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Old 03 March 2008, 11:48   #1
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Max Engine Offset?

Installed my 280 kg F250 Yamaha with a starboard offset of 1.5" as per osprey's guidlines. With just me in the boat (starboard seat), side to side trim was OK - may be a slight lean to port. When my mate of similar weight sat in the port seat, the lean to port at speed was more noticable. I have now installed the FT9.9 auxillary (45kg) on the port side (where there is more room of course) and now the lean to port is worse. The closest I can put the engines together is 20.5" centre to centre

Calculating moments 280 kg offset by 1.5" is 105 Nm. 45 kg offset by 19" is -108 Nm. I have therefore totally reversed the effect of offsetting the main engine!

The options I see are:

1. Shift the whole lot to startboard. I think I can move the main engine by 2" easily. This would give me a net moment of 31 Nm. Less than I originally had with the main engine alone but better than what I have. This is the easiest mod to make and leaves me with the most room for trim tabs. Only four holes to fill. Is 3.5" too much engine offset?

2. Swap the aux over to the other side. This would give me 351 Nm or more than three time what I had with the main engine alone.

3. Centre the main engine and swap the aux over to the starboard side. This would give me 230 Nm or about twice what I had before the auxilliary.

Going to do some test work with weights as soon as I can but I was interested in whether anyone else had ever tried to calculate what righting moment might be required.

Last two mods would make the auxilliary clash with the dump trunk and I would probably need to shift it to the port side. Bit reluctant to drill more holes. Always thought that the dump trunks were a pretty simple but basic solution to the problem. I would have thought a big bore valve mounted on the inside of the transom would be better - anyone tried anything line this?

Richard
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Old 03 March 2008, 14:05   #2
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Were you observations with mate on board at rest or whilst under way? the torque from the prop may be making it worse if underway. I was under the impression the offset was 1" for every 100 hp, Personally I would mount aux engine somewhere else rather than drill more holes in the transom though, or alternatively ask mate to reduce his BMI.

The one experiment I did try was centering a 150 hp Mariner on a Ribtec 6.5m which Ribtec set up with 2" of offset. Putting the engine on the centre line made not a jot of difference to the handling that could be detected. It did mean the boat floated level and drained properly when it rained though, before it always leant to port.

Pete
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Old 03 March 2008, 14:09   #3
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Fit trim tabs!!!
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Old 03 March 2008, 15:21   #4
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I didn't think it was anything to do with righting moments per se, but more to do with prop torque. Obviously I know they're linked to some extent, but it'd be nice to know what the formula is. Seeing as I'll be mounting a 200 suzuki fairly soon!

I suppose the question is.... what is the moment applied by a prop at wot given a certain amount of slip I suppose and it's diameter.
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Old 03 March 2008, 15:41   #5
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Quote:
Originally Posted by Richard Selman View Post
I would have thought a big bore valve mounted on the inside of the transom would be better - anyone tried anything line this?

Richard
Yes. I had two trunks and removed them. I replaced with two, screw expanding rubber plugs inserted from the inside. Each one has a lanyard so they do not get lost. When you want to plug up , just insert and put a couple of turns on the wing nut and your done. Usually sold at a plumbing shop as winterising plugs. Cost about 7 bucks each.
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Old 03 March 2008, 16:01   #6
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Quote:
Originally Posted by Richard Selman View Post
Installed my 280 kg F250 Yamaha with a starboard offset of 1.5" as per osprey's guidlines. With just me in the boat (starboard seat), side to side trim was OK - may be a slight lean to port. When my mate of similar weight sat in the port seat, the lean to port at speed was more noticable. I have now installed the FT9.9 auxillary (45kg) on the port side (where there is more room of course) and now the lean to port is worse. The closest I can put the engines together is 20.5" centre to centre

Calculating moments 280 kg offset by 1.5" is 105 Nm. 45 kg offset by 19" is -108 Nm. I have therefore totally reversed the effect of offsetting the main engine!

The options I see are:

1. Shift the whole lot to startboard. I think I can move the main engine by 2" easily. This would give me a net moment of 31 Nm. Less than I originally had with the main engine alone but better than what I have. This is the easiest mod to make and leaves me with the most room for trim tabs. Only four holes to fill. Is 3.5" too much engine offset?

2. Swap the aux over to the other side. This would give me 351 Nm or more than three time what I had with the main engine alone.

3. Centre the main engine and swap the aux over to the starboard side. This would give me 230 Nm or about twice what I had before the auxilliary.

Going to do some test work with weights as soon as I can but I was interested in whether anyone else had ever tried to calculate what righting moment might be required.

Last two mods would make the auxilliary clash with the dump trunk and I would probably need to shift it to the port side. Bit reluctant to drill more holes. Always thought that the dump trunks were a pretty simple but basic solution to the problem. I would have thought a big bore valve mounted on the inside of the transom would be better - anyone tried anything line this?

Richard
I am in the throws of the same question. My offset is almost 2 inches on an 8.5 Hull, but because my helm is to port, dual batteries and AUX motor head is on port also, I still get a cant to port when under way at speed. I can trim this off but the hull position is not correct and fails to use the bows' ability to slice water at this trim, so the aux is getting reversed, and the motor has been moved a further inch to starboard, but I havent tested the new config as yet.

Interstingly, I tried a 4 blade prop as advised by Barrus, but its had no effect, other than give better grip in corners etc, and paddle wheel effect has not been reduced, despite their advice it would be.. although in some installations I dont doubt it would.. but it didnt in mine. I'm intersted to read your calculations

See if I can find a photo
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Old 03 March 2008, 16:46   #7
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Codprawn's solution is probably the best/easiest option. You may be able to move heavy items (Battery etc) to starboard in order to offset the port list. I located my dual-width console and starboard seats 2in to the right of the boat centreline which resulted in perfect trim. All based on experience and a little bit of guesswork, please don't confuse me with righting moment measurements!!! Finally, 1.75 in engine offset should be about right.
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Old 03 March 2008, 17:13   #8
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When i first started building boats, had a technical sheet from fletcher boats.
Depending on the hull/engine, there was upto 3" offset.
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Old 04 March 2008, 11:58   #9
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All healing observations are underway. Boat sits quite level when at rest or at displacement speed.

I will fit trim tabs but I just wanted to correct the fundamental problems as best I could before addressing any residual with tabs- did not want to be driving around with the port tab hard down all the time and the starboard one doing nothing.

Also when the boat leaves the water it falls off to port making the landing harder than it should be - don't think the trim tabs will fix that! (unless they are really big!).

Richard
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Old 04 March 2008, 12:10   #10
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Also when the boat leaves the water it falls off to port making the landing harder than it should be - don't think the trim tabs will fix that! (unless they are really big!).
Actually, I think they should. The roll while airborne is not due to unbalanced lateral weight, but rather due to forces acting on the hull at the time it leaves the water (once in freefall, everything will fall at the same rate.) If you've countered the roll at the time the boat leaves the water, it should travel pretty flat until it finds water again.


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