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Old 12-05-2014, 06:06 AM   #1
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Engine offset

Single engine outboard. Mount in middle or offset?
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Old 12-05-2014, 08:14 AM   #2
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Old 12-05-2014, 09:27 AM   #3
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Engine to middle, battery and oil tank to starboard
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Old 12-05-2014, 07:37 PM   #4
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there are different opinions out there...
I'd prefer it in the middle. If you would give it any offset the next question would be how much. Starting symmetrical and then shifting the weights to counteract the wheel effect of your prop seems like the way to go If you have a (normal) righthand turning outboard that means shifting the weight to starboard side...
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Old 12-05-2014, 08:47 PM   #5
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Well, im starting a fresh with new transom. Currently engine (250ho) is offset couple of inches. Manufacturer puts them all in the middle, but this boats a bit more old school. I am trying to keep her mnimal, so battery, and oil tank, steering helm, fuse box will be my only real ballast
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Old 12-05-2014, 09:24 PM   #6
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You can't get any more minimalist than mine and OCRDA's Bernicos, and I've had no tendency to roll to port with any engine, height or prop combo I've tried so far. More by luck than judgement though!
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Old 13-05-2014, 09:23 AM   #7
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Old 13-05-2014, 08:32 PM   #8
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Engine to middle, battery and oil tank to starboard
Any particular reason for the battery and tank to starboard?
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Old 13-05-2014, 09:27 PM   #9
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I've offset all of mine by 1 inch... But then I'm not a hardcore speed nut case...
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Old 13-05-2014, 09:36 PM   #10
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Interesting topic.

The easy answer to whether to offset or not is a plain yes or no.

More interesting is why?

Does the hull design, engine height, propeller type, rig setup, conditions, desired handling characteristics, targeted top speed, etc. play any role or change any of the assumptions behind the scene?
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Old 13-05-2014, 10:11 PM   #11
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This is a topic that really interests me, and I've thought about it an awful lot.

The reason a boat would have a natural tendency to roll to port is because of the propeller's rotation. Every action has an equal and opposite reaction, so the propeller spinning clockwise is going to want to spin the boat anti-clockwise. Of course with the boat having a much greater mass than the propeller the propeller wins, and the overall effect is pretty small, but still enough to cause a problem sometimes.

By mounting the motor off centre, all you are effectively doing is shifting weight. The weight of an outboard is quite a large percentage of your overall weight, so moving it slightly can change the balance of the boat quite a lot. If you now imagine a boat with the motor shifted quite a bit to the starboard side, that uses a propeller that has a tendency to provide lift, you're lifting the starboard side of the boat more than the port side, causing more roll to port, counteracting what you're trying to achieve by shifting the motor in the first place!

Another thing to consider is that the tendency to roll to port is exaggerated when trimmed in. Once you're trimmed level, or out, the roll reduces a lot. On flighty boats that get a bit wild when trimmed out, yet roll to port when trimmed in, adding some weight in the bow would help. You can then use more trim, without getting flighty, and with more control which should mean more speed overall...

The above is just my interpretation, based on a lot of thinking and some experimentation with my own boats.

Pretty much every high performance boat built and rigged today, be it a drag racer, offshore boat, ski racer boat, are mounted central. It's far easier to add a few kg of ballast than it is to guess how far over to mount an engine.
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Old 13-05-2014, 11:33 PM   #12
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Moving off centre a little bit also changes the leaver point. Imho its not just the fact that it moves the weight, but more that it moves the leaver point making it harder for the boat to roll, as its not applying force on-axis. Take it to the extreme, if you were to mount it on the side of the boat, it couldn't possibly make the boat roll at all, it would just try to lift or lower the boat instead..

I have to say however I've been playing with smart tabs, and those have a MUCH MUCH greater effect than off setting the engine.
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Old 14-05-2014, 05:26 AM   #13
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Quote:
Originally Posted by Paul E View Post
This is a topic that really interests me, and I've thought about it an awful lot.

The reason a boat would have a natural tendency to roll to port is because of the propeller's rotation. Every action has an equal and opposite reaction, so the propeller spinning clockwise is going to want to spin the boat anti-clockwise. Of course with the boat having a much greater mass than the propeller the propeller wins, and the overall effect is pretty small, but still enough to cause a problem sometimes.

By mounting the motor off centre, all you are effectively doing is shifting weight. The weight of an outboard is quite a large percentage of your overall weight, so moving it slightly can change the balance of the boat quite a lot. If you now imagine a boat with the motor shifted quite a bit to the starboard side, that uses a propeller that has a tendency to provide lift, you're lifting the starboard side of the boat more than the port side, causing more roll to port, counteracting what you're trying to achieve by shifting the motor in the first place!

Another thing to consider is that the tendency to roll to port is exaggerated when trimmed in. Once you're trimmed level, or out, the roll reduces a lot. On flighty boats that get a bit wild when trimmed out, yet roll to port when trimmed in, adding some weight in the bow would help. You can then use more trim, without getting flighty, and with more control which should mean more speed overall...

The above is just my interpretation, based on a lot of thinking and some experimentation with my own boats.

Pretty much every high performance boat built and rigged today, be it a drag racer, offshore boat, ski racer boat, are mounted central. It's far easier to add a few kg of ballast than it is to guess how far over to mount an engine.
+1
My thoughts to.
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Old 15-05-2014, 09:00 AM   #14
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Thanks guys for sharing your thoughts.

I must say I can't speak of any personal experience. It's been 30 years since I had a boat that fast that this question would have made any difference. That's why I'm asking.

To say the least, there seems to be very strong opinions for and against offsetting, depending who you talk to or which forum thread you read through. What’s strange is that there is very little in writing on the subject. I guess it all depends and that, at least for me, is the fun part. I'm sure I'll enjoy most of the summer experimenting with different setups and props.

I found one good article on setting up your rig from several sites. It has lengthy and detailed descriptions on all other setup parameters, but as far as center/offset goes the description is very short and states that centerline is the best place for the engine. Here’s a link to the document from one of the sources.

http://www.killcaremarina.com.au/ind...questions.html

I also found an old thread on the subject on Boatmad from about 8 years ago.

http://www.boatmad.com/forum/f5/offset-outboard-5903.html

Now the question, also evident in the previous thread, still remains why the Marshans, Phantoms, Rings and alike most all were originally setup with engines about 2” to the starboard side. There must have been a good reason for this. The 2 Phantoms I’ve seen locally also have the engines both 2” on the starboard side and both chaps say they drive well over 70mph dead straight. What I’ve gathered so far is that offsetting the engine was done to offset prop torque and the eventual chine walk… Yet many of the racers seem to favor a centered setup. I guess it’s a bit faster with a centered setup.

However, I just discussed the matter with an old ex-racer locally. He probably realized I didn’t have a clue what I was talking about so he explained it to me in layman’s terms. “Try standing straight up and place your right foot directly behind your left foot. Then bring your right foot just beside your left foot. Now which do you feel more comfortable with?” LOL. Go figure… Trade secrets I gues
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Old 15-05-2014, 04:14 PM   #15
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Quote:
Originally Posted by Carrera View Post
Thanks guys for sharing your thoughts.

I must say I can't speak of any personal experience. It's been 30 years since I had a boat that fast that this question would have made any difference. That's why I'm asking.

To say the least, there seems to be very strong opinions for and against offsetting, depending who you talk to or which forum thread you read through. What’s strange is that there is very little in writing on the subject. I guess it all depends and that, at least for me, is the fun part. I'm sure I'll enjoy most of the summer experimenting with different setups and props.

I found one good article on setting up your rig from several sites. It has lengthy and detailed descriptions on all other setup parameters, but as far as center/offset goes the description is very short and states that centerline is the best place for the engine. Here’s a link to the document from one of the sources.

http://www.killcaremarina.com.au/ind...questions.html

I also found an old thread on the subject on Boatmad from about 8 years ago.

http://www.boatmad.com/forum/showthread.php?t=5903

Now the question, also evident in the previous thread, still remains why the Marshans, Phantoms, Rings and alike most all were originally setup with engines about 2” to the starboard side. There must have been a good reason for this. The 2 Phantoms I’ve seen locally also have the engines both 2” on the starboard side and both chaps say they drive well over 70mph dead straight. What I’ve gathered so far is that offsetting the engine was done to offset prop torque and the eventual chine walk… Yet many of the racers seem to favor a centered setup. I guess it’s a bit faster with a centered setup.

However, I just discussed the matter with an old ex-racer locally. He probably realized I didn’t have a clue what I was talking about so he explained it to me in layman’s terms. “Try standing straight up and place your right foot directly behind your left foot. Then bring your right foot just beside your left foot. Now which do you feel more comfortable with?” LOL. Go figure… Trade secrets I gues
So does that mean we should all get twin set ups, and have them side x side? coz standing with a wide stance feels better than standing one foot behind the other.
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Old 15-05-2014, 04:33 PM   #16
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So does that mean we should all get twin set ups, and have them side x side? coz standing with a wide stance feels better than standing one foot behind the other.
That was more or less a joke

I don't know the right answer to the "center/offset dilemma", if there is one. What I'm looking for is just a few facts on the pros and cons for both sides. So far it seems it's more just a matter of opinion rather than based on any facts... I'm sure there are many experienced guys around who could enlighten us on why the initial setup from "factory" was most often to offset the engine by 2". And again there are many racers who today favour mounting the engine dead center. It would be interesting to know some of the underlying facts and what has changed in thinking over the past 20 so years...
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Old 15-05-2014, 04:55 PM   #17
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I know it was a joke.
Engine suppliers also used to recommend that the cavitation plate was level with the keel. Know days people are putting the prop shaft level. Just wonder if this has any bearing on it?
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Old 15-05-2014, 11:11 PM   #18
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So does that mean we should all get twin set ups, and have them side x side? coz standing with a wide stance feels better than standing one foot behind the other.
You might keep up with Twin engines !!
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Old 16-05-2014, 01:16 AM   #19
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You might keep up with Twin engines !!
Or even one that isn't virtually detached from its mid section.
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Old 16-05-2014, 09:54 PM   #20
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You not in for Torquay then Andy ?
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