Make sure that you get your tongue weight correct. If you don't the trailer will pull like crap or even be dangerous. I think Boatmate puts 7% of total package weight (Boat and Trailer Combined) on the tongue. You can get it close by centering the tandems on the old axle centerline but, I'm guessing that you'll have to push the axles back a few inches to load the tongue correctly. I recommend tying the two axles together as a unit at the desired spacing with a piece of square tube "heavy tacked welded" to both of them. Put the square tube directly under the current frame rails like a spacer between the axles and frame. Align the centerline of the tandem unit to the old centerline clamp it to the frame and measure the tongue weight. If it is too light or too heavy, un-clamp, adjust, re-clamp, and measure.
Make sure that the "tandem unit" is square and centered side-to side and make sure that every time you clamp the unit to the frame you make sure the measurement from the front of the tandem unit at the port frame rail to the front of the tongue equals the the same measurement on the starboard side. Doing this makes sure that your trailer tracks straight behind you. Once you get it right, mark your new centerlines and then determine how you want to affix them.
Good Luck! if you have any design questions, I'll be happy to help.
Are these torsion axles the short ones that bolt on a frame or full length axles similar to what you would use with a conventional leaf spring set up?
Full width torsion axles.
If it were me I would build an angle iron frame to mount the axles to and bolt them to the frame of the trailer to make them adjustable in case you miss the placement of the axles. If you are interested in doing this I can walk you through it. Today I removed my dads boat from the water for the season, his lift is giving him grief, and after towing his 19ft deck boat I remembered how crappy his trailer pulls. It needs more tongue weight, my dad says to me "so that's not normal?" At every bounce it was clunking on the ball.
Well since I'm away. My father-n-law is about to start the project of switching over my trailer to a tandem axle. I think I have all the measurements that I will need to be able to get this done. He is calling boatmate trailers today to get the fenders ordered. Figured this would be a good time to take care of this project before putting the boat away for the winter. He is suppose to send me pictures as the project goes along. I'll put them up on here so you guys can check it out. I'll definately be happy to have this done. Just last week the trailer blew a tire and left my wife and son on the side of the road for awhile. Atleast with tandem axles she could have gotten to a better spot to be able to get the tire changed. I do appreciate all the help that I have gotten from the guys on here. Especially Al.
Definitely keep us updated. Thanks.
Got the axle assembly under the trailer and bolted in place. So if you remove 10 bolts and disconnect the brake line the entire assembly can be removed from the trailer. Currently says that the trailer sets nose heavy without the boat on it but he can move it by himself with one hand so it seems to be balanced. Still have to bleed the brakes and then go and pick the boat up for fender placement. Not sure how much the boat will flex the axles. The fenders were ordered from Boatmateparts.com and should arrive today. Pretty excited. Can't wait to see the end results. Hate that I was gone during the entire deal tho. I have been waiting for awhile for this project and can't wait for it to be done.
Got the boat back on the trailer. Says it towed great. Needed the boat back on the trailer so could install the fenders. Have sense ran into some problems but hopefully can figure out how to fix the issues and continue on.