School me on outboard engine placement/set up for twins

[MARCYTECH]

Winter project time.1989 hallet vector with twin Mercury outboards. Why am I doing this? Current set back is 26” with 10” tabs. The boat porpoises a lot if trimmed out. I think it could be set up better with less setback and bigger tabs.

I’m gonna be re doing my transom and engine brackets. Switching to dual jackplates I think. My thoughts are to mount the jack plates so that at the lowest setting it’s where my engines currently sit (I have no adjustment)

My question… would I want them lower than where they are currently for any reason?? Rough water handling or anything? My goal here isn’t top speed on glass. It’s maximum comfort for all water conditions.

I tried to draw to the best of my chicken scratch ability all my measurements currently. This is a twin engine boat so there’s an extra factor here.
Center of props is 1” below most bottom part of the V
If measuring straight forward from the lower unit prop centerline is 3” below the point in the hull that’s inline with center of lower unit. I hope that makes sense.

Second part of question is how close do I want the engines? Currently they are 24.5” center to center. They are clam shell cowlings so they can’t be too close but they can be closer for sure.

Thanks in advance!

 

[NeedlesRat]

What about ditching the twins and setting it up with a newer single? Might increase performance, efficiency, and reliability.

 

[ChrisV]

450r

 

[Perlmudder]

From what I have read, you want the cavitation/anti-ventillation plate to be right at or above the water when at a fast cruise speed. Most of the time porpoise is caused by the props being too deep. With the amount of set back you have, and the pictures attached I think you are probably too deep with the props.

 

[DrunkenSailor]

Agreed probably too deep maybe too wide and probably the wrong prop. That flotation bracket is more than likely where most of your problems are coming from. It is deeper than your tabs. I would be curious to see what's in contact with the water at speed. If your tabs are touching the bracket is too.

 

[MARCYTECH]

Thanks for the responses everyone.
It’s gotta stay twins. I love this boat for the twin factor for a few reasons.

As for the porpoising, would I be wrong to assume the amount of offset is also throwing the balance of the boat off which is adding to the porpoising? I tested a theory and put 150 pounds of lead wake bags in the bow and it helped TREMENDOUSLY. you could actually drive the boat on chop and trim the boat out. But to me, that’s a bandaid and I’m not ok with it haha.
Also with the props being so far away from the tabs my tabs really don’t do anything no matter where they are set.

As for how much is in the water I do have some pics from videos but nothing really showing the engines in detail.

 

[Perlmudder]

That boat is sick with twins. I would not change that. Balance is also a part of what causes boats to porpoise. If it were me, I would raise the engines to see if that helps. If it solves the issue great, if you do and still have a porpoise I would try to relocate weight forward like the batteries. I would not want to run with ballast that is unnecessary. Propos also make a huge difference and could help as well.

 

[MARCYTECH]

That boat is sick with twins. I would not change that. Balance is also a part of what causes boats to porpoise. If it were me, I would raise the engines to see if that helps. If it solves the issue great, if you do and still have a porpoise I would try to relocate weight forward like the batteries. I would not want to run with ballast that is unnecessary. Propos also make a huge difference and could help as well.

Well I’ve already started pullin engines to re do the transom set up anyway.

what I’m thinking is 12” bobs jack plates and 20” tabs.
My thought would be to mount the jack plates so that at their lowest setting is where the engines are currently. From there I’ll have 5” of adjustment on the jack and even more in the engine mount holes if needed

Good plan?

 

[Perlmudder]

That sounds solid to me. Lots of adjustment options then.

 

[DrunkenSailor]

It doesn't surprise me that the tabs don't do anything they aren't in the water. I think your plan is to ditch the flotation bracket to reduce the setback. Will the boat float the weight of the motors without the flotation bracket? If it will I would ditch that thing. If it won't maybe change to a smaller flotation bracket or raise that up to get it out of the water while underway. Once you make that change you may not need longer tabs.

 

[MARCYTECH]

It doesn't surprise me that the tabs don't do anything they aren't in the water. I think your plan is to ditch the flotation bracket to reduce the setback. Will the boat float the weight of the motors without the flotation bracket? If it will I would ditch that thing. If it won't maybe change to a smaller flotation bracket or raise that up to get it out of the water while underway. Once you make that change you may not need longer tabs.

I’m sure it’ll float the engines. At about 320lbs a piece they can’t weigh more than a blown big block with either v drive or outdrive running gear which vectors typically have.
Rex marine also had a twin engine vector with no flotation bracket just straight jack plates like I want to put on.

Good point about not needing longer tabs. I’ll consider leaving those alone

 

[STV_Keith]

A couple of thoughts here:

1. With those blunt nose gearcases and side water pickups, you won't be able to go up very much before having water pressure issues. Jack plates do give you the ability to mess with engine height and allow for shallow water maneuvering, which is helpful.
2. If the boat porpoises, and moving weight to the front helped, then it sounds like your CG is too far back, at least with the props you are running now. How much of that 26" setback is bracket vs jackplate? Sounds like you need to shorten the overall by at least a few inches. I'm sure someone smarter than me can help you do the math based on the distance, weight you added, center of gravity, etc.
3. The other thing that can affect this is the rake of the prop. More rake (higher degree) generates more bow lift. Lower rake generates less bow lift aka more stern lift. You need to find out how much rake your current props have. Simply going to lower rake may have helped the porpoise issue.
4. Generally, putting the engines closer together is preferred for performance. You do lose a little response to maneuverability trying to spin the boat on axis as you don't have as much leverage any more.
5. Are those single ram Bennett tabs? How long are they? They might not be long enough to leverage the boat with the engines so much further back.

 

[hallett21]

WOT Marine has the most up to date experience with outboard vectors. Either in re rigs, re powers etc. I’d talk to Kyle and Cole on what they think.

Outboard vectors need to run a little tab (flat) at pretty much anything other than 60+. I’ve played around with a heavy cooler in the bow of mine and talked with Kyle and the general consensus is that they all could benefit from a ballast tank. I think the outboard vectors need 24”+ tabs.

My short Bennetts work fine burying the nose at slower speeds but I think they are angled too deep at higher speeds. Meaning I think they cause too much drag vs a longer tab running flat (or even up a degree or 2) would apply just enough pressure to stop the hop.

This can obviously turn into a can of worms real fast lol.

Vectors in general want to run pretty flat and level to the water. It takes an insane amount of trim to try and fly the nose like a modern day outboard ski race boat.

Are you going to power the jack plates?

Excited to see this project unfold!!!

 

[hallett21]

As far as motor height I’m running 1” below the keel with a 250 Optimax with a torque master lower unit. Motor is on a 12” bobs jack plate.

Your setup with twins and different lowers is gonna be different. I’d just start at 1- 2” below the keel and spend the day raising and lowering the motors.

 

[hallett21]

Any updates?!

 

[MARCYTECH]

The boat got taken apart and is patiently waiting to be dropped off at WOT. Kyle and crew are going to fill all the holes in the transom and paint it inside and out. They’re also going to mount the bobs jack plates. After that I’ll bring it home and put it all back together, wire new jack plates and so on.
We talked a bunch about engine heights and stuff and have a good game plan!
still deciding on what tabs to run. Kyle recommends a shorter single ram Lenco tab than I think I should run. I wanna be in the 21-24” tab region I think.