trimming motors

trevz

Senior Member
hi guys

what is the best way to trim motors on a cat when out at sea. i heard that this effects the boats handling tremendously.

the advise i got was to trim the motors out until it cavitates and then drop it a little as the cats handle best with the bow up, is this true.

Also what is the best way to trim the motors when riding into the swell and also with a following swell.

thanks guys
 

Fin-S

Sealiner
Quite an in depth subject and much depends on the boat, the motors, mounting height, sea state, are you running for comfort, speed, safety, launching etc. I would recommend asking a competent examiner to accompany you on your boat and take her through a series of trials at different settings and explain the results to you. Much of this should have been covered in your skippers course? The advise you have been given is quite common but generally wrong as it negates hull dynamics and hydro dynamics and assumes a flat sea. I would generally start from the opposite end - motors trimmed fully in. Set your cruising rpm and measure your speed. Start adjusting the trims and watch the effect on your speed. Once you have found the maximum speed at a given rpm (probably about mid position on the trims but dependant on all of the above factors), you can then experiment with different rpms. You ideally should have at least 4 positions for the trims memorised, lowest planning speed, most economical cruising speed, flat out position, and then the difficult one - the most comfortable, which will tend to be somewhere in the middle and is very dependant on sea state. You can then add launching and beaching positions, head and following sea positions and also lateral trim positions where you will want to lift the windward side slightly. In saying that however, a good rig should require minimal adjustment except in extreme circumstances. The main thing to remember with a following sea is to run at least slightly faster than the sea and trim the bow up so that you do not dig a sponson onto the trough. Do not trim too far though or you will create more drag at the stern than you are trying to lift.
 

orange

Senior Member
I am very green when it comes to this, but all I can really add is that it is, as Fin-S says, very boat dependant. I have found that it even goes as far as what size props you have. With my boat the biggest suprise was when I changed from 15 inch to 17 inch props as this cocked up everything needless to say I went back to 15's. Try to find someone with the same setup and speak to them.
 
Top