change the engine if reduction is 3.13 ?

[boayan]

thank you for your help and time (sorry for my french english)
Just bought an new sail boat aluminium 33" (light about 2500kg)
Am very fast sailing easy 5-6 knts with little wind but so slow with the very old yanmar with a reduction gear of 3.13 (part of the engine cannot be changed)(3LM-sold for fishing boat in japan in the 80s)
supposed to be 40hp at 3000 rpm
at 2500 just over 5 knts reaching a maximum
but already at 4 knt at 1500 rpm
i change the 3 blades propeller 16/18 to 18/18 and am better about one knot
but still very slow for such a light boat !
what you think ?
merci beaucoup

[Ziggy]

Boayan,
If the engine is really 40 hp, that's huge for such a light boat. But the reduction gear and low max engine rpm require a very big prop diameter. Do you have room for 21x19? You should get ~7 knots at 2500 RPM.

[boayan]

thank you for your answer
yes this is huge and that's why the final result are frustrating...5 knt !!!

I don't have space for more than 18" ( less than 2" above )...
that is why I was thinking about an other engine, but I want to be sure it will make the speed much better
meanwhile as there is not too much load on the engine ; do you think I can run on 2500 rpm as a cruising speed (still no smoke) ? the original panel goes up to 4000 and I found on the Internet 40 HP at 3000 /45 at 3200...
thank you again


a bigger prop anyway will be bad for sailing...(18 is not so good already)

[never monday]

repower

that thing should make 5kts with 10hp

[Ziggy]

Originally Posted by boayan

I don't have space for more than 18" ( less than 2" above )...
that is why I was thinking about an other engine, but I want to be sure it will make the speed much better

Download propcalc (http://www.alberg30.org/maintenance/MechanicalPropulsion/Propeller/propcalc.xls) and play around a bit with reduction gear ratios and engines. In general, higher shaft RPM allows you to use a smaller prop, but with less efficiency.

Originally Posted by boayan

meanwhile as there is not too much load on the engine ; do you think I can run on 2500 rpm as a cruising speed (still no smoke) ?

It is said that a diesel likes to run at ~80% to 85% of max rated output. Your problem is that the prop is too small to load your engine to anywhere close to its rated output. But running at reduced output shouldn't damage the engine, beyond eventually fouling the valves and cylinders.

[boayan]

thank you very much for your time and advices , but unfortunately I cannot open your link as alber30 is private and cannot open without password
thank you

[Alan Wheeler]

~7 knots at 2500 RPM.

You won't get 7 kts from that hull if she is 33ft LOA. What is the actual WLL Boayan?? I would say 5.5kts is going to be closer to your boats actual speed. You will always struggle to make the "on paper" hull speed. You will get close, but the last 1/2kt is very costly.
Plus how are you measuring speed. Don't rely on a paddle wheel giving you a number as being accurate. A GPS is the better for accuracy.
Prop diameter is not going to help you in achieving speed. Pitch is the critical element. Also remember The engine is 40Hp at 3000RPM. It will be perhaps 20Hp at 2500RPM(ruff guess). Now as Pat said, a 10Hp engine will push the boat through the water at 5kts. That maybe correct, but 10Hp or in this case, 20Hp is going to struggle to spin an 18'' blade. That's a big lump of wing in the water. So a whole lot of energy is going into just trying to spin the blade. 40Hp will handle it, but you are not achieving 40Hp because you are not getting 3000RPM. You maybe better going back down to the smaller diameter blade, but ensuring you have the correct pitch for the RPM and water speed. It will add to your sailing performance, the speed should not be any less, you will just notice getting up to speed a little slower.
The rules are,
You MUST be able get max RPM in neutral
You MUST be able to get to Max continuous RPM(-10% max) in gear.
The propeller's pitch is what gives you the speed through the water.
The propeller's pitch MUST be calculated for you max continuous RPM/hull speed
The propeller's diameter is efficiency only. The bigger the diameter, the more power it takes from the engine and the more power it takes from you sailing. Sailboats are always a compromise in fixed propellor efficiency, because you want to be most efficient when sailing. So the smaller the diameter you can get away with, the better.

[Alan Wheeler]

If you give me water line length, I can run the numbers for you and let you know what prop pitch you should require.

[boayan]

thank you again for your help

sorry I am not very clear
when I changed the prop diameter from 16 to 18, I got about one more knot at 2000 ( 3,8 to 4,6)(with a max of 5 at 2500) pitch is 18 in both case.
with the 18/18 prop 4 knots is reached very fast at 1400 rpm

the WLL is about 31 without moving and 33 moving at 3,5 knots , an interesting fact that may talk to you a lot , at 4,3 water start to fill the skirt ( either sailing or with the engine)....

thank you very much

[Alan Wheeler]

That means the boat is "squatting" down due to it dropping into the trough of the wave it produces. This does not actually increase the WLL. At least not by much. The skirt part of the hull is too high for it to add boyancy and increase the distance the water has to flow. Increasing the distance lengthens the wave and thus raises the speed. So the boat speed will be governed by the WLL of 31ft. Squatting means the boat is reaching it's maximum speed and you will never make it go any faster. To do so means the Bow has to rise up over the bow wave ans sinks the stern down even further.

[Alan Wheeler]

So here is a close estimate. If you can get the engine to 3000RPM, and you have a ratio of 3.13:1 and the boat theoretical hull speed is 7.47kts, the pitch of the prop needs to be around the 11.5" mark. This is based on a slip of 20%.
The big prop is going to have less slip. It grabs more water, so you have less loss, so you don't need as much pitch. If you had a smaller prop, then you could have more slip which would mean more pitch. But most slip is in slow speeds when the prop is the least efficient. As the RPM builds, the efficiency increases. Hence you got a little more speed with the bigger prop when the engine was still turning slow. A rule of thumb is 10% slip at high RPM, which would equate to a pitch of only 10.5". 18" is just far to much. It is overloading the engine, causing you to lose 50% of your Hp and by turning the blade too slow is causing you to lose 25% of your efficiency of the prop, coupled by the engine having to turn 18" of surface area through the water which is probably robbing you of another 50% of what Hp you have left.

[Ziggy]

Originally Posted by Alan Wheeler

You won't get 7 kts from that hull if she is 33ft LOA

With 31 ft waterline one should easily get over 7 knots. The key parameter is SLR = speed/sqrt(waterline length), and with appropriately sized engine a displacement hull should achieve SLR of about 1.3. For Boayan's boat that's just over 7.2 knots. An "appropriately sized engine" requires about 1 hp to push 550 lbs of displacement. Boayan's boat displaces 2500 kg or 5500 lbs, and after he loads her up with a few cases of wine and other provisions she may be displacing 8500 lbs, so in theory he would need 15 hp engine. In practice, it's good to have some reserve, and a 20 hp engine would normally operate at ~75% max output, which is just about right. With a 40 hp engine, the boat is really overpowered.

Originally Posted by Alan Wheeler

So here is a close estimate. If you can get the engine to 3000RPM, and you have a ratio of 3.13:1 and the boat theoretical hull speed is 7.47kts, the pitch of the prop needs to be around the 11.5" mark. This is based on a slip of 20%.

Au contraire, mon ami! To first order, slip is a function of propeller speed through water. At 7.47 knots, the slip is ~44%. Thus the required pitch is quite a bit greater.

Originally Posted by Alan Wheeler

A rule of thumb is 10% slip at high RPM, which would equate to a pitch of only 10.5". 18" is just far to much.

To get the slip down to 10% you need to be planing along at over 50 knots. Boayan's boat looks really fast, but that may be a bit much even for a French rocket like his.

Originally Posted by Alan Wheeler

It is overloading the engine, causing you to lose 50% of your Hp and by turning the blade too slow is causing you to lose 25% of your efficiency of the prop, coupled by the engine having to turn 18" of surface area through the water which is probably robbing you of another 50% of what Hp you have left.

If you really have 40 hp at 3000 RPM and 3.13 reduction gear, you should be getting over 200 ft-lbs of torque at the propeller shaft--a veritable tugboat that should have no problem swinging a 21" prop. The real problem, Boayan, is that your boat is overpowered, with a slow turning shaft, and no room for a prop large enough to match the engine's output with a sensible pitch ratio. I think you would do nicely with a 20 hp Yanmar 3YM20 with a standard gear box.

Originally Posted by boayan

thank you very much for your time and advices , but unfortunately I cannot open your link as alber30 is private and cannot open without password

This appears to be a weird bug affecting links in messages on cruisersforum. The correct link is http://www.alberg30.org/maintenance/MechanicalPropulsion/Propeller/ (copy and paste the link into your browser) The site is not protected and it is open to the public.

In exchange for all this free advice you can invite me to spend a few weeks (years?) on your beautiful island!

[boayan]

thank you very much
yes I would love to invite you but I have to find the money to buy the 3YM20
...

[Alan Wheeler]

To first order, slip is a function of propeller speed through water.

Yes correct. But the propeller has no idea if the water passing over the blade is speed of rotation of distance traveled forward. Propulsion is a simple law of Physics. For every action, there is an equal and opposite reaction. The propeller tries to "fly"(because it is a wing) through the water. The water is not a solid immovable object. So a water "columb" for want of a better word, is moved aft. The mass of water moving aft results in the force opposing that movement acting on the blades and the boat floating in that fluid with little restriction, moves forward. The actual loss at low speed is not slip of the blades as much as it is "slip" of the moving mass of water against the non-moving. Providing the propeller is rotating at speed.

With 31 ft waterline one should easily get over 7 knots.

No that is not quite correct. Once again, that is theoretical on paper speed. But hull shape has a lot to play in the actual. It is correct that the formulae is about hull length and wave length. But it is about where the hull has an affect on the transition of the hull length. Especially in sailing hulls, that WLL is often designed at a point of heel for a starter. But that is a long tangent to take. However it is easy to say that the last 10-5% of boat speed takes 95% of the engines power.
Weight actually has very little to do with hull speed. Mass is resistance to inertia. It simply takes an applied force longer to act on the mass to achieve speed. A simple display is the ability of pulling a 10,000ton container ship toward you with your own weight on a mooring line at the dock. If it was about energy, a person could simply not do it. A 5 Hp motor could get the same weight to close to hull speed as a 40Hp motor. But the 5Hp would not have the spare 95% the 40Hp does to take it to that last 5% of hull speed.
This is why there are two formulae around for working out hull speed. I have explained this in past discussions. There is the normal everyday theory formulae and another of only 1.34x the square of the WLL. That is the one most professionals now use as it describes that last % of as a waste of time to try and calculate into the speed.
If you go take a look in the Study Hall, you will find some good info that may help in understanding more.
As for me if I were Boayan, if I had the money to spare, then a new smaller engine and correctly propped would be a good investment. But it is never cheap nor simple to do so. 40Hp is way over kill, but it does allow some extra reserve for other purposes like generation and AC and so on. If money and the enormity of the job was a concern, the first thinking I would be doing is trying another prop. It will be the cheapest and hey, it will quickly prove or disprove theories.
I can't stress enough the need for a correctly matched prop to RPM. I see it time and time again and those that have changed have always noted major improvements.