Porpoising

I repowered my 1448 Alumacraft F7 with an 83 Johnson 25hp.

I took it out for the first time today and it's plenty fast, but porpoises pretty badly above 25mph.

I had the trim pin in the middle hole and moved it one hole toward the transom to see if it would help but it did not.

It's fairly heavy in the front, there's a group 29 battery, bow mount TM, and a small casting deck. Over 100lbs. The gas tank, myself, and the motor are all that's in the stern. The bow stays down once I get on plane.

It's a short shaft motor and the cavitation plate is level with the ribs on the bottom of the hull, about 1" lower than the flat part of the hull.

I do not plan on traveling much faster than 25, but what is the best way to alleviate the bouncing?

Stingray hydrofoil stabilizer. It’s a 14ft tinny’s best friend. You will hear from those who love them and those who don’t. Listen to the people who have actually used one. I have had 2 different 14’ boats with 20 and 25 hp. I put stabilizers on both. I have found it gives transom lift which will help back heavy boats or allow lighter boats to run a higher trim than they normally would. Eliminate porpoising (unless you trim too high). Plane at lower speeds, get on plane without ridiculous bow lift, help with cavitation. Basically it’s the one product I’ve used that actually does everything it says on the package.
Or you could always upgrade to a 16 footer....


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Ditto .. get a foil. The SE Sport 200 from Sport Marine tests as the best foil on the market ... and is ideal for that motor!

DO NOT get the clip attachment ... bolt the sucker on.

A boat at speed caries the bow out of water even when the stern rises up on plane. If you do not have enough hp or have too much weight up front the bow will fall and bounce. Move weight back and carry the bow at speed. You don't want the bow crashing down. Most think the bouncing is a light front and it could be if it's set up horribly wrong but the culprit is probably because it's not able to hold the bow up so it crashes down. Fix that problem with less weight up front and trim. You need to try one notch lower on your trim. One size does not fit all on trim and you may wind up in the bottom hole while your buddy winds up in the top. If you wind up blue in the face with trim and weight management then make a small flat fin out of 3/16 sheet aluminum. They work better than the bulky drag inducing plastic contraptions that are often twice as large as you need. Keep cutting or triming away the aluminum fin to get as little as you need and no more. They do work well but you fool with weight and trim first and then a fin if needed. In other words don't cover up a poorly set up boat with a fin. Add the fin to fine tune a good thing.

It could also be trim angle that is causing the porpoising. My Lowe was porpoising and I added the transom wedges and moved weight forward and now I can run full throttle if I want to. I have a jet so it's a little different, but when I first got the boat, it had a 40 hp prop motor and it was a lot worse with the porpoising than the jet.

I would try weight distribution before I would one of those whale tails. Another thing you can do if your boat has the small tabs at the back you can make very small adjustments to them and you will be surprised how much difference it will make.

The foils many times will actually slow your WOT on many motors.

Stumpalump said:

A boat at speed caries the bow out of water even when the stern rises up on plane. If you do not have enough hp or have too much weight up front the bow will fall and bounce. Move weight back and carry the bow at speed. You don't want the bow crashing down. Most think the bouncing is a light front and it could be if it's set up horribly wrong but the culprit is probably because it's not able to hold the bow up so it crashes down. Fix that problem with less weight up front and trim. You need to try one notch lower on your trim. One size does not fit all on trim and you may wind up in the bottom hole while your buddy winds up in the top. If you wind up blue in the face with trim and weight management then make a small flat fin out of 3/16 sheet aluminum. They work better than the bulky drag inducing plastic contraptions that are often twice as large as you need. Keep cutting or triming away the aluminum fin to get as little as you need and no more. They do work well but you fool with weight and trim first and then a fin if needed. In other words don't cover up a poorly set up boat with a fin. Add the fin to fine tune a good thing.

Click to expand...

I have the trim set all the way toward the transom now. Had it in the middle at the beginning. I may try to move it all the way out and see what that does, if my thinking is correct that should raise the bow higher.

The boat ran beautifully with the 9.8 Merc that I had before at 17-18mph. Weight wise the boat is set up the same, although the 25hp is 30-40lbs heavier.

The more you trim up, the higher it will try to carry the bow, and holeshot will suffer. I don't think that will stop your problem though. Do everything you can before you put a bandaid (hydrofoil) on it. Those things are quick fixes (term used loosely) for people who refuse to setup their rigs properly. No offense to anyone running one, but it's true.

The weight of your boat is overriding the motor/prop's ability to carry it, thus the bouncing up and down (as described in a previous reply). Moving weight forward is rarely the fix for this since that weight is then exaggerated under the lever principle. HP is one way to deal with it, but since that's not what the discussion is about, prop selection is next on the list. At some point you will reach a point of diminishing return with prop selection, then HP will be the only option.

Not sure what prop you are running, but having some cup added to it should help carry the bow a bit better at lower trim settings. It will keep the water on the blades longer, improving bite, thus the ability to carry the weight. The amount of cup needed could be a little bit, or it could be a lot. Maybe even a different prop design all together. You might even find you need to go to a lower pitch with more cupping to get the rpm right while maintaining enough bite. It's a complex formula, one where a prop "guru" can make all the difference.


Engine height is another variable, but it sounds like you are pretty close there, without a lot of options to change it unless you have more mounting holes to use, or a jackplate to adjust. Look at the prop shaft height in relation to the bottom of the boat. The anti-cavitation plate thing is another, less specific, way of looking at it. On my rig, admittedly very different in all respects, I run the prop shaft 3.75" below the hull. Basically, that's as high as I can before I start getting water pressure fluctuations. Then I propped accordingly. This instruction comes from my setup guru, and is the same for just about any boat he deals with. Set the engine height as high as possible BEFORE water pressure is an issue, then, and only then, start changing props. And other than flats rigs, which present a whole other realm of challenges, he has never admitted to needing any kind of foil or other drag inducing parts to get a rig setup properly.


Having said all that, I understand that adding a water pressure gauge and tachometer might not be realistic for some rigs. But without this valuable info you are pretty much flying by the seat of your pants. That can get frustrating and often expensive. Thus the market for products that fix one issue while hurting in other areas.

I picked up a Sport SE 200 Hydrofoil from BPS last week and installed it. Took it on the first trial today.

It works great. No more porpoising, was able to go wide open at 28mph. Boat feels stable and not as twitchy as it was before.

I think I can get more speed out of it if I raise the motor until the foil just skims the surface.