Question On Trim

Hi All

I'm doing some planing calcs for a little aluminium dinghy, approximately 1.7m long (5.57 feet if my imperial isn't wrong). I was just wondering what a typical trim value for a boat this size may be whilst planing? 5 degrees? 8 degrees? more?

Cheers

 

From what tribe is the pygmy coming, that will drive the boat ? Seriously, 5.57 feet is not a fit with the size of human beings, if I was to move to the stern, the trim angle would probably be 58 degrees !

 

haha possibly, I could make it larger, nay should make it larger. Ok I'll alter the question. How bout for a typical 12 - 13 footer

 

Something around 5 degrees would be pretty normal, imo. It depends on various factors, but at a nice clean plane is about right.

 

awesome, along the line of what I was thinking. Cheers buddy

 

Depends on the bottom shape, speed, load etc. In a small boat, the placement of non-fixed weights ( also known as passengers) can alter things considerably, but the range of trim in a planing boat is considerably influenced by the fact that planing lift is greatest at the leading edge. You move weight forward, the leading edge moves forward too, moderating the influence of weight shifts underway.

 

Yeah, this is a hugely difficult question to answer without an idea of many determining factors. The CG, CB, general hull form employed, weight distribution, damn and whole crap load of things, need to be known.

5 degrees is the start of the pretty steep range and you'll likely want tabs, if you're that much bow up. 3 degrees is a better target, but how will you measure this and what "planing calcs" are you performing?

 

Five degrees is a bit steep alright, but with a little twelve footer and the typical weight distribution, especially one-up, it would be hard to get it much lower, probably. You certainly see some crazy angles before they plane.

 

Optimum trim angle is usually considered to be that which creates minimal combined drag from wave making and surface friction. Most planing hulls, that is hard chine with straight aft buttocks, will develop this minimum drag at 4 1/2 to 5 degrees and deep V hulls will have a higher optimum trim angle than flatter bottom ones. There are many reasons to choose a lesser trim angle.

Weight and longitudinal balance plus adequate power to get on plane determines whether your 12 footer can plane, provided the hull shape is a good one. There is no theoretical reason why your 5.57' boat will not plane at a trim angle of 5 degrees but it is not so easy to do.

 

For a boat of 12 feet or less, trim can be easily adjusted by sitting forward or aft. I often use a tiller extension for my dinghy when I'm alone. Otherwise, my dog in the bow does fine.