weight and boat performance

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atlowers

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We had a new engine fitted and this engine exactly the same weight sits 3 inches further back towards stern. This has had a dramatic effect on boat trim and performance. Boat sits 4" deeper at stern. It is only 23ft. Lifting engine to adjust position of mounts is a big job so my question is, would adding weight in anchor locker compensate or would the extra weight just compound my problem. Now find it more difficult to get on plane and she wants to come off plane much easier.
 
It may only sit 3" aft but it's the distance between the centres of gravity of the old and new engines which count and I suspect that is greater than 3" if it's made so much difference. Are you sure they weigh the same? As sailorman says, try removing unnecessary weight from the stern or at least try moving weight forward. I guess moving the fuel and water tanks are impossible so maybe you can move other heavy items like the batteries forward. Also you could try fitting fixed or adjustable trim tabs to lift the stern which should make planing easier. I would look at these things first before simply adding more weight forward (eg anchor chain) as adding weight overall will make the boat slower and less fuel efficient
 
Mmm... there's a couple of things which just don't stack up.
Firstly, being a 23' planing boat, she has a sterndrive, doesn't she? If so, moving the engine 3" forward is not just a "big job", it's almost impossible. And I can't imagine which different engines of similar weight and fitted on the same leg can affect the boat COG significantly, to start with.
Secondly, the 4" waterline increase is huuuge anyway (if the weight is really the same), even assuming that the engine COG is further aft, as Deleted User suggests. Of course that depends on the hull shape, but by the same token I wonder if a couple of persons sitting on the swim platform wouldn't sink the boat!
In any case, ballasting a planing boat is never a good idea by definition.
 
Yes its a stern drive and I do have trim tabs. I have posted before about the change in leg trim ie I can now only trim leg out very slightly before loss of speed and plane. the whole leg and engine is a new for old amnd I am assured the unit would be slightly lighter if anything. I noticed the drop at the stern from the waterline and had to raise the boot line 4". the engine mounts on the engine are pre positioned but there are additional 3 fitting position options. I can and do use tabs to help with getting on the plane and was considering dropping from a 19" prop to a 17" but dont really want to lose top end. The boat is a falson spc23 by the way with yamaha 370sti hydra drive. The change in trim we noticed straight away and were given potential reasons which clearly were not the case. The whole boat has a heavier feel and with the boat out of the water I have now confirmed no water retention or additional weight (whch would need to be great to have caused the change) and I am confident that the engine is 3" further astern. Guess I will just have to put up with heavier feeling boat and slightly inferior performance but having brand new replacement set up I was hoping for the oposite effect.
 
Moving an engine of the same weight 3 inches would make a very minor difference but not a significant difference, It sounds like the new engine or gearbox is heavier.

From a performance point of view either the power or torque is less or the gear box ratio or prop pitch are wrong?
 
Some more details methinks?

Sounds like the new engine (or drive? is heavier) a small difference in position as has already been stated should not make that much difference.

Either that or different power / torque characteristics.
 
old engine and leg 1994 yamaha me 370 sti with hydradrive leg new engine 2007 yamaha me 370 sti with hydra drive leg. same prop(19"). I am told by yamaha engineer that weight is no different and gear ratio is the same.
He also said that there was a tweek on the engine that could enhance power and which will be done when the boat is back in the water and perhaps previous owner had had the original engine similarly tweeked. I will patent this when I find out exactly what it is. Assume it is fuel pump?
boat sitting lower in water I put down to extra weight immediately but yamaha have insisted this is not the case, I have not added anything to the stern and definately have not taken on water anywhere so engine position seems only reasonable explanation as it is clearly further back.
 
Then the question is, if it's exactly the same engine and drive, why did the engine have to be moved back 3"? Have you checked the position of ancillaries like the alternator and starter motor? Have these changed on the new engine? Just trying to find reasons for the trim change.
Having said this, thats a pretty big lump for a 23ft boat. How much does the boat weigh and how much the engine weigh?
 
[ QUOTE ]
if it's exactly the same engine and drive, why did the engine have to be moved back 3"?

[/ QUOTE ]Precisely. And not just WHY it had to be moved back", but also HOW?!?...
That said, not such a big lump actually: 370 stands neither for cubic inches nor hp, but just for 3.7 liters: 160 or so hps, IIRC.
I've seen 23 footers with a couple of similar engines, and I helmed a single screw 22' with almost three times such power...
 
the engine mounts have alternative positions where they fit to the actual block. The boat position of the mounts is of course the same. The new engine came with the mounts already fitted and quite reasonably the new engine was simply dropped in and lined up accordingly. Had the mounts on the engine been moved to the alternative fixing points available on the engine this would have moved it 3" forward. I assume there is an amount of play for the fitting to the leg but have no idea if this is correct. This slight change in position is the only reasonable explanation for the boat sitting deeper at the stern. I just do not know how to best redress the affect on boats feel as being heavy in the water which it did not have previously. I am wondering if prop reduction is best possibility.
 
The drive and transom mount dictate the position of the engine?

I think what was meant above was that the same engine and drive could therefore not be mounted further back?

I suspect that there must be phsical size and weight differences between the old engine and new engine even if they are called the same? ie. the new engine must be shorter and the engine/drive package perhaps a little heavier?
 
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