Can transom height worsen porpoising?

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67Dart273

67Dart273

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Took the so called "new boat" for a ride today, first time at speed. It wants to porpoise. According to "the internet" sometimes mounting an outboard a bit higher can help performance (speed or handling), so I did not notch the transom at all--yet. Short shaft outboard 35hp Johnson, did 30 on the little 12' Crestliner. This is a 14ft, not a great deal heavier. Did 24mph

I don't remember I think there's about 2" + difference between the old notch and the current top of the new transom

So I played with tilt, moved cargo in the boat, but it still "wants to." I'm going to notch the transom 1" and see where that sits, any ideas?

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I never found much affect when messing with prop height. (Unless we are talking chopper, exposed props. Stinger drive)

Prop angle and pitch hell yes.

I think more so its a pitch, angle and or ballast issue.

Can you go one position negative on the shaft angle? (Shaft angle in tward transom?) That or move weight just forward of center.

Whith you and the engine in the rear she is gonna want to be bow high. I think you need to plant more hull in the water.

Edit- just noticed it has forward controls but i would still try one click negative before i cut the transom.
 
i was always told that t e higher you get the outboard the faster the boat will go..just dont get it too high, the water pick up must stay in water..........................................jeff
 
It looks to me like the engine is mounted too high. Rest a yard stick or another suitable straight edge along the keel on the bottom of the boat so that it makes a straight line aft to the outboard. With the motor trimmed so that the propshaft is parallel to the keel, the anti-ventilation plate should be just about even with your straight edge, and thus even with the boat bottom at the keel.

Like this...

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Yours looks like it may be higher than that...

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Mine was level with the keel and worked fine w/o stabilizer.
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Yeh it's pumping OK. Seems to aereate on a hard corner, but it may be prop clutch slipping, I had some trouble with that last year. I had it as far in as it would go and as much as 3 holes "out". (Pull the pin insert in hole type deal) There is not much more weight I can move up, I had the little 3.3 Merc laying right behind my seat, 2 or 3 gal of fuel and the toolbox up in the bow, and a tank "amidships" with maybe 3 gal of fuel. Not sure if I can pull the pin clear out and run it or not
 
Can get wedges as well.
Straight edge on hull, anti-cavitation plate can be a little above the line, but trimming motor up will cause cavitation on turns. If you trimmed too much off transom then you can at least shim it up inside the clamp.
Buddy couldnt get enough negative trim and installed wedges to correct it.
 
Will add, the dol-fins do help. Gets out if the water quicker,and adds a little stern lift.
 
I'd save a fin for last I think, it gets up quick, and not a very heavy boat. No sub floor, it's just a minimal shell with bow decking I doubt it weighs 300. I can lift either end even in my old age LOL I may look into "wedges"

I'll see if I can add a photo of the bottom of hull/ ventilation plate relationship

"It's gonna rain" (and cold!!) tomorrow
 
If you're not the original owner, I'd make sure the bottom isn't deformed.
If you can turn it over and check with a straight edge, may tell you something.
But yes, easiest thing to do is to adjust trim closer to transom.
If you already have it trimmed all the way in, the transom may not be angled back enough.
No way of knowing what the boats been through before you got it!
 
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If there's enough space, I'd also try loosening the engines clamps and put as much spacer as will fit at the top edge of the transom. Before the water test I'd tie the engine to the boat in case engine comes loose. And no hard cornering!!! This is just to see if it helps the porpoising problem.
 
sccachallenger said:
If you're not the original owner, I'd make sure the bottom isn't deformed.
If you can turn it over and check with a straight edge, may tell you something.
But yes, easiest thing to do is to adjust trim closer to transom.
If you already have it trimmed all the way in, the transom may not be angled back enough.
No way of knowing what the boats been through before you got it!
Click to expand...

Yes ^^^^^ adjust the trim closer to the transom.
 
I have a 14 foot boat with a 25hp Johnson on it. I've only had it out a few times but have noticed it's always nose up when I'm at low speed or even higher speeds. I have moved the pin a few times but just not enough to really test it out. I did a "re-seal" on this engine this winter and painted it. I finally got it tuned last night and she runs pretty darn good. As soon as the weather stops being Wisconsin weather I'll get it out again.
 
Well it's been a hell of a week, I won't go into. Thanks for all the help and ideas here. I'm calling it "fixed" for now. Got two rides, and cut the transom down 1" I was getting a LOT of spray 'specially on the port side of the stern, actually kicking spray into the boat.

"What I dun"

Checked hull with straightedge, heck there's not 1/2" of any change until it starts to slope towards the bow.

Cut transom down 1"
"Rigged" a temporary under-bow shelf and tied the fuel and toolbox up in there. It actually had a bit more fuel in the can than last time. There is not much else I can move up front. I have a bin for personals, only weighs ?30? lbs. It sits about "midship" Battery is limited by cables, I left it back in the stern. Electric backup motor stays on the mount. I keep the 'mergency 3.3hp Merc laid down behind the front seat. So there's not much else I can get up front

"I think" (maybe hoping) the spray was reduced by cutting the stern. The anti-aereation plate is nearly even with the bottom, perhaps less than 1/2" above hull bottom. Experimented with tilt, and a piece of steel wedged in the mount, finally left it "native" mounted on the closest hole to the stern.

Boat will bang on 27-28 mph which is only 2-3 mph slower than the same outboard on the little 12ft Crestliner. I'm happy for now. Heck of a nice day(s) last 2 days. More rain comin' Tue-Wed maybe Thu

The "bad" is I had a little 3hp Johnson I cleaned the carb and tuned up ignition, it seems to have "junk" in the fuel. So it will come apart again, maybe "boil out" the fuel tank which is gravity. If I get it reliable I will sell the 3.3 Merc

Photos here are before I notched the transom down 1". Original cut went down another inch and maybe 1/2

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HELL of a nice day, ugly old boat. I'm still deciding whether to keep, paint "or not" the "new" boat
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Newest member, 67 Johnson 3hp, can't get it to run right yet, dirt in carb??
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Great to hear! So no porpoising?
That does look like a lot of spray, really hosing down your electric!
Thanks for all the pics, old outboards used in freshwater sure look nice for their age!
And yea, with all the extra weight, and the bigger boat, the speed seems about right.
Thanks for letting us know what worked!
 
^^Bear in mind the spray is before I notched the transom an inch. I "may" go further. Most of the outboards I've, uh, ......."accumulated" aren't that pretty. But they ARE cheap entertainment. Wish I had more room LOL
 
Weather has been "up and down" today did not rain, but was not particularly warm. Tomorrow appears to be gonna be the only decent day all next week. I'm hoping to "go for a ride." Today I built some guide-on rub rails to (hopefully) aid in one man recovery

Used 1" square tube for the verticals and some larger tube to create a "hitch receiver" for the 1", that was welded under the rear frame rails, so they are somewhat adjustable and removeable. 1 1/2" PVC pipe on the outside

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67Dart273 said:
^^Bear in mind the spray is before I notched the transom an inch. I "may" go further. Most of the outboards I've, uh, ......."accumulated" aren't that pretty. But they ARE cheap entertainment. Wish I had more room LOL
Click to expand...

Come to Georgia. I'll cut you out an acre. Or two.
 
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