TomCat Fuel consumption

[jritsema]

I am a new TC owner and would appreciate any feedback regarding the fuel efficiency issue. I was unable to locate any reference to TC on the Boat Test site. The rate of fuel/RPM is important, but also wonder about the average plane speed and typical cruising speed most skippers prefer, assuming a relatively flat sea.
Thanks
Jeff

[jritsema]

I have further questions as well:
1. Do you recommend the use of Permatrims? My boat does not have them but seems to get up on plane readily and seems to hold its "attitude" steadily?
2. What alignment of the motors is best. I am told that a slight toe-in is best and that the rooster tail created is an issue regarding how far aft it is placed. I can imagine that this significantly affects the efficiency of the motors and fuel consumption indirectly as well.
Thanks
Jeff

[thataway]

The Boattest.com site for the Tom Cat is here. Hit the "test results" button to see the table:
Best was at 3000 RPM, 20 mph, 17.8 knots, 6.4 gal/hr, 3.2 m/gal, 2.8 NM/G
Top speed: 6000 RPM, 47.3 mph, 41.1 knots, 30.5 gal/hr, 1.6 m/gal,1.4 NM/G

These are always optimistic. Cats are weight sensitive, but I run heavy boats.
My best mileage was on the run between my home and Apalachicola. Often I did this about 200 miles in one run, not stopping--or with one stop--at a cruise speed of 22 knots, and got 2.1 to 2.2 miles a gallon.
I had Suzuki 150, swing bigger props, with higher pitch, and top speed was just under 50 mph on the GPS in smooth water @ 6000 RPM

We put Permatrims on as an experiment to see if we could get the boat to plane at a lower speed. They did help, but at 10 knots there was a huge wake, and my overall impression is that the Tom Cat does not need Permatrims--and I am a fan of Permatrims.

Planing speed, with the Permatrims, I was seeing early planing at 10 mph, but not until 12 was I really on a plane. Without, I would say that 15 mph is starting a decent plane. The Tom Cat is a planing cat, and does not run at the intermediate speeds (between 8 and 15 knots) as well as a displacement cat. When cruising with trawlers, I often would run ahead, and then anchor and read or fish for a few hours until my friends caught up...especially if there was chop.

Toe in of a couple of degrees. Rooster tail, is going to be present in most cats--not really an issue, unless towing a dinghy or water skiers...which I don't recommend--although I have towed dinghies at times. In rough weather, the faster speed seems to get a better air cushion effect. I have gone up Perdido Bay in 2.5 foot chop at 35 mph a number of times.

Great boat, My wife reminds me every day, how foolish I as to sell ours!

[gulfcoast john]

Jeff
Bob beat me to it, but I get about 2.0 MPG real-world combined for TC255 planing speeds between 21-30 MPH (3300-4000 RPM depending on engines, props pristine or not, bimini, bottompainting, pristine bottom or not, and loading). From 4000-6000 RPM my fuel meter jumps from 13 to 31 GPH per engine. 150g of gas alone is 900 lbs so don't go around full if you don't need to. Everything from Honda 135's to Suzuki 175's have been rigged. The air temp, windage, alignment, currents affect it, but not enough for me to care about. (I'm told engine alignment is engine-specific, not boat specific, and may be best left to a dealer). A 15% efficiency difference between engines is par (the counter rotator has an additional set of gears and the depth sounder should be to stdbd of the RH prop which wouldn't work...small stuff). Plan on years of experience to get things dialed in the way you like (I like 4-blade 17p props which are slower but better torque and control and less vibration at my boat's favored 26MPH @ 4000 RPM). Etoh free boat gas (use Puregas.org) has been a surprisingly small % of the cost of ownership the past 3+ years compared to $100/night for Key West marinas and 'toys' and expert annual Yamaha 5-Star maintenance.
You want the engine AV plates to be awash at your fav planing speed. She likes a LOT of trim up.
The boat design is very forgiving of imperfect drivers. Don't sweat the small stuff until you have mastered getting on plane, recognize over trimming/ventilation (which doesn't hurt the engines) and throttle-only steering around the docks. Hiring an off duty cat Sea-Tow captain to show you the tips and tricks would probably be a good investment.
Have fun climbing the learning curve!
John

[jritsema]

Bob and John
Thanks for the thorough responses. Will give me a good place to start.
Jeff

[BTDT]

[thataway]

[gulfcoast john]

John/Jeff,
I also put out a massive wake at 10 knots, defined as what kids fishing in a skiff would say if I swamped them at that speed. I agree with Bob...Don't run a TC255 at speeds between 8.8-16.5 MPH (I'd say 20MPH @3000RPM, just where BoatTest found the maximum efficiency, which is always going to be the same as the minimum solid planing speed) . At 10 knots you're smack in the middle of the No Man's Zone transition between max displacement speed of, say, 8MPH and planing. You can't see it from the helm, but the stern will be way low in the water, water up over the level of the engine cowling seals (I don't know how but they seem to keep the water out of the engines even when they're 3 inches underwater ) and entire bracket underwater, all of which generates a huge wake even without Permatrims. Around here, Mississippi boat drivers would be yelling and throwing their empty beer cans at you. (They never seem to run low on them, though they run out of gas all the time). In my Regal monohull the wallowing, unstable feeling while in No Man's Zone was obvious, but the TomCat is very stable and stays pretty level climbing over the bow wave, even with the bracket and engines drowning. For me, the most challenging aspect of driving the TomCat is getting on and off plane and avoiding those speeds (and/or RPM's) between minimum wake displacement (7 MPH for me) and a good solid planing (20MPH for me). That's the reason for my advice to Jeff on this issue. I still can not tell if I'm in the transition zone just by 'feel' or the horizen or even the inclinometer, I have to see the huge wake or RPMs or GPS speed or get hit by beer cans. Again, the design is very forgiving of imperfect driving. Bob noted he ran Suzi's with much different gear ratios, props and pitch on a lift-kept boat without bottompaint.
Coming off plane too abruptly drowns the stern more than getting on plane does (I had Eileen drive so I could see it all) and for slowing off plane, throttling down by 50-100 RPM per second seems to work best for my boat (excluding emergencies of course). To get on plane from 6 MPH with full trim down, I advance the throttles quickly to 3100-3300 RPM, let it settle out for a few seconds, then trim up 1 bar every few seconds. RPM and speed will increase without any further throttle to 3400-3700RPM (4 bars on my Yamaha gauge). Too much trim will cause speed to stop increasing or even drop off and the noise of ventilation. RPM may increase in the frothy water but speed won't. You can add throttle or decrease trim then. Above 4000RPM fuel burn really starts to take off. Best cruise (most fuel efficient) is around 3000RPM for most all well designed trailer planing boats, including this one, whether single or twins, outboards or sterndrives, cats or monohulls. (I may not have convinced Bob of this yet). Of all the hundreds of past planing boat tests I've seen, 95% had best cruise at 3000RPM (a handful at 3500 RPM). NOT ONE AT UNDER 3000RPM. Well-designed planing hulls will start to try to climb out of the hole at 1500 RPM, pushing a wall of water to do so by 2500 RPM. It's a great boat and you're going to love it. Since you should spend under 30 seconds at 9-10 knots out of every 100 engine hours per year, your huge wake at that speed is no big deal. Take the permatrims off and try it, but don't be disappointed if you still swamp skiffs between 8.8-19mph and burn more fuel to boot than at 20MPH and 3000RPM (and with a much smaller wake at that speed). Stay under 6.5 mph for no wake zones.
I invite doubters to find any planing, prop-driven trailer boat report for which Best Cruise is not 3000 or 3500RPM and send me a link. It's physics, and power ratios, and stuff the engineers figured out years ago. There are no secrets in marine engines/engineering that haven't been discovered.
John, watch the bracket while someone else gets on plane at different speeds, you'll be impressed. Great Boat!
Happy boating, everyone!
John

[thataway]

John,
Both the C Dory 22 and 25 by battiest.com criteria were far better at 3500 than at 3000.(neither were on a plane at 3000 in the Boatest.com studies.) There are multiple factors, including design of hull, HP, weight carried, prop, altitude, etc. Absolutely, there are going to be a lot of outboard boats with the best efficiency at "Boat Test" with the 3000 to 3500 range, but they are all very light boats. As they are loaded up--the RPM often go up for best performance. For example my C Dory 25 with the 130 had to be run at close to 4000 to get on a good plane when loaded for an AK trip. At Lake Powell, everything becomes much different because of the altitude. Even my 22 with the correct prop, needs close to 4000 to be on a good plane.

So folks are going to find different RPM's which are best for their specific circumstances.

For example my Caracal is a single engine planing cat (unusual to have single engine cats because of ventilation problems.), which has hull bottom lines very similar to the Tom Cat. The beam is 8 feet, with an OAL of 18 feet, the tunnel clearance is a little more than the Tom Cat. The ride is very similar to the Tom Cat--including in rough water.

Most of us who have this boat and either the 115 or 140 Suzuki run find that they run best at about 4000 RPM--and many times I go on up to 4800 RPM.

With the Tom Cat, I had the impression that I was running my engines slower than those who had Honda's for the same speed.

[jbdba01]

And all of the above is why I hang here in this forum; great details and comments.

I wish all the forums I hang out in were this good.

[BTDT]

Thanks for the great input. The more hours I spend with my Tomcat the more I like it. I have always been 'wake-sensitive' towards others so was truly shocked with the wake I created in the 8 to 11 speed zone. I now spend most of my time above 2800 rpm or at no wake speeds.

Encountered an afternoon thunderstorm last week and was very impressed with how the Tomcat handled the wave action. Chop was about 2 to 3 feet with winds head on. At about 23+ knots the hull seemed to lift up on an air cushion and traveled across the tops like I was on smooth water. For the helicopter pilots, it reminded very much of translational lift.

Having radar was also nice as it helped me keep track of the 'blow-boats' that had no choice but to stay out in the weather.

[Discovery]

The others have covered the subject well. We cruise at 3800 to 4000 RPM at around 20 MPH. This gives us 1.8 to 2.2 MPG. We often slow cruise at under 2000 RPM on one motor at a time. Speed here is up to 6 MPH, depending on tides and current, and 5 to 6 MPG. We have Honda fins on the motors. They are smaller than the Permatrims.