Engine Trim

K

Kerry and Lisa

New member
My wife and I flew to Fort Myers about a month ago to sea test a 24 Tomcat at the Bulldog Boatwerks. We purchased the boat and it will be shipped to Galveston in about a week. We ordered a number of modifications--radar arch/rocker launchers, sink, solar-powered vent, extra battery storage, bimini, windlass, etc.--that are just about complete. It has been a good experience doing business with the Boatwerks' personnel, including Vivian, Chris and Brady.

My wife Lisa and are new to boating and will have much to learn when the boat arrives. Meanwhile, I've been reading all the original owner's manuals for the GPS, 24 Tomcat, Wallas, portable head, twin 100 Yamaha TXRX's, VHF radio, helm controls, etc.

Could someone explain the meaning of "trim." There are switches on the dash for raising and lowering the starboard and port engines, along with guages indicating their position from all the way up to all the way down or vertical. Additionally, there is one up/down power trim switch on the throttle handle which I assume trims both engines simultaneously.

Is "trimming" making slight changes in the angle of the engines relative to vertical, at whatever cruise speed one chooses, to adjust the bow height and, therefore, the efficiency of the engines? How does one know when the best trim setting is attained?
 
You are correct in your assumptions about what trimming is with regards to the engines. For twin engines (as will be on your cat), you will have two adjacent throttle controls each with it's own trim adjustment - the outside throttle will typically have the trim adjustment switch on starboard side while the inner most throttle with typically have the switch on the port side. In addition to trimming with the engines alone, some people also choose to install trim tabs. Trim tabs are the boating equivalent of elevators on a plane - e.g. they are tabs that extend from the stern into the water that can be individually raised or lowered to push the corner of the stern up while underway. Trim controls are always set up so that each side can be adjusted independently to compensate for weight imbalances in the boat.

With the cats, many people have added "permatrims" or similar fins to their engines. These are essentially wings that bolt onto the lower unit of each engine to give it more surface area for trimming. With these on a cat, some would claim there is little need for trim tabs and there is not a lot of space on a cat to install tabs anyway. Do a search a permatrims to learn more.

Now - how to determine where to set the trim on the engines...
First - NEVER raise the engines too high while they are running. They water intakes for cooling must at a minimum be kept below the water line.

Second - the proper trim will depend on conditions and your goals at the time. If you are in rough water, going slowly and you want to keep the bow down, you will trim the engines most or all the way down. If you are cruising along in reasonable water, you typically want to adjust for maximum fuel efficiency or speed. On my 22, I usually adjust to maximize speed at a given throttle setting. When you are going from a stop to a start, it's better to have the engines trimmed down a bit and to trim the up as or after you get onto plane (it takes a lot longer to get on plane if you start off with the engines trimmed up). I also adjust my trim by visual inspection of the water flow off the back of the boat. When have the engines trimmed down too far, the extra drag creates a lot of spray between the two engines. When the trim is right, I get a nice V-shaped hole in the water off of each engine.

In practice, you'll find out very quickly how to best trim the engines for a given condition/goal simply by playing with the adjustments while underway. Just don't make rapid changes to one side while traveling at high speed and you'll be fine.
 
I purchased my Tomcat from Bulldog Boatwerks thanksgiving before last and they were easy to deal with would recomend them to anyone. My boat has been great only a few issues that I have had to deal with in 450 hours of operation.
JoeC
 
I don't know how your particular outboards are rigged, but most have a "2-stage" engine positioning range. The first is a slower, lower geared movement that adjusts the "trim" , the up and down tilt while you are moving and with the engine in the water. The second is just the old, coarse movement which raises the engine out of the water. Sometimes one set of switches will be limited to the tilt adjustment only and the other would have the full range of movement. Different detents, etc, can be used to help prevent lifting the drive completely out of the water while the engine is in use.

Good luck on your new boat!

John
 
Kerry and Lisa:

Wow- another Tomcat in the Galveston area! I have a Tomcat 255 that I own jointly with my friend Charlie. We keep it in the water in Seabrook. We bought it about 6 months ago from Wefings in Apalachicola, and we have had a blast! We come down to Galveston often, especially when we have friends visiting- take them around Pelican Island in the Bolivar Roads channel, then through Gaveston Harbor past the Elissa, under the Pelican Island bridge, then to Offat's Bayou...usually tie up at Boudreaux's for lunch. We have also been up every Bayou in Galveston Bay and are looking forward to some quality fishing starting next month.

I look forward to meeting up with you. I have been in the Galveston area for almost 30 years.

I will also send you a private message- watch for it.
 
Thank you all for your very helpful information and suggestions. Shy of actually experimenting with trimming the boat once it arrives, I think I now have a solid grasp of what trimming is about and how it's done.

It is readily apparent to us why so many describe the C-brats as friendly and helpful. We look forward to a long association and hopefully meeting many of you in the not too distant future!

Kerry
 
I have a 2007 255 TomCat
I had a monohull 22angler before..w/trim tabs
Two different animals
Trim down ....On the monohull.... trim tabs were on to LOWER the bow down to take on wind waves or chop...flat bottom

Trim up..On a Cat there are no ..and cannot have trim tabs...no space..interfere with twin props..........................to address Wind Waves you run at neutral trim or UP trim to take on chop...got to keep that tunnel open...get Permatrims..for your ouboards ...they really enhance the use of trim up or down on a cat

Dick
TomCat 255
Shriley Mae
 
As Squidslayer says, the Tom Cats (or any cats) run differently than the monohulls do--and what works with trim on a monohull may not necessarilly be the best on a cat.

As Les Lampson and I have pointed out--there is a difference between what an outboad or I/O trim does (including with Doelfins, or Permatrims--which only accentuate the usual trim--or adjusting the prop angle with the surface of the water and helping to channel the prop wash downward slightly. The trim tabs (which you will not have on a Tom Cat for several reasons, work only on the hull attitude. The outboard or out drive trim works both on hull attitude (accentuated by the add on plates) and the angle of the prop wash from the water.

At least on the Tom Cat 255--(the TC 24 should be similar--but I have only ridden in one for about an hour)--the cavitation plate of the outboard should be right at the surface of the water--the prop is riding higher than most mono hulls ride. Thus the fins--either cavitation plate, or Permatrims etc are only working on the surface of the water, not as some foils work deeper on the water.

You want to be careful to keep the bow up, at least to some degree when running down waves. If you trim the bow up too much, the engine will start to cavitate, the RPM will go up and at some point the speed will drop.

Normally you trim both engines the same--as noted above, my engines can be either locked together, or used one by one. If I have to trim out balance of the boat, then I will make one of the engines trim down more than the other--but still lock them together.

You want to bring the boat on a plane, then trim up until the RPM have started to rise, back off on the throttle and keep the RPM the same. This will be the most effecient speed. With the Permitrims, you can trim the bow down slightly and keep the boat on a plane at a lower speed--but you don't want to trim the bow down as in a CD 22--that will bury the bow, and cause hull slam.

Hope that helps--you have to get the feel of it, and then it is a reflex.
 
Bob & Kerry,

Glad to see another Tomcat come aboard. My TomCat stays at my house south of Anchorage, but when I'm working I'm just north of you up in Kingwood flying out of IAH airport.

This is my second boat, trimming is CRITICAL on my first boat, (an aluminum monohull) for ride in the waves, speed, and so forth. We even added one of those fins to it.


I've found on my 24 TomCat that my trim position (at least with my Honda BF90's) doesn't make a lot of difference. In fact my boat doesn't even have trim position gauges installed. The bow moves up and down only slightly with trim changes. The previous owner told me he just trimmed the tops of the motor cowls flat with the boat. You probably got the best advice further up this thread about finding your highest RPM available setting with trim. We don't run this boat wide open throttle, so I don't have the advantage of finding the highest RPM setting with trim while wide open. I haven't felt the need for a permatrim system yet on this boat.

We added fuel flow meters last summer as soon as we bought this boat. One of the flow transducers never worked and we didn't get it fixed until this winter. I think I should see a little of the effects of trimming on the MPG portion of the gauge function. I'll try and make a post if I find anything of interest.

Welcome,
John
 
Bob and Karry! Welcome and you will love that boat. Time in the drivers seat will be your best teacher and guide on this particular hull. Ruff waters as stated above, trim the motors up just a bit. But, on the smooth inland waters, I found better economy by triming back down a bit...as to ride the hull ....the whole boat hull, not the flat parts on the motors, but the entire boat flat... got about 1/2 more mpgs. Basically, all you are doing is keeping the back corners/end of the sponsons running "shallower" in the water by running flater, thus simply reducing over all drag. Not driving the bow down into the water, but just running at/or just barley off the water in front, as to keep the boat running as flat as possible. I ran my bow too high...buy just a bit for the 1st season...and only figured this out when running on a gathering with other boats. Now... on a car getting 25 mpgs...that is no large change. On this boat, on long trips on inland waters, it likes to run flat"er". The TC24 and the TC255 are weighed differantly in a couple of ways as the weight of the cabin changed, the length of the cockpit changed, and having the Armstrong brakets rather than mounting the motors right to the hull is a bit differant. These all minor differances, I would bet, make the trim needs for the TC255 differant than your TC24. Anoter change is the weight on the stern (ass end). Most guys powering the TC255s run pairs of 135/150s as apposed to your pair of 90s... and most more than likely have more batteries...refrigerators, generators and things on board... so... you are just going to have to play with YOUR TC24... You will love it... I put a bunch of hours/miles on my TC24 and it was powered with a pair of Honda 90s, two batteried, no generator, no refrigerator, no Armstrong motor mounts.

Enjoy and welcome aboard the Brat site. Hope to see you out on the Texas gulf late next fall/early winter.

Byrdman
 
Back
Top