Mariner 30 prop

[secondopsman]

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

[secondopsman]

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

[xs 400]

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

[secondopsman]

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

[PeterM]

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 ?

[secondopsman]

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

[jyasaki]

Originally Posted by secondopsman

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

[secondopsman]

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

[jyasaki]

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

[demondesmond]

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

[secondopsman]

Thanks for the reply Desmond.
Yes your are 100% correct with us too there is no normal load anything from myself to 5 up .
25l fuel tank and battery are mounted underneath the rear seat.
I too found the best posistion for the trim pin was 2nd from bottom.
In the handbook Mariner have the WOT at 5150rpm - 5850rpm.
They have list 6 props

3 x light duty
3 x 252 x 360
3 x 252 x 332
3 x 250 x 288

3 x Heavy duty
3 x 250 x 260
3 x 249 x 236
3 x 260 x 210

Non of these seem to match my current prop 9 7/8" x 10.5" (250.82 x 266.7).
Do agrre the best way to go is get the tiny tach fitted (when it arrives) and go from there.
Its just a PITA as i can only try it when im on holiday as im miles from the sea

[demondesmond]

Pete,
when you engine is mounted on the boat where is the cav plate in relation to the bottom of the keel? Also without the engine running, drop it into gear and try rotating the prop. It's rare but sometimes the prop can spin on the hub. Have you had the prop off the spline?

Des

[secondopsman]

Hi Des
I have had the prop off to check the bush and it looked fine.I even put it back on and in gear then put a screwdriver through the splitpin hole and i cant make it spin but i did manage to bend my screwdriver
Pictures below show engine on boat but Doel fins have now been removed.
Distance from top of transom to centre of prop is 24".

[Nos4r2]

You won't be able to spin the hub by hand even if it's totally had it. Mark the bush and prop,use it and see if the marks move relative to each other.

[demondesmond]

Nos4r2 option of marking the prop and checking for movement next time your out is the best option and it was unlikely that you would be able to spin the prop by hand but if for not out for a while...........Set up looks ok to me. If anything it looks a bit low but raising your engine won't help if you still have the rpm issue. To get the boat on the plane try moving some of your weight forward. I used to stap my 25ltr tank to the floor under the bow bag (as far forward as it would go) and use the bow bag to contain any heavy items (anchor / chain etc). This used to work great even when towing the kids on the toys.

Des