Aerotec 420 Transom mod, +55 mm

[C-NUMB]

Here finaly some pictures regarding the 55 mm transom mod of my Bombard. As the increase in height is as much as 55 mm and having a 55 kg engine, decided to go for a permanent solution, target to get it as strong as original.

Used marine plywood, epoxy glue, epoxy putty, epoxy primer and epoxy polyurethane finishing paint. Started the project by removing everything that is possible to remove from the transom.

Drilled "oversize" all old holes, to reach fresh plywood. Holes filled by hardwood plugs and epxy glue. Plugs sanded down so that they where covered with epoxy putty when even with transom. The extension part was made by glueing marine plywood to achieve 30 mm thicknes, equal to the transom.

Then by using a milling tool(newer used one before so needed to practise on som leftower wood) made a 8 mm wide and 20 mm deep "gap" in both transom and to the new extension part. Then made a 36 mm deep and 7 mm thick "counter part" to put in to the gap. Glued with epoxy. To further increase strenght, made from 12 mm marine plywood supports on both side of the transom, agin glued with epoxy.

Everything covered with primer and finishing paint. On top of the transom put a aluminum piece to reduce the risk of water going into the transom due to wear of the paint where the engine is attached. As always in my project, heavily underestimated the amount of work envolved, but still quite happy with the outcome. The mod did not add much weight, feels wery strong. Now engine is at about correct height, might be even higher but this is enough for now.

[C-NUMB]

aditional pictures

[Jimbop]

Hi C-Numb

We have had our Aerotec 420 for about 9 years now. I experimented with engine height and finished off with a chock height of 3cm. This means I just get away with leaving the engine plate in the same position,the engine clamps sitting just under the raised edge of the top of the engine plate surround which helps hold the engine in position.

Raising the engine has solved all of the water coming over the transom but I still get a lot of spray behind the boat at full speed with a Tohatsu 30HP (about 30mph). Very impressed with your permanent transom modification, I'm not quite brave/skilled enough to start hacking into my pride and joy! I did fit the flipup/down launching wheels which have been brilliant for launching/recovery.

Having looked at down at the spray when going fast, it seems that as the V of the bottom of the transom is concave you get a tunnel of water each side of the lowest part of the V which hits the spray coming forwards off the engine as it goes through the water causing the spray to come straight up.

Very interested for this reason in your trim tabs as I think it would hold down the water being released from the transom and stop the water collision and spray issue.

Have you found the trim tabs have helped and please can you tell me the make and size? I assume the tabs fold flat to the transom so you can still roll the boat up? I haven't bothered with a trailer as like the convenience of getting the boat and engine in the back of the car or the boat rolled up on the roof rack.

Overall have loved the boat and have had some great years in it, many more to come hopefully! An electric pump made all the difference to getting the floor hard enough which is really needed for it to perform. I have been out in some fairly choppy conditions and the boat has performed very well keeping up planing speeds in all but the worst. The V-shaped hull stops a lot of the slamming other SIBs suffer from. Need someone with a bit of weight (and good at holding on) in the front of the boat though when going into bigger waves!

Only problem I have had is having to reinforce the back edge of the inflatable floor as it was rubbing against the transom and wearing away. I had this professionally done and it would be a good move for anyone looking to buy one.

Happy Boating! Any thoughts on trim tabs appreciated.