How does increased set back affect stability?

[Stilldouble]

When adding to the set back of the engine, from no set back to approx 12" including jack plate. What is your experiences of how the stability is affected at low, medium and high speeds? Naturally, it differs from one boat to another, but in generally?

The boat is a 21 ft offshore racer/cruiser. The weight is approx 1100 lbs.

Best regards,
/Stilldouble

[Largemouthlou]

Prop will be back in cleaner water, so will have better bite and stability.. Remember the further ya go back the more you have to raise the motor to get bck to your original height.. In most cases you can run the motor a 1/4 higer for each 2" of setback.. Louis

[Trikki1010]

Also, like a fulcrum, you are changing the "Pivot Point" off of the transome to whatever your setback is, so it's almost a bigger boat w/ less wetted surface;) :cool:

[David]

The only time I added set back to a boat, it decreased stability. But the boat went faster, and was still quite easy to control.

[Stilldouble]

How far did set back the engine? What kind of boat was it?
Did you get the feeling of the boat being "slippery", or was it reduced stability side-to-side?

Best regards,
/Stilldouble

[David]

I added a Land n Sea jackplate to a Hydrostream Virage. Still stable. Then I added another 5" of setback, and experienced chinewalking. I could get rid of the chinewalking by raising the motor enough.

With the extra setback the boat ran freerer and carried weight better. It was a little faster with just me aboard and a lot faster with more passengers.

[Stilldouble]

Interesting. Just so I understand this correctly, I need some clarification. Was it with the engine more tilted forwards, or with a higher position on the jack plate that made the chine walking to go away?

Best regards,
/Stilldouble

[David]

Raising the engine vertically high enough on the jack plate made my Virage completely stable.

I now have an Allison SS2000, and raising the motor makes the boat easier to drive. It still takes steering to go straight, but less steering. At least going from full down to 1/2" below the pad helps. The reputation of Allison is that bad things can happen with too much height, ie blowout.

And raising the motor made my Voo Doo easier to drive.

So I am 3 for 3 on easier control with more height. The downside is that there is not always enough bow lift with the engine high.

[Stilldouble]

If I understood you correctly, your conclusion is that you found a delicate balance here. Adding setback increases the bowlift, by itself, but this is also causing a demand for a higher position of the engine (to keep off chine walking), by itself reducing the bowlift.

Optimum performance of bowlift, engine height and propeller choice seems to be correlated in some way. Have anyone done some experimenting with different propeller types, and at the same time trying different engine heights to get maximum performance of speed and lift, still keeping the chine walking away? If so, what was your conclusions, what prop performed best etc?

Has anyone done these experiments?

Best regards,

/Stilldouble

[mercpunk]

ITS MORE THAN SAYING HEY YOU NEED A 26 FOUR BLADE CLEAVER AT 1/8 INCH ABOVE THE PAD WITH 2 DEGREES POSITIVE TRIM :eek: LIKE YOU HAVEN'T SAID WHAT KINDA MOTOR OR MOTOR'S YOU HAVE AND HOW MANY RPM'S

[Stilldouble]

Hi mercpunk,

Well, that is true. But the question was what OTHERS has done, experimenting with THEIR rides and what conclusions THEY made doing it regarding these parameters.

But once again, your right. Their conclusions are not necessarily adaptable for me. The reason I asked was that usually in these kind of experimenting it usually gives a lot of guidance of what does works, and does not work.

Best regards,

/Stilldouble

(A swedish saying; "the wise man learns from his mistakes, the even wiser learn from other´s mistakes. Sharing experiences is the foundation for faster learning")

[David]

Stilldouble, the experimenting is a lot of what performance boating is all about.

I change engine height and props all the time, as do a lot of others on the board. There is often more speed in set up than in adding HP. You can get directional guidance on this board, but the truth is in the actual testing.

A power jackplate was essential with the Virage. I needed the prop low for lift below 60 mph, and high for stability above 60. Over 60, the tunnels started to provide lift. My Allison could easily be run with the engine high all the time, at least solo. With people on board I get better bowlift with the prop lower.

Where in Sweden are you? I used to go to Sweden when I worked for a prior employer. We were near Gothenburg.

[Stilldouble]

Hi David,

If I sum up the conclusion so far for my ride, from this thread and others. To most likely setup for success for my actual ride is as follows, at least has the best potential to start with (All this discussion has been very important to me due to a major rebuild will take place during this winther):

1) To add 10-12" set back of the engine, to accomodate for the basic shape of the hull (deep V) to get most of the lift needed. Today the engine is mounted directly to the stern.
2) Previously I had not understood the importance of changing engine height during the "flight". I saw this as a matter of adjustment limited to the needs occuring when changing prop. My initial idea was therefore to use a manual jack plate. Now I understand more of the dynamics around this, and will naturally choose a hydraulic one.
3) The prop I will try first will be a Tempest Plus, 26p. The change from the current five blade SS 21p will probably be dramatic. The holeshot will definitely change, that is for sure, demanding more action from the trim tabs. My stern is also quite narrow.
4) Initial engine height will be 2" below the pad, quite low to start with. We will see what will happen to lift and stability when the speed is getting higher. Here I definitively see the need for a hydraulic jack plate. If the líft is sufficient, then there (most likely) will not be any problem with stability when the speed picks up and the engine is being adjusted upwards gradually. Thanks to the three quite agressive rails on each side and the added set back, my guess the lift will be enough and there will be no problem to raise the engine gradually, from the initial 2" below the pad.
5) In order to increase safety, all trim controls will be pedal operated, and also the throttle will be pedal operated.
6) The shape of the trim tabs will also be modified to become more narrow and longer, maybe even with fins at the far back of them. If trying with fins, they will mounted in a such a way they are easy removable for modification, or not using them at all it it does not work out at all.

This will be my initial setup to start experimenting with. From that I will start to try out more and more to see how the boat reacts to different changes, to maximize performance and handling. Just as David suggests.

Any comments from someone regarding this approach for how to move forward?

David, since the last two years, I live in Malmö. A lot of open sea between Malmö and Copenhagen (Denmark)... Öresund. For those persons interested in performance boating around here, "Copenhagen Race Boat Club" (I do not know the exact name of the club) has some really potential triple digit rides, bare in mind we are not talking flat out lake rides, but quite rigid big boats with massive amount of powers, very well set up rides.

Best regards,
/Stilldouble

[Techno]

Something that caught my eye when I first noticed this post is the use and weight of the boat. I assume this is bare hull? because my boat rigged should weigh about 1250lbs. Or did you make a mistake and meant 1100 kilos about 2600 lbs?
This makes a big difference in how much jackplate and what its going to do.
I don't know where my boat balances but I do know the tongue weight of the trailer was 30 lbs. Now with the motor off, and it was on a 6" jackplate, it now is about 250 lbs. Move 400 lbs of engine back a few inches and it really changes where the CG of the boat is.

I stopped lifting the tongue by hand after I weighed it.:D

[Stilldouble]

Hi Techno,

You are perfectly right. 1100 lbs is the weight of just the boat, without the engine. The light weight comes from a light Kevlar composite hull and an absolut minimum of interior.

I will do some basic calculations of momentum on my boat. I your case with the trailer, there is huge impact from where your CG is positioned in comparison to where the axle of the trailer is located. But the fact remain in your conclusion; The engines position has a very strong impact on the overall CG of both boat and engine.

Best regards,

/Stilldouble