How to boost my economy on my Tomcat 255

C

cmetzenberg

New member
So i've had Kanaloa for a couple months now. My average fuel burn is about 1.8 nm/gal. I run at about 4k rpm @ ~20 kts. I find i have to keep the engines trimmed down to get her at peak efficiency and ride (not used to that). She already has trimming fins on the lower units. So what have others done to boost performance and economy? Should i get my propellers tuned? Figure out how to mount trip tabs? Add ballast to the forward hull compartments?
 
Motors should be mounted so the anti ventillation plate is right on the surface when running at speed. In comparison with the Suzuki's, that is a low speed for 4K. What is your WoT RPM and speed?

Also best to have stern lifting 4 blade props. We did not have to trim bow down--but had 4 blade stern lifting props. Suggest contact Ken at Prop Gods. Suzuki uses much higher pitch than Honda, because of gearing of lower units.
 
A good question to help diagnose or make suggestions would be what brand and HP of motors ?
Are the motors mounted at the correct height ? [integral motor cavitation plate bottom should start at level with bottom of hull measured with straightedge .They will be well below waterline at rest .
Are they Permatrims [not foils ]or Doel Fin type [foils]
Is there bottom paint ?
Current prop data ? [you cant assume they were optimized ]
That info will help to diagnose issues or let the community make suggestions .
Marc
 
WOT= Wide open throttle. You always want to state the speed and RPM at WOT when asking questions about props. Also the diameter and pitch are helpful--the rake and cup are also important--and that is the reason I suggested Prop Gods.

You have to have someone else drive the boat, and see if the anti ventillation plates are just at the surface. With the Tom Cat one needs to look specifically, rather than rely on the bottom of the boat--since there is a setback of 24" with the Armstrong Bracket on the Tom Cat 255. (Your phot album shows 150 hp Hondas--my experience is that the Hondas are not going to be as fast as the Suzuki 150, but it may have to do with setup of the boat.)

Marc's points are well made. The Permatrims are better than Doelfins, But I question if the foils are really necessary on the 255. I added Permatrims to get a lower planing speed. It achieved this, but didn't really improve the ride, or make the boat more effecient. I also found that motors trimmed all of the way down did not give the best economy or ride in my boat.

Also don't assume that props are optimal--if you get "the" set of props, then take them to a "Prop Scan" or other laser scanning service to get the exact pitch, and be sure that the props are symetrical.
 
Kanaloa:

From the sounds of things, you are burning about 11 gph to do 20 knots in a Tom Cat 25.5 with Honda 150s. A trifle slow perhaps, but pretty close to 'normal'.

Depending on circumstances, that is very similar to what I have obtained with an identical Tom Cat over the past 5 years, and several hundred hours of operation in a carefully maintained, bottom painted boat. So, you may not have anything really "wrong" at all.

But, perhaps your boat is just a knot or two slow. Before spending any money, you might try the following:

Are your speed readings coming off the GPS or a water speed sensor? I have found the latter to be somewhat unreliable. Check your GPS speed in one directly, then turn 180 degrees and run the other. Average the speeds, and compare that to your water speed sensor. Trust the GPS numbers.

The most efficient engine trim position in my boat is about a 2 on the Honda engine trim gauge which has 5 positions, the latter being engine highest. The correct trim position varies slightly with weight, and is worth 1 - 2 knots. So get your GPS going and try different trim positions at different power settings, and log the results. Take your time, as a small engine trim change can all of a sudden make a big difference in speed.

Check to see where your engine mounts are set. From the factory, the engines were mounted too high out of the water on my Tom Cat. Lowering them by one bolt hole (about 1.5 inches), made some difference, and also resulted in more reliable cooling water intake. Before doing that, the over heat light would come on from time to time, shutting the effected engine down.

Keep the boat as light as possible. Cats are very weight sensitive, and a Tom Cat 25.5 with full tanks has some 800 lbs of fuel on board, or enough to run for 10 hours. There is no reason most of the time to carry that much fuel, and you will definitely notice the difference at say half tanks. Same goes for the water tank, which holds another 240 lbs. In addition for some reason when we put 'stuff' in the boat, it tends to stay there. With time this can also add a lot of weight, so sort through what you have on board and eliminate what you really don't need.

Pull the hatch covers off the aft engine mount brackets and check to see if they are full of water. The covers have a seal that tends to leak, which can slowly fill the bracket up. The phenomena tends to be worse on the starboard side, because a Tom Cat is slightly heavier on that side to begin with. The engine mount bracket can hold a good 30 gallons on each side. At 8 lbs per gallon, you could be carrying another 500 lbs back there...a bad place to have the weight if attempting to get the boat on a plane.

All of the above could add nearly a ton of weight to your boat, and that you will clearly notice in terms of speed and fuel burn.

As you have suggested, keeping the boat 'bow' heavy does seem to help get it on the plane faster, but am not sure it makes it run faster. In mine, there is 200 feet of anchor chain in the bow locker, plus another 100 feet of line, and it does plane out a bit faster than others I have been in. But not enough (other than for anchor chain reasons), to go to the trouble of adding weight in the bow.

Finally, when new the boats usually came set up with sea level operation in mind. There is a considerable difference in prop requirements between sea level salt water, and higher altitude fresh water. In mine, the sea level props will not plane the boat in fresh water at 3000 feet, regardless of throttle setting. If you are operating in fresh water, well above sea level, you might want to try a smaller prop.

Good luck.

Kevin Ware
El Gato Thomas
Tom Cat 25.5
 
I don't have a Tomcat but I feed a 150hp outboard. I invested in a fuel management system which tells me what mileage I am getting at different trims, loads and speeds. With two engines even more reason to see what you are burning as gasoline is not getting any cheaper. Learning to live with 5 to 8mph speeds helps an awful lot. The difference a displacement speeds on our boat loaded between 8 mph and 10mph is a full mpg. We normally run between 22 and 25 mph cruising conditions permitting. We get better mileage traveling at the faster speeds rpm wise is sweet spot at 4200 to 4400. We watch the fuel monitor and adjust the motor trim, trim tabs, load the boat evenly front to back as much as possible and adjust rpm's to achieve the better mpg. Safe Travels.
D.D.
 
checking the bracket for water, interesting; i'm going to look into that. The hull paint is good so frictional loss should be minimal. As for how i gather my numbers: actual volume of fuel consumed between fill ups compared to gps trip log. Note: i use nautical miles for distance measurements and speed. I'm going to load up some pictures of how the engine is mounted for you all to analyze.
 
Fuel flow meters can be checked accuracy by starting with full fuel tanks, use the boat and refill the tanks then compare numbers
 
On our recent trip to Lake Powell we traveled 190 miles and used 84 gallons of gas ( 2.26 MPG ). Most of the running was at 19 to 20 knots. We have Honda 135's and the Honda fins ( like Doelfin's ) on both motors. We get 20 knots speed at 3800 RPM's with Honda 17" pitch stainless props. This is at 3500 feet elevation with full 150 gal of fuel and 40 gal of water, with a cruising load of food and goodies. At sea level we get just a little bit better MPG.

I usually run the motors trimmed up a little ( don't have degree's indicated on the Honda trim meters ). Play with the trim and watch the speed and RPM's. We have Floscan fuel flow meters and use about 9 GPH at 3800 RPM.

We ran some with Kevin Ware on this trip and I think got better mileage, but not sure why. I checked the Armstrong bracket for water and it's dry after 850 motor hours.

Going slow I can get 3.5 MPG or more. Under 2000 RMP my flow meters indicate about 2.5 total gallons per hour used. At 2000 RPM with both motors running I am indicating 7 to 8 knots speed. After 800 hours using the Flowscan meters I have observed a less than 1% discrepancy in the fuel used and actual fuel required to refill the tanks. For some reason my port motor is more efficient at low RPM's, under 2000, and the starboard motor more efficient at 3800 RPM's.
 
i'm only getting GPH off my flow meters. I'm pretty sure my pitometer tubes are blocked so i don't really have any real time speed to mpg comparisons. I've blown some compressed air through them so hopefully they work next time in the water.
 
[qus and that the engines are mounted at their lowest position.



Hole #1 is the one far est from the water
and
Hole #4 is closer to the water


Are you saying it mounted in hole#4 or #1?
 
Piot tube speeds are notoriously inacurate. The cats do better with counter rotating engines (something you cannot practically change), and mounted the opposite as in inboard boats--ie the right hand prop/engine on the port side, and visa-versa, gives more stern lift. Part of this may be why the boat I owned had top speeds in the high 40 mph range, and we were cruising in the mid 20's at 3200 RPM. Our best economy was about the same as Brent's: about 2.2 mpg on long runs and about 25 knots.

Still would like to see what you top speed and WOT throttle numbers are, with the current props.
 
you were getting speeds in the 40s out of a tomcat?! My props are set up as you described (inward turning). When i'm home next I'll get the WoT speed and RPM. Also I run pretty heavy most of the time so that isn't going to help the numbers. Bob, did you get to take a look at that picture of the engine? Not sure if you can fully tell from there but does that look to be mounted at about the same height?
There are two other Tomcats in Santa Barbara with honda 150s. I should touch base with the owners and compare numbers.
It just occurred to me that many of you may be running ethanol free fuel, i don't have that option in california. This also could reduce the mileage.
 
That's a really good thread. It was linked here last year and is what inspired me to raise my engine to see if it would improve things overall. Looking over the transom at my anti-ventilation plate area at speed showed it looking just like the "wrong/too low" photo. And, as they say will happen, when I trim the engine up at all, it slows the boat down (in other words, full down trim is always the best position, which really shouldn't be, but is). I have a Permatrim which seems like it would exacerbate the "dragging a bucket" phenomenon with trim-up and the engine too low.

I moved my engine up 1.5" (was in second from top hole; is now in bottom hole so "all the way up"), but have not tried it yet to see the effects. I'm really curious.

Here is my engine "before" - looks an awful lot like the "too low" photo, doesn't it.

IMG_1985.sized.jpg

Sunbeam
 
CMetz,
Agree 100% with everything Kevin Ware advised. If your aluminum brackets have iron drain plugs like mine, coat the threads with Tef-Gel to prevent galvanic corrosion...they are dissimilar metals immersed in saltwater. My line..."Every marina is just one big battery...someone's underwater metals are going to disappear."
During the off season, we take everything off the boat (including cabinet doors) which re-enforces his 'ah-ha...I had no idea this was on the boat' moments and encourages some realism. Your truck can carry around extra wgt and stuff without any performance degrading compared to your boat.
Agree also with getting your used props scanned and trued tho they look perfect (1 blade of mine had a significant bend and minor dings can make a big difference).
I run only ethanol free gas (it's a well known secret that Yammi's need it).
It's SO much easier to appreciate subtle control changes and see the AV plates if you have smooth water.
Each engines trim motor may trim at a slightly different rate, so your individual engine trims may not be identical while you trim using the single 'both' button.
Search site for Squidslayer's experience with Honda 19 vs 17 pitch props in the Pacific.
I wasn't using enough trim the first 10 trips, to paraphrase Marc Grove, the 255 likes lots of trim up. Now I'm at 50% of max on Yamaha gauges.
With experienced crew in smooth water, open rear door and with trim full down accelerate to 3200 RPM or so. Then trim up in 1-2 second increments while crew writes down RPM, TRIM, GPS SPEED with each 1-2 second (200 RPM) change. RPM and speed will increase without further throttle. Give each change 10 seconds to take effect and make some gentle wheel right to left inputs. As you get towards optimum, the higher resistance to turning to the right (in my boat) will decrease, even with power steering (this may be affected by which motor is your counter rotating one...mine is to port, Marc says he rigs his Tom Cats the opposite for better speed). Have crew take pics of AV plate position at 4k RPM and adjust individual engine trim if one is underwater.
Eventually you will trim too high and the engines/props will make more/different noise (hard to describe it, but it will be obvious they are less happy) and RPM may increase (props in frothy rather than solid water) but MPH will start to fall off. You can hear it much better with the door open in smooth water. This ventilation (not cavitation) will not harm the engines for this experiment and your engines have a rev limiter.
Trim back down and throttle up to WOT and trim for max speed and note max RPM, which should be as specified for your Honda 150, preferably in the higher 50% band (5400-6000 or whatever).
You have the props and engines of TC 255 Hull #1 (see report on BoatTest.com), so you can't be far off except bottom paint, prop dings, loading, insufficient experience to date, etc.
The 255 consensus here seems to be (to paraphrase Dr. Bob) there is not much difference in fuel burn between 3 and 4000 RPM.
Per Marc Grove, the Honda BF150 gets great economy by not kicking in the BLAST til 4400 RPM or so at the expense of a little low end grunt (but again, at better economy).
To emphasize the obvious, the main 'economy' of the 255 is the ability to trailer it (getting 13+MPG at 60 MPH through a thunderstorm with my GMC 2500HD) to close to where you want go, then launch and get 2 MPG or so, is way better in the long run than a what a $500k, 11.5 ft wide, SeaRay 370 with twin 300HP Verados can do in 6 weeks but only if the weather is great.
George Sass Sr (Viewfinder) downsized from a 45 ft custom boat to a 255.
I defer in advance to any input offered by Dr Bob, Marc, the 99% more experienced than me etc.
I do know you have a great boat and we hope you are really, really enjoying it!
Cheers!
John
 
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