catamarn with stern drive engine

Certainly not my best drawing, but I guess you understand the intention.
In this particular case there is very little lift surface from the hulls so the load on this trim plate will be huge.

 

Cummins Mercruiser told me to raise up the postion of propeller,in my opinion it could be the best solution for this case,
The other solution is to change to propeller shafting system

 

Like you say, it needs a new bottom, the buttocks are uncool & maybe the leg height could be wrong too. Jeff.

 

Extending the sponsons as you've illustrated is a "band aid" approach, you need to address the running surfaces to straiten the buttock lines aft & drop the engines/legs to match.

 

In the absence of proper pics, what can be said is the catamaran part of it is nearer to 30 feet than the 38 feet quoted, and did he say 12 tonnes ? That sort of weight in a 9 metre cat is well on the way to sinking it ! Hardly surprising it drags it's aarse ! If Mercruiser say raise the leg height, and the assembled experts here say straighten the buttocks, how about fit some wedges to the bottom of the sponsons right aft and attack both issues at once.

 

Good points, esp on the weight & length, apparently this cat is a sick one.... overweight, worms, bent buttocks, short sponsons with blunt snouts & a really bad case of spoondrift.
I was involved in adding wedges to a cat once.... no actually three times....or is that x 2hulls = six time to the same boat, the last set where like whale tails & then it ran slightly bow down....... & then we got to take them all off! & the acceptable operating speed was lowered.

 

I suppose the addition of an ample bustle kissing the static waterline aft, applied to each sponson, could help the cause, but my experience with overweight planing vessels is that it is literally an uphill battle to get a satisfactory fix.

 

are the cav plates on the drives under the bottom and by how much?
If you cut off the last 30% of the hull and fabricate a new section with a straight keel you will be way in front but that doesnt mean you have a proper boat.

 

The buttocks all rising from amidships forward worry me as much as the stern, that is going to push the centre of dynamic lift more forward than it might otherwise have been.

 

cats dont plane....rule number 1
http://stott.customer.netspace.net.au/devilcat.htm
those large vessels do not change attitute from zero to 50kts
and if I can find it on the internet I can show you a very early wave piecering passenger cat that tried to plane (had bow lift anyway not to contradict myself))and then nose dived in Melbourne in the 80's with many broken arms and legs
NA's were just learning then.

 

Dynamic lift is, or can be, a significant factor in a cat this size, it is not a ship.

 

sure it can have both hydro and aero lift but it doesnt plane and both of these are much more critical to a cat than a mono

 

I want to use the propeller shafting system as below on the next boat,does it would be works well?

 

What is the width of the sponsons, chaser, particularly at the stern ? I guess they are wider than a 220 Mercruiser diesel !

 

Well lets not get into a definition debate of "planning" here... but in the general sense of the word YOU ARE WRONG, cats can and will plane if certain design criteria are met...

However, there is a distinct difference between "high speed displacement hulls" (as shown in your link) and planning hulls - which is where i think you are getting confused and demands a rather lengthy discourse in order to explain the difference. Regardless, the DLR and of this boat in question, rules out any chance of an effective slender body high speed displacement hull form, therefore it must plane (or at least try to climb over the hump) with massive amounts of thrust if it will ever exceed Fn = 0.6, and obscene amounts of the same if it will get anywhere near 23kts...

If this boat displaces 12T, im curious what the outboard powered version displaced in order to acheive 23kts from 500Hp at lets say 62% (at best) efficiency for arguments sake? Ill bet it was considerably lighter than 12 tonnes, and still quite poor performance regardless from that much HP...