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Boat stops going at high RPM

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Ezybao View Drop Down
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    Posted: 16 July 2018 at 16:16
Hello,

We own a 2011 Hanse 445. Just a couple weeks ago, we wanted to see the top speed of the boat while the sea was flat and there was no wind. We increased the rpm gradually; waited for the boat to gain speed and then increased it a little more. To our shock, at around 2800 RPM (which is almost max for us), the boat started slowing down, as if the propeller stopped turning and stopped pushing the water. The RPM remained the same, and there wasn't a huge increase in the noise, nothing like changing the gear to neutral from the 4th while going with a 100km/h in a car. 

I know the propeller was changed once, around 4-5 years ago, and I am not sure but it could have been renewed once again since then. What could be the cause of this issue? We tried it out once or twice again after that and it was always the same result. Could it be the propeller hub?

The engine is D2-75 - Volvo Penta.

Thank you in advance for your input. 
Alican


Edited by Ezybao - 16 July 2018 at 16:21
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Konstantin View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Konstantin Quote  Post ReplyReply Direct Link To This Post Posted: 16 July 2018 at 20:06
What kind of prop do you have? We have fixed 3-blade on our 445 with D2-75. Once after switching to reverse while the prop was still spinning its momentum separated the metal part from the plastic insert so it started free-spinning under load... so we have to change it and have this one rebuilt. There was certainly a change in sound - almost like the gear disengaged and the engine sort of lost its load.
Another possible reason for this is all the bubbles around the prop at max RPM from cavitation. The prop can not absorb all the horse power the engine is producing and so it starts creating a foam of bubbles around it, so no propulsion forward. You can google max prop HP absorbtion curves for D2-75 (there is a brochure on VP website with it). Ours is certainly well below in terms of what D2-75 can produce at hight revs.
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Rock View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rock Quote  Post ReplyReply Direct Link To This Post Posted: 16 July 2018 at 21:57
Could indeed be cavitation.
Would suggest you have the wrong prop for your boat/engine setup.
What speed were you at, at 2800 rpm?
Hanse 400e "M-square2" #0241
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Ezybao View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ezybao Quote  Post ReplyReply Direct Link To This Post Posted: 17 July 2018 at 11:02
Hello Konstantin, 

Many thanks for the information. 

Our propeller is Fixed 3-blade 18x16 alum. I not sure about cavitation, because I am almost sure that we have done the max rpm & speed experiment before and didn't encounter any problems. If it was cavitation, wouldn't we see a tube of bubbles coming out of the prop? 

We have been talking to experienced mechanics and there seem to be two main opinions. One is focused on the prob hub. Their solution is to renew the prop. They are asking us to do the same experiment, gradually increasing up to 2800 rpm, in reverse gear. This seems quite dangerous. Maybe we can try it out while we are tied up in the marina. For them, if the same thing occurs in the reverse, it is almost certainly the hub.

The second opinion is the transmission plate (or is it marine gear damper in English?). 

Because the problem occurs in very high RPMs, we are not sure if we need to do something about it right away, or should we wait for the season to end. During regular cruising speeds, everything is fine. Of course, we don't know how long it will take for the situation to worsen and start happening in 2000-2200 RPM. 

Regards
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Ezybao View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ezybao Quote  Post ReplyReply Direct Link To This Post Posted: 17 July 2018 at 11:04
Hi Rock, 

You can see the prop specs in my reply to Konstantin. 

Our speed was up to 9 knots just before the problem occurred, which seems normal.

Regards
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Konstantin View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Konstantin Quote  Post ReplyReply Direct Link To This Post Posted: 17 July 2018 at 15:25
I've got the same prop - take a look at the image attached showing the prop with damaged, slightly separated internal knob/plastic insert. When this free-spinning happened I dived to check it and the prop shower a bit of play when pushed back and forth. New prop by contrast is dead fixed, no play at all. When I dived on the prop I was not able to turn is though, but clearly it started to free-spin at high revs.
If I were you I'd dive on it and check if it sits on the shaft tightly. If there is some play then the plastic part is damaged or separated from the metal part. I'd then take the prop off, underwater, and change it or have it fixed. It is not that it didn't work in our case when motoring, the trouble was it started to free-spinning when, say, you want to slow down reversing, the time you want your propulsion the most.

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Captain Dan View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Captain Dan Quote  Post ReplyReply Direct Link To This Post Posted: 24 July 2018 at 17:03
Check Prop is clean, a few shells and some growth can give you RPM but no SOG.

VP Standard bush prop hubs are very strong, i doubt this has come apart, I would check the prop is clean first and go from there.

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John Economides View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote John Economides Quote  Post ReplyReply Direct Link To This Post Posted: 30 June 2019 at 07:38
Have a similar problem and posted about on http://www.myhanse.com/loss-of-boat-speed-at-high-engine-speed_topic11436.html

Have not resolved the issue yet. Did you have any progress with yours?
John
Hanse 445
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Ratbasher View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ratbasher Quote  Post ReplyReply Direct Link To This Post Posted: 30 June 2019 at 17:40
Sounds like you were trying to exceed hull speed! According to the spec your boat has a LWL of 40.03ft/12.2m which gives a max hull speed of just under 8.5kts. As you were doing trials in flat calm water on a still day you weren't surfing down waves and the engine doesn't have the power to push the bow up on to its own bow-wave. The only other way to exceed hull-speed is to increase your LWL which you'd only be able to do when heeled over. Given the conditions you describe, if your log was reading more than 8.5kts then it needs calibration.

At least it seems you got an answer to the question you asked yourself: how fast can it go. If it's maxed out at 2800 try running it up to 2600/2700 and see if you can maintain 8.5kts. If you can then you don't have a problem. As Scotty said in Star Trek: "ye cannae break the laws of physics".
Rat

H400 #311 'Wight Leopard', cruising UK & Ireland

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