Porposing Boat Question??????

[Cane Pole]

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]

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]

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]

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]

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]

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]

Don't forget to include my fat butt

[Barnacle Bill]

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]

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]

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..