Tomcat propellers

E

Edward J

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C Dory Year
2008
C Dory Model
255 Tomcat
Hull Identification Number
cdo25514b808
Vessel Name
Sandbagger
Hoping for some advice for my Tomcat props. I've read many posts and looks to me every boat slightly different. I'm hoping to improve fuel economy and cruising speed. No fuel meter.
Here's what i have:

2008 Tomcat, twin honda 135's, three blade aluminum 14.8x17 honda props, bottom paint and 5 batteries in stern locker. I have not been on other Tomcats but mine seem to ride stern heavy.

RPM's and Speed (mph)
2000 8.1
3000 11.7
4000 20.4
4500 22.7
5000 25.5
5400 28.7 WOT

My speeds seem low compared to other Tomcats, I'm guessing the bottom paint and stern heavy is causing this. I'm thinking a new propeller could help. Thanks

Cheers Ed Jonson
 
Yes way under performing in comparison to what the TomCat 255 I owned with twin Suzukis and top speed at max RPM at just under 50 mph. Cruised in the mid30's.

The Honda 135 are going to be the same as the 150's until you get in the upper speeds and RPM, when the valve train shift takes over, I found that all of the Honda powered Tom Cats were slower than what I had on what I finally settled on.

Unfortunately the Suzuki's have a different final gear ratio and I was running 4 blade Power Techs at 22" pitch.

Are your props 3 or 4 blades? Aluminum or Stainless? Honda brand? You should be getting WOT RPM closer to 6000 RPM.

Your engines should be counter rotating and places the opposite of conventional placement. Conventional would have rt rotation on Starboard, and left hand rotation on the Port side. To get the most stern lift, you want the opposite, with Left hand rotation on Starboard, and Right Hand on the port.

If someone sold you both right hand rotation, you will not get the maximum left and speed/hole shot and handling.

Rotation inward, gives more stern lift and that is the reason for the reversal of placement.
 
Hi Edward,

Ten years with our TC255 has shown that it is quite sensitive to loading. Five (group 31?) stern batteries may be a record at 375 pounds. We get two nights at anchor with three group 31 FLA running the 3.0 CF fridge but turn down the thermostat with the last opening of the fridge door that night. Now adding solar to help.

Gas weighs six pounds per gallon, so full tanks is 900 pounds. Don’t cart that around on days that you don’t need a 300 mile range. We try to stay under ¼ tank when cruising locally. An extra 400 pounds makes a huge difference in a 26 foot boat.

There’s so much storage that ‘stuff’ tends to get put on but never taken off. We’ve found it helps to take off EVERYTHING at end of season to re-evaluate the need for the next Loop segment…don’t carry a propane heater around in the summer.

Open the engine bracket access plates…they can leak and allow the bracket to fill with water, which is another eight pounds per gallon and a lot of gallons can quietly live back there and can’t drain out. I’ve replaced the access plates twice and put silicone lube on the gaskets, which dry out.

Even a very slightly slimed or fouled bottom also makes a huge difference.

If you are comfortable cutting gas lines, the Garmin GFS10 gives a more accurate accounting…
https://www.anchorexpress.com/Garmin-GF ... 0%24137.99

The boat likes a lot of trim up at cruising speeds. At times myTrim gauges are approaching the ‘trailer’ position. You can safely experiment with finding the highest trim up that gives the highest speed and RPM but just short of ventilation (not harmful) …you’ll hear it, and the RPM will shoot up momentarily but rev limiters protect the engines. Too much trim down at cruise won’t allow the boat to plane on the flatter aft sponson sections resulting in slower speed, lower RPM and wasted gas.

I’ve experimented with just about every combination of three and four blade aluminum and SS props from 17 to 19 pitch. In our more common ‘heavy cruise mode’ our five star Yamaha tech has said don’t be afraid to go down to 15 pitch with extra stern lift if needed to get closer to the upper end of the 5,000-6,000 WOT ideal. She’s a “big ass girl that likes to squat.”

Before doing that, check the issues Bob noted and each of the above. Hope something here is helpful.

Cheers!

John
 
Bob and John,
Happy 4th of July BTW to you and all C-Brats members. I don't want to distract from Ed's question about the right prop for his Tom Cat 255, but I have always wondered how counter rotating Suzuki 140 motors would do on a Tom Cat 255. The motors are light for the power they produce at higher rpm. I would think they would do great on a Tom Cat 255 and help keep the weight off the transom of the vessel. If the vessel would be kept at a modest weight and with only 2 passengers, would they be acceptable for overall performance? Thanks for your reply. Gary
 
Hi Gary,

I hope I didn’t come across as advising ‘the right props’ for Ed’s boat. The ‘right props’ are the ones that allow the engines to achieve the RPM’s at WOT that the engine manufacturer recommends (5000-6000 for my F150’s) when the boat is loaded the way it most commonly is. Higher end is better. High altitude lakes often require a lower pitch.

I think all the surviving outboard manufacturers make great engines. According to Suzuki, twin DF140B = 838 pounds, vs 1058 for the 175 or 200. Last I heard, the factory was approving 4-cylinder outboards to 175 HP but not 200 HP even if the same weight for the TC255.

https://www.suzukimarine.com/outboards/115-200hp/

So bottom line, twin 140s would only save 220 pounds grand total over twin 175s (or 200s if the factory approved it).

For perspective, the TC255 has a 30 gallon fresh water tank. At eight pounds per gallon, that’s 240 pounds which I can (and do) easily shed by merely turning on the sink faucet and draining it overboard. Presto! 240 pounds lost, not just 220!

I’m sure the Suzuki 140 will drive the TC255 as well or better than any of the competition. I have no idea what the price differential would be, but I did ask my tech how much it would be to repower with trade-in to twin Suzuki 175…answer $30,000 plus rigging. But he said if we keep up the premier maintenance on the F150’s, they will likely last more years than we have boating left, even if we are as extraordinarily lucky as the Austins have been in that regard.

Ed:

Make sure your fish boxes in the cockpit aren’t holding rain/splash water (it can’t drain out unless you manually turn on the fish box macerator pumps). If you never, ever put fish in them, you could disconnect the plumbing pipe at the macerator pump and let in lay in the bilge, where the automatic bilge pumps can pump out the fishbox water.

Before each trip, pull up on each aft bilge pump float arm to ensure they work and empty any excess bilge water.

Bottom Paint is 40% copper and heavy…don’t re apply just because some is flaking off, wait until the underlying barrier coat (usually white epoxy) is starting to show.

Lithium batteries are the wave of the boating future, but I’m waiting until the first ten folks who got a ten year warranty on their batteries actually collect on it.

My first three year major errors were hauling around WAY more gas than we needed and not giving nearly as much trim up as the boat needed when at cruising speeds.

Happy Birthday America!

John
 
DayBreak":22frmm5s said:
Bob and John,
I have always wondered how counter rotating Suzuki 140 motors would do on a Tom Cat 255. The motors are light for the power they produce at higher rpm. I would think they would do great on a Tom Cat 255 and help keep the weight off the transom of the vessel.. Gary
Click to expand...

Gary, I owned a single Suzuki 140 for over 15 years on my 18'Caracal Catamaran.The 140 is the 115 block, and only gives more "power" when at the top end of the power curve. My feeling is that the 140 would be underpowered on the Tom Cat 255, and the weight back there is easy to put on or take off (batteries, gear, extra water, etc. Even crew.

My 225 was anything but light. Even then I would get close to 50 when lower water/fuel was present. It would not be unusual to run up Perdido Bay in a chop at 35 mph. I had a 130 on my first C Dory 25. I considered swapping that with the 140, and as I looked at the blocks, displacement and power curves, my decision was no--not any better than the 130--where the 25 was underpowered, the way I run boats (heavy).
 
Thank you John and Bob for your answer and explanation. The Suzuki 140 is a smaller block. To not skimp on power and looking for other ways to keep the weight off the transom is good advice. Thanks again. Gary
 
Thank you all for your comments. A little history, my Tomcat is fairly new to me about 14 months. Boat came with 3 house batteries and two starter batteries. It also came with 12 v hot water tank so that's probably the reason for 3 house batteries. I was thinking of changing out the propellers and than eventually new lithium batteries but now i'm thinking new lithium batteries and then check speed and rpm's and then possible new prop's. so couple questions:

if i remove the 12 v hot water tank i'm assuming two house batteries would be sufficient? I do have solar panels.

can lithium batteries be used for starter batteries? I get conflicting info with internet search.

Thanks. Ed Jonson
 
There are some LiFePO4 (Li) batteries which can suffice as starting batteries.
I suggest that you do a good search on the internet, including "The Hull Truth" and Will Prowse.

The reason I am not giving you the several names of batteries which can be used for a starting battery, is that there is a lot more than just showing a battery in as would be done with a flooded Lead Acid starting battery (or two). You need to read, and understand what the limitations are, what charging can be used, both mains power, solar and engine. There a lot of complex interactions and dangers still there for example damaging the outboard engine's charing circuit...
 
I can't help you with you Honda's but can only provide some info on my boat with 2009 Yamaha F150's, 500 hours, no bottom paint.

My left engine prop turns counterclockwise, my right engine prop spins clockwise, so opposite of what Bob said is preferred.

My boat at 3,500 rpm runs in the mid 20 mph range. I am almost always in the snotty PNW open Pacific and loaded very heavy so don't pay too close attention to rpm/cruise since I'm constantly adjusting for big waves.

In smooth water, my boat achieved 46 mph flat out, 3 adults, 4 kids, half fuel day trip (lightly loaded}.

Regarding batteries, my boat has 2 starts, I believe group 24, they're small, and one group 31 house. The house is going away this fall and will be replaced with a 100 AH Lithium with a Yandina Lithium isolator. When the starts are due for replacement, they'll get replaced with a single Group 31 start.

Regarding your question on Lithium starting batteries, I know some battery manufacturers are now offering Lithium start batts. The issue with a "normal" lithium is for one, they don't offer the cold cranking amps required for outboards and two, when a "normal" lithium battery is full of charge, it says "I'm full, no more charge thank you" and it takes itself offline. This offline can/will nuke your boats charging system. And 3, if you happen to do any cold weather boating, lithiums don't charge when they're cold. So in theory, you could start your boat several times that day, but the battery would be too cold to charge when the engine starting depleting the battery. They've worked around that now by installing heaters inside the battery that use it's own juice to keep the battery above it's min charge temp (usually right around 32F but they're all a bit different). So on the last issue, kind of like a Tesla as far as using their own energy to keep warm. This works great, but sooner or later, if left alone in the cold, they'll use all their own juice just to keep warm.

I'm a big fan of solar and had it one my last boat and my RV (also with lithiums). I currently have 3 lithiums on my TomCat to run my trolling motor. After watching my Victron Smartshunt tell show me how fast my outboards charge my lithium troll bank, I don't think I'll bother with solar. Super quiet engine burns 0.5 gallons per hour at high enough idle to charge. Lithiums just gobble the power. So instead of adding solar or more batteries, consider filling your tanks with a few more gallons of gas and using the engines to generate some quick, easy, quiet power to replenish your new lithium house battery.

Rob
 
A comment about the water heater. Several C Dory owners did what I did and put a Bosch 2.5 gallon quick heat water heater outboard of the galley sink. This frees up a full area under one of the dinette seats, plus cuts down on the wasted water you use to get the hot water to the sink or shower. I don't understand why C Dory plant does not do this or at least offer it an option. It would be cheaper for them. And better for the boat owner. The cost at Home Depot is about $180.

I also had the Bosch heater set a fairly high tempature. I have a mixing valve for the shower, so that there is plenty of hot water, much faster than with the large ones 6 or 12 gallon heater.
 
I agree with all the advice above, especially from Bob and John. I do not have experience with the 255 but I have some suggestions based on my experience with my Tomcat 24. I think you have a lot to gain by getting rid of some of those batteries. Ask yourself how you are going to use the boat. If you are primarily going to stay at marinas than all that house battery capacity is not needed. I find that the boat rides so much better when it is light and especially so when it is light in the stern. Even if you do not gain much speed, the boat ride and handling will improve. Also, take a look at the engine height. I notice many boats have the engines mounted lower than necessary. This increases drag and slows the boat. Lots of articles can be found discussing ideal engine height.
-Jeff-
 
Just want to give an update on my stern heavy Tomcat. I had 5 batteries in the stern locker, moved them up under the helm floor and now have two starter and two house batteries. What also changed is my outboards were mounted to high. I could never figure out why trimming the performance decreased, higher rpm's and lower speed. The motors were lowered by 3 inches and with less stern weight its like a new boat. i now plane at lower rpm's / speed and WTO speed increased, all with same props.

Thank you all for your advice. Ed
 
Ed,

What's your speed now? I used to have Honda 135's on my boat with two 6D batteries in the stern and getting to 40MPH was straight forward. I've since repowered with Honda 140's (they don't make the 135's anymore) and I've reconfigured so that I just have two group 24 batteries in the stern. Still get around 40/42MPH at WOT. My guess is that most of the impact on your speed/planing came from lowering the engines.
 
Roger i think your correct most of my improvement was from lowering the motors. There is a improvement in ride and much better self bailing with batteries moved up. My WOT speed now is around 33 mph with 5200 rpm's, still low but better then before. What prop's are you using? Thanks Ed
 
Edward J":qxuhwvvm said:
Roger i think your correct most of my improvement was from lowering the motors. There is a improvement in ride and much better self bailing with batteries moved up. My WOT speed now is around 33 mph with 5200 rpm's, still low but better then before. What prop's are you using? Thanks Ed
Click to expand...
That suggests you are still over prop’ed. What is the current prop? Blades? Cup & pitch?
 
Hi Bob, my props are 3 blade aluminum 14.8x17. Ed
 
Edward J":2t0e9dkd said:
Roger i think your correct most of my improvement was from lowering the motors. There is a improvement in ride and much better self bailing with batteries moved up. My WOT speed now is around 33 mph with 5200 rpm's, still low but better then before. What prop's are you using? Thanks Ed
Click to expand...

I'll have to check next time I'm at the boat. They are the stainless steel props that the prior owner put on the boat and I've never had a reason to change them so I haven't paid much attention to the specs.
 
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