Porpoising after repower

[muddstopper]

first post to this forum
I retired and bought a used 1995 pro 17 aluminum tracker. I don't know the weight, but its light. When I bought the boat it had a 4cyl 2stroke, 40hp merc, with a 10.5x13 pitch prop. I tried different things, raising the motor two holes to try and get just a little more speed. I could run 30mph, with gps, but the prop was bad to spin out in turns. I tried a 11x13 ss prop and picked up 2 mph and it stopped the prop spinouts in turns. I still wanted just a bit more speed so I bought the 60hp merc that I borrowed the prop off of and mounted on the boat. The 89 merc, 60hp motor had a whaletail on it when I got it so I left it on for testing.My speed jumped to 37.4mph, but rode like a bucking bronco. I can lower trim to stop the porpoising, I can run 35mph, smooth as a baby bottom. I started out in the second hole with mounting as that seemed to put the ventilation plate close to the bottom of the boat. As it turns out, Motor is still running low in the water. I removed the whaletail yesterday and hit 38p with two people, but it wasn't a pleasant ride, and the boat tried to porpoise at around 35mph, unless I kept the motor tucked almost all the way under the boat. Hopefully, that is enough background info.

I realize, that having a extra person in the boat could make the porpoising worse because of weight, but I cant really kick them out of the boat and the boat design just doesn't lend itself well for moving weight around. My plans are to raise the motor to get the ventilation plate where it should be, but I am also considering a bigger prop. with the 11x13 ss prop, at wot the rpms is hitting 5800, I want to get those numbers down to where I am cruising at around 5000-5200 and wot at around 5500. I don't really care about more speed, but I don't want to be riding around and bounceing all the time either. So my question is, will raising the motor help with the porpoising and how will it affect my rpm's. I suspect rpms to go up. If so, just how much more prop will it take to get the rpms back down to reliable ranges.

One other thing, When at idle, the motor will bogg to the point of dieing if I don't ease into the throttle. Kind of like it is flooding itself to death.. Once the boat is moveing, it seems to run just fine. idle is around 1000rpms and it will idle all day long, its just when I hit the gas full throttle that it chokes down. Getting the boat moving 5mph or so and then I can throttle all I want. Would this be a timing issue?

 

[Scott Danforth]

move some weight forward. put a cooler in the bow.

your carb needs work

 

[ondarvr]

What HP is the boat rated for?

​Low cost aluminum boats don't always respond well to more HP and higher speeds due to the limited ability to build one with a good hull design. They can also get bent slightly out shape, and this can cause all kinds of handling issues.

​Raising the motor should reduce the porpoising, it may also increase the RPM's, the exact amount of each won't be known until you try it. As for props, that's just a guessing game at this point, you'll need to make other adjustments before putting on a different prop, then you may be able to make even more adjustments.

 

[Scott Danforth]

Should be rated for 60hp

 

[ondarvr]

And like SD said, move stuff around, about 90% of the time you can solve the problem in a small boat by doing this.

 

[muddstopper]

there is really no good way to relocate weight, unless I start cutting out decks. These boats dont come with a lot of storage options up front. The boat is rated for 60hp. As for carb problems, I assume this response is referenced to the motor bogging at hole shots. this is sort of a iffy topic. When I got the boat, it wouldnt open up at all, couldnt even get the boat on plane. Previous owner claimed having the carbs rebuilt. Maybe, but who ever did the rebuild didnt connect the bottom carb with the top one. A easy fix once the problem was found, but I also found wires laying on condensers that didnt have the retaining nuts in place. I Am not a boat mechanic, but I have years of equipment repair, its just taking me a little time to get up to speed about working on boat motors. The iffy part of the carb question has more to do with just what else was tinkered with on this motor. If someone can post a link to a repair manual, I can go thru and double check things like carbs and timing settings before I spend a lot of money bolting on new parts. I did do a compression check on the motor and every cyl had 120psi of compression, so I am guessing what ever the problem is, its a external bolt on part, but which one.

 

[jimmbo]

Sounds like the carbs need the idle mixture richened slightly. If that doesn't fix the bog, float setting may be low, the syncing of the carb to timing is suspect, or the carbs are still dirty internally.
You say the boat doesn't porpoise until you trim the engine out. Just how far are you trying to trim out? You probably aren't going fast enough to warrant very high trim angles. If the boat, properly loaded, is bucking and the AV plate and prop thrust are parallel to direction, then perhaps you have a hull problem, such as a rocker

 

[Scott Danforth]

there is really no good way to relocate weight, unless I start cutting out decks. These boats dont come with a lot of storage options up front..

So move the cooler you have in the back of the boat up to the bow. Move the anchor to the bow. The weight doesnt need to be in a compartment. It needs to be forward where it is

 

[muddstopper]

So move the cooler you have in the back of the boat up to the bow. Move the anchor to the bow. The weight doesnt need to be in a compartment. It needs to be forward where it is

I understand what you are saying, but My cooler full wont weigh 5 lbs and I dont have a anchor. I did add another battery and placed it in the floor in front of the consol. I dont carry a lot of gear. dont need much more than a fishing pole and a box of crickets for panfishing. Dont carry anything of any real weight and most of it is stored in the forward compartment

I plan on raising the engine this week. I'll go ahead and tear down the carbs at that time, if I an get a kit for them in time. Definately going to change fuel filters, the lines dont even have clamps.(lights go off in my head as soon as I typed that) could be sucking air. That I will fix first thing tomorrow.