Help on what I think is Porpoising 2010 Sylvan Prosport 1600 w/ Evinrude Etec 75

trickynick

Cadet
Joined
Jun 30, 2015
Messages
13
What is happening is that I cannot trim the boat without it hopping in the water. When i begin to accelerate the motor is all the way down as the boat get on plane and i try to trim the motor the boat begins to hop violently at which point i have to trim all the way down to gain control. I just recently changed the prop to a 3 blade aluminum 13.5 x 15 it was a solace 12.75 x 17 4 blade aluminum prop. The Rpms with the motor all the way down were 5900-6000 WOT which I know are high for the motor but I would like to cure the hopping first and be able to trim the boat properly to see what the true RPMs are. The only thing I can think of is the height of the motor, it is at the second hole from the top, just not sure which way to go either lower it one hole down or raise it 2 holes up and I have been getting different opinions of which way to go from the marinas here in Ontario.
 

dozerII

Admiral
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Oct 25, 2009
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6,527
First off :welcome:, the anti cavitation ventilation plate ( right above the prop) should be level to 1 inch above the bottom of the boat, trim the motor all the way down, you may have to lower the tongue, then place a straight edge on the bottom center of the hull in front of motor to verify the height. If this is right then you can check your weight distribution, if it is all in the back it can cause propoising, to check have some one sit up front, if that helps you may have to look at moving batteries or gear around. If you can't do that a set of Nauticus Smart Tabs will fix you right up.
 

Pony

Rear Admiral
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Jun 27, 2004
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4,355
First off :welcome:, the anti cavitation ventilation plate ( right above the prop) should be level to 1 inch above the bottom of the boat, trim the motor all the way down, you may have to lower the tongue, then place a straight edge on the bottom center of the hull in front of motor to verify the height. If this is right then you can check your weight distribution, if it is all in the back it can cause propoising, to check have some one sit up front, if that helps you may have to look at moving batteries or gear around. If you can't do that a set of Nauticus Smart Tabs will fix you right up.

This is where you need to start....assuming the motor is at the right height after doing this, then start looking at where weight is in the boat.

I have 5 group 27 Deep Cycles in the boat, so if my fishing buddies sit towards the back I will porpoise too. As soon as someone moves up to the front it evens back out generally.... Otherwise a small trim adjustment at that point will work.

I am guessing, and I'm sure some of the experts here can chime in to help, that once the porpoising issue is fixed you will be looking at a different prop if you are getting that many RPM trimmed all the way down
 

airshot

Rear Admiral
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Jul 22, 2008
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4,219
Also get under your boat and sight down the keel from back to front. The keel should be straight, not have any curves going up or down, it should remain level and straight until it starts the rise at the bow. Some aluminum boats have a hook in them from the factory or from rough pounding and some glassers develop a hook (curve) in the keel from structure defects. If the keel is straight then you can remove that from your list of what could cause porpoising. That hook in the keel can definitely cause your problem if it is there.
 

airshot

Rear Admiral
Joined
Jul 22, 2008
Messages
4,219
Also get under your boat and sight down the keel from back to front. The keel should be straight, not have any curves going up or down, it should remain level and straight until it starts the rise at the bow. Some aluminum boats have a hook in them from the factory or from rough pounding and some glassers develop a hook (curve) in the keel from structure defects. If the keel is straight then you can remove that from your list of what could cause porpoising. That hook in the keel can definitely cause your problem if it is there.
 

trickynick

Cadet
Joined
Jun 30, 2015
Messages
13
Thank you for the info I will check the height of the motor this afternoon and if the anti-cavitation plate is roughly 1 inch above the hull I will buy the Smart Tabs. The weight should not be a issue as the boat is rated for a 90 four stroke the etec i have is 80 lbs lighter and the passenger weight in about 500lbs and the boat is rated for 670. one battery is under the passenger console the other is in the back and we only take what we need for that lake so minimal fishing equipment. I look at the Smart Tabs online and have not seen any bad reviews and it does make sense so there goes another $200 to get it running properly. HOPEFULLY. I will definately repost after i check and try things out.

Thanks Again.
 

Watermann

Starmada Splash of the Year 2014
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Jan 12, 2013
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iboats has the smart tabs for right at 100 for the factory blemish ones.
 

Alumarine

Captain
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Feb 22, 2005
Messages
3,684
Or, maybe there's no need to trim the motor up.
What are you hoping to achieve by trimming it up? You don't want more rev's.
 

trickynick

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Jun 30, 2015
Messages
13
Trimmed all the way down is a pain in the ass to steer the boat with WOT. So I would like to get the boat on plane and trimmed properly
 

WIMUSKY

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Sep 26, 2009
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Have someone sit forward of the helm on the deck. If the porpoising stops, you have a weight distribution issue.....
 

Watermann

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You should run with the thrust bar all the way at the bottom hole and trim up from there so the motor is in full tuck when pulling out of the hole and then trim up until it starts to hop and bump trim down to settle it down.
 

trickynick

Cadet
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Jun 30, 2015
Messages
13
Just a update. I purchased installed and tested the Nauticus smart Tabs today and all I have to say is WOW. On setting 3 the boat has almost no hull lift from the hole shot and it planes out beautifully and best of all the porpoising issue is gone. Thank you to everyone for your suggestions and I look forward to talking to you on iboats
 

Watermann

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Hey thats great to hear!

I love my Nauticus smart tabs, it's cool how you boat turns with them and you'll notice the low speed wander is gone as well..
 

trickynick

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Jun 30, 2015
Messages
13
It's true. I have lost a couple of mph on top end but my rpm is at 5100 at WOT. I am running a 13.5 x 15 3 blade aluminum prop. Any suggestions to get it to 5500-5800 rpm
 

Watermann

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Try adjusting the tabs one setting lower and see how the boat handles. Were you getting those high RPMs prior to the trim tabs? See if the lower tab setting increases RPMs and speed first. Prop pitch will raise RPMs at about 200 per inch decrease.
 
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trickynick

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Joined
Jun 30, 2015
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13
Agreed I was getting the high rpm prior to the tabs. So I will fine tune them first and deal with the prop later. Thanks for the help
 
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