Hi guys, Is there a relationship between the height of the outboard on the transom and the trim height when running? I get 4 bars on the trim and have my outboard height at one hole left from max engine height. Question: If I was to raise the motor to the top transom height would that reflect on the running trim height, maybe 3 bars on the trim and still low enough not to create ventilation. Trying to get a grip on raising the motor one more hole without it being an experiment. Hope this question makes sense to some one in the Grady group. Thanks, trapper
engine height
- Thread starter trapper
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I'm pretty sure the trim sensor is mounted on the engine and has no relationship to the hole you use to mount the engine. Find the sensor and I think you find your answer.
Thanks BigI Yes I know where the trim is but the relationship I am try to determine is this. My anti-cavitation plate is approx. 1 1/2 inches above the keel on my 208 Grady. I do understand the higher the outboard is on the transom the less trim will be possible due to earlier ventilation in raising it. Just trying to figure out that with four bars up on trim, raising the outboard to the last whole would understandably be less trim but would it also less drag at cruising speed? I may be splitting hairs at this point, also it may not be a given until one raises the motor to find out. To raise it one more (last) hole may also not make a big difference in the overall performance. My original question stemmed from packing the 100 lb. kicker on the transom increasing the weight on the stern and therefore running a little deeper. My anti cavitation plate does not ride on the surface at this point, although it can be made out through the turbulence. Apologies if I am flogging a reluctant horse, but just could not help it. Cheers, trapper :hmm
The trim angle is measured by a sensor with a lever that rubs against a cam on the shaft that the powerhead tilts on. Since the cam can be adjusted in many positions, there is no real absolute meaning to the bars. Usually you set the cam to read zero bars when the motor is completely tilted down even though that may be past vertical. . Bottom line is that the bars are all relative. You need to learn to 'feel' the trim. With respect o mounting height, the trim sensor has no relationship at all. The Grady rigging manual will give recommended mounting heights and motor spacing for twins if so equipped but there are factors that might make other settings better suited for your setup.
As a start, when on plane at your typical and realistic cruising speed and with your typical load, you should be able to trim the motor for maximum speed as measured by your GPS without the prop experiencing cavitation and with the cavitation plate a bit below the water line. If you adjust the trim angle and watch the speed, as you near optimal trim, the speed will increase while the revs stay the same. At some point as you increase the trim angle, the speed will slow decrease. That should happen before the prop cavitates.
As a start, when on plane at your typical and realistic cruising speed and with your typical load, you should be able to trim the motor for maximum speed as measured by your GPS without the prop experiencing cavitation and with the cavitation plate a bit below the water line. If you adjust the trim angle and watch the speed, as you near optimal trim, the speed will increase while the revs stay the same. At some point as you increase the trim angle, the speed will slow decrease. That should happen before the prop cavitates.
Thank you seasick, That is how I arrived at the fourth bar on the trim, by doing exactly as you have suggested. The question still is, Is there any way of knowing, without raising the motor one more hole, what the results would be? Yes whether the trim bar on the gauge is 2,3 or 4 on ones boat, you arrive at what your optimum speed is in relation to rpm by doing as you mentioned. My thought, once again being... If the motor was raised on more hole, it would change the previous trim setting as you would start cavitation earlier on the trim bars( higher motor).So motor height would have a bearing on the height of the trim angle. But there is no way of telling the results of raising the motor without doing it.All boat manufacturers tend to go with the same theory " anti ventilation plate level with the keel. I believe that changes with the strapping on of a 100 lb. kicker etc. On that note I will give this a rest and thank you for all your input on the subject. trapper
trapper said:
The question is complex to say the least. The method of finding the optimal trim angle ( as it relates to speed) is true for calm sea conditions and straight line boating. The best trim for other sea conditions varies depending on the seas. Trim tabs play a role too since they will shift the effective motor height by lifting the hull or pushing it into the seas
If there were one simple law it would be that the less of the hull that is in the water, the faster the boat will go for a given thrust. It's a simple drag situation. So in theory if the entire hull were out of the water but the prop for the most part was in the water, speed would be maximized but stability would be a problem
Many performance speed boats ride on very little hull surface and can do so because they have the thrust needed to lift the hull.
Now if more of the outdrive (LU) is under water, there is more drag. If too much is out of the water, thrust will be lower and turbulence higher. Exactly where the sweet spot is not easy to calculate.
Clean basically undisturbed water flow into the prop is a factor and as the motor get mounted higher, the keel and hull can create turbulence in the water flow and affect performance.
My suggestion is to refer to the manufacturers rigging guide as a decent starting ( and maybe final) setup. The boat manufacturers know a lot more about their hull characteristics and rigging than I do
There is a question that I am note sure of the answer. Perhaps someone knows. Folks often talk about mounting a motor in a different hole or what hole is your motor mounted in?
I wonder if the holes drilled in the transom are always at the same height.
If you buy a boat and the dealer is going to power and rig are there holes already there from the factory. I would suspect not since the factory can't be certain what motor will be mounted. In that case, do the dealers use a template or the measurements in the rigging guide or do they just line up the motor , mark and drill?
I wonder if the holes drilled in the transom are always at the same height.
If you buy a boat and the dealer is going to power and rig are there holes already there from the factory. I would suspect not since the factory can't be certain what motor will be mounted. In that case, do the dealers use a template or the measurements in the rigging guide or do they just line up the motor , mark and drill?