Why setback?

[m3rb]

I've heard two rationales for setback: 1. it moves the center of gravity back so the bow rides higher. 2. It gets the prop into "cleaner water"

#1 I can buy, if you've got a boat that likes that...V's I suppose, but sounds dangerous for tunnels? But why not just trim up instead?

#2 I don't get. In theory, the hull puts some energy into boundary layer water, and the closer the prop is to the hull, the more of that can be recovered. In other words, the water right behind the boat is moving along with the boat, and if the prop is pushing off that, you get more thrust than if it's pushing water that is more still. (this seems to be called "wake effect" in some marine engineering papers)

Is anyone really confident that, all else being equal, they've gotten more speed out of more setback? If so, why do you suppose that is?

[flabum1017]

#2.......... The water comming off the bottom of the boat is aerated. The further back you go, the less disturbed water you get to bite onto.

[CharlieB]

picked up 4mph with 10 in setback and 6 mph with 14 from bolted directly to hull Much better bow lift less chine walking and I was able to lift the motor higher which probably is the main contributer to the speed increase,

[Raceman]

Of course it varies from boat to boat, but generally speaking in addition to moving CG (less trim angle required to lift bow if CG is moved rearward, because of change of position itself AND more leverage on fulcrum), the engine can be run higher the further back it goes, so less drag because of less gearcase in water.

[Specialized P4-12]

the ideal trim angle for any v hull boat.. is 3 degrees... so i have been told by a naval engineer who i have been working with on a hydroflight system to put on a 26 sutphen... it is difficult to achieve good bow lift at 3 degrees on a boat that has no setback.. further back you go the angle of attack changes and reduces the trim needed to set the boat prop at 3 degrees.. check out this stuff... http://hydroflight.com/

[Coupe9050lx]

Setback made a world of difference on my boat.. Made it ride completey different.. It enabled me to get more lift at more of a neutral trim angle.. Less boat in the water= MPH in my case..

[j_martin]

the ideal trim angle for any v hull boat.. is 3 degrees... so i have been told by a naval engineer who i have been working with on a hydroflight system to put on a 26 sutphen... it is difficult to achieve good bow lift at 3 degrees on a boat that has no setback.. further back you go the angle of attack changes and reduces the trim needed to set the boat prop at 3 degrees.. check out this stuff... http://hydroflight.com/

Blanket statements like that are baloney. Just think about it.

If you're running a fishing plow where the water meets the boat about 1/3 of the way back from the bow, you put the lunch box, fat grandma, fuel tank, whatever you can up front so the attack angle of the boat is best.

When you're running with the water hitting the pad about a foot in in front of the transom, you need to hang some weight off the back to get it to balance right. In other words, then it'll take less power wasting overtrim to get the bow to fly where you want it.

It all depends on the particular boat, the power available, what it's carrying. (or who)

The stuff about the layer of water going the speed of the boat. That's all called laminar flow theory. The layer in question is inches thick, probably pretty insignificant as prop feed. The biggest issue with the prop is getting a clean bite as close to the surface as possible to limit the friction of gearcase and prop slippage.

hope it helps
John

[Specialized P4-12]

everybody has there opinion....

[largecar91]

Just trimming the the motor up to get the hull "loose" wastes forward thrust. You will use horsepower to lift the boat instead of push it forward. The setback enables the boat to carry the nose with less trim.

[m3rb]

Thanks for the explanations...I can see the benefit of large setbacks for V-hulls. For the LTV I recently got, though, I'm inclined to start with 4 inches...which I think I remember seeing in a post here was what Summerford recommended.

The hydroflight stuff looks really interesting. I wonder why it still seems to be a research project, since it was successfully demoed in the early 90's?

[Raceman]

Thanks for the explanations...I can see the benefit of large setbacks for V-hulls. For the LTV I recently got, though, I'm inclined to start with 4 inches...which I think I remember seeing in a post here was what Summerford recommended.



Actually, Ruark Summerford didn't like setback on the STV hulls. He thought they should be run with the engines mounted flat to the transom, BUT we're talkin' different hulls here. I had 3 STV ModVP's at the same time. The first two had 20" motors mounted straight to the transom. I got the idea that I had to put 15's on em, so that required 5" of setback to keep from butchering the boat. The Merc 15 has 2 extra inches of setback built into the swivel bracket, so in reality, this change set the motors back 7 inches. There didn't seem to be any significant speed increase, maybe a mile or two at most, and this could've very well been mostly from taking 5 inches of the cowl out of the wind. (hold your hand out at a hundred and feel the drag, and then multiply that times about 20 for the hole the engine punches being reduced by 5 inches times the width. Anyway, the downside to that change was that the boats BOTH had a much greater transitional hop as it made the change from purely hydro dynamics to hydro/aero dynamics (started trapping air). The yellow STV in my signature had about 10" of setback to accomodate the champ mid section which is a FULL 5" shorter than a 15, or 10" shorter than the std length. This boat also hopped more at mid range than the old 20" motors straight on the transom, but once through the hop, better hang on.

I don't think that it's practical or accurate to try and apply a blanket trim angle to all boats in anything but theory, and in all honesty would have to question how much experience and practical knowledge a Navy engineer would have on the dynamics of the type of high performance boat we're dealing with here. I would think that theoretically speaking a zero trim angle would be the most ideal if the boat itself was designed to provide adequate lift to unwet the optimal amount of bottom to run it's fastest. If you look at a 3 point hydro for instance, the fast ones are always run with the engines tucked UNDER, so the theoretical optimal trim angle goes out the window there too.

[Mark75H]

I don't put much faith in your explanation of #2 ... more likely in my opinion that the hull pushes water down and water rebounds a little behind the boat ... the farther back the prop is, the more likely it is to be where the water has rebounded. No boundary layer or wake effect movement involved.

The higher the prop is, the more it is pushing the boat from directly behind the boat rather than on a lever under the boat ... more efficient to push straight than with an offset lever. Moving the prop back, to the higher clean water is the advantage.


Combine that with the improved center of weight and you have 2 good things going on.

Tunnels have used setback since the 1970's. The back of the sponsons is the axis carrying the weight. Leaving the motor forward causes you to waste energy lifting the hull (either by prop design, thrust angle or aerodynamic lift). The weight of the motor can benefit from a free ride by moving it back.

All the fastest three point hydros are now running negative propshaft angle and super high rake props. This even applies to taildraggers. I think this is due to efficient use of the thrust from the prop. Remember that the thrust is a diverging cone behind the prop. At zero or positive trim you are sending a large part of that cone up into the air rather than straight behind the boat. Pushing harder against water, which is denser than air, is more efficient. On hydros and tunnels, the down angle also gives considerable tail lift, letting the boats run more aerodynamically efficient with less wing angle of attack across the body of the boat. The energy spent lifting the tail with a down-angled high rake prop is not a burden, because the improved efficiency of the set up yielded more free thrust than is being spent lifting.

[Forkin' Crazy]

I think an aeronautical engineer would have a better clue to what was going on with a high performance boat than a naval engineer would have.

I always thought that the more level the prop shaft while at speed the more efficient. If you have to use trim to lift the bow, you loose efficiency and thus are loosing speed. Move the motor back, and the bow will be easier to lift. My boats run with a tad bit negative trim.

[Dave S]

Depends on the prop ... hp ratio... hull type...driver...and luck. Mostly luck. In finding the rite combo.;)