I am a new owner of a 1989 Tartan 31 Sirena in Jersey City, NJ. I am seeing a maximum speed of 4.1 knots at 2800 RPM with a Yanmar 2GM. Is this normal for a boat this size with this engine?
Question about boat speed under power (Tartan 31
- Thread starter zmarks
- Start date
Probably need a different propeller. What do you have now in terms of diameter, pitch, no. blades, fixed or folding? Check with a prop supplier to match prop with engine hp.
jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 22,820
Before you start doing anything, insure your boat speed is being properly measured. Are you measuring speed over the water with a water flow sensor or over the bottom with a GPS unit? If you are doing it with a GPS unit you need to make two runs with the 2nd being in the opposite course of the 1rst. Average out the speed of the two runs. Your boat should be doing close to 4.5 knots at 2,400 RPM. What is the maximum RPM you get when running at wide open throttle? If you cannot attain 3200 RPM either the bottom of the hull needs cleaning, the engine has lost power or you are over propped. The size and pitch of a proper propeller should allow for the engine to approach 3200 RPM, but not to exceed it. The RPM measurement should be taken by a calibrated tachometer to be reliable. Do you know how many hours on that 30 year old engine? A well used engine may not have the power to push the boat's prop. A fouled hull by marine growth will increase the drag and affect RPM and boat speed. Use a process of elimination to determine the cause and start with the simplest and least expensive possible causes. Confirm that your speed and RPM readings are accurate and reliable. Check and clean the hull bottom if necessary. Check that the prop is also clean of marine growth. Identify the Type and Size of your current prop. 2 blades, 3 blades, foldable? If after you have exhausted all possibilities you still cannot get a gain in speed then use the size of your current prop to figure out what lower pitch would give the engine's rated RPM. The boat's are slow as they are so don't squirm about a few fractions of a knot, but do try to attain the rated RPM as the engine will run more efficient at around 3000 RPM.
Start with the simple things.
Are you getting an accurate speed measurement? During slack water use a stop watch to time how long it takes to go a known set distance at a given rpm. Turn around and go back, timing the run. Do the math, does your math match the knot meter's reading? If it doesn't recalibrate the knotmeter.
Is the hull clean? Prop clean?
What is the motor's rpm at wide open throttle (WOT)? How fast can you go at WOT? How fast are you going at 80% of WOT? Reporting speed and rpm as a percentage of WOT is more useful than just rpm. RPM is different for different motors and transmissions. The engine's manual should have the specs on WOT.
Are you getting an accurate speed measurement? During slack water use a stop watch to time how long it takes to go a known set distance at a given rpm. Turn around and go back, timing the run. Do the math, does your math match the knot meter's reading? If it doesn't recalibrate the knotmeter.
Is the hull clean? Prop clean?
What is the motor's rpm at wide open throttle (WOT)? How fast can you go at WOT? How fast are you going at 80% of WOT? Reporting speed and rpm as a percentage of WOT is more useful than just rpm. RPM is different for different motors and transmissions. The engine's manual should have the specs on WOT.
Thanks to all for the quick and informative responses. Let me answer a few of the questions I should have included in my original post.
The bottom and prop are clean as the boat was just launched 3 weeks ago after new bottom paint
The speed of 4.1 knots I referred to is boat speed from a mechanical knot meter and not SOG from the GPS
The engine is 30 years old with 1400 hours
RPM at wide-open throttle is 3100 RPM
The prop is two-blade and folding
The bottom and prop are clean as the boat was just launched 3 weeks ago after new bottom paint
The speed of 4.1 knots I referred to is boat speed from a mechanical knot meter and not SOG from the GPS
The engine is 30 years old with 1400 hours
RPM at wide-open throttle is 3100 RPM
The prop is two-blade and folding
How different are the two measures, SOG and knot meter speed?
The calibration could have changed or if a really old knotmeter, it could be at fault.
Has the prop ever been refurbished? it is possible that the blades are not opening completely or the stops are worn and they open too far effectively changing the prop's pitch?
Have you noticed any difference while powering in reverse?
The calibration could have changed or if a really old knotmeter, it could be at fault.
Has the prop ever been refurbished? it is possible that the blades are not opening completely or the stops are worn and they open too far effectively changing the prop's pitch?
Have you noticed any difference while powering in reverse?
When motoring in areas with no current or tide the SOG and knot meter speed are the same.
Since I am new to this boat for 6 weeks, I cannot comment on power in reverse. I seem to have enough power and control to drive the boat in reverse when docking.
When the boat was on the hard in April, I greased the prop internals and opened and closed the feathering. The operation seemed normal and blades moved freely.
Calculation of hull speed comes out to 6.83 knots which I have been able to achieve on a beam reach in 12 knots of wind.
Calibration can be ruled out.
What brand of prop? Maxprop? Did you disasseble the prop to grease it?
Yes it is a Maxprop. I did not disassemble the prop but removed the grease fitting bolts and used a grease gun to inject until the excess grease came out.
Did you do a sea trial before buying the boat? It is possible the prior owner had the pitch set incorrectly. Can you contact him and ask if he had the same issue With slow speed?
Contact PYI and ask for the correct pitch. They will give you correct angle and the settings. Changing the pitch is pretty easy and pretty easy to screw up. The first time I did it in under an hour. I’ve seen it done in less than. A half hour. The boat does need to be hauled to do this easily. An experienced diver might be able to do the job, but I’d haul the boat.
With a static LWL of 26 feet, and moderate beam, once you get the prop properly pitched, you should be able to motor at around 6.5 kts in smooth water, at your motor's stated "cruising rpm's" Check you engine manual for that information. You will often sail faster than that, BTW. Downwind on a breezy day you may exceed 8 kts at times.
One other thing, do not get seduced by the ease of reading out "speed" on a GPS instrument. That is SOG , and useful to predict when you will arrive at some distant point. Good to know, but what you need to have to evaluate everything to do with sailing and motoring basic information, is speed thru the water.
As others have hinted, check on the accuracy of the boat's KM.
One other thing, do not get seduced by the ease of reading out "speed" on a GPS instrument. That is SOG , and useful to predict when you will arrive at some distant point. Good to know, but what you need to have to evaluate everything to do with sailing and motoring basic information, is speed thru the water.
As others have hinted, check on the accuracy of the boat's KM.
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Any chance of getting info from the previous owner? If he experienced the same then the prop was never properly pitched. If he had 'normal' speeds under power (5.5-6.5 knots) and the boat is clean, somehow the pitch has been messed with... As mentioned setting the pitch is relatively simple but you do need to pay attention to the instructions and line up the letters and numbers just right. The MAX prop is a beautiful bit of engineering.
Max prop only makes feathering, right? So it must be feathering. I think there are a few things going on ... 2GM is a little undersized for this boat. Max RPM at 3,100 is low. Under load, you should be getting 3,400 with that engine (3,600 in neutral). You may be losing compression. Do you see black smoke at WOT? The prop could also be under-pitched (possibly to partially hide a compression issue?). I'm not a diesel mechanic, but I would consider all of these issues.
My boat is a little heavier and I have 3GM30f in her. While I do get 3,400 rpm under load, I'm not getting hull speed in calm water with slack current, but I do get over 6 knots so it is close enough for me. I have Max Prop 3-blade feathering and the pitch just looks low to me. I've questioned myself about adjusting the pitch but I haven't done anything about it yet.
My boat is a little heavier and I have 3GM30f in her. While I do get 3,400 rpm under load, I'm not getting hull speed in calm water with slack current, but I do get over 6 knots so it is close enough for me. I have Max Prop 3-blade feathering and the pitch just looks low to me. I've questioned myself about adjusting the pitch but I haven't done anything about it yet.
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And how accurate is that 2800 RPM ? Have you calibrated your ship's tach with an optical tach ?
No guarantees when the price is this low but a little research would be in order.
Even more reason to suspect your tachometer may be the problem
.If he is really short on RPM (yes, "if"), one thing to look at is the throttle handle and linkage. Wear and tear and faulty adjustment might keep it from opening all the way. If the thing is supposed to be in the 34-3600 range, 2800 is way, way off, no?
Good Points... about "assumptions"...
I bought an inexpensive laser tach to check on our new tach install (part of new engine install) in 2018. Engine panel instrument was spot on, and it's good to know that.
Doing a two-way speed run to check on calibration of the boat's KM is also a great idea on a new-to-you boat.
Cables for shift and throttle that are old can get sticky and sometimes break. I replaced both of ours about ten years ago due to binding. At about 30 years there are lists of parts to be replaced on any boat, even the top tier boats like a Tartan. (The basic structure is superior, but all systems decay at a constant rate.)
Another place to check on assumptions is wiring connections. Start by removing, one at a time, and cleaning up and re-tightening ever terminal screw. Including the back of the engine panel. Surprising how many little things will start to work a bit better or faster... !
While your basic 110 household electricity really wants to move and sometimes zap you... Twelve Volt DC electrons in 20+ year old boat circuits sort of saunter up to each connection or terminal strip (both plus and minus sides), hesitate and discuss whether they are interested in going further, and then saunter on to the next one. I have measured heat in old OEM circuit connections with a laser pointer temp sensor, and it's not reassuring or comforting. At all.
Anyway, you have a marvelous design from a high quality builder. Take care of her and she will take care of you!
I bought an inexpensive laser tach to check on our new tach install (part of new engine install) in 2018. Engine panel instrument was spot on, and it's good to know that.
Doing a two-way speed run to check on calibration of the boat's KM is also a great idea on a new-to-you boat.
Cables for shift and throttle that are old can get sticky and sometimes break. I replaced both of ours about ten years ago due to binding. At about 30 years there are lists of parts to be replaced on any boat, even the top tier boats like a Tartan. (The basic structure is superior, but all systems decay at a constant rate.)
Another place to check on assumptions is wiring connections. Start by removing, one at a time, and cleaning up and re-tightening ever terminal screw. Including the back of the engine panel. Surprising how many little things will start to work a bit better or faster... !
While your basic 110 household electricity really wants to move and sometimes zap you... Twelve Volt DC electrons in 20+ year old boat circuits sort of saunter up to each connection or terminal strip (both plus and minus sides), hesitate and discuss whether they are interested in going further, and then saunter on to the next one. I have measured heat in old OEM circuit connections with a laser pointer temp sensor, and it's not reassuring or comforting. At all.
Anyway, you have a marvelous design from a high quality builder. Take care of her and she will take care of you!
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