Boat pulling to right and porpoising...help!?

[Canaan]

I just got a 17.5ft Alweld boat w/ the 90hp merc this past weekend. It pulled to the right pretty bad before coming to plane, then eased up some but still pulled.

I moved the fin above the prop to the right (it was dead center), and that helped quite a bit. Took it back on the water and was getting up to 43mph @ ~5200-5400rpm (via gps) but could only VERY slightly trim up before it would start to porpoise pretty bad. That was with just me in the boat, no gear.

I then went and got a Stingray Hydrofoil and mounted it on the outboard. The boat got to plane faster out of the hole and would plane at a lower speed. Held ~38mph @ ~4600rpm. But it now pulls to the right MORE!!! (not as bad as it originally did before adjusting the fin, but more than without the hydrofoil)

Also, I still can't trim the motor out much at all before porpoising??? The motor has to be trimmed nearly all the way down or the front will hop.

It has to permanent (welded) trim tabs on the back, maybe they should be bent?

Here are a few pics of how the outboard is set up now.

Thanks for any help you can give!!!

 

[Canaan]

Re: Boat pulling to right and porpoising...help!?

one more pic.

 

[dockwrecker]

Re: Boat pulling to right and porpoising...help!?

The motor looks a tad low maybe. The cavitation plate should be about an inch higher than the bottom of the transom. That's probably where the porpoising started. Your steering compensator behind the prop looks like it's set straight on, that can be adjusted to take some of the right hand pull out. The little tabs welded on the transom are useless, way too small to do anything even if they were bent down a bit. Grind them off, throw away that fin (sorry you just paid good money for it), raise the motor a bit, work with the adjustment of the steering compensator and if none of that works, put at least Smart tabs on it and set the starboard tab ram one notch lower than the port ram.

 

[Maclin]

Re: Boat pulling to right and porpoising...help!?

Looks like to me you have some room left to move the tab more?

I think your problem mainly comes from those hardset tabs. I would look into removing those or at lest rendering them neutral, then see how it performs (just be ready for anything out on hte water). Then look into getting the lightest Smart Tabs on there in place of those small hardset ones.

The Smart Tabs act like shock absorbers and can help balance the stern and many times can help with a pulling problem. They are definitely the answer for porpoising.

I also think that foil will cause other problems because it can only "work" from the middle of the boat hull like a teeter totter pivot, and the hull has to "fall" left or right even if it is a subtle movement. It will also affect how the boat turns and usually not in a good way.

Tabs are at the edges of the stern and just by design add stability. And Smart Tabs react to the conditions and thus help even more.

 

[Maclin]

Re: Boat pulling to right and porpoising...help!?

Looks like dockwrecker and me were preparing posts at the same time along the very same lines.

Hey dockwrecker, my nickname is "WaveSlayer" after an on the lake adventure I keep trying to get everyone to forget:redface: Does your screen handle have any signifcance? Just curious

and good catch on the cav plate height...

 

[dockwrecker]

Re: Boat pulling to right and porpoising...help!?

My name was earned and bestowed upon me by a Park Ranger when I was teaching cat sailing 30 years ago. Long story...
But not to hijack the OP's thread here....

 

[dpoff]

Re: Boat pulling to right and porpoising...help!?

It loks like the fixed tabs on your boat are at different angles.
Port side lower than the STBD? Maybe even them out?

 

[Canaan]

Re: Boat pulling to right and porpoising...help!?

In the pics of the outboard trimmed in, it is not totally trimmed in. The way the boat is setting right now I can't trim all the way in. It can come in some more.

I will pull the hydrofoil and turn the fin above the prop further to the right.

I have 3 higher positions to mount the motor. How much higher should I do w/ it?

Thanks for the replies!

 

[Maclin]

Re: Boat pulling to right and porpoising...help!?

It is hard to tell from the pics on the height adjustment, this statement from Dock's post...."The cavitation plate should be about an inch higher than the bottom of the transom"... is correct. Get a straight edge and see how all of that lines up on your boat.

The not being able to trim in all the way could be an issue also, what keeps it from going in? A jack plate may be the answer there, it fits in between the transom and engine, moves the engine back some and allows for more adjustments up and down also.

 

[dpoff]

Re: Boat pulling to right and porpoising...help!?

I believe the reason not being able to trim all the way in is the boat sitting on the trailer in the driveway. My old boat was the same way. I think he can trim "all the way in" when the boat is on the water.

 

[Canaan]

Re: Boat pulling to right and porpoising...help!?

I believe the reason not being able to trim all the way in is the boat sitting on the trailer in the driveway. My old boat was the same way. I think he can trim "all the way in" when the boat is on the water.

Yes, when its in the water I can trim all the way in. The boat trailer is just too low to do so in the driveway. I will drop the tongue and get a pic w/ a straight edge under the boat.

 

[Canaan]

Re: Boat pulling to right and porpoising...help!?

I just went out and rolled the jack back and let the trailer rest on the tongue. Got the back of the boat up high enough to trim the motor in full.

Here are a few pics.
Does it look like the motor is mounted to low? Is this my most likely cause or the porpoising and not being about to time the motor out?

 

[dockwrecker]

Re: Boat pulling to right and porpoising...help!?

Looks like it could go up maybe 2". Hard to say. Get the cavitation plate at the same angle as the bottom of the hull and measure. It will be hanging at it's lowest point then.

 

[Canaan]

Re: Boat pulling to right and porpoising...help!?

Thanks dockwrecker.

Looks like I will be lifting the motor. Gonna try 2 pegs higher.

 

[dpoff]

Re: Boat pulling to right and porpoising...help!?

Hell! In pic two, it looks like the drive is tucked in to close and I would think that would cause porposing.
After reading your original post that is not the case in your desrciption.
Maybe someone else has an idea to get your boat right.
Let`s get this boat running right fellow boaters!

 

[Canaan]

Re: Boat pulling to right and porpoising...help!?

Hell! In pic two, it looks like the drive is tucked in to close and I would think that would cause porposing.
After reading your original post that is not the case in your desrciption.
Maybe someone else has an idea to get your boat right.
Let`s get this boat running right fellow boaters!

Yes, the drive angle does tuck in.
Correct me if I'm wrong (and I likely am), but usually when you trim in fully....isn't the drive roughly parallel to the bottom of the transom?

On this boat, it seems to be far beyond level w/ the transom bottom when trimmed in full...but here is the kicker.
When I trim the motor out (when on plane), I can only trim to slightly above 'level with transom bottom' before porpoising starts.

????

I used to have an old mid 80's Tidecraft Spitfire V15 w/ 70hp merc (actually posted a thread here about rebuilding it, but after messing w/ the boat for awhile a realized the hull was trashed, so I bought this 'new-to-me' 17.5') I could get to plane w/ that old Tidecraft, trim out w/ the 70hp and never see porpoising unless i got stupid w/ the trim. It topped out around 33-35mph though.

I'm really liking my new fishing boat...no leaks, no wood to rot, and engine runs like a sewing machine, but would like to have it run more efficiently if possible.

Any further help would be greatly appreciated.