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SimonDewhurst
Joined: 13 Mar 2012
Posts: 14
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 Posted: Thu Jul 12, 2012 7:45 pm Post subject: Pod issue and mounting height |
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Hi All,
First time posting on the forum, i have read a lot of topics so thanks for all the great information.
I have a few issues i would like some opinions on.
I purchased an Aluminum Boat Hull (21foot) which originally had a single 235HP Evinrude 1985.
I have since had a full cabin built on the boat and also done a fair bit of work to allow me to put my new engines on.
2x 115HP Mercury / Mariner Optimax 2004.
I took the boat out for its first run and quickly realized all is not well with the motor setup.
It would get up on the plain very very quick but then soon as you started to trim the motors up very slightly it would take off (picking up water) then start proposing and cavitating. it was very twitchy in the controls and very unstable. It only got upto about 30KPH at WOT.
We decided to try some bigger pitch props, went from 17" to 20".
It was a little better but still very bad, gained another 10KPH in speed but the handling and general characteristics of the boat are way off the mark.
I have since talked to the chap who fitted the motors (Very reputable dealer and also very helpful cant thank him enough for all the help) he looked at the bottom of the pod setup and thinks it could be because the pod sits about 100mm above the hull so its not a hull extension pod its giving the props dirty water and creating a sort of air pocket that's why its doing as described above.
I was just wondering what other people might think, i was thinking why we couldn't drop the motors down so the cavitation plates match the bottom of the hull not the bottom of the pod as thats how they are currently setup. He has suggested we might need to add to the bottom of the pod so its a hull extension.
Has anyone else had any experience with issues like this?
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Deepsix
Joined: 31 Aug 2010
Posts: 1355
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| Posted: Thu Jul 12, 2012 8:00 pm Post subject: |
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What do you mean "it picks up water"  What make of boat ? What happened if you didn't trim the motors out ? Pics would probably help. |
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SimonDewhurst
Joined: 13 Mar 2012
Posts: 14
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| Posted: Thu Jul 12, 2012 8:11 pm Post subject: Re: |
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| Deepsix wrote: |
| What do you mean "it picks up water" What make of boat ? What happened if you didn't trim the motors out ? Pics would probably help. |
So to get it up on the plain the motors have to be all the way down other wise it will cavitate.
Once its up if you trim the motors up every so slightly the boat will jump forward and take off, "picking up water" might not of been the best selection of words, it seems to have a lot of drag when getting up onto the plain then once up you trim the motors up it will take off but then start to cavitate and bounce around in the front end.
It seems to be getting a lot of prop slip.
If you dont trim the motors up it will just labour very hard and not have any speed.
Make is fully custom, hull is about 15 years old but the rest of the boat is new.
I dont have a pic at the moment, but will get one tomorrow.
Here is a drawing of the current setup (MS Paint goodness)
The cavitation plates on the motor are aligned as per the picture.
https://www.dropbox.com/s/zmflb77k7hfoiyt/BoatDesign.jpg |
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Deepsix
Joined: 31 Aug 2010
Posts: 1355
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| Posted: Thu Jul 12, 2012 8:21 pm Post subject: |
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| Fair bit of weight hanging back there with not much to support it, you haven't mentioned what make it is, unless it is pretty beamy in the rear you may have overdone the pod idea. Power aplenty for a 21 foot aluminium seemingly, but how much extra weight has it put on with the cabin and pod ? |
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SimonDewhurst
Joined: 13 Mar 2012
Posts: 14
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| Posted: Thu Jul 12, 2012 8:28 pm Post subject: Re: |
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| Deepsix wrote: |
| Fair bit of weight hanging back there with not much to support it, you haven't mentioned what make it is, unless it is pretty beamy in the rear you may have overdone the pod idea. Power aplenty for a 21 foot aluminium seemingly, but how much extra weight has it put on with the cabin and pod ? |
Agree is as added some extra weight to the back, the boat was built originally with the pod, all we did was make the back engine mount surface longer to accomodate the 2 engines. It has had a big cabin added on the front which would of added some weight over the middle / front of the boat. and also the fuel tanks are now in the floor up the middle / front as they use to be in the back. so it should of evened out the weight.
It sits a little duck ass at rest but nothing more than my old boat did and nothing that look out of place.
Make is custom, a guys built it 15 years ago, he built 5 of them and i've talked to people with other ones and they have told me its very stable and good handling. but they do not have twin 115's!!
its about 2.3m wide. |
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seacarp 1
Joined: 07 Mar 2012
Posts: 597
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| Posted: Thu Jul 12, 2012 8:42 pm Post subject: |
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shitloads of factors involved
had the same problem with a mono hull single motor
the motors need to be mounted lower by the sounds of it they also may be giving each other dirty water
race boats have a seperat ram and bracket that allow trimming of the motors up and down not just in and out these brackets are not that expensive and very handy when your transom pod doesnt allow any further adjustments
yhere is a bloke on here called jaygeee ireckon hes a dick but he is very informed in boats large alloys by the look of things are his specialty i think he trawls the posts but if you message him im sure hell help you
if you are in vic i can reccomend a bloke at basymarine in carrum his name is bryan andrews hes a wiz been around forever knows his shit i worked at a boatyard with him 27 years ago and all the ocean race dudes took their boats to him
on here but i think jaygeee is your man dont mention my name to him but
cheers |
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SimonDewhurst
Joined: 13 Mar 2012
Posts: 14
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| Posted: Thu Jul 12, 2012 8:48 pm Post subject: Re: |
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| seacarp 1 wrote: |
shitloads of factors involved
had the same problem with a mono hull single motor
the motors need to be mounted lower by the sounds of it they also may be giving each other dirty water
race boats have a seperat ram and bracket that allow trimming of the motors up and down not just in and out these brackets are not that expensive and very handy when your transom pod doesnt allow any further adjustments
yhere is a bloke on here called jaygeee ireckon hes a dick but he is very informed in boats large alloys by the look of things are his specialty i think he trawls the posts but if you message him im sure hell help you
if you are in vic i can reccomend a bloke at basymarine in carrum his name is bryan andrews hes a wiz been around forever knows his shit i worked at a boatyard with him 27 years ago and all the ocean race dudes took their boats to him
on here but i think jaygeee is your man dont mention my name to him but
cheers |
Thanks for your comments, i think its down to the mounting height of the motors, i will be dropping them down a few notches this weekend and seeing what the results are. if that fails the last resort will be to fill in the bottom of the pod to match the hull, not a massive issue but just one id prefer not to do. |
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Deepsix
Joined: 31 Aug 2010
Posts: 1355
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| Posted: Thu Jul 12, 2012 8:49 pm Post subject: |
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| If those optimax 115's are a V6 you have got a lot of weight out there. |
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JayyyyGee
Joined: 04 Feb 2012
Posts: 2159
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| Posted: Thu Jul 12, 2012 8:53 pm Post subject: |
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| Quote: |
It seems to be getting a lot of prop slip.
If you dont trim the motors up it will just labour very hard and not have any speed.
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Prop down not up
You're trying to achieve speed & props aren't biting
Pods are a tricky thing to setup correctly made mention of it in anutter thread angle of pod is very important get it wrong & you'll have woes
Without seeing or driving goat to see what it's doing tis hard to guess cause once you opt to pod a goat you change axis if that is correct description of the props
May very well be wedges are needed behing motors cause it sounds you dont have enough neg trim contrary to what you may think & have described
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SimonDewhurst
Joined: 13 Mar 2012
Posts: 14
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| Posted: Thu Jul 12, 2012 8:53 pm Post subject: Re: |
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| Deepsix wrote: |
| If those optimax 115's are a V6 you have got a lot of weight out there. |
No they are 3Cyl so 170KG's each. |
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