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Setback theory

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Old 10-03-2010, 07:05 AM
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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 the bracket. Plus Latham 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
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Old 10-03-2010, 08:16 AM
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Calculating the numbers is good, but in the end you will run it too see how much effect the second gearcase and prop had on overall tail lift or bow lift.

Just leave some room for adjustment and let us know how it runs.
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Old 10-03-2010, 08:57 AM
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Hi!

Thanks 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 setup. What strikes me is that the engine weights impact. The setback does not have that effect I 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.

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

Last edited by Toffen; 10-03-2010 at 09:04 AM.
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Old 10-03-2010, 02:58 PM
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Check this website!

http://www.fastcompanymarine.com/fas...ny_product.php

This is my brother in laws company and he designed and developed the setback himself. Have been in the test boat and you couldnt imagine the difference between having the plate at 0 inches to 14 inches.
I think it gained something like 3mph.
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Old 10-04-2010, 09:43 AM
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changing the motor will also change the Trim, and in turn change the VGC. it will also alter your CG and with make the hull more tender.

Stability calculations arent as easy as a worksheet. expecially on a small vessel, small weights matter. When doing stability calculations on ships, we have a book specifically for that boat that the builder and marine surveyer put togething with all the needed inputs and ratios for calculating triim,
tipping center, TPI, KG, CG, LGC, VGC, TCG, Displacement, and some others.

do you know your original LCG, and VCG?
Exact displacement?
What KG are you looking to achieve?

i think you are looking to hard into the numbers, just throw the v-rod on it with minimal setback with the jackplate installed. and work from there. i
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Old 10-04-2010, 12:48 PM
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Like Tuffboat mentioned,the single gearcase vs the 2 gearcases you had before cannot be overlooked as its probably more important than the CG and all other things you mentioned .. also the centerline propshaft is very important to have enough bowlift ,put the gearcase too high and the boat will run flat with a large roostertail,and probably more rpm and less speed .. too deep and the lower unit causes tremendous drag wich slows you down,what I found on Phantom hulls like yours is the centerline lower unit 2 inch under the pad/bottem works good and gives good bow lift when using a 4 blade prop with lots of cupping .
See if you can someone made a video while you running the boat at topspeed to see the hull angle and roostertail ,and engine angle .
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Old 10-04-2010, 01:44 PM
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Default Setback theory

Hi!

Thanks for all input

When running with twins the proshaft height was approx 1,2 inches below with the engines mounted directly on to the transom. When running WOT, the set up required positive trim, but the rooster tail was almost non existent. I used 15 inch mids, so the propheight could have been 0,3 inch lower in a perfect world. One hole further up (0,75 inch) was too high. Lost bowlift and speed.

When I ran the boat with a single Promax 300 between 1998-2006 I used a CMC SS jack and a spacer, giving a total of 7,5 inch total setback. The boat was very sensitive to propchoice and engine height. Optimum propheight was as you said, approx. 2 inch - 2,5 inch below the bottom. Best prop was a old style Tempest A45 25 pitch prop. The boat ran 76-77 mph at 6,150 rpms and 1:1,75 gears.

With the twin set up the top speed has been 86 mph spinning a 4 bl Hydromotive Quad X-O/B 25 pitch at 7,400-7,500 rpms and 1:1,87 gears. I did 87 mph spinning a labbed Mirage Plus 25 at 7,500 rpms.

So the slip factor is good on both set ups.

As you say the Phantom runs very flat. Classic V hull without steps and no notch.

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, ToffenG


Quote:
Originally Posted by stirling View Post
Like Tuffboat mentioned,the single gearcase vs the 2 gearcases you had before cannot be overlooked as its probably more important than the CG and all other things you mentioned .. also the centerline propshaft is very important to have enough bowlift ,put the gearcase too high and the boat will run flat with a large roostertail,and probably more rpm and less speed .. too deep and the lower unit causes tremendous drag wich slows you down,what I found on Phantom hulls like yours is the centerline lower unit 2 inch under the pad/bottem works good and gives good bow lift when using a 4 blade prop with lots of cupping .
See if you can someone made a video while you running the boat at topspeed to see the hull angle and roostertail ,and engine angle .

Last edited by Toffen; 10-04-2010 at 02:23 PM.
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Old 10-04-2010, 08:35 PM
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I did not know about the Pro max, you have a baseline with a single so your prop rotation on the twin is irrelevent. Question is do you think you would like more bow lift than what you had or not.
Also, could I suggest a Bravo 1prop for great bowlift and out of the hole performance over the Tempest (faster too).
Since your only talking about a couple of inches either way, just get it in the water and send pics ASAP.
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Old 10-05-2010, 06:28 AM
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Hi!

It would be great to have more bowlift than running my Promax 300. So I think 350 sci will be fine with 4 inch jackplate, which is the smallest you can get handling these big outboards. If I need more, I can add 2 inch spacers.

I also see that most likely the boat will not sit too deep in the water as with the twin 2.5 EFI SS engines using the 350 sci. Thats good, because I do not want the water to run into the splashwell (as it does with the twin set up) when still.

I will post some pictures of the rigging process. It will be started up in a couple of months, as the summer period definetly is over here in Norway. Its just me not letting it go yet

Cheeers and thanks! Toffen
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