Cannot trim motor up - Porpoising

[atengnr]

Re: Cannot trim motor up - Porpoising

1968? Hummmm......Hook possible, DH?

Im not quite sure what that means....Hull shape distortion?? Anything I should look for??

 

[Dhadley]

Re: Cannot trim motor up - Porpoising

That's what I'm thinking Kenny.

Atengnr, your boat will have a small hook in the hull at the rear. That's to help get the boat on plane. One problem we're "seeing" is the motor's anti-ventalation plate being below the bottom when parallel. When the plate is parallel to the bottom it should be no lower than even. With that much diameter, and assuming the prop is in "as new" condition, it could probably be up more than that. For now, I'd suggest we move the motor up so the plate is even and retest.

Now, between the time the prop was on the old motor and the newer motor, was it reconditioned? Or changed in any way?

 

[dreamchaser]

Re: Cannot trim motor up - Porpoising

Sorry for budding in ........This ?? is for Texasmark.....Mark in ur reply U Refer to "is to run away from it" ... are U suggesting with more power that the porpoising would stop??? pls explain to me .thks .Leo

 

[atengnr]

Re: Cannot trim motor up - Porpoising

That's what I'm thinking Kenny.

Atengnr, your boat will have a small hook in the hull at the rear. That's to help get the boat on plane. One problem we're "seeing" is the motor's anti-ventalation plate being below the bottom when parallel. When the plate is parallel to the bottom it should be no lower than even. With that much diameter, and assuming the prop is in "as new" condition, it could probably be up more than that. For now, I'd suggest we move the motor up so the plate is even and retest.

Now, between the time the prop was on the old motor and the newer motor, was it reconditioned? Or changed in any way?

Prop is in pretty good shape and was not changed at all. It was merely transferred to the "mew" motor.

The only thing changed was an aluminum plate was added to the back of the transom (my request) for added strength of transom.

One observation: When I look back at the motor at speed, I notice a stream of water spraying on both sides of the motor, out and up from the prop area. Is this because the engine is mounted too low in the water??

When I remount the motor, should I move to a 17 inch pitch prop, given that it runs at 5800 rpm with the 15 right now??


Thanks for all the replies.....

 

[Dhadley]

Re: Cannot trim motor up - Porpoising

The spray is indeed, as you suspect, from the motor being too low. I wouldn't change props just yet. Raise the motor and test. Then change props accordingly. One change at a time. That's critical.

 

[Texasmark]

Re: Cannot trim motor up - Porpoising

Hey dream, dunno about other folks but I play a lot with my little tin lizzie. Just for kicks I will trim out pretty good before I punch out of the hole.

When I do, the engine comes out screaming (great). As I get to, onto, and somewhat past plane, the boat starts/is porposing, amount depends on speed and tilt amount.

To straighten her out, I just hammer down on the throttle and she pulls right out of it.

When I come off top end, with the trim out, it will start porposing as I slow down, the slower (with nothing else changed) the worse it gets.

To stop it I can speed back up or trim in till it stops.

HTH

Mark

 

[atengnr]

Re: Cannot trim motor up - Porpoising

OK. I plan to tryout raising the motor tomrrow (hopefully the weather cooperates in MI). I also will bring the 131/4x17 prop with me and swap at the lake.

I will report back tomorrow with the results (hopefully).

Thanks for everyones help thus far.

 

[walleyehed]

Re: Cannot trim motor up - Porpoising

I can't speak for Dhadley, but I "think" what he was saying is raise the engine 1 hole at a time and test with the prop you have on it now...you will know when you have reached the limit. At "that" point, try another prop, of different pitch or higher quality(preferred) and see what you get.

 

[atengnr]

Re: Cannot trim motor up - Porpoising

I can't speak for Dhadley, but I "think" what he was saying is raise the engine 1 hole at a time and test with the prop you have on it now...you will know when you have reached the limit. At "that" point, try another prop, of different pitch or higher quality(preferred) and see what you get.

Yep, will definitely test one change at a time here. Im figuring that raising the motor will bring revs up to where I need a 17"er....

 

[atengnr]

Re: Cannot trim motor up - Porpoising

OK. Heres the results:

1) Raised the motor by 1 hole (1inch?). It ran 6000 rpm at about 33.5 mph. It was around 32.5 mph before.

It now can be trimmed out a bit before it begins to porpoise. I think the anti-cavitation plate is still .5 inch below the hull....Should I raise it again and try??

2) I ran it with a 131/4x17, and runs about 5500 rpm at 34.5 mph

SO, should I stick with the 17"er and try raising the motor another notch???

Thanks.

 

[walleyehed]

Re: Cannot trim motor up - Porpoising

I'd say go up another hole and test...that'll put the AV plate 1/4" above hull so there won't be any water pressure issues, so I say go another...
Dhadley will be back on tomorrow, so see what he thinks and go from there..this is his thread so I'll watch from here...8)

 

[Dhadley]

Re: Cannot trim motor up - Porpoising

Yep, definately go up. That plate has to be at least even if not slightly above. Nice job with keeping track of the changes and results.

 

[atengnr]

Re: Cannot trim motor up - Porpoising

Yep, definately go up. That plate has to be at least even if not slightly above. Nice job with keeping track of the changes and results.

Thanks. I will try and make it out there this week.

 

[atengnr]

Re: Cannot trim motor up - Porpoising

Well dont laugh too hard, but in a mad rush today, i planned to raise the motor another nothc and run it to the lake for a 15 minute ride.....Long story short, I ended up jacking up the motor then re-installing at the same height it was at... I realized it when I got to the lake....

I will raise the motor correctly and retest this week if weather is reasonable (no snow!)

On a side note, based on the info here, I switched to 87 octane fuel for this ride, while I had always used 93 previously. It perhaps may have spun a bit higher - 50 rpm 5450 vs 5500, though this could easily be due variability in water conditions, etc.

 

[Texasmark]

Re: Cannot trim motor up - Porpoising

Well, you'll never know cause we are assuming the next time you are at the lake the engine will be jacked up and all bets are off.

Mark

 

[Dhadley]

Re: Cannot trim motor up - Porpoising

It wasn't a mistake. Subconsciously you made the correct choice to make one change at a time.

 

[atengnr]

Re: Cannot trim motor up - Porpoising

Well, theres only one more hole left to raise this motor, so knowing that now, I think I can manage to put the bolts in the last hole on the bracket.

 

[atengnr]

Re: Cannot trim motor up - Porpoising

OK, I ran the boat with the motor one more inch up (as high as it goes). It gained perhaps .5 mph (at 35 mph, 35.5 at times), but no improvement in ability to trim it out. So, I can trim it about 1.5 mph worth before it porposes....

There was one significant unexpected change: The ride of the boat improved significantly....

I wish I could raise the motor more....

So anything else I should try??

Also, Im running about 5500 rpm with the 17". Should I stick with this or drop back to the 15" (probably would be right at 6000 or 6050)??

Thanks everyone for your help...I gained about 3.5 mph from this exercise.

 

[walleyehed]

Re: Cannot trim motor up - Porpoising

The option to add some setback is always there, but we don't know what it would do for porpoise.
See what DH thinks....a high quality 15P may be in your future..perhaps a worked 15 to increase efficiency...

 

[Dhadley]

Re: Cannot trim motor up - Porpoising

I think we may have some positioning issues going on here. Hopefully he can fill everyone in on what we've been talking about offline.