Fuel consumption of Tom Cat 255 at low speeds

[thataway]

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 .

[Bad Boy]

Bob,

Thanks for the technical information. I think you also have the Permatrims, correct?

Only 45 hours on my 150HP Mercury Verados and I swing 17"P x 14.65" diameter props to get the wot rpm within range. It amazes me that you have 21.5" P props. Are you able to achieve the WOT rpm within specifications?

I can get 2.3-2.5 mpg at 4200 rpm. I have not yet gathered data as you have, but I plan to before and after I install the Permatrims. However, I have noticed similar inefficiences in the 7-12knot range. At this time, I have yet to experiment with one engine.

Your information in valuable and gives me a point of reference, even if with different engine/prop configurations. What I am happy to report is that I am getting idential fuel consumption on both engines. Three times, I have filled both tanks within .3 a gallon of each other.

[drjohn71a]

Dr. Bob,

Thanks for the careful evaluation.

When I lift one engine at slow speeds, my hydraulic steering tends to go awry and wander and groan - esp. if the autopilot is 'on'. Do you have that problem with the Suzukis?

Also, I just came in from a long cruise at slow speed with 1.5 gal/hr for 6 smph, for about 4 mpg = averaged both legs.

I was running in the 1400 rpm range and just over.

John

[thataway]

The Suzuki's have much lower gear ratios--and thus allow the swinging of both larger diameter props and a much higher pitch. This was one of the attractions of the Suzukis--and I found that they handled slightly better than the Honda's which I sea trialed in another boat. (But there was much more weight and the engine trim could not be easily altered i the boat with Hondas).

Yes, I should have mentioned that I do have the Permatrims--and I kept the engines at mid range (on the longer hash mark at the 90 degree point. I did not vary the trim angle. My impression was that the fuel effeciency was not changed much by the Permatrims. This was also confirmed by the same fuel consumption by the flow meters. I get the same mileage as you do at the higher speeds (Miles per gallon--I did these tests in knots--can be easily converted to staute miles.)

In the past, it seemed that the steering was labored with one engine up--this time I had the non running engine, just up above the trailering position. There did not seem to be much "laboring"--but I was steering mostly a straight course--avoiding much maneuvering. I did check the alignment after the tests, and it was still correct.

Each boat and motor combination will have slightly different results (engines, trim, weight etc)--but the general trend will be similar.