trapper said:
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
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