My grandchildren have returned to California, and I had to take the Tom Cat 255 down the bay to pump the holding tank. I had loaned the boat to a friend to tow his disabled mainship 34 200 miles back to his home, so one engine had 1.7 hours more on it than the other, and I wanted to run on one engine to equalize the hours. This was a perfect time to look at the low speed performance of the Tom Cat. I have Navman (Northstar) fuel flow meters, which have been calibrated during the burning of several thousand gallons of gas, so I feel that they are within a few percert of being perfect. The two guages agree. I feel that below a gallon an hour that they may not read as well as over a gallon an hour. The GPS readings were taken with a beam wind on both sides (coming and going, with less than 0.1 knot current --again minized with runs in both directions).
There has been some discussion in the past, that the Tom Cat 255 does not have a true catmaran hull form at low speeds, because the tunnel between the hulls drags in the water, making the boat more of a "barge" type mono hull at low speeds. When the boat is on a plane, the tunnel is clear (air compressed in the tunnel helps to give the smoother ride of the cat).
I did not record at speeds of less than 5 knots, because of the probably in-accuracy of the flow meters at lower speeds, but postulate that at lower speeds that the boat is even more effecient.
One engine--the other engine out of the water to reduce drag--Suzuki 150, with 21.5" pitch and 16" diameter props
RPM / speed in knots / gallons an hour / Miles a gallon
1500 / 5 / 1.1 / 4.5
2000 / 6 / 2.1 / 2.86
3000 7.5 8.2 0.91 WOT
Two engines at same RPM / / GPH both engines/ MPG both engines
1500 / 5.8 / 2.2 / 2.6
1800 / 6.6 / 3 / 2.2
2000 / 7 / 5;3 / 1.3
2200 / 7.4 / 6,6 / 1.1
2400 / 7.7 / 7.6 / 1.0
2500 / 7.8 / 7.8 / 1.0
2600 / 11.8 / 8.2 / 1.4
2800 / 15 / 9,2 / 1.6
3000 / 20 / 10 / 2.0
The Tom Cat is handling like a monohul "barge" at speeds less than planing. Assuming that the Length Water Line is 22 feet, the sq root is 4.7--and this is a very easily driven speed. 1.34 x sq rt LWL is max displacemenet speed--and would be 6.3 knots--and anything beyond that speed would take large amounts of power (fuel)--ie with one engine, the max speed reached is only 7.5 knots, and conventional formula would say that you are using 114hp to achieve this speed--yet it only takes this same hp, with two engines (more blade area and the boat is reaching a planing speed to come on a plane! At 5 knots the boat is only using 16 hp and at 6 knots about 30 hp. with my boat/engines, props, 2600 to 2800 is the critical RPM where the boat begins to plane--putting the throttle to achive 2800 RPM with both engines, will bring the boat slowly onto a plane, and it will stay on there down to 2600 RPM--then it will drop to 2350 and down to 9 knots, if you don't give it more throttle.
Bottom line--if you want effeciency from the Tom Cat you need to keep the speed less than 6 knots--even better at 5 knots. Or you need to go to a planing speed--where the same "mileage" is achieved at 20 knots as at 6.8 knots....Incidently, I had a similar sized deep V express with a 300 hp I/O Duo prop and the best mileage I ever saw was 1.6 miles a gallon at 24 knots. The Tom Cat achieves 2.0 plus miles a gallon at planing speeds--so it is more effient than a similar sized, weight and HP monohull at planing speeds.
Every boat, and set of engines will have slightly different numbers--if the tunnel was clear, and there was no wing deck drag, the Tom Cat would be more effecient at the intermediate speeds acting like a semi displacement cat at those speeds--such as 9 knots. But the way our boats are--anything between 6.5 and 11 knots is very ineffecient--and I think we can pretty much debunk the "trawler type of mileage"myth at these speeds .
There has been some discussion in the past, that the Tom Cat 255 does not have a true catmaran hull form at low speeds, because the tunnel between the hulls drags in the water, making the boat more of a "barge" type mono hull at low speeds. When the boat is on a plane, the tunnel is clear (air compressed in the tunnel helps to give the smoother ride of the cat).
I did not record at speeds of less than 5 knots, because of the probably in-accuracy of the flow meters at lower speeds, but postulate that at lower speeds that the boat is even more effecient.
One engine--the other engine out of the water to reduce drag--Suzuki 150, with 21.5" pitch and 16" diameter props
RPM / speed in knots / gallons an hour / Miles a gallon
1500 / 5 / 1.1 / 4.5
2000 / 6 / 2.1 / 2.86
3000 7.5 8.2 0.91 WOT
Two engines at same RPM / / GPH both engines/ MPG both engines
1500 / 5.8 / 2.2 / 2.6
1800 / 6.6 / 3 / 2.2
2000 / 7 / 5;3 / 1.3
2200 / 7.4 / 6,6 / 1.1
2400 / 7.7 / 7.6 / 1.0
2500 / 7.8 / 7.8 / 1.0
2600 / 11.8 / 8.2 / 1.4
2800 / 15 / 9,2 / 1.6
3000 / 20 / 10 / 2.0
The Tom Cat is handling like a monohul "barge" at speeds less than planing. Assuming that the Length Water Line is 22 feet, the sq root is 4.7--and this is a very easily driven speed. 1.34 x sq rt LWL is max displacemenet speed--and would be 6.3 knots--and anything beyond that speed would take large amounts of power (fuel)--ie with one engine, the max speed reached is only 7.5 knots, and conventional formula would say that you are using 114hp to achieve this speed--yet it only takes this same hp, with two engines (more blade area and the boat is reaching a planing speed to come on a plane! At 5 knots the boat is only using 16 hp and at 6 knots about 30 hp. with my boat/engines, props, 2600 to 2800 is the critical RPM where the boat begins to plane--putting the throttle to achive 2800 RPM with both engines, will bring the boat slowly onto a plane, and it will stay on there down to 2600 RPM--then it will drop to 2350 and down to 9 knots, if you don't give it more throttle.
Bottom line--if you want effeciency from the Tom Cat you need to keep the speed less than 6 knots--even better at 5 knots. Or you need to go to a planing speed--where the same "mileage" is achieved at 20 knots as at 6.8 knots....Incidently, I had a similar sized deep V express with a 300 hp I/O Duo prop and the best mileage I ever saw was 1.6 miles a gallon at 24 knots. The Tom Cat achieves 2.0 plus miles a gallon at planing speeds--so it is more effient than a similar sized, weight and HP monohull at planing speeds.
Every boat, and set of engines will have slightly different numbers--if the tunnel was clear, and there was no wing deck drag, the Tom Cat would be more effecient at the intermediate speeds acting like a semi displacement cat at those speeds--such as 9 knots. But the way our boats are--anything between 6.5 and 11 knots is very ineffecient--and I think we can pretty much debunk the "trawler type of mileage"myth at these speeds .