Jim Smith 100' Sportfish backing down... a street?

Whew! Beautiful vessel.

Like the lighting on the trailer.....

109' and yes, Jim Smith is about a mile from the water and always have trucked them to the Hinckley yard I think it is to be launched, also need to get the power company out to remove power wires in order to do so. For the 2-3 yachts they build a year, it seems to work for them and has for decades. Jim Smith also builds a few smaller yachts that don't need all the fan fare which the article doesn't state.

I'm thinking that is the underwater exhaust.

It's most likely where the recessed trim tabs will be mounted, obviously once it's off of the trailer.

That's where mine are on my sportfish and where are the exhaust ports on the stern? Recessed tabs are directly behind the wheels, not far to port and starboard. Mine don't have openings to the side just facing to the aft but I assume this vessel has 2600 hp mtu. Those engines need to breathe and the fiberglass box under the deck that forms the exhaust would interfere with tabs and rudder if only discharging toward the stern.

Must have small bypass otherwise she'll shake at idle if exhausted underwater ANYWHERE on the vessel. Underwater exhaust is an upgrade for sportfishing boats, it's much quieter and doesn't blow water back in your face while backing down. My new boat has app. 2 x2 box under rear deck that has a cut out thru the hull. Cats can take higher back pressure than mtu. Hence my belief that's the reason they went out the side also. Once launched they can adjust back pressure by slightly enlarging the vertical side of the hole. Like 1/4" makes a big difference. One of the drains at waterline on the jim smith should be the bypass. This is what recessed tabs look like on most new sportfish boat if they didn't use fin style tabs.

I would say that indent is to break up the water coming off the stern to eliminate a vacuum and reduce drag. I would expect it to carry under the bottom to also house the trim tabs (again to reduce drag as well as keep them out of the way).

Actually opposite is true. The hull directly behind wheels and rudder is extended to prevent cavitation pitting at high speeds. Laminar flow around all the running gear is important to prevent pitting and additional drag. That little bump is all it takes.

I suspect that the cutout on the corner, the recessed trim tabs and that extension are all pieces to that puzzle.

Sportfish don't use underwater exhaust. At least I don't know of any. They come out the stern.

Jim Smith was a giant pioneer in the speed game on sportfish and leaders in pushing the speed envelope. He invented the cold molded process of using no fasteners in the finished hull to save weight that is still being used today. As well as being the first to use epoxy in building the hull. His first boat was a 1959 35' with a tuna tower with an elevator that did 44 knots. In the 70's he was building 57-59' SF with full tuna towers and only 8v92's at around 700 HP or less and all those boats did just under 50 knots WOT.

John Vance the current owner of Jim Smith has kept up that tradition since Jim passed away, even though the equipment on SF's has gotten much heavier (ice makers, freezers, watermakers, etc. etc. ) and still an innovator and have built the largest cold molded sportfish in the world and are still extremely fast and efficient for their size and power. Here is an excellent article on the history of fast sportfish and how Monterey(now ACY) and Tribute learned to build boats from Jim Smith.

https://www.*****************/copy-of-review-86-sapelo

To give you an idea of how far advanced these guys were. Monterey built in 1989, a 80' Sportfish, fully outfitted for fishing with a tuna tower, and the boat did 48.6 knots with twin diesel engines and shafts.

Sportfish do use under water exhaust but they exit under the stern not amidship. Jim smith wasn't the only innovative builder of his day. My parents founded Lydia yachts and built the first boats ever to use kevlar. They built Jim ***** a 60' sportfish with minimum speeds in the contract. My dad sweated that until sea trial day. She did a couple knots more than required and was an awesome vessel. She had the big Detroits also.
Funny story, Jim loaned my folks a Toyota while the boat was being built. Said he just signed deal to be distributor for SE united states. The car was really not very good and had already started to rust in less than 2 yrs. My father told him so and said you might have made a mistake kn those cars. Jim made that deal work out pretty well himself and he got another nice boat.

Ahhh, Jim M..... I worked for him for a small period of time in 2003. He seemed like a very nice guy. It's amazing the people you run into. I am familiar with Lydia's and they were built next to the Roosevelt bridge in Stuart. I thought all of the Lydia's were built out of Aluminum? I'll PM you as there was a very big Lydia SF 85'/90' that was super fast that I've heard about, but never got the details on. Some SF use underwater exhaust but it seems more rare.

That is a traditional exhaust as seen on most sportfish boats. Not an underwater exhaust. The very top is above the waterline. So no bypass needed. Still need water box( silencer) to reduce noise and exhaust temp. That design is louder while running, takes up room in engine compartment and will spit seawater at crew and angler while backing down on billfish. While backing down hard the waterline is at covering boards, engine builds back pressure until it burps seawater everywhere.
So take the boat in photo and bend exhaust pipe down at 90° before exiting transom. That is underwater exhaust for sportfish. You need bypass or boat rumbles pretty bad. Owned two 73' trawlers with ue. One had bypass, one didn't. What a difference!! Since these were less than 20knot boats the exhaust exited at rear bulkhead in engine room about 12' astern of stabilizer fins.
Can't do midship underwater exhaust on fast sportfish boats that aerated water plays hell on the props and rudder with cavitation pitting. Don't know about other hull shapes and exhaust location.