How Fast Does Your Boat Motor?

[Uncle Bob]

Originally Posted by TooCoys

THAT is good information! Never heard that before. Thanks!

I would do a little research on the prop spinning vs locked concept, you may find that it is not such a good idea.

[TooCoys]

Originally Posted by Uncle Bob

I would do a little research on the prop spinning vs locked concept, you may find that it is not such a good idea.

Interesting! this post describes the same thing I was hearing. And when I was hearing it, we were moving quite fast. Much faster than motoring!!

[skipmac]

I think a lot of this has been mentioned but I'll go ahead and put all my comments here.

1. Slipping. If you have a problem in the transmission then the engine could be turning and the prop not. Just like a bad clutch in a manual shift car when you rev the engine but the car doesn't go.

2. Cutlass bearing. This is to support the prop shaft, keep it in line and prevent vibration. Unless something is jammed in it or similar issue it will not prevent the prop shaft from turning. If worn it can make vibration under power worse.

3. Prop turning under sail. This is common with most sailboats. How it works will depend on the transmission. Some transmissions you put in gear and the prop stops turning. Some (like my BW hydraulic) putting in gear doesn't stop the prop.

4. Engine horse power. 8 hp should be enough to move you at least 4-5 kts through the water in calm conditions IE no strong wind on the nose.

5. Clean prop. Your photos look OK. Very unlikely the prop would foul up in two weeks so probably not an issue.

6. Other boats passing you. How fast do they pass you? Do they zoom by or slowly pull ahead?

Questions.

1. What rpm is the engine turning? Can you get it up to max under power? Will it turn much faster rpm in idle than in gear?

2. Was the boat faster under power in the past?

3. Have you done anything related to the engine recently like new fuel filters, adjusting throttle cables, etc?

[thomm225]

Originally Posted by TooCoys

The prop was just scraped when I hauled it out two weeks ago. This is what it looked like before.

I know sailboats are slow, but when I'm being passed by other boats my size, its starting to concern me. I know the engine is fine because it runs without missing a beat, but its almost like the prop isn't turning like it should be. Almost like its slipping.

Should I be able to get 5 knots out of an 8hp engine on a 7000lb boat? Could it be that I'm underpowered?

I can get 5 knots motoring boat speed out of my 5 hp 4 stroke outboard.

Sometimes faster, sometimes slower depending on tide, wind, waves.

My boat's displacement is 6600lbs. It's a Bristol 27.

[Sailmonkey]

if you were motoring out the kemah channel and the tide was running in, the standing waves by the boardwalk will make all but the longest highest powered and heaviest sailboats crawl through. Also, even though your prop was scraped two weeks ago, it's still not that clean when you consider the extremely low pitch and the low power of the engine. even the slightest growth on there will will cause you to be very sluggish.

[Calif.Ted]

Yanmar has issued several TSB's that state the transmission should be left in neutral when under sail.

YANMAR MARINE USA CORP.
101 INTERNATIONAL PARKWAY
ADAIRSVILLE, GA
30103
TELEPHONE
(770) 877-9894
FAX
770-877-7565

BSITE WWW.YANMARMARI



dvisory Number: MSA08-003
A

:
DATE February 8, 2008
rs and OEMs
TO: All Marine Distributors, Deale
Sailing
SUBJECT: Gear in Neutral WhileODELS: All Sailboat Engines
M


We continue to get questions regarding the correct gear position while sailing with the
engine OFF. This advisory is issued as a reminder; Yanmar requires that if sailing with
the engine OFF (not running) the transmission shifter must be in the neutral position or
internal damage to the gear or sail-drive will result. This damage will not be covered by
Yanmar’s Limited Warranty. Please instruct customers and dealers who deliver the
sailboat to the customer, of the correct (Neutral) position for the marine gear while
ailing.
s

If the customer desires that the propeller shaft not spin while sailing, either a folding
propeller, shaft break, or other suitable device may be used. However, Yanmar accepts
no responsibility for the selection, installation, or operation of such devices. Please also
refer to Marine service advisory “MSA07-001_Yanmar Sail Drive Propeller Selection” for
dditional information.
a


If you have any questions regarding this advisory please contact a Customer Support
epresentative at (770) 877- 9894.
R



MARINE SERVICE ADVISORY

[Cheechako]

Your boat has a 22 ft waterline length, based on that you should motor at a little over 6 knots in flat water.
-Put the boat in gear and go as high rpm as it will go. How high is that? Is it within 15% of the max engine rpm rating?

[TrentePieds]

Quote: "I've got a 1980 27' with a Yanmar 1gm10 in it, and it motors extremely slow."

Your engine is a tad wimpy for your boat. It's rated at 10HP at 3,400 RPM or thereabouts. Actually, the output is more like 8HP. I would consider 15Hp minimum and 20HP better.. But there are lots of 27-footer that make it in and out of their harbours/slips on just a grody 9.9 HP mercury or other "egg-beater". I grew up with boats that had no engine at all so don't think that all is lost :-)

Go about this systematically and be clear-eyed about how much money you want/can afford to spend on the boat from now on. The investment so far you should consider what accountants call "sunk cost" i.e. a money outlay that you can not expect to recover in a sale.

Worst case scenario dealing with the engine is a complete replacement with a brand new modern engine. Allow $15K for that. If that becomes necessary, I would go for 20HP if I were you because NOTHING goes to weather better than 3200RPM :-)!

Now let's go back to the basics:

1) Your boats bottom is clean. 2) your prop is clean. So far so good.

Do you have the right prop for the engine? Do you know the specifications of your prop? The best prop for your boat would be a right hand turning three bladed prop of 11" diameter and 6" Pitch. With that prop you can expect a speed through the water (at Wide Open Throttle -"WOT") of just over five knots. To get seven knots out of the boat you need about 17HP.

To change the prop if it is wrong for the boat/engine combo, will set you back about 1K, but you must consider that just a "tuition fee" :-) If necessary you should do it when you change your cutless bearing.

If your existing prop is very close to the above stated dimensions you have more serious problems: It's unlikely that your tranny is slipping. You can only know that by measuring the RPM of your shaft and comparing it with your engine RPM compensating for the tranny's reduction ratio. For little money you can buy a stroboscopic tachometer that will read your RPM remotely without having a physical connection either mechanical or electric to the engine/shaft. And you won't have to call in a mechanic to do the job.

If you have a chance to take her out again before you haul, go to WOT and note the RPM on your tachometer. If you don't have one, you need to fit one, but for now you can by with the aforementioned stroboscopic job. You absolutely need to know your Engine RPM.

If you can tell us your engine RPM(WOT) and your prop dimensions we'll be far better able to help you.

You ask if propellers can slip. I take it you mean on the shaft. HIGHLY unlikely since in addition to the taper there is a key that makes a mechanical connexion twist shaft and prop. But all propellers have a trait called "slip". It arises from the fact that propellers (sometimes called "screws") work in a fluid, not, unlike machine screws, in a solid medium. We can come back to that later if you like. The prop dimensions I gave you above are compensated for slip.

TrentePieds

[TooCoys]

I don't have a tach. I've been looking for instructions on how to install one, but I'm coming up empty handed. If I can't figure out how to install it on my own, it will be a few months before I can continue to diagnose the problem.

[MarkSF]

"Sometimes faster, sometimes slower depending on tide"

Just to be clear, the tide does not change your speed through the water at all. It does change your speed relative to the ground.

[MarkSF]

Originally Posted by TooCoys

I don't have a tach. I've been looking for instructions on how to install one, but I'm coming up empty handed. If I can't figure out how to install it on my own, it will be a few months before I can continue to diagnose the problem.

The first thing to do is to determine if you have a problem. Which means measuring your speed THROUGH THE WATER. To do this you need to subtract the effect of current.

Some ways to do this without a knotmeter (never seen an accurate one, anyway) but with a GPS :

1) Wait until there is no current.

2) If you are in a narrow, restricted, body of water like a river or narrow estuary, it is reasonable to assume that the flow will be in the direction of the river or estuary. In that case, you can motor in one direction, turn around 180 degrees and motor in the other, and then take the average of the two speeds.

3) Use the local current table to estimate the current. Unfortunately, prone to error, as the tables do not take into account seasonal variations other than tides (ie, river in flood). The other week the current was 4 kts, and the tables said it would be 2. Reason : lots of flow down the rivers leading into the bay.

[KP44]

Sounds like your prop is very inefficient or something else in the drive train is not working. If not the prop, check your transmission. It may be slipping or working sporadically. I imagine when you do the cutlass bearing you will check the entire drive train.

You should be motoring at 4 knots or more, and hull speed over 6.5 knots. Hull speed is a function of waterline length, and in knots will be approximately 1.34 times the square root of waterline length in feet. Hull speed is the max speed, not the normal speed.

[seabreez]

Originally Posted by TooCoys

The prop was just scraped when I hauled it out two weeks ago. This is what it looked like before.

I know sailboats are slow, but when I'm being passed by other boats my size, its starting to concern me. I know the engine is fine because it runs without missing a beat, but its almost like the prop isn't turning like it should be. Almost like its slipping.

Should I be able to get 5 knots out of an 8hp engine on a 7000lb boat? Could it be that I'm underpowered?

With a look at the picture and this could cause enough resistance that you would feel no push from the engine. Clean it very good and try a run between two bouys and do a timed run and check the mph. Then try and compare with another boat the same as yours... try this first to see. The hull has to be cleaned also.

[thomm225]

Originally Posted by MarkSF

For the prop to turn in neutral, the boat has to be moving relative to the water - i.e., being driven by the sails. This would be normal.

If you were sailing down wind and against the current, the prop could turn if it was in neutral even if the boat wasn't moving

[TrentePieds]

TooCoys:

Start your posts with a quick quote (or a "handle") so we know if you are talking to a particular person :-)

If you don't have a tachometer permanently fitted in the boat, don't waste money and effort on one at this stage. For thirty bux you can buy one of these:

https://www.amazon.com/Neiko-20713A-...obe+tachometer

And that's good enuff for the present purpose. Because if is NOT mechanically connected to the engine you can measure both the RPM of the engine and the RPM of the shaft with one of these. Once you get used to the sound of your engine at different (known) RPMs you hardly need a tach other than your ear :-)

And don't think that you need a knotmeter to tell your speed. That is another one of those newfangled doo-dads that sailors can well do without.

If you want to know your speed through the water toss something biodegradable like a banana skin off the bow so it hits the water a little forward of the boat, the "count elephants", from the moment it passes your stem until it floats past your transom. Now follow this :-):

There are 6080 ft in a nautical mile, so at 1 knot (nautical mile per hour) you move 6,080 in 60 minutes, or 6,000/60 = 100 feet (close enuff) per minute. At 6 knots (which is as fast as you are likely to go, at least till you get more experience) you go 600 feet in a minute =10 feet a second. Your boat is 27 feet long, call it 30. So it will take 3 seconds for the boat to pass by the banana skin when you are going "about" 6 knots. if it takes 4 seconds, you speed through the water is "about" 4 knots. If it takes 6 seconds your speed is "about" 3 knots.

Why the elephants? Cos counting aloud "one elephant, two elephants, three elephants....." a a normal deliberate speaking speed takes one second per elephant.

In a small boat such as a Cherubini, you just don't NEED instruments. They are nice to have, but you don't NEED them :-)!

The one exception to that is a compass. But the BOAT doesn't need a compass. I have a set of binoculars with what's called a "bearing compass" built right in. In my coastal waters, that's all I need. Some other time when you've got your present problem handled, we can talk about how you "get by" with just a bearing compass when you are out of sight of land. :-)

TrentePieds