Poor boat speed

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Burrix

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I have a 29 ft sailing boat equipped with a Yanmar 2GM20 with sail drive.
The boat has 8m LWL and the weight is about 3000 kgs. She is a cruiser racer, very fast under sail.
The problem is that the speed at 2800 rpm is around 5 kts, with the bottom cleaned. I think this is a poor speed as I know and can see other boats, heavier, with other motors with also 18 hp going faster at 2500 rpm.
What can be the solution? Anyone has the same problem? This is normal for Yanmar 2GM20 with sail drive?
 
Assuming that there is nothing slipping in gearbox or drive shaft. (frist thing to check)(also consider tacho may be wrong). High engine speed at low boat speed indicates a prop that is too fine or too small. If it has always been that way then perhaps consider a courser prop. If it has changed recently then a check of the prop condition might be advisable. Around here that weed can grow so quickly. If the prop etc are all in good shape you might rmemember that a fine pitch prop will be ideal if you find yourself wanting to motor into a big wind and sea. It is like a low gear on a bicycle. It is all compromise and you might really prefer to be able to motor in calm seas at a decent speed with not such high RPM.
good luck olewill
 
William_H said:
Assuming that there is nothing slipping in gearbox or drive shaft. (frist thing to check)(also consider tacho may be wrong). High engine speed at low boat speed indicates a prop that is too fine or too small. If it has always been that way then perhaps consider a courser prop. If it has changed recently then a check of the prop condition might be advisable. Around here that weed can grow so quickly. If the prop etc are all in good shape you might rmemember that a fine pitch prop will be ideal if you find yourself wanting to motor into a big wind and sea. It is like a low gear on a bicycle. It is all compromise and you might really prefer to be able to motor in calm seas at a decent speed with not such high RPM.
good luck olewill
Click to expand...

I agree with most of your post but re the fine pitch for waves--
Are you sure you have that the right way round? I changed to a coarser prop to get better drive in chop.
But mine was an autoprop & kept pitching fine & loosing drive in chop. A coarser version gave better drive in waves
 
Burrix said:
I have a 29 ft sailing boat equipped with a Yanmar 2GM20 with sail drive.
The boat has 8m LWL and the weight is about 3000 kgs. She is a cruiser racer, very fast under sail.
The problem is that the speed at 2800 rpm is around 5 kts, with the bottom cleaned. I think this is a poor speed as I know and can see other boats, heavier, with other motors with also 18 hp going faster at 2500 rpm.
What can be the solution? Anyone has the same problem? This is normal for Yanmar 2GM20 with sail drive?
Click to expand...
Welcome. Your posts will be bit slow as you are new, so don't think you are being ignored.

Need more information, but starting from basics. You should be able to achieve at least 3400 rpm and over 6 knots. If you can only get 2800 your prop is too big and needs to be reduced in size, either diameter or pitch or both. If you can easily get over 3400 but poor boatspeed then your prop may be too small. You can check the recommended prop size by putting your boat data into Propcalc on www.castlemarine.co.uk Boat speed is also influenced by prop type and asking 2 blade will probably give less speed than a 3 blade. However you should be able to achieve over 5 knots with a standard fixed 2 blade at 2500 if it is the correct size.
 
+1 re the condition of the prop. My Yanmar 3 went like a dog recently (having previously changed the prop), I thought that the prop wasn't up to the job but thought I would dry her out against a wall. Prop completely barnicalised (that's a real word btw ...). Quick clean and all was well.
 
Hi I have the same engine and sail drive as you with a folding 2 blade prop on my X-302. in fkat water and clean bum flat ou she will touch 6.5knts, (slack time on GPS) waves and a dirty bum will slow down to 5.5knts.

one time I had a lot of worm groth and could only get 2knts and a lot of black smoke.

Know nothing about props but in comparison to my boat you do seem a little slow,

hope this helps
 
Could also be clogged mixing elbow, maladjusted injector pump, dirty injectors, restricted fuel supply etc.
Can you check the dimensions of the prop?
Is 2800 rpm as fast as it will go, or just your choice of cruising RPM?
Does it smoke? and what colour smoke?
 
Your maximum hull speed will be about 6.5kn. 2800rpm is cruising revs for your engine. (reasonable speed and economy) Max revs is probably 3400ish. I'd guess cruising speed is about 5.5kn. If the prop and engine have not been changed and performance has deteriorated, the prop is probably fouled or the engine needs an overhaul. As William says, is your tacho correct? What happens if you increase revs? More smoke, increased speed, more fuel used?
 
Thank you very much for your replies which exceeded my expectations.
I’m trying to reply to all of you.
I bought the boat (used) in Spain. She is a Quantum 29, designed by Soto Acebal (Argentina).
As you can see in the attachment, the bottom is quite modern, and really she is fast under sail, both up and down wind. Being so, with only 3000 kgs, I expected higher speed with an 18 hp motor.
2800 rpm is the value I think the adequate for cruising, so I understood from reading posts in this forum. The max rpm according to Yanmar is 3600. The almost 5 kts I refer in my post are reached at 2800 rpm and with the bottom clean. It is not the case by now, as she was launched 6 months ago. At least 1 kt is lost, in these conditions, which is normal.
I don´t know the propeller length, neither I can guess from the image that I also attach ( a little bit out of focus).
There are some questions asked in your posts that I can´t answer today, but tomorrow I will see the max rpm I can achieve and the color of the smoke
Knowing that Clive with probably the same LWL and similar, if not higher weight, can achieve 6.5 kts I´m more confident that a solution is somewhere.
IMG_6490.jpgIMG_6501.jpg
 
You should really be able to achieve better speeds. My boat is quite similar: 33ft, 3000kgs (X-99). Engine is a 10hp Bukh with sail drive and a two-bladed folding prop. Even with that small engine she does around 5.3 knots at 2.500 rpm and close to 7 knots when under full load, of course only in flat water.
I'd think your prop is not clean.
 
Prop sizing is a dark art, and with the cost of replacements its well worth making sure the rest of the mechanicals are in good order. Engine running properly, exhaust elbow not clogged etc bearings not dragging on the shaft and absorbing power, or saildrive internals in good order and not causing power absorbing drag.

A simple rule of thumb on prop sizing is that if the engine will easily attain top revs, usually before the throttle reaches the fully open position, but not pushing the boat to the expected speed, and when first moving off needs revving to get her moving, then the prop is too small. At the other end of the scale an oversize prop will set the boat off at 2 or 3 knots at tick over (which makes for interesting close quarters maneuvering in the marina!), but will not allow the engine to develop full revs at full throttle.

Working hard, the undersize prop will continue to allow the engine to run at full revs, but will lack 'guts' to push her through the sea and weather. The over size prop will cause the engine to labour increasingly, and fall away more and more from full revs as conditions worsen. Of the two undersize is preferable because as the load increases the engine can still produce full power, but at the cost of full speed in calm condtions.

Props are sized not only by the diameter, but by the angle of the blades, which is known as 'pitch'. There is a direct correlation between pitch and diameter so that a 16x12 is virtually the same as a 15x13 or a 17x11. Adding pitch and diameter gives a 'size' so our examples above are all size 28. The most critical measurement is the pitch, as this determines the efficiency and speed at which the propellor transmits power at a given water speed.
Diameter can be easily measured, but determining the pitch requires careful measurement and calculation. the size is usually stamped somewhere on the boss or hub of the propellor, diameter x pitch.
 
Today I went for a day sailing and took the opportunity to check some questions aroused in this forum, related with the motor:
At full throttle the motor runs at 3600 rpm, without any smoke. 3600 is the max rpm, accordingly to the manual. The speed was 5.6 kts, with flat water and a bottom with 6 months in water.
Accordingly to Oldharry, this means that the propeller is undersized.
The Yanmar SD 20 saildrive manual refers 3 recommended propeller’s diameters: 14” for 1GM, 15” for 2GM and 16” for 3GM, but says nothing about the pitch.
As I don´t know the diameter of my propeller, there is the possibility of undersize (14” instead of 15”). By the other side, if it is 15”, maybe I can change to 16”. What do you think Oldharry?
Anyone knows if Yanmar propellers come with different pitches, or they are unique for each diameter?
 
That is not a standard Yanmar prop. From what you describe it is too small, either diameter or pitch. You need to find out the size and make of the prop. The size may stamped on the boss, but that type of prop has replaceable blades so you may need to contact the maker or dealer to get the size. They should also be able to calculate the correct size for you and ether modify the blades or supply new ones.
 
Thank you Tranona. Once the boat comes ashore I will try to check the make and diameter. For now I´m going to google for Yanmar propellers and see what do they have to sell
 
The standard props are fixed 2 blade aluminum. The folding prop you have is better for sailing performance. You just need to get the right size blades to improve motoring.
 
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