an efficiecny prop

[Mike C]

To the engineers in the group, is this the correct way to calculate slip and how efficient you are using the engine prop combination.

for a 13 dia x 8 pitch
engine speed is rpm 3600
this gives how many inches per minute the boat would go if there was no slip at all.
there is about 6000 ft in a NM
this equals 37 knots
if the CD-27 is moving at 6 knots this means there is a slip of 84%

[tartansailor]

Sorry about the duplication.

[tartansailor]

Don't you have any gear reduction?

8"X3600revsX1ft.X60min.X1Knot all divided by 1revX1min.X12"X1hrX6000ft.

You see all the units cancel out except knots.

Do you have any cavitation? that would be my concern, as the slippage seems a bit on the high side.

Dick

[Boyd]

You prop should not turn much faster than 1500 rpm at full throttle, hence the transmission ratio of about 2 for most small boat engines.

What are you trying to figure out? Slip? which is a value of very limited use? Or evaluating your prop's performance?

Boyd
s/v Tern
CD30 MkII
Fort Lauderdale, Fla.

[Mike C]

Yes!, there is a 2.62 deductioon gear on this Yanmar

Don't you have any gear reduction?

8"X3600revsX1ft.X60min.X1Knot all divided by 1revX1min.X12"X1hrX6000ft.

You see all the units cancel out except knots.

Do you have any cavitation? that would be my concern, as the slippage seems a bit on the high side.

Dick

[Mike C]

You prop should not turn much faster than 1500 rpm at full throttle, hence the transmission ratio of about 2 for most small boat engines.

What are you trying to figure out? Slip? which is a value of very limited use? Or evaluating your prop's performance?

Boyd
s/v Tern
CD30 MkII
Fort Lauderdale, Fla.

[Mike C]

Boyd,
I am not getting any reasonable numbers for this trip I took with a new motor and paint job. I am trying to work out a table that will show both fuel comsume and condition of the bottom/prop. This is a new Yanmar 2 cylinder 15 hp. I'm told I can go up to 3600 but I don't have enough hours yet to go that high.

Example, my trip from Titusville to Jacksonville (FL) I used 9.5 gallons with a new painted bottom. I thought that just barely moving "drifting" I would be getting better than that. According to GPS I was averaging 4.2 knots for the 283.5 NM. Note there was some wind and current variations but I tried to keep everything as neutral as possible. Like traveling in the quiet times in evenings, 11pm to 4am, and at slack tide. I estimated extra weight at 300 lbs for the 20 gallons water and for the food.

Mike

You prop should not turn much faster than 1500 rpm at full throttle, hence the transmission ratio of about 2 for most small boat engines.

What are you trying to figure out? Slip? which is a value of very limited use? Or evaluating your prop's performance?

Boyd
s/v Tern
CD30 MkII
Fort Lauderdale, Fla.

[Boyd]

Hi Mike:

I have spent quite a bit of effort trying to pin down exactly the same numbers for my boat. I changed props recently so the reason I collected the data was to get my prop pitch optimized.

To accurately determine boat speed vs rpm, I made a chart derived from numerous runs in both directions (one against the current and one with) on a known course at constant rpm intervals (200 rpm). I took both GPS and Knotmeter readings. This takes several hours to get right and gives a good idea what speeds to expect at specific rpm's in flat water with little current/wind effect.

Since then I have kept careful records of fuel usage vs rpm and engine hours. One source of error is the fuel gauge is linear and the fuel tank is wedge shaped. This results in maraculous fuel usage with a nearly full tank and poor usage toward the end. I am in the process of making a fuel tank chart relating the tank readings to the actual fuel remaining which should help stabilize the numbers.

At this point it seems that my Westerbeke 21A uses about .38 gph at 1200 rpm (loafing), .55 gph at 2000 rpm, and .625 gph at 2500rpm (crusing). My max is 3000 rpm.

Several things which could account for your aparent excessive fuel usage are:

1. The tank shape and the dificulty in determining when its actually full.

2. The difference between speed over ground and speed through the water. My knot meter and GPS rarely give the same values. My knot meter is calibrated as well as those things can be. A bit of growth on the little impellor makes a big difference.

3. Its a new engine and tight so fuel consumption may be higher until it loosens up a bit.

4. Your prop is not pitched correctly.

Hope this helps.

Boyd
s/v Tern
CD30 MkII
Fort Lauderdale, Fla.

[Jim Davis]

You should be doing hull speed, sustained cruising, at about 2800. At 3600 it will be guzzling the fuel. The rule of thumb I use is to be able to just hit max RPM while the boat is tied along side (static load), and cruise at about 75%.

[John Vigor]

Example, my trip from Titusville to Jacksonville (FL) I used 9.5 gallons with a new painted bottom. I thought that just barely moving "drifting" I would be getting better than that. According to GPS I was averaging 4.2 knots for the 283.5 NM.

Mike

Mike, are you saying you used only 9.5 gallons to travel 283 miles?

John V.

[Boyd]

I didnt do the math on his consumption, which comes out about .17 gph. A very respectable consumption indeed. I just read the tone of his comments as a complaint about excessive consumption.

Perhaps I should pull out the calculator prior to engaging brain.

Methinks something is still amiss with these numbers.

Boyd
s/v Tern
CD30MkII
Fort Lauderdale, Fla.

[tartansailor]

Your CD-27 has a hull speed of 6 knots which if you do the arithmetic is equal to 607.7 feet per minute.

Now lets start there.

Lets determine your prop rpm needed to achieve hull speed. (ignore slip for now)

607.728 ft./min X 1 rev/8" X 12"/ft. = 911 prop rpm th get hull speed.
so
911 prop rpm X 2.62 gear reduction = 2388 engine rpm to achieve hull speed.

Now you want to cruise at engine max TORQUE not max hp. There is a hugh difference in rpm there.
It seems to me that 2388 is a bit much, but may be you have a high speed diesel

Look at the torque curve for your engine.

Tell us the rpm corresponding to max torque, and a prop engineer (who knows something about slip loss for displacement hulls) can suggest a starting pitch for your engine.

Dick

[John Vigor]

Boyd, a useful rule of thumb to remember is that a diesel engine uses roughly 1 gallon of fuel per hour for every 18 horsepower generated. So if you are cruising at half power with an 18-horsepower motor, you'll be using half a gallon per hour, and so on, roughly pro-rata.

My Westerbeke 13 on my CD27 with a 12 x 9 propeller uses about 0.56 gallon per hour at 2,300 rpm and 5 knots. Flat-out at 2,900 rpm it gives me 6 knots and uses 0.75 gallons an hour--a 34 percent increase in fuel consumption for that one extra knot.

And, incidentally, slippage on a CD27's propeller, as for most full-keel cruising sailboats, will normally be in the range of 45 to 50 percent.

Cheers,

John V.

[Mike C]

John,
I love that line about putting the brain in gear ( before hitting the send button).
an explaination: When I did the trip I had only two real world measurements the RPM and the GPS readings everything else was from calculations. Plus - I know I used 9.5 gallons because that is how much it told to refill the tank to the same mark. what I was trying to say is that I thought I should be getting better use out of the diesel.

Example, my trip from Titusville to Jacksonville (FL) I used 9.5 gallons with a new painted bottom. I thought that just barely moving "drifting" I would be getting better than that. According to GPS I was averaging 4.2 knots for the 283.5 NM.

Mike

Mike, are you saying you used only 9.5 gallons to travel 283 miles?

John V.

[John Vigor]

Mike, firstly, the credit for that saying goes to Boyd, not me.

Secondly, if you're really getting 29 miles to the gallon I am truly amazed. Just be thankful and don't touch a thing.

Cheers,

John V.