Setback theory

**Toffen** · 10-03-2010, 06:01 AM

Hi!

Just a quick question. As we all know a jackplate gives you less drag and cleaner water, and a more optimum LCG (longitudinal Center of Gravity).

I read back in time in BassAndWalleyBoatMag that if you added 1 inch setback it corresponded to 1/12 of the weight of the engine. So if you have a 300xs of 229 kilo and 7,5 inch setback, it corresponds roughly having an engine of 372 kilo (229/12*7,5 + 229) directly on the transom plus the weight of the jackplate itself (CMC SS 30 kg). I am unsure about this theory.

The reason for asking is that I am to mount a Verado 350 sci on my Phantom 25, and I have to figure out the correct setback. I plan to use a HD Bobs of 4 inches. The Verado weight in 300 kg / 12*4 + 300 is roughly 400 kg plus the weight of the jackplate (16 kg).

My two Merc 2,5 EFI SS are mounted directly, but have 2 inch setback built in. Plus steering, extra battery, pumps, etc etc. Thats is 350 kg / 12*2 + 350 is roughly 410 kilo plus all the extras (approx. 50 kilos)...

Thanks!

Cheeers, ToffenG

**Toffen** · 10-03-2010, 08:09 AM

Hi!

I received a calc sheme from my brother in law. He is an engineer. I will try to attach a version of it.

In the calc scheme you can change weight of the boat, LCG without engines, engine weight, setback, etc - and you will get the LCG with that particular engine setup. What strikes me is that the engine weights impact. The setback does not have that effect I first believed. For instance, if you have an engine of 300 kg mounted directly on the transom, this corresponds to mounting an engine of 265 kg on a 8 inches setback bracket (on my 25 feet narrow hull).

So If I mount a 350 sci on a 4 inch setback bracket, the LCG will be 1,34 meters from the transom. Mounting a 300xs on a 7,5 inch setback bracket will give an LCG of 1,44 meters from the transom. With twin 2.5 EFI's and two inch built in setback, the LCG is 1,27 meters from the transom. I have to figure out how deep the boat will sit with the 350 sci. The twin set up sits 3 inches lower in the water compared to the PM 300 I had. I think it will be somwhere between, approx. 2 inches lower than the PM 300 ..?

Speak soon.

Cheeers, Toffen
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**roosterds650** · 10-03-2010, 08:26 PM

My head just exploded.

**Dave Strong** · 10-03-2010, 11:16 PM

The only way to know for sure is bolt the new set-up on and see what happens.

May need adjustments to get set-up just right.
That kind of math is way over my head.

Dave

**Toffen** · 10-04-2010, 02:36 AM

Hi!

I understand. Not my intention to do that! My dillemma has been that my boat is very narrow, only 1,5 meter / less than 5 feet. Despite the fact the boat is 25 feet long. So its very sensitive to weight aft. Not at speed, but when it is still in the water. So the question has been if the 350 sci is too heavy in the first place, especially taken into consideration that the COG is 2-3 inches more aft than a 300 xs. But I have decided to go for the 350 sci, so now its more to find the correct set up

Cheeers, Toffen

**transomstand** · 10-04-2010, 05:59 AM

Originally Posted by Toffen

But I have decided to go for the 350 sci, so now its more to find the correct set up

That's a very "American" attitude

Toss the calculations, "Let's hang that big boy on there, and see what happens"

**JR IN JAX** · 10-06-2010, 01:42 PM

It would seem like a narrow long hull would have a long CG ARM and it would be more amplified by the decreased weight per linear foot as you get towards the transom, due to the narrow hull. If you could use a scale and a sliding leverage point, you could figure out the CG moments for each foot of the hull as measured from the CG [forward]. The result of these measurements would only be useful while the hull is at-rest due to the influence of aero and Hydrodynamics on the hull at speeds.
Clear as mud?

**WATERWINGS** · 10-06-2010, 02:11 PM

Originally Posted by roosterds650

My head just exploded.

**specboatops** · 10-06-2010, 02:52 PM

Originally Posted by roosterds650

My head just exploded.

Mine too hahaha

Chris

**bigboy** · 10-06-2010, 04:48 PM

Toffen, sounds great!

I would like to see this calc scheme...

What parameters do you have to fill in?

Thx

**Toffen** · 10-06-2010, 05:03 PM

Hi!

Its an excel scheme, so pls send me an pm with your email, and I will send it to you.

Cheeers, Toffen

**John.Vaught** · 10-07-2010, 03:01 AM

Don't forget that all big Verado motors have a little setback built in.

Why don't you just hang it on the transom first get some testing in and then put your jackplate on and test again and see if things change for the better. I know one thing if it looks like it's going to sink with a motor on the transom just wait untill you put a 15" rapid jack on it.

I wish I had a pic of an old boat we were playing around with so you could see what I am talking about. It looked like it was flying on the pad just sitting in the water with no one aboard and if I wanted to pull the cowling off it I had to find a shallow spot because if I stood in the back of the boat water started coming in.

**Toffen** · 10-07-2010, 04:27 AM

Hi!

Yes I know

The 350 sci has 2 inch more aft center of gravity than a 300xs. Approx 2 inches. However, the cowling is only 4 inches higher than my old Promax 300. It looks bigger... The reason for me looking at the LCG is the fact that the boats narrow transom and limited freeboard. However, I have found out that with a 4 inch setback I should be fine. Per date I have twin 2 x 2,5 EFI SS engines, and they weight much more incl. steering, batteries, tie bar, catchtanks, etc etc

Cheeers, Toffen