Porpoising on re-powered boat. Need Advice

[srr5008]

I am looking to get some advice on fixing a porpoising problem that I am encountering. I recently re-powered my 16’ tracker boat from a 25hp to a 50hp motor. It gets right up on plane, and runs fine as long as my motor is trimmed all the way down. As soon as I start to trim my motor up however, it starts jumping pretty bad. I am assuming that the added weight of the motor (80 more pounds) combined with the extra power is causing this. From what I am reading, many people fix this through the use of hydrofoil fins or trim tabs. Is one better than the other for my situation? Some people swear by the foils, but from I read, they shouldn’t be touching the water once you’re on plane – so I fail to see how they can help eliminate porpoising? And my concern with the trim tabs is that I don’t have a lot of real estate on the back of my boat, and I have to keep my transducer mounted somewhere too. Lastly, I was wondering if anyone had any trailering issues with tabs like the “smart tabs”? It looks like my bunks would rip something like that off?

Sorry for the long-winded novel, but if anyone could share recommendations, thoughts, or experiences it would be much appreciated.

 

[kofkorn]

How high up is your cavitation plate from the bottom of the boat? Is there any chance you can lower the motor a bit??

 

[srr5008]

Quite honestly I didn't look at that. There is no opportunity to lower the motor (it's as low as it will go). I do know that the length down to the cavitation plate was within 1" of the motor that was on the boat originally.

 

[srr5008]

Just checked - the cavitation plate is dead level with the bottom of the boat. That is with the motor mounted as low as it can go.

 

[juggernot]

Why do you need to trim it up, if it performs OK all down? The motor may actually be to low and the cavitation plate may move below the keel as you trim it out causing drag and porpoising. Look carefully at the relationship of the plate and the bottom of the boat as you trim it out on the trailer, I imagine you'll see it actually arcs down at the front when trimed as it's likely pointed up when it's tucked against the transom.

 

[surfman]

Proposing is the result of too much bow lift, is there a way to redistribute weight? Also, I do recommend smart tabs they work well and if your trailer bunks are sticking that far past your transom just trim them off. I would also try raising the motor one bolt hole, that may not work but you do want the motor mounted as high as possible for less drag, although it sounds like the motor height is not an issue.

 

[srr5008]

juggernot » Yesterday, 23:12[/url]"]Why do you need to trim it up, if it performs OK all down? The motor may actually be to low and the cavitation plate may move below the keel as you trim it out causing drag and porpoising. Look carefully at the relationship of the plate and the bottom of the boat as you trim it out on the trailer, I imagine you'll see it actually arcs down at the front when trimed as it's likely pointed up when it's tucked against the transom.

I will have to look at the position when trimmed. And I'd like to be able to move it, mainly because I think you can tweak top speed and efficiency by playing with the trim. Also, the fact that it is porpoising tells me that something is wrong, and I'd like to fix it.

Proposing is the result of too much bow lift, is there a way to redistribute weight? Also, I do recommend smart tabs they work well and if your trailer bunks are sticking that far past your transom just trim them off. I would also try raising the motor one bolt hole, that may not work but you do want the motor mounted as high as possible for less drag, although it sounds like the motor height is not an issue.

I'll take it back to the river with a second person and have them sit up front to see if weight distribution helps. I suspect this is part of my problem. And my bunks hang out 2" past my transom and need to be redone anyways, so it would be easy enough to trim them. I guess my concern is more the smart tabs competing with my transducer for space. They give you specific measurements for mounting transducers, and I am sure the smart tabs have similar recommendations - so I am worried about conflicts.

 

[jethro]

srr5008 » 23 Jun 2014, 14:09[/url]"] Some people swear by the foils, but from I read, they shouldn’t be touching the water once you’re on plane – so I fail to see how they can help eliminate porpoising?

I can't speak to any other experience than my own, but my boat came with a 25hp Mercury with a foil already installed. The foil does not come out of the water when I'm on plane. But even with the foil, if I put the trim in the wrong place it will dive or porpoise.

 

[SquiggyFreud]

surfman » Yesterday, 06:13[/url]"]Proposing is the result of too much bow lift, is there a way to redistribute weight? Also, I do recommend smart tabs they work well and if your trailer bunks are sticking that far past your transom just trim them off. I would also try raising the motor one bolt hole, that may not work but you do want the motor mounted as high as possible for less drag, although it sounds like the motor height is not an issue.

Agree on weight distribution...can you slide a battery forward? A small change can make a great difference, it did for me.

 

[onthewater102]

I'm going to whole heartedly disagree with the comments on the porpoising being a result of too much bow lift and that weight redistribution is the answer.

Porpoising is the result of the boat's thrust vector being out of line with the natural travel vector of the boat on its plane, and the resulting back and forth "porpoising" motion comes from the increasing downward force from the changing leverage of the bow as it is raised higher and higher until it overcomes the power of the motor and is pulled down only to rebound again.

Putting weight in the bow only increases the load on the front so that it constantly overpowers the misalignment of the motor's thrust vector with the angle it needs to be at relative to the bottom line of the boat. This applies excess stress throughout the entire hull of the boat concentrated on your transom. Think of it this way - you wouldn't load up a trailer to weigh it down so that a misaligned wheel will stop vibrating - that's exactly what you're doing by adding weight to the front.

Your issue is that you're at the bare minimum depth with your cavitation plate, so your trim adjustment is just barely stable in its lowest position (which may or may not be the sweet spot for top speed/ efficiency etc.) and any further tampering is causing an issue.

Unfortunately without a longer shaft motor you may be stuck. A hydrofoil isn't going to fix the situation - it just helps you plane faster by adding a vertical lifting force to the stern end of the boat so that the lift to get on plane isn't coming exclusively from water resistance acting against the tapered front of the boat alone. You mentioned planing wasn't an issue.

 

[onthewater102]

But as juggernot mentioned - you've got it working fine - so don't play with it from there.

 

[surfman]

onthewater102 » Today, 12:11[/url]"]Porpoising is the result of the boat's thrust vector being out of line with the natural travel vector of the boat on its plane, and the resulting back and forth "porpoising" motion comes from the increasing downward force from the changing leverage of the bow as it is raised higher and higher until it overcomes the power of the motor and is pulled down only to rebound again.

Like I said too much lift, anyway, by any chance is there a trim pin in the bracket that would prevent the motor from actually trimming all the way down? If there is one, remove it. It seems hard to believe that with the motor trimmed all the way down it would be in the correct attitude. Usually a motor trimmed all the way down will cause the boat to plow pretty bad, but not all boats are the same.

 

[onthewater102]

Bow lift should come from surface resistance - acting on the bottom of the boat - not from upward torque exerted on the transom by the engine, but yes, in the most basic sense the bow was being lifted too much.

But weighing it down is the wrong solution.

Not to take away from what he said in that hopefully there is some lower position for the trim hydraulics to mount in which might give more travel for the motor...

 

[turbotodd]

Lot of guys have put a hook in the hull, within the last 5 feet or so of the boat (toward the stern) to solve the propoising. Kind of crude but it works.

Also, where the transom meets the hull, there is usually a weld right there. If the weld is not protruding below the bottom of the boat, it'll be more likely to porpoise. I've actually put some JB weld on mine to stop the porpoising-it worked great. Increased top speed slightly. Adds slightly more drag but gives the transom a bit of lift which helps control the bounce apparently.

I was going to put a little hook in it, but man I just can't bring myself to beat the bottom of my boat up with a big hammer.

 

[juggernot]

I'm no expert but I have researched the issue w my own porpoising problems. Smaller lightweight boats ( Alum., Tin ) and flat or shallow V keels are more prone to porpoise than similar, heavier glass hulled boats and solving the problem is often a trial and error process. Some stern heavy and high powered lightweight boats may never be made to perform like a similar sized glass boat w similar power to weight ratios or respond to trim like a heavier boat. Trimed all down most lower units/cavi. plates are pointed up slightly to the surface of the water and push the bow down as they push the boat forward. The cavitation plate should be on top of the water at planning speed and visible when viewed at speed on faster, high powered boats. If it is pointed upward then when it's trimmed out the front edge turns down and that can cause porpoising as the water moves over the top of the plate, the prop is now pushing down on the stern pulling it under the water and forcing the bow up, then the plate/prop comes to the surface turned up by the rising hull and the bow drops again and this is repeated until the throttle is lowered or the trim is backed down. Some weight forward in a lightweight, stern heavy boat will help but not stop porpoising if the motor is not mounted at the optimum height on the stern and if that is what is causing the problem. I myself have had good results raising my motor and switching to a stern lifting 4 blade prop. I can now trim twice as far before porpoising w a light load and may raise my motor again, but it really performs well now. I've read and have been told the ideal setup is when the cavi. plate is slightly or a few inches above the keel when trimmed parallel to the keel and the top of it is clearly visible when viewed at planning speed as it rides on the surface of the water spilling off the hull. again I'm no expert but it makes sense to me.

 

[juggernot]

duplicate deleted

 

[JL8Jeff]

My 1652 porpoised really bad with the 40 hp prop motor. I couldn't get past 1/2 throttle. I was going to put a fin on it but decided to get a jet outboard. Even with the jet it porpoised and I ended up putting on some transom wedges which basically will trim the motor in farther. I still had to put some weight up front to help and it's much better but still not perfect. My old 17' Wahoo with a 90 hp Merc had the fin on it and I didn't like it so I took it off. It would start to porpoise so I ended up putting it back on and it was fine with the fin.

 

[richg99]

Smart tabs do so many good things...in addition to often fixing porpoising. Do some more reading and visit their website. Richg99

 

[srr5008]

I appreciate all of the advise and the variety of thoughts and opinions. It sounds like step #1 will be to get it on the river tonight and see where my cavitation plate sits in relation to the water surface while on plane. I have room to raise it, if need be. I'll also have a buddy sit up front, to see if weight distribution will help. If raising the motor doesn't fix it, weight distribution might be the best "quick fix" until I get a chance to research smart tabs.

 

[richg99]

You can always fill a couple of five gallon buckets ( covered and tied down) with water. Move them around to see how weight shifting helps or hurts. Each gallon is around 8 lbs. Richg99