TC 255 Hull Speed?

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peterbrownell

New member
Was just wondering what the true "hull speed" of the Tomcat 255 is. I know the formula is 1.34 times the square root of the waterline length but was wondering if the unusual hull shape of the Tomcat makes a difference. With a waterline of 25.5 feet, the theoretical hull speed should be something line 6.8 knots. Any thoughts on this one?
 
The waterline is more like 27+ feet because 25.5 doesn't take into account the "Armstrong Bracket" on the stern. That gives you about 7.02 knots.

Why are you trying to calculate that? The boat will not plane at 8 knots, it's still trying to come up out of the displacement mode. I think mine falls off plane at about 12 knots but I always kick it way up so as not to spend too much time "wallowing".

Charlie
 
That formula gives you the speed, in knots, of a wave trough moving through the water, corresponding to the length of a displacement hull being powered along and generating the wave trough. It's not an exact limit, but rather the location of a very sharp knee in the power curve as a function of speed. In other words, if you try to go any faster, the power required gets silly.

That formula is for sailboats, trawlers, tug boats, etc.

Planing hull boats follow a different curve. You can get to something like hull speed and then add more power and climb out of the water and plane on top.

-Jeff
 
I'm clear that once one gets a boat up on a plane, the efficiency curve is a totally different animal. Generally, the hull speed of a boat works out to be the fastest displacement speed one can efficiently travel at. Increased power beyond that speed (short of attaining planing speed) will produce very small increases in speed thus miserable efficiency. For example, I'm sure it would be terribly inefficient to cruise a Tomcat at 10 knots.

As we travel the rivers and have the time to go slowly and smell the roses, we were just wondering what the most efficient and comfortable displacement speed might be for a Tomcat.

Best
Peter & Linda
 
Peter, I have found that about 2000 RPM is very comfortable and quiet and gets me to 6+ knots. Not sure of the fuel flow as I don't have any meters on my Cat. If you really want to just save gas, shut one engine down and lift it out of the water. Beware though, with one engine up, steering is difficult unless you uncouple the engines with the liquid tie bar.

Charlie
 
If you want to find the uppermost most efficient displacement hull speeds, just 1.) pay attention to your rpm/engine noise/effort , and 2.) look at the boats wake and wave making. No need to have fuel consumption meters, or a Ph.D in Fluid Dynamics. Sometimes we overlook the obvious. :lol:

Joe. :teeth :thup
 
On our recent trip in the Broughtons, we were keenly aware of fuel usage due to the high prices. We often cruised on one engine with both engines down. We achieved 3.5-4.0 nmpg at about 6 knots. When destinations are close, the potential speed of the TC is immaterial compared to economy.
 
We have just spent 8 weeks on the TomCat. Most of that time cruising at 6 to 10 knots. The most efficient speed was at 1800 RPM. That gave 6 to 10 knots, depending on the current. The Flowscan fuel flows show about 1.25 GPH for each motor at 1800 RPM. When we trolled for Salmon we would use one motor at 600 RPM. That gave about 1.5 to 2.5 Knots, again depending on current. The Flowscans hardly recognize any fuel flow at 600 RPM. I have kept records of all miles traveled and fuel consumed. When we get home I will report on the entire trip.
On edit. We checked in at PT. Roberts Customs today. The seas were 3' to 5' and we were 12 miles from Blaine and our trailer. I ran 45 degrees into the waves at 20 to 22 knots and made the last 12 miles in under 40 minutes. When you need to go the speed and capability is definitely there on the TomCat.
Brent
 
On our recent 8 week trip to British Columbia we traveled by boat for 1172 statute miles.
The total fuel consumed was 417.33 gallons, for an average of 2.8 statute miles per gallon. Much of the time we traveled using both motors running at 1800 RPM. This gave us 6 to 10 MPH depending on current. When fishing for Salmon we trolled at 600 RPM on one motor @ 1.5 to 2.5 MPH. We ran for not more than 100 miles at cruise speed of 25 MPH. We were heavily loaded with food, water, clothing and fishing gear. In hindsight we had too much of everything, but thats the way we travel.

Fuel cost for the boat was Just under $2000.00. Gas in Canada ranged from $1.17 to $1.49 per liter. Gas to fill back up at home was $3.75 per gallon.

We were in a marina 19 nights, two nights on public docks, the rest on the hook. Marina's ranged from $.85/ft/night with power included (Blaine), to $2.00/ ft/night (False Creek YC, Vancouver). The most expensive 30A power was $20.00/night at Echo Bay. Showers ranged from free, to $5.00 each. We used Campbell River, and Port McNeil as the major resupply points. Another good spot was Heriot Bay. The cheapest Canadian fuel was in Campbell River, $1.17/L. The most expensive was at Blind Channel, however we heard that Refuge Cove was even more.

The trip was very enjoyable even considering the weather. June was cooler and wetter and possibly we would delay the starting until July next time. We experienced all types of seas, from glass calm to 3-5 ft waves on several occasions. Roger (Dreamer) always said the most dangerous thing on a boat was a schedule. Using that philosophy we were able to avoid most rough seas. On those occasions, the travel was at most just uncomfortable. The number of great, interesting people we met were innumerable. Overall, boaters seem to be terrific people. We look forward to our next trip--where ever that may be.

Brent and Dixie
 
Brent, what kind of mileage do you get on plane? When I first got my TC I was regularly getting 1.8 to 2.1 mpg which did not seem to vary much from 14 to 25 mph, as measured by my Yamaha electronics. Now I am lucky to get 1.7, even with all the cruising crap off the boat. I don't know if it is the non-copper bottom paint, engine tune or what. How critical is the parallel setting of the engines? Any suggestions from you and other TC owners would be appreciated!

Warren
 
Warren, is that calculated from gas usage and mileage? Or from flow meters? If it's from electronics, could it be a calibration problem??

Looking forward to getting my hands on THATAWAY which has flowmeters connected to the Suzuki NEMA network. My Suzis don't have that capability on CAPTAIN'S CAT and I hesitated to put the "propeller" in the gas line to measure flow.

Charlie
 
Captains Cat":hezjdhsl said:
Warren, is that calculated from gas usage and mileage? Or from flow meters? If it's from electronics, could it be a calibration problem??
Click to expand...

It's from the integrated Yamaha Command Link system. AFAIK there is no calibration, as it measures the fuel being squirted into the cylinders. Maybe Matt or Marc can comment further?

Warren
 
Captains Cat":zzwhsjwr said:
Warren, is that calculated from gas usage and mileage? Or from flow meters? If it's from electronics, could it be a calibration problem??

Looking forward to getting my hands on THATAWAY which has flowmeters connected to the Suzuki NEMA network. My Suzis don't have that capability on CAPTAIN'S CAT and I hesitated to put the "propeller" in the gas line to measure flow.

Charlie
Click to expand...

Yes, fuel consumption in terms of volume/time (gph) is more meaningful than volume/distance (mpg), unless you're always boating in conditions of zero wind/tide/current.

My knowledge of the higher power four stroke outboards is minimal at the moment.
Do all the larger manufacturers now use a NEMA protocol buss and are they still on 0183 or has everything moved to 2000?

I assume from your comment that reading fuel flow is just a matter of connecting an appropriate reader?

If the above assumptions are correct, do you know which year the 100+hp engines started generating this information?
 
One of the dealers that handle multiple brands of engines will have to chime in on the details of the networks they use and the info they provide. I do know that current engines, at least Suzuki, provide multiple info that can be displayed on the multifunction displays as gauge info with pointers, etc, etc. Oil pressure, temp, water temp/pressure, and all sorts of stuff you probably don't need to know.

From about 2005> I would guess it's available, not sure of the format, NEMA, etc that they use, may be different for different brands of engines.

Good hunting..

Charlie
 
Joedee and I just spent 2 nights on Martha's Vineyard and Woods Hole. That is roughly a 170 nautical mile trip for us. I have not yet calculated my numbers for this one but last year during a similar trip I averaged 2.02 nmpg (note nm) running at 22-24 knots and 3600 rpm.
 
Back to the original question--the Tom Cat is a planing cat, and will have different characteristics than a displacement or semi displacement cat. Some of the best curves are on Malcom Tennant's site: (He was killed several years ago, but his work remains).
http://www.catamarans.com/news/2006/04/ ... arison.asp

Generally the "displacement" formula does not apply to long thin hulls (such as two of them or three of them in cats or tris. It is not a hard line in semi displacement hulls either. The true displacement hull, will not go any faster than xx no matter how much power you apply (as already noted some sail boats, like the WestSail 32 or some tugs, and trollers etc.

The Tom cat is also a little different, in that there is not full tunnel clearance aft when underway at slow speeds, so there is some increased drag there.

The question is already answered, and that is slower the better as far as efficiency.
 
We have Floscan fuel flow meters. They have proven to be off by about 5%. Always showing more fuel was consumed than was actually used. If the meters show 100 gallons used, I can never get 100 gallons back in the tanks, usually about 95 gallons. This has been the case from day one, and has continued for more than 600 engine hours. I always compare the meter reading for fuel used to actual fuel replaced into the tanks. Until I refuel I consider the gas gone, and never plan on using the 5% error.

Our twin Honda 135's don't use fuel at the same rate from engine to engine. The port motor uses more fuel at low RPM's than the starboard motor does. The starboard motor uses more fuel at higher RPM's ( over 2000 RPM ).

In the calculations for our summer in the Broughtons, there was no way to calculate for the time spent idling the motors. There would be a small increase in mileage if idling could be accounted for.

We get about 2 MPG at planing speeds, 14 to 30 MPH. The problem with cruising boats is that over time you constantly add things to the boat. We have just put on bottom paint and all the Broughton calculations are with bottom paint. We have never just used a tank of fuel at cruising speed. So I can't verify fuel mileage at cruise speed. At 4000 RPM the Floscan's show a combined flow of around 9 GPH. In flat seas we will usually be going around 22 MPH at that RPM, or better than 2 MPG. Most of an average day's use is at 2000 RPM or less, and if fishing often with only one motor on at a time. I do try to keep the engine hours as equal as I can.

Next trip will be to Lake Powell, at 3550' elevation. So the mileage will vary.


Brent
 
Brent, Flowscan meters are the ones that have the propeller sender inline with the fuel line, are they not? I'd think the electronic ones hooked into the NEMA bus might be more accurate but I'm not sure!!

Charlie
 
Speaking from theory, not direct experience, if you want a cat with more efficient displacement hull speeds, get a displacement cat instead of a planning cat. The long narrow hulls are easily driven, and the tunnel clearance is greater by design.

We have a 24 foot pontoon boat (besides the C-Dory and Sea Ray), and the thing is effortlessly driven compared to a monohull, and that's with a 40 hp 4-stroke, although, of course, the light weight is also a big factor in efficiency.

Joe. :teeth :thup
 
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