I'm sure this topic has been discussed and if so please point me in the right direction. I am in the process of having a custom boat built and will be running a 37 EFI GTR. The boat will be 18x48. I am trying to decide whether to leave the bottom square in the back or to have an angle cut on it. My last GTR boat had the angle to aid in reverse I'm assuming. Just looking for ideas on the pros and cons of having the angle on the bottom or not. ( speed, reverse capability, general performance loss/gain). Any advice appreciated. Thanks

I do not recommend the cut back at the transom. You lose planning surface with them. The only thing they do is help reverse a little bit. You spend WAY more time going forward so That's my take on it.

Leave it squared off. The cut back hurts more than it helps in every scenario except for going backwards.

On a wide boat prob wont affect it as much. On a 48 it will prob draft more water at rear of boat

My boat performs great and I had the 45 degree cut back added to the build. It was the first time Alumitech had built a boat with a hunt deck and the 45 cutback. Turned out great and couldn't be happier with the performance.

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I don't think it kills you on the top end as much as low speed stuff and stepping a load in deep water. Those extra several inches of planing surface missing directly under the heaviest part of the boat lets the boat squat instead of forward and up.

Based on the math and the way water is shoved when trying to plane out, on an 1848 with loss of 2" of length with the angle, figure on needing 1 more inch depth of water on hard bottoms without gator glide reducing drag to plane out in the same distance as without that cut. Harder to calculate on soft mud, but wont matter as much. Not a hard and fast measurement, but a decent guideline based on lift lost, weight of the motor, etc.

Get outta here with that logic nonsense....

-I'm gonna try it anyway

Yeah - shoot me that formula that shows draft is based upon bottom being hard or soft

An extra inch of draft

Boats are about displacement of water - that cutback may cost you 10 lbs or so of lost
displacement

Draft isn't different, friction is. Less drag on the hard bottom with gator glide means more speed under same power, which translates to more effective lift for same surface area. Don't need quite as much standing water depth. Soft mud also has less drag than hard bottom typically does. There are exceptions due to the type of soil the mud is created from, but a general idea can be had from this.

And that 10lbs or so displacement isn't a big deal at rest. I'll not argue that at all. It won't be an inch of draft. That's also never what I was talking about.

Think of it like this: 2 inches extra width on a boat at rest means a boat will draft less water. I'm sure we can agree on that. But it can also mean the boat doesn't squat as much when trying to plane. Is that an effect of draft (which comes from displacement), or lift (an effect from surface area) because of the resistance to compression of water underneath? I'd wager at a certain (and fairly slow) speed, it ceases to become a function of displacement and transitions to lift due to surface area. Yes, likely a certain speed where both are working together, but that will be a small speed range.

You're hung up on draft, giga, and that's not the concern here at all. That loss of surface area (not displacement) is where that cutback causes the difference. Draft doesn't mean anything when the boat is dragging across the high spots. Lift does. You lose lift when you remove flat bottom at the very back of the boat, whether it's width or length.

I'm not positive how much, how drastic, or exactly how measurable, or even how to measure it. But it stands to reason that there is something to be gained by maintaining or even adding to the planing surface at the rear of a boat when talking about the transition from sitting, idling, and "climbing" up onto plane. Once the boat has reached a point where it's traveling faster than the water can fill the void left by the hull behind it "on plane/step" then I think the returns on this theory are diminishing. Definitely after a certain speed.

-FORGET DRAFT- I think this would aid the hole shot and load carrying ability of a hull that lacks planing surface. I'm sure the gains are directly proportionate to the hull dimensions and even drive length and angle. A hull that might not "carry a load well" usually just has a hard time getting on plane, after that, the playing field evens with respect to Hp and overall hull size.

I plan on experimenting with this theory with my own 1860 hull. I don't think it's severely lacking. These hulls are known for their ability to carry weight well. But for my personal use, I could benefit greatly by being able to stay on step at lower speeds due to some of the terrain I run, as well as being able to get on plane quicker and in shallower water. I do think my transom design hurts me in these areas. In theory, instead of the "cut back" traditionally put on a GD hull, I'd like to see something similar to what PD uses on their factory hull design with a small lip or tab extended past the face of the transom. Ironically enough. There are a lot of PD hulls that roll out of GD's boat shop. That's no secret. I do wonder if one would ever leave there built like I imagine.... In the mean time, I'm going to do my own experiments with it and see what comes of the concept. What little bit I stand to gain may prove to be huge for someone who's working with a narrower hull or different chine design trying to accomplish the same thing.

In 10 foot deep water that is mostly true....

In 4" shallow water once the hull gets going 3mph a different set of rules apply
because of the water squeezed between your 18 foot long hull and the bottom
and the 2" to 4" of a cutback makes dang near no difference...

If anything a square chine vs tapered vs round makes 10x more difference than
the tiny cutback does under these shallow water takeoffs...

I say this because I have owned and driven all three taper designs...

I don't agree Giga-

The surface area lost is in the most critical part of the boat. Where the thrust is being applied and where the majority of the weight is sitting. Once the boat is moving at your "3mph" this becomes much more of a factor, the shallower the water gets.

I get where you're coming from but from time to time you have to take your head out of the GoDevil box. You can use a different pair of overalls at times to skin the same cat.

All around, there is absolutely no issue with the Go-Devil cut back transom theory. But there's no way you can argue that having that length gone is better or even as good as having it there.

I drive a long thin round chine --- when going any speeds at all over 5 to 10 mph
as you run into shallows the hull angle changes --- it gets flatter and flatter the
shallower the water gets....

It is much less dramatic in square chine or wide hulls --- but in shallow water all
hulls flatten out --- this is what lets them run faster in the shallows than the deep...

The surface area pushing against the water at whatever point is what matters. These boats don't defy physics, dude. More surface area at the rear of a boat from added length create a longer lever to get more planing surface in the water towards the front, which causes the rear to not need as much lift anymore, since more surface area is getting direct lift. I'd counter that 2 inches at the rear of the boat would have more effect in shallower water, not less, because the pressure from the water would be greater on those last 2 inches than in deeper water, therefore providing more effective lift.

Everything about a hull design is a compromise

Heck that is why so many of us have 2 or 3 boats !

Summary - cutback has certain assets just like different taper designs

But don't say it adds an inch to draft at rest !

I didn't say that. Never did. You read that and took that argument out of it. My post was always concerning the attempt of a boat to plane. While moving.

And cutback has 1 asset. Less resistance going backwards. A second possibility is debateable, and would vary with each setup, and be very difficult to quantify. Better water flow characteristics at the prop. If there are others, I'd be interested to hear them.

Same here. I was never here to debate the merit of a cutback transom. I only was trying to give reason why a square or even extended design has certain useful advantages. The cutback being on the GD boats is a bit ironic in my own opinion... It's supposed to be there to aid in moving backwards among other things. I had to walk outside and look again, but my GD doesn't have reverse

We're posting the same damn thing all over the top of each other lol

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Is that flat? It's probably the angle of the pic but it looks like it still has some rise to it.


And clean that damn salvinia off that trailer!!!!!!!!!

I had the cutback at the rear of my boat and had this plate added to delete it. I have noticed my boat runs flatter in the water and I don't have to shift everything in my boat to the front to get it to stop hopping on plane. Adding this to the rear of my boat was the best thing ive done to the boat so far it jumps on plane now and gets up on step faster in shallow water only drawback I've found to doing this is it slides a little more in the turns, but it did that already.

Hate to jack the thread much but it looks like the op has his answer so about this salvinia.
Here it seems to be an every other year or every three year occourrence it just comes and goes. What I mean is some years we don't see it any and some years it's horrible, is that the way it is where ya are?


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No, once it's here, it's here and only gets worse for the most part. There are chemicals that slow it down and there are weevils that eat it but nothing that has been proven to do away with it at a faster rate than it can multiply on its own. I floating patch of that stuff can double in size in a matter of days. Eventually it covers the waters surface and chokes out everything under it.

Around here, get caught launching a boat with some salvinia (or any vegetation for that matter) on the trailer, you'll be going home with a ticket.

Is the water where you're at tidally effected?

Sounds good to me!!!

Not tidal where I'm at and like I said it comes and goes here


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