Thanks for your feedback. You post an interesting issue. I will reply to this after disclosing my experience with Gori today.
We just hauled out yesterday to put the GORI.
First Observation No vibration at all
Second The speed in normal forward is lower than autoprop
Third The speed in overdrive mode is more or less the numbers of the Autoprop but NO VIBRATION
Fourth In reverse the stopping power is good an the yacht is pretty easy to steer stern to ( mediterranean mooring
style). Not as good as the AUTOPROP.
The speed at 3000 rpm
which is the crusing speed of the 3800 rpm Yanmar
HTE was 7,6 knots in Normal mode and 8,6 Knots in Overdrive. we didnt rev up more as we were inside the port of palma and the traffic was heavy.
No pulling of the wheel
to port in forward.
This is our first obvervation and I promise to write a more in deep feedback in a couple of months and after checking how can I use the overdrive mode.
I consider the normal mode speed a bit deceptive.
I was e mailing with GORI plant directly as I considered the size of the prop recommendeed by the importer in spain
(20 by 16) was under size following my calculations as per the propeller handbook recommendations (22 by 15) .
They insisted on that size, If you want more speeed use the overdrive mode they told me.
I still think the prop is bit undersized and they dont like to recommend a bigger size because the potential problem of a overloading the engine in overdrive mode.
The real fact is in a 3800 rmp engine you dont rev up over 3200. I'm talking about real life. Some recommendations of crusing revs up to 90-95% of the max revs seems unrealistic to the best of my crusing experience.
So in my calculations I tried to match the engine to the prop maximizing the power absorved by the prop at that rpm (3200). That calls for a 22 by 15 prop.
But Sune from Gori didnt like the idea. So I ordered a 20 by 16.
Concerning your post
we didnt have a bump problem on the hull, We literally were shaked on deck
by the vibration. So we can feel or heard bumps....with the AUTOPROP we feeleed like the yacht is gonna be broken in pieces. So we have a much bigger problem that simple hull bumps
Of course I know what you are talking about and my next weekend project
will be insulationg the engine. The insulation
in my Hanse is a bad joke.
First the foam didnt cover the stair door in the companionway
. The engine room is full of absurd holes and doors all because Hanse wanted to install a 14 KW generator
over the engine which makes engine service impossible.
I already removed the generator
bracket ( 50 kilos of square galvanized tube) and now I can even acces the water
pump (unreachable before taking the gen bracket out).
We feeled the noise was a bit down also just by taking the gen bracket apart ( resonance problem?). the bracket was screwed to the engin e bed
just one inch clearance to the engine mounts.
So next I will install a new engine room insulation
closing some doors and holes.
I have received some samples from a firm called acoustica. They are in Australia
and not representeed in Europe
up to date. The advertised insulations graphs are outstanding. i have never seen 25 dbs on 150 Hz.
you can check this at www.acoustica.com.au
The samples I have jus received are excellent. Anybody have ever used theese materials? Any feedbacks?
And after that I expect to hear my "bumps in the hull" . Now the noise is too heavy to hear although I feel they are there.
The Hanse hull is epoxy
not Polyester the noises are transmitted diferently. while sailing you can not hear the water
flowing in the hull , the yacht is in complete silence inside. That makes engine noise more stressing when started on.
Ernest,,,,I would like to know if you have used any products with success to treat the hull bang problem. The stern beds are over the propeller hull area and I have plenty of space to treat the hull surface inside.
I checked the Gori prop clearances to hull and skeg we have 35 % blade to the skeg and 25 % tip to the hull. I think they acceptable.
I was thinking into a redesign of the shaft as the rudder
blade is far away so we have space to place a brand new strut bearing and a longer shaft which will make clearances much bigger and will put the prop closer to the rudder
and far away for the skeg deadwood.
in that way I can make a glass work fairing the deadwood as per the prop hadbook recommendations.
BUT NOT THIS YEAR
Originally Posted by ernest
I have read about your hull vibration problems and I do not believe that Autoprop is the only offender. You will always have a pressure pulse in the hull when a propeller blade passes an obstacle. In you case (as in mine) you have a flat underbody and I can fully understand that your bottom "buckles every time the propeller blade passes. That will happen with any propeller and the blade shape and the distance between the blade tip and the bottom of the hull will determine how serious your problem will be.
What I am saying is that you might have the same problem with your Gori and one partial remedy would be to put an inch or so with some heavy gunk inside your boat in the area where the prop passes. Quit a common treat as far as I know.
My own experience with a fixed twobladed prop made me literally sick!
At certain rews I started to have a cold sweat and a nausea. Changing the motor
rews slightly fixed the health
problem but with my hand I could feel the bottom vibrating.
Bringing down an oscilloscope and an accelerometer and measuring the vibration frequency directly showed pulses two times per revolution of the prop shaft.
When I changed to a threebladed Volvo-Penta folding prop the problem dissapeared completely. I dont know if you are familiar with that product but it has scimitar shaped blades which are optimized for silent vibtationless operation. If you look at modern submarine propellers for extremely silent operation you will see something similar.
Your theory about the blade slightly changing its pitch
is however still interesting and thought evoking.
Half a year ago I was determined to change to an Autoprop. I am going to chnage to a stronger engine so I have to change my prop anyway.
Now I am not that sure anymore and I really appreciate all the trouble you are taking in sharing your experiences with this forum.
The patent application would be interesting reading even if you probably will not find any answers there about vibrations.
I am however still interested in background theory about the propeller.
Have you pursued this matter?
Best regards ...