changing large inboard/jet drive to twin outboards

I'm involved in a project to modify a 33' aluminum 250hp inboard Detroit Diesel/Hydro Jet driven boat. We would like to remove the inboard and then build a transom to hang twin 300hp outboards on. It will involve building an aluminum transom to attach to the 1/4" plate on the back of the boat that is there now. I have questions (being new to boats but not new to welding and fabrication work) as to what all I will need to tie into to gain enough strength when we attach the transom. Anyone done this type of thing and what pitfalls should I try to avoid. Any advice appreciated...

 

Twin 300 hp outboards is one hell of a leap from a 250hp inboard diesel jet. What is the desired cruise speed, and is the hull suitable for the increased speed, shape-wise ? You may be out of the water more than you're in it. I think you have to be convinced the design shape is up to the job of handling this power, and the shift in weight balance, before attending to the structural issues.

 

Agreed a 240% increase in power is a huge jump. Unlike a car where you can triple the HP and get more speed, a boat reaches a maximum point of useful application, before a huge need is required, for just a few more MPH and then the drag wall hits you and just chews up fuel, instead of making you go faster.

What are the goals of the huge HP increase?

 

Typically, an aluminium boat with a diesel inboard won't be a deep vee, and not designed for the kind of ballistic trajectory re-entries that 2x300 hp can create, in changing from a boat into a projectile.

 

engine change

OK I understand it's a bit unusual modifying a boat this way and here are some of the considerations. It's a V-hull to flat bottom design hull. According to manufacture able to handle 30 knots. In it's earlier life used as a fire boat with extra weight such as pumps, an extra diesel to run them etc. Plans are to use this boat for a "fast ferry" overseas in missions use. That means it will have to handle the weight of hauling 20 people for day trips maybe up to 10 hrs max. We'd like to see 20knots out of it without running the engines near capacity to get more longevity. Currently it will not exceed 10 knots and won't plane out with the 250hp/jet drive combo. This picture insert is not the boat but one that's very similar.

 

If your interested in selling the old used jet pump, send me the details on it. Thanks, Stan

 

If you are carrying 'passengers' make sure the applicable regulatory agency (USCG, etc) is okay with gasoline. Many are not.
Determine why the speed is so poor. Bad hull form?, undersized jet?, overweight boat?, etc...

 

300 hp outboards are geared for speed, not 20 knots cruise. Could prove expensive to feed, and having 20 people aboard won't help. I am baffled as to how it won't plane, if the hull is good for 30 knots it must have had to satisfy a specification to reach a greater speed than 10 knots, to be in service as a fire boat, and now it has the fire-fighting equipment stripped out ? Has the engine installation been changed ?

 

We were informed that when the fire department owned this boat, they tried changing the jet drive impeller and exhausted their options with the current drivetrain to make this boat plane. At this point all the fire fighting pumps and gear have been removed along with the V6-92TI Detroit Diesel. We are currently removing the Ultra jet and it will be up for sale as will the engine. Part of the desire for twin engines is to make it safer for ocean service and to be able to get "home" on one if need be. Sounds like twin 250s may be a better choice but wouldn't it be better to have the power available if you need it and not have to use it then to be underpowered and not able to plane? It will need to be able to handle quite a bit of weight in it's next duties. Thanks for your comments...