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Post by rene460 on Aug 26, 2014 9:51:31 GMT
This thread has been very interesting to follow, due to the range of propellor recommendations from one manufacturer (so we assume same blade shape, all 3 blade etc.) for similar size boats with similar power engines but different engine speeds and different reduction gear ratios.
The power absorbed by the propellor is determined by propellor speed, so I would expect all the 18 x 14 props to be driven at the same speed. However the gear ratio for the 3000 rpm engines gives a much lower propellor revs than the gear ratios quoted for the 3800 rpm engines. The 18 x 13 prop on a 3000 rpm engine would seem to be even less of a load, yet still attached to a 55 hp engine.
Am I missing something here? At the very least, all the critical data, as listed by Trafika II above, needs to be double checked with the manuals and the prop manufacturer before a purchase.
Rene460
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Post by Spritz on Aug 28, 2014 18:22:27 GMT
Must received a mail from flexofold..... But still I do not understand........advices? ? here under the mail: My mail: Dear Marlene, On a English Sailing forum someone wrote: Post by Tafika II on Mar 20, 2013 at 23:49 When I contacted Flexofold, I told them my Yanmar engine is 54 hp and gearbox is 2.36:1 reduction in forward. They we would recommended a 3-blade 18x14-3R propeller, which what I purchased. I am happy with the acquisition! If they are now suggesting a 15 pitch, maybe they can explain why. Replay: Dear Antonio It seems to me like Tafika II is having a 4JH4E for shaft (54 HP@ 3000 RPM – 1:2,36) and yes there we would recommend a 18x14-3R You have a 4JH5E for SD , 54HP @ 3000 RPM – 1:2,32) we would still recommend a 18x13-3L, but if you would like to take of the top of you RPM’s with (200-250RPM) then you can take a 18x14-3L. It’s all about personal preferences! Our recommendations are made on behalf of our calculations and feedback from the shipyards – what is working best. Hope this could explain and answer your question Best regards/Mit freundlichen Grüssen/venlig hilsen Marlene Storm Now again, please advices and/or suggestiona? ??
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Post by rene460 on Aug 29, 2014 8:42:39 GMT
Hi Antonio,
It it becomes clearer if you compare just two. You and Trafika II both have 54 HP, 3000 rpm engines but a different gear ratio.
If you divide the rated engine speed (3000) by the gear ratio, your propellor is turned at 1293 rpm when engine is at 3000, Trafika II propellor turns at 1271 rpm with 3000 rpm engine speed. You can see Trafika II's excellent documentation of his performance in his earlier post.
If you select the same propellor you will require more power from the engine but have the same power available. It does not look like much, but you have to compare speed^3 or something like that. Reducing the pitch compensates for your higher speed in order to not overload your engine. Marlene has suggested that with 14 inch pitch your engine will max out at about 2500 rpm (only the prop manufacturers can really calculate this, but you can rely on them when you have a specific recommendation like this).
My mechanic insisted that the correct prop should allow the engine to max out at about 10% above rated speed. You should check this with your mechanic, but on this basis, he would say that the 14 inch pitch is too much for your engine because it would max out at only 2500 compared with rated speed of 3000. I know that most of us do not often run the engine at rated speed, but that is a whole other question. At least on this basis we have maximum engine power available to us if we ever need it, and we save fuel running at lower engine speed.
I hope that clarifies things for you. You are dealing with a reputable manufacturer, and my suggestion is to carefully check all the data you have supplied for your engine and gear, run it all past your mechanic and go with the flexofold recommendation. It makes sense compared with Trafika II due to the gear ratio, and no real option in between.
rene460
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Post by electricmonk on Aug 29, 2014 22:11:58 GMT
I would be wary of any prop that "stalled" the engine, its a false economy to over prop. If the engine reaches max power at 3000rpm but the prop wont let it get beyond 2500rpm then there is something seriously wrong, at the very least you will never get the benefit of all the HP the engine should develop, at worst you will soot up the exhaust elbow along with the cylinders and injectors. Size the prop on the continuous rated output of the engine. i.e. match the prop to engines prop curve. is what I would do.
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Post by Spritz on Aug 31, 2014 8:57:29 GMT
Dear Rene, i apologise but I do not completely understand your comment: If you select the same propellor you will require more power from the engine but have the same power available. It does not look like much, but you have to compare speed^3 or something like that. Reducing the pitch compensates for your higher speed in order to not overload your engine. Marlene has suggested that with 14 inch pitch your engine will max out at about 2500 rpm (only the prop manufacturers can really calculate this, but you can rely on them when you have a specific recommendation like
what do you mean? I would like to have the Same motor speed (8.3-8.5 knods) at the same range (2500-2600 rmp) and highest spead (over 9 knods at 3000 rmp), from you message I do not understand if in your opinion I will have these results with Flexofold,
could you you please clarify your thoughts? Thank you
best regards
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Post by rene460 on Aug 31, 2014 10:17:03 GMT
Hi Antonio,
No need to apologise, it is a complex subject made more difficult by our different languages. I will try to clarify.
First, when you say "motor speed", I assume you mean boat speed under motor. When I say engine speed, I mean engine revolutions per minute. And propellor speed in revolutions per minute is different to engine speed due to the gear ratio, propellor is slower than the engine. Dividing engine speed by the gear ratio gives the propellor speed and is an exact calculation.
But there is no direct relation between propellor speed and boat speed, unlike your car, for which road speed can be calculated from wheel speed. So I do not know if you will achieve the speed you hope for at any specific propellor revolutions per minute.
The propellor produces thrust to push your boat along. For a given thrust force, measured in Newtons, the boat speed is dependent on the length, mass, shape and cleanliness of the hull surface and even wind resistance from the mast and rigging. The propellor thrust is determined by diameter, pitch, number of blades, blade area, propellor surface shape and smoothness and similar design factors.
Our discussion is about propellers, not about boat speed. The aim is to select the propellor that effectively uses the engine power to create thrust. Because your gear ratio turns your propellor at a higher speed than the gear ratio on Trafika II, you will need more power if you select the same propellor. But your engine has the same 54 hp, so your engine will be overloaded, and unable to reach its rated speed and hence will produce less power, less thrust and most likely less boat speed. To compensate for the higher propellor speed, Marlene is proposing less pitch. The 13 inch pitch propellor will require less power than the 14 inch pitch at the same propellor speed, and allow your engine to develop its maximum power despite the different gear ratio. I expect that this propellor will give you similar boat speed to the one selected for Trafika II, but I am not able to provide a precise calculation.
I hope that this clarifies for you. I fear that I am getting too technical, my wife says too long winded, so if you need more clarification perhaps you could send me a PM with some more detailed questions on which part I need to clarify further. I am happy to continue the discussion if it will help you.
rene460
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Post by Spritz on Aug 31, 2014 10:51:56 GMT
Dear Rene, thank you, now is much more clear!!!!!
i will send you PM ...
best regards
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Post by toniocask on Jun 5, 2015 14:06:52 GMT
Same engine of Tafika : yanmar 4jh4ae 54 cv At 3000 rpm,30 mm shaft no SD. Flexofold wrote me that they reccomend a 20x13 3 blades instead the 18x14...... I've the email of yesterday..
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Post by rb on Mar 11, 2018 13:35:40 GMT
Hi all...I know this is an old post. However, this is a current challenge for me. We just purchased a 2006 SO42DS with a fixed blade prop. We are replacing the fixed prop with a flexofold. Flexofold recommended the 20x13 for best forward and reverse performance. Our engine is a 54hp Yanmar, 2.36 ratio, shaft drive. I will let you know how it works after we install and put the boat in the water in April.
Currently on the hard in Michigan waiting for the weather to worm up.
Cheers
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Post by Spritz on Mar 13, 2018 7:12:34 GMT
For ANYONE with folding propeller and a YANMAR sail drive
please remember that, when sailing, you have to put reverse for max 1 min to allow the propeller to “close” than you have to put NEUTRAL even if you destroy your cones.
is a specific instruction from YANMAR
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Post by NZL50505 on Mar 17, 2018 6:34:17 GMT
Hi all...I know this is an old post. However, this is a current challenge for me. We just purchased a 2006 SO42DS with a fixed blade prop. We are replacing the fixed prop with a flexofold. Flexofold recommended the 20x13 for best forward and reverse performance. Our engine is a 54hp Yanmar, 2.36 ratio, shaft drive. I will let you know how it works after we install and put the boat in the water in April. Currently on the hard in Michigan waiting for the weather to worm up. Cheers This is v interesting to watch because we seem to have Tafika and rb who both have identical setups (54hp engine & 2.36 gearbox ratios) but using different prop specs (18x14 vs 20x13). I’m planning to swap for a Flexofold this winter so will wait with great interest the results of rb’s tests from his 20x13 compared to Tafika’s 18x14 experiences.
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Post by fakinx on Apr 25, 2020 16:34:38 GMT
And now we have 3rd different recommendation for the same engine & gear. Flexofold in my case recommends 3-blade 18x15-3 R. Now I’m really confused. Any update from toniocask or rb?
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