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Old 04 June 2012, 12:54   #1
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Country: UK - England
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Mariner 30 prop

Evening guys
Well i finally made the effort and im down in devon with the sib.However i seem to have a problem and i would like some input from wiser peolpe please.
I have a quicksilver HD430 sib with a Mariner 30 two stroke.When i get it on the plane it seems to run fine then rev up really high(As if when a clutch slips in a car).
I have checked the prop and got some numbers of it which are 9.7/8 x 10.1/2 f.(3 blades)
The engine was used by the previous owner to power an 2.5m SIB so im wondering if he may have changed the prop to a smaller one.
I have experimented with the trim and this made no difference.
Just wondered if anyone knew of the correct prop size please and i will ride round Devon tomorrow trying to track one down.

ON THE PLUS SIDE
we encountered about 20 dolphins and the kids havent stopped talking about it for the past 2 hours.
Thanks in advance
Pete
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Old 05 June 2012, 13:51   #2
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I did some Internet trolling today and it seems that I do have they correct size prop according to this website
Prop Guide, find the perfect boat prop. Easy to use Propeller Guide.
Could it be the doel fins causing this problem?
I have measured from the transom to the centre of prop and it's 23" so is the correct short shaft for the boat.
It's just as if the prop comes out of the water and there is no resistance and it revs higher.
I'm at my wits end now
Pete
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Old 05 June 2012, 14:43   #3
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dont dispair

9 7/8 is the width in inches. sounds ok for a 30hp ( I have a yam 2 stroke with a 9 7/8 type f prop)

what sounds wrong is the pitch too little at 10 1/2

when i had a honda t40 sib the engine had a 9 7/8 x 13 f

the noise you describe sounds like over revving. check your manual but my engine wide open throttle (WOT) is 4500 to 5500. you sound like your doing more than this.........not good

you need a rev counter (easy option is a tiny tach 48 easy to fit) and check th revs

type f props go......10 1/5, 11 1/4 , 12, 13........each 1 increase reduces revs by 150 to 200 rpm
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Old 05 June 2012, 14:59   #4
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Thanks for the reply
We're away at present and I have just rung the inlaws and asked them to fetch the manual and ring me.
I'm hoping I can pick a prop up in devon somewhere so I can try it before I go home.
Pete
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Old 05 June 2012, 15:34   #5
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Have you tried it with out the doelfins ? Could be they are lifting so much you are pulling air under the front of them at speed ?

They could be doing something odd to the water flow.

Worth a go for the few mins to take the 4 bolts out ?
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Old 05 June 2012, 15:57   #6
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That is my plan in the morning if the waves die down a little
I have just had a walk down and it's rough out there.
Once I leave on fri I won't be using it again until August when we go to Cornwall so would like to bottom it before then.
Pete
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Old 05 June 2012, 17:00   #7
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Quote:
Originally Posted by secondopsman View Post
It's just as if the prop comes out of the water and there is no resistance and it revs higher.
Close. It sounds like you've got a ventilation problem. Prop gets close enough to the surface that the descending blade pulls air with it, and you lose hydraulic force on the blade.

Trimming in may help (assuming you're not in all the way now), lowering the motor definitely will. Changing props (different brand = different blade shape) might make a difference (though it's a crap shoot as to better or worse.)

The doel fins may be lifting clear of the water at speed, which would get you back to the anti-ventilation plate as your only defense (aside from depth.) Any chance you can take a look back and down while you're running? Hard to see what's going on, but if you see the entire base of the fins front-to-back, they're not doing any good venilation-wise.

Luck;

jky
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Old 06 June 2012, 14:26   #8
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Quick update
Doel fins removed today and took for another test and it seems a lot better but the sea was rough and couldn't get on the plane but it didn't rev up like it did before.
I do still think its a little under propped so a tiny tac has been ordered and I will search for a selection of props to take with me to Cornwall and I can experiment.

Thanks for the replies.
Pete
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Old 07 June 2012, 00:31   #9
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If you never got on plane, the test is moot.

Not on plane, the motor is sitting low, of course you won't have ventilation problems.

Try again when you can run the way you want to normally.

jky
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Old 09 June 2012, 17:03   #10
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Pete,
sorry it's taken so long from initial contact but here's my tupence worth......
Everything you have been told so far is correct. You do need to know what RPM your current prop is making and you need to know what Mariner says your WOT (Wide Open Throttle) revs should be. Altering the pitch of the prop will increase or decrease the rpm as stated by xs400 above. This in turn will allow you to choose the correct pitch for your new prop. You should ensure that you boat is loaded as for 'normal' use (which is easier said than done if you use your boat in the same manner that I used to use mine. 'Nomal' could be anything from me on my own going fishing to four up exploring to towing a doughnut / wakeboard).
In answer to your question to me I have two props that I used on my 3.8mtr Zodiac Futura S / Yamaha 30HP 2 stroke combo. 9 7/8 *12 and 9 7/8 *14 which I swapped depending on what I was doing. In practice you should always carry a spare prop anyway. I suggest that you do not simply plump for the same pitch as myself or xs but work out from the above what you need. If you need a second opinion once you have your current prop rpm and your Mariner WOT operating rpm post it up and we'll have another chat.
Having said all the above, hieght of the engine on the transom (and therfore cav plate position to the water), how you load your boat (how much wieght you have in the bow either people or equipment (fuel tanks / anchors etc)) and as you have already illuded to trim all make a huge difference. If you have power trim you should trim all the way in for take off and then trim up for top speed. This of course is not possible with manual trim so you have to compromise a little. I used to use the second hole up on my manual trim Yam 30 which seemed to give the best overall results. It's difficult to know what else to suggest without knowing the specifics of your set up. You could post pics of engine position which might be helpful. Give me a shout if there is anything else and I'll try and help if I know the answer.
Sorry if you already know about all the above but I seemed to have gone into download mode.

Des
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