Is a eurotransom more efficient, faster

[Bullshipper]

I have a pilothouse catamaran is running 50 mph with twin 225 outboards with 25" shafts. The motor is all the ways down, and because she is light, the prop is still a little high with the motor sitting as low as she can go. So with a 1000 liter fuel capacity, 2 100 liter livewells and a 80 liter water tank I am running the hull 1/2 full of liquids to get her to draft more to keep the props from ventilating as the hull draft decreases about 10 inches when she is cruising along at 32 knots.

I will be producing a center console style that will be about 600 lbs lighter, so I am wondering if I should just go to 30" shaft motors for more money, or put a small step that the rear of the transom
measuring 4" high x 10" long to permit the water coming off the bottom of the hull to rise sooner and faster to the prop, and if there might be some small gain in efficiency by decreasing the running surface by about 3.6%.

Here is a pic of the 28x 9'10" cat.

 

[fallguy]

California or Mexico? Where is the slip? I zoomed in San Carlos Yacht Club in the Baja!

I'd jackplate it. Offers more flexibility if you are unsure. Jackplates and long legs also offer greater safety from drowning the obs.

 

[Bullshipper]

I am in Mexico.

I would think that euro transoms shorten the running drag surface & help bow lift, so there must be a reason that most of the cats and mono's employ them.

Longer motors and jack plates cost more, are heavier, and take longer to get. These are minus' in my mind.

So if there is a slight efficiency gain with less equipment, less cost and fuel consumption I think those are +'s.

 

[baeckmo]

Could you give us a picture "on the dry" from the side , with engines down so we can see the avp height re bottom, and indicating the static wl. Also, what props are you running, and is the keel line straight?

 

[Bullshipper]

I believe the pics I posted are the best I have in the water to show this but the transom is 27" tall, the cavitation plate is 1.5" above the bottom of the keel with about a 12" setback from the bottom of the keel, the hull drafts 16" at the transom at rest, and the cavitation plate is runs right on the surface with neutral trim at 30 knots when the hull is on on plane.
There is no rocker on the bottom, nor does it have steps, and the hull leans in slightly on high speed turns. It weighs 5650 lbs rigged with motors dry and was tested at 8161 after adding passengers, liquids, gear and fuel

THIS BASICALLY THE SECOND SHAKE DOWN RUN ON THIS HULL WITH 16x18 ENERTIA ECOS,BUT RPMS HAVE NEVER GOTTEN CLOSE TO 6000 EVEN WITH SMALLER DIAMETER 17'S.

DATA FROM ONE OF THREE RUNS TODAY
GAS 69% full, 190 gals
20 Gal of Fresh Water
Livewells Full at 25 gallons each
2021 Merc 4 Stroke with 35 hrs, Gear Ratio 1.85:1
Passengers 4, tools, gear
Test weight
8287
Temp 90F Sea/wind 1FT, 15 KNOTS IN BAY
Sneezing - Very little to none
SPLASH ON MOTORS none
Permatrim depth below water at Rest 7" and at Cruise Speed on top of water
COMMENTS
Trimability Good, motors carry bow well for dry ride, tilting in slightly even on high speed turns
Very Dry ride, even standing outside of cabin
Boat handles 2-3's very well at 35 mph with dry ride in head, beam and following seas. Still cannot get full rpms after checking and replacing filters and adjusting throttles but hull is 4 mph faster than best previous test at same rpms.
Slip numbers are a way too low and do not make sense as calculated. I suspect previous owner modified these used props adding a lot more pitch than marked, or tachs are not reading correctly.

RPM MPH SLIP
1500 7.50 45.7%
2000 14.50 21.3%
2500 19.90 13.6% on plane at 14 mph
3000 24.20 12.4%
3500 31.20 3.3%
4000 37.00 -0.4%
4500 40.00 3.5%
5000 45.00 2.3%
5200 48.00 -0.2%

Hull got 49 mph at 5400 rpms with 1100 lbs less liquids so she is not that weight sensitive for a cat imo.

 

[Bullshipper]

 

[Bullshipper]

 

[Bullshipper]

Looking at the rear step of this 100 mph Skater answers my questions.