SF32 - 4kts under motor

[phil765]

Hi, new owner of SF32 1999 (also first boat) – our cruising speed with motor is < 4kts and max about 4.5kts & not much power for stopping and reverse, so thinking something not right?



We have the 2GM motor (recently rebuilt) start and revs just fine although no tacho – previous owner swapped to Radice folding prop – bottom should be pretty good as cleaned recently and around 5-6kts under sail.



Is most lightly prop undersize or not opening correctly? Can pitch on these be changed? Any advice on expected speed?

[zaphod]

Is it possible the prop is fouled and is not fully opening?

Does your engine run up to max rated rpm?

It is possible the prop is undersized, but you will have to get some measurements to verify that.

[rene460]

Hi Phil, welcome to the forum and congratulations on your purchase.  That should be real fun to sail.  

There are three components which determine the speed you get under motor, propellor, engine and hull.  Usually propellor sizing gets the first attention, but there are a few points in your question that make me think further.


First, achieving five or six knots under sail is not pressing the SF32 very hard, and should be easily achievable under motor as well.

The hull model, engine size and two bladed prop combination makes me think the previous owner was interested in all out racing.  My boat is a SO30i so somewhat smaller, though we are heavy for our length, and the boat was supplied with a 3YM20, around 16kW.

You said 2YM.  Have you checked the power rating of this motor?  Was it the original spec for the boat?

We achieve just on 7 knots under motor, wide open throttle, with everything new and clean.  Hull needing a clean at the moment we do about 6 knots wide open throttle.  (Due for a haulout sometime soon after this lockdown ends.). With your longer hull length, you should achieve a little more with the appropriate engine and propellor combination, if motor cruising is your aim.

Then the propellor.  A two bladed folding propellor is a common choice for those who want minimum drag when sailing.  But it is a compromise for thrust, especially in reverse.


I am thinking there is possibly nothing wrong at all, but you have bought a fast racing boat, with a minimum weight engine, low drag propellor, intended only to get out of the pen to the race course.  The main question is, only 7 knots under sail?  Possibly a bit slow for a thoroughbred, but perhaps there was not much wind.

I believe we have a couple of Sunfast owners on the forum, one of them may chime in with a comment.

To enable us to help you work it all out, can you please check the power rating of the engine, and the specified engine for the boat.  This information will help narrow down where to look.  Also tell us a bit about yourself and your sailing experience.

I really think you have a great boat, and exciting times ahead, even if you never want to race.  I will comment more on that aspect later.

rene460

[phil765]

Thanks for replies & info



I’ve no tacho atm so I can’t verify the rpm (hoping to fit one another post coming 😉) – the engine doesn’t seem to be labouring though it certainly sounds like its able to rev freely, even when opening the throttle wide (at which point can get around 4.5kts).



The motor is 2GM20 @ 18HP – it is the original motor which was recently reconditioned professionally.

It is our first boat so we are picking our days carefully 😊 - I’m sure in the right hands it is capable of a lot more under sail!



The boat was previously used for Thursday night racing so as you say last owner probably opted for lower drag.



We are more interested in cruising also its quite tidal here, coupled with a reasonably long fairway into our marina so often being below 3kts is a pain!



Assuming current prop is not fouled and opening correctly – any advice on a better suited prop?

[rene460]

Hi Phil, I can understand that low speed will be frustrating with a long fairway, particularly against the current.  However, remember that motoring tends to be limited by the hull length which determines the maximum reasonable speed through the water.  A head current will always slow you down.

The first thing to check with the propellor is whether it is fully loading your engine.  For this you need to read the rated speed from the specs.  It seems to be an older model, in keeping with the boat age, and I was not able to find it on line in a quick search.  Then you need to know the actual wide open throttle speed, if the tacho is a bit further off, it’s worth buying a digital non-contact hand held tacho.  They can be bought in an electronics store or possibly an auto parts store for around $50 and no doubt less online. They involve sticking a reflective tape to the pulley on the end of the crankshaft and pointing the laser light at the tape.  Obviously you will need a helper to do the test with a hand held unit. Though in the long run you will want the permanently mounted unit.

Yanmar recommended correct propellor sizing is one that allows the engine to reach about 5 to 10% above rated speed at wide open throttle.  Watch the tacho as you try this, and don’t go above 10% over.  Some forum members prefer a slightly bigger prop than this would indicate as they normally run well below max rpm for noise and economy reasons, though it’s not necessarily best for the motor.

This test tells you how well the propellor is matched to your motor.  If it easily reaches 10% above rated speed before wide open throttle, don’t push it further, but this means you can use a propellor that absorbs more power.  But that two bladed prop does not have much blade area, and a three blade folding or a fixed propellor with three generous blades might be more efficient in terms of generating thrust with the available power.  

It would also be worth measuring at least the diameter of the propellor and the tip clearance from the hull.  Unfortunately this requires diving or lift out unless you can find it in the documentation you received with the boat.  And also check the ratio of the gear unit between the engine and shaft, as the prop manufacturer needs to know the actual propellor operating rpm.

Then contact the manufacturer.  They are the best able to tell you what options they can offer.  It would also be worth contacting one or two other manufacturers to cross check both the selection and the cost.

I have a feathering prop, and it has plenty of thrust, including in reverse for stopping.  A fixed propellor is less expensive, most trouble free and probably the most thrust for a given power due to the large blade area.  But the prop manufacturers will explain the options with more authority.

I must say I am envious of your SF32.  My previous boat was an all out racer, and my wife and I were quite an anomaly sailing it only for cruising.  But I can assure you sailing fast is fun.  Pick your weather until your experience builds, and reef early and hard.  It will always be easier to handle with less sail area, and when the wind is light, you have options.  So get the inside comfortable.  A bar-b- que on the back rail is highly recommended, and have fun.

rene460

[zaphod]

Jul 28, 2021 8:58:27 GMT phil765 said:

Thanks for replies & info



I’ve no tacho atm so I can’t verify the rpm (hoping to fit one another post coming 😉) – the engine doesn’t seem to be labouring though it certainly sounds like its able to rev freely, even when opening the throttle wide (at which point can get around 4.5kts).



The motor is 2GM20 @ 18HP – it is the original motor which was recently reconditioned professionally.

It is our first boat so we are picking our days carefully 😊 - I’m sure in the right hands it is capable of a lot more under sail!



The boat was previously used for Thursday night racing so as you say last owner probably opted for lower drag.



We are more interested in cruising also its quite tidal here, coupled with a reasonably long fairway into our marina so often being below 3kts is a pain!



Assuming current prop is not fouled and opening correctly – any advice on a better suited prop?

Once you have a functioning tach you will have a better idea what is going on. If the engine was recently removed and reinstalled it could be a simple matter of a misadjusted throttle linkage.

It is also possible that the previous owner put the wrong prop on the boat, but normally it is not a choice between low drag and motoring speed. You can get both. In fact my Flexofold prop actually improved my motoring speed over the 3 blade fixed prop the boat came with. A lot more engineering goes into low drag props than fixed props.

We can go into greater details about the pros and cons of different low drag props if you determine that is the problem, but first things first...get that tach working!

[sitara]

You can use a hand held tacho (mechanical drive on the crankshaft). A 1GM I had on an old boat would not rev out correctly. The mechanic checked fuel supply, we tweaked the prop pitch and still no good. Found the fitted tacho was reading 500 rpm low.

[phil765]

I ordered a non-contact RPM meter off ebay not sure how accurate it'll be but should be close enough.



We've got the Yanmar engine panel without the tacho - think I can get a pulse pickup added to the Hitachi alternator will look into that at some point for a more permanent solution.



The engine does sound/feel like its revving hard with the throttle wide open - I certainly wouldn't be comfortable running it like that for any extended periods.

[alenka]

Phil,

The 2GM should be giving you little more speed than you are achieving that is for sure.  We had that engine in our last boat - a Hunter Legend 290 and could get in excess of 5.5kts easily.

The one time we had trouble was after we had transited an area of intense weed and we had lots of the stuff wrapped around the keel, not the rudder or prop. A quick dip might find an old tarp or something similar stuck to yours.

[Andrei]

Debris around the keel would slow sailing speed too, not just motoring - so unlikely given the description.   I had a situation a couple of times when kelp around the folding prop was preventing full opening - both motoring speed and max rpm suffered quite a bit.  This could be the root cause here, but in my experience it was also accompanied by strong vibrations.   Seems like getting a working tachometer is the first step, then a look underneath might be needed - to check the state of the prop and possibly to measure its dimensions.


 

[alenka]

True Andrei but a sail full of wind is likely to be more power than an 18HP Yanmar at an undetermined throttle setting. 

[Don Reaves]

Jul 29, 2021 10:22:20 GMT phil765 said:

I ordered a non-contact RPM meter off ebay not sure how accurate it'll be but should be close enough.



We've got the Yanmar engine panel without the tacho - think I can get a pulse pickup added to the Hitachi alternator will look into that at some point for a more permanent solution.



The engine does sound/feel like its revving hard with the throttle wide open - I certainly wouldn't be comfortable running it like that for any extended periods.

If you have a smart phone, you can use a frequency meter app to determine your engine's RPM. I use one called PanoTuner. It's available for iPhone and Android, so maybe you could use it.

The program displays the frequency of the sound picked up by the phone's microphone. It's typically used to tune a guitar or other instrument. But if it can hear your engine, it should be counting the number of times your cylinders fire each second. You have to convert the frequency in Hz to RPM, which you can do with the following formula:

    Engine RPM = frequency * K

    K = 60 * 2 / # of cylinders

The 2 is due to the fact that each cylinder in a 4-stroke engine fires only once every two revolutions. The 60 converts seconds to minutes. You compute K just once for your engine.

My engine has 3 cylinders, so K is 60 * 2 / 3 = 40

For your 2GM, K = 60

So read the frequency, multiply by 60, and you have the RPM. The advantage of this method is that you can usually get the frequency right from the cockpit. Plus, if you already own a smart phone, it's free.