More Tomcat observations/chat

Doryman":2idqz05l said:
...

Any ideas about what might be going on? I tend to fill my tanks when at the fuel dock.

Warren
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As I'm sure you know, full tanks on the Tomcat means 900lbs of gas. That will most certainly account for some decreased mileage compared to numbers taken with less total weight.

The status of the bottom paint will dramatically affect mileage as well. I learned this lesson a few years ago on a former boat. I had some slime on the hull, not a sweater or anything, but some green slime. It reduced my max speed by 10mph! I thought for sure I had an engine problem, I never dreamed some slime could cause that much friction. I spent 20 minutes with a power wash and got my top speed back.

I have twice made the trip from Salem to the Isle of Shoals and back. One time I carried the dink on top, one time I did not. There was no measurable difference for me in the fuel usage. I'm eye balling the fuel gauges for my calculations thus far so it's certainly not precise. Next year with fuel flow meters I will have more accurate data.
 
I agree, I don't think that the dink makes much difference--but I do have Radar, a hailer, two antennas, and a Camper canvas--all of which will make wind resistance at higher speeds--but should not make much differnence other than weight.

Most of us trailer or lift keep the boats, so the bottoms should be clean. (I used to wet and the bottom of my racing sailboats with 600 wet paper before important races--and wipe with terry cloth before not quite so important races. )

Although weight makes a difference--it would make even more difference at low speeds, if the tunnel were clear--but at least in most boats I have seen, the tunnel is occluded aft at speeds below a plane, regardless of loading--the more loading, the more surface area of the bridge deck is exposed--and more drag.
 
Dr. Bob et al,

Mine was not a strictly controlled, scientific test. The winds were shifting from S to SE to N and around, although I did try to double back and average the readings.

My high speed top end and mpg are less now than early on - prob' due to the bottom paint and algae and excess equipment.

As you mentioned, the readouts, even on the Honda guages, blink on and off when under 1 gallon per hour. On some occasions, the dial would read 2.0 mpg reading both guages, but one or the other would blink on and off reading single engine use.

The main reason I posted was that my personal experience is that I can cruise from around 7.8 to 8.6 mph and still get about 3 gallons per mile flow. As you mentioned, 5-6 mph increases the economy to nudge 4 mpg.

This may sound crazy, but I am wondering if the very heavy stern (with the genset, extra fuel, bbq's, ice chests, etc, hold part of the bow high and somewhat out of the water at slow speeds.

Also, I am using statute miles.

So, don't put a great deal of weight on my figures, I just putt around here
and daydream most of the time without the dire need for exact figures that you ocean-going craft require.

Thanks again for the data,

John
 
Hmm--I could put the bow up with the Permatrims. I'll have to check that. I know if I put the bow down to force the boat on a plane at low speeds, it decreased the mileage (increase fuel consumption at the same speed) Also the Test runs of the Tom Cat showed some extremely optimistic numbers:

The Boat Test.com Two Honda 150's 3 people and 1/2 fuel, no water or gear.

RPM...........stat. MPH......Gal/hr......Miles per gallon
1000..........6.7.................1.2............5.5
1500..........7.4.................2.1............3.5
2000..........8.6.................2.9............2.9
2500.........11.9................4.8............2.5
3000.........20.5................6.4............3.2

On a plane, they get 50% more than we are getting (and the 2 to 2.2 is a very common nunmber. I did some mileage checks when the boat had no gear on it--but there were full fuel tanks, which were closer to the 2.2 Statute miles miles per gallon.
 
Just made a 200 mile run back from Corpus Christi to Clear Lake- fully loaded boat with two people: 1.96 to 2.12 statute miles/gallon. The mileage did not change between 3100 and 3700 RPM; most of the time we were running at 30 mph even. Our Navman has not been "officially" caliberated, but seems to be pretty accurate out of the box.

Our boat has twin Suzuki 150s with Permatrims and is set up the same as Bob's boat.
 
Matt, Using only one engine, either engine, I must turn the engine significantly to keep a straight course. Perhaps the smaller Honda props are the reason for this.

I could find no speed at which using one engine at a particular speed used less fuel than using two engines. Using two engines at slow to moderate speeds used at least .2 to .4 gallon per hour less than going the same speed on one engine.

When my TomCat was new, with no bottom paint and a freshly waxed hull, I, too could obtain the 2.2 mpg rate at speeds around 30 mph, BUT now that the hull is painted, heavily loaded and algae coated, I can no longer go 40 mph and usually run in the 1.86 mpg rate at high planing speeds.

John
 
drjohn71a":tnxxxepi said:
Matt, Using only one engine, either engine, I must turn the engine significantly to keep a straight course. Perhaps the smaller Honda props are the reason for this.....

John
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Well that's interesting John. Perhaps it is the props but I can't get my brain around the physics of why. Do you have to correct in the same direction, regardless of which engine you use, in order to go in a straight line?
 
Here is a somewhat related question about the operating temperature of my Yamaha 150s.

Under any sustained RPM under about 3500 my temp gauges read something like this:

------|------

that is 50%, exactly in the middle of the scale (there is no actual numeric readout)

Over 3500 RPM for more than say 5 or 10 minutes the temp gauges change to:

----------|---

that is 75%, halfway between the middle and the top

Obviously, an engine at 4000 RPM is working harder than one at 2500 RPM, and thus generating more heat. But what I am unsure about is whether the higher-RPM temperature is normal or abnormal. Any thoughts?

Thanks.
Warren
 
matt_unique":33z86x2g said:
drjohn71a":33z86x2g said:
Matt, Using only one engine, either engine, I must turn the engine significantly to keep a straight course. Perhaps the smaller Honda props are the reason for this.....

John
Click to expand...

Well that's interesting John. Perhaps it is the props but I can't get my brain around the physics of why. Do you have to correct in the same direction, regardless of which engine you use, in order to go in a straight line?
Click to expand...

I won't answer for John but I would think that, because the engines on a TC are quite far off the centerline, using only one would require it to be turned to keep the boat heading straight. If using the stbd engine, you'd have to turn it to starboard, and vice versa... :idea: :?:

Or did I misunderstand the question?

Charlie
 
Captains Cat":2bfe9fb4 said:
matt_unique":2bfe9fb4 said:
drjohn71a":2bfe9fb4 said:
Matt, Using only one engine, either engine, I must turn the engine significantly to keep a straight course. Perhaps the smaller Honda props are the reason for this.....

John
Click to expand...

Well that's interesting John. Perhaps it is the props but I can't get my brain around the physics of why. Do you have to correct in the same direction, regardless of which engine you use, in order to go in a straight line?
Click to expand...

I won't answer for John but I would think that, because the engines on a TC are quite far off the centerline, using only one would require it to be turned to keep the boat heading straight. If using the stbd engine, you'd have to turn it to starboard, and vice versa... :idea: :?:

Or did I misunderstand the question?

Charlie
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Hi Charlie,
I mentioned in an earlier post that running with one engine, even wide open and getting up on plane, I did not have to make the helm corrections some have mentioned with the Tomcat. I'm trying to figure out why John would need a significant correction to go straight when I do not.
 
Perhaps it may have to do with the directional rotation of the props. I believe Matt has LH rotation on the starboard side and RH rotation on port side. Is that correct? It would be interesting to know what John's Tomahawk setup is.
 
My rotation setup is the same as Matt's.

I did fill up with fuel for the third time and now the average total use of fuel is pretty much even between the two engines. One time was even use, one time the Port tank took a bit more and the last time the Starboard tank took the same bit more. It all averages out even.

Don't spend too much time worrying about any discrepancies in my readings. I only posted them because I seem to be getting better mpg around 8 mph than some others and worse mpg at 28 mph.

Again, my TomCat has been immersed almost two years now with rare times out, in mostly fresh water and is heavily loaded, so it would not be a surprise if the numbers were different from those with differing conditions.

John
 
Bad Boy":19wsxpdr said:
Perhaps it may have to do with the directional rotation of the props. I believe Matt has LH rotation on the starboard side and RH rotation on port side. Is that correct? It would be interesting to know what John's Tomahawk setup is.
Click to expand...

That is correct, starboard engine is LH rotation, port engine is RH.
 
Re: Having to correct the engine position to go straight:

If I run the Starboard engine with the Port engine up and clear, the boat goes to Port. I have to turn the the Starboard engine significantly to Starboard to get the boat to run straight, and vice versa for the Port engine only driven situation.

I am having a tough time figuring out how one can go straight with an engine headed straight when only one engine is operating.

At any rate, there is a .2 to .4 gal per hour cost to running one engine at 5-8 smph on my boat compared to running two engines at the same speed. I am thinking this cost is due to the degree of correction required.

John
 
drjohn71a":3k3is0z8 said:
Re: Having to correct the engine position to go straight:

If I run the Starboard engine with the Port engine up and clear, the boat goes to Port. I have to turn the the Starboard engine significantly to Starboard to get the boat to run straight, and vice versa for the Port engine only driven situation.

I am having a tough time figuring out how one can go straight with an engine headed straight when only one engine is operating.

At any rate, there is a .2 to .4 gal per hour cost to running one engine at 5-8 smph on my boat compared to running two engines at the same speed. I am thinking this cost is due to the degree of correction required.

John
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The non-running engine would cause drag and certainly pull you to some degree. When I did my single engine test, I trimmed the other engine clear of the water.
 
I trimmed the non-running engine clear.

I still am amazed that you have no steering disturbance by running only one engine - even with the other clear of the water.

In my experience, you could drive the boat home by just running one engine or the other. It seems to defy the laws of physics and decades of personal experience in twin engine craft that running only one screw makes no difference in the steering tendencies. Should they re-write Chapmans?

John
 
Many twin engine "craft" also have rudders. A destroyer can go "straight" with only one screw turning but the rudder has to compensate for the thrust not coming from the centerline.

I'll try my TC next time I get underway.

Charlie
 
drjohn71a":16t835v0 said:
I trimmed the non-running engine clear.

I still am amazed that you have no steering disturbance by running only one engine - even with the other clear of the water.

In my experience, you could drive the boat home by just running one engine or the other. It seems to defy the laws of physics and decades of personal experience in twin engine craft that running only one screw makes no difference in the steering tendencies. Should they re-write Chapmans?

John
Click to expand...

Ha ha, I don't think they should re-write Chapmans, but for whatever reason I do not have to make significant corrections to go straight. Perhaps a slight correction to account for the fact that the thrust is not coming from the centerline but very negligible. I mentioned I ran at several power settings including wide open and planed off. There must be something going on with our difference in props or counter-rotation or something.
 
I have to also correct for the engine being offset, It is only a little more than 1/8 turn, but there is definate pressure, and the boat would do circles if I didn't correct.The twin screw trawler I owned had to have rudder correction when running on one engine.

Matt, Did you say that you got on a plane with a single engine? That is just not possiable with my boat. I think I have tried all combinations of trim.--maybe I didn't trim the bow up enough--I'll have to try that.

What was your RPM Vs speed on a single?

I cannot get max RPM with the single engine. I do have lower pitched props and I could try that to see if it would plane the boat---however, I believe that it would be severely lugging the engine.

Since we are all farily close at planing speeds, I suspect that the lower speeds have to do with loading, attitude of the boat, and the accuracy of the flow meters. I did one fill up to fill up (about 200 miles), mostly at 7 knots and my flow seemed within 5%. But as you go lower, it will be less accurate.
 
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