitive. A better, but much more expensive solution would be a set of Smart Tabs. BPS carries them. Other than that, unless someone else knows another cure, you will have to learn the max up position. Raise it until it starts porpoising and then back it off a little.

Cane Pole
02-12-2005, 09:04 PM
All that weight aft will certainly contribute to the problem. Can you move any of that gear forward? If not, a doel type fin would help, but they make trimming very sensitive. A better, but much more expensive solution would be a set of Smart Tabs. BPS carries them. Other than that, unless someone else knows another cure, you will have to learn the max up position. Raise it until it starts porpoising and then back it off a little.

Very little up trim with current motor bill. It is only 40 hp. Maybe 60 hp will make some difference here. I just dont know, and dealers avoid my questions about trim with maybes.

Don G
02-12-2005, 09:10 PM
Very little up trim with current motor bill. It is only 40 hp. Maybe 60 hp will make some difference here. I just dont know, and dealers avoid my questions about trim with maybes.

CP, I had the same problem with my Lund last year. With motor trimmed down it planed out but would plow water. Slightest trim up & it would start bouncing. My motor was 1/2" too high. I let the motor down 3/4" to the next set of holes in the mounting bracket and it ran out perfect.

I learned this from our Jet outboard business. You wouldn't believe that 1/16" would make a difference but it will with jet outboards. Placement of motor on transom is a critical thing if you're looking for performance..

Pooch
02-12-2005, 10:58 PM
After years of jumping logs and running up on stumps in the Mississippi River backwater, my boat started porpoising like that, due to the bottom of the boat becoming concave. I noticed it one day, while the boat was on the trailer. Not saying that's what your problem is, just something anyone could be on the look out for.---Pooch

fishin3
02-13-2005, 08:09 AM
Hey Tom have Fatboy ride in the front to even out the weight ratio. LOL. If I trim up too fast or not give enought throttle my boat will start bouncing too I either give more power or trim down a little.

KARL
02-13-2005, 11:21 AM
..typically salesmen don't know jack squat about real life performance...I would talk to mechanics and get their views...salesmen know $$ and "cents"...mechanics know performance and "sense" ....I know this to be true in auto world anyways---being auto mech....I go along with fishin3--adjust throttle and trim for desired ride attitude..also motor position on transom as others have stated... :cool: .good luck with your seach and keep us posted :)

Speck1
02-13-2005, 11:43 AM
It seems to me that certain boat models (mostly older boat models) are more prone to porposing than others. Take for instance the Hydra Sports boats. They were bad to bounce. I thought it was a bad hull design that caused it rather than the motor. And of course the F Model Alumnacrafts that we used for so many years. They would bounce too so we would stick the gas cans up front and it would help alot. The newer boats are so smooth and stable, I have a new G3 and it is silk on the water and has a good range of trim available before it ever would start to bounce. I think every boat has a sweet spot and unfortunately it isn't running wide open. It will be maybe 70% of maximum range. But then you are only giving up a very few mph and the gas mileage gets a whole lot better in the sweet spot! Speck

tallboy
02-13-2005, 02:49 PM
This thread caught my attention because I had the same problem.Back in '95 I bought a new 14x70 jonboat and a 25 hp motor and trailer from a local dealer. The dealer put a prop on the motor and off I went. I soon found that my boat also porpoised at full throttle. Adjusting motor angle did not help. Called the dealer and explained the problem,he was of no help at all.He said,"it's a jonboat,what do you expect?" I was advised by someone to try raising/lowering the motor on the transom,and in my case raising the motor did help.Now my boat only does it if I hit a big wake and get it started,then I have to slow down to stop it or it just keeps on doing it. I know there are some good ideas posted here,I think if you get out on the water and experiment around you will find one that helps. Moving weight foward will help especially if no one is sitting up front.

Also the most common props used are cupped props,designed to provide bow lift. I read an article once from a boat dealer set-up/serviceman who said that a light-weight aluminum jon does not need extra bow lift and cupped props can actually be detrimental to performance on one,maybe a possible reason for porpoising? I don't know,something to consider.

Moose1am
02-13-2005, 05:05 PM
Tom:

What is the Shaft length of the motor. Look at the prop and see how far it's located below the bottom of the boat. Transom height of the motor can make a difference also. Has the motor slipped down on the transom due to loose bolts or attachements? Maybe rising the motor or lowering the motor a bit will help. It may take some testing. Mark the position now with a pen or a marker and then try raising and lowering it by 1/4" increments and see if that helps. Better yet ask a reputable dealer how the motor should be mounted on your particular boat. It's all about the angle of the thrust. Also check on that I boats forum that someone in here posted. I hear that they are very good at this stuff. Give them the boat model number and year and do the same with the engine. Maybe some expert will have a good answer on how to mount the motor to your boat so that it's smoother.

It took me a while to get my boat trimmed right. If your boat worked in the past then something has changed. Check your prop for nicks and cuts and see if the motor mount has moved.