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Old 02 December 2008, 08:33   #1
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Prop hitting air in the water...

Well don't know how to putt this exactly in english, but on tight high speed corners my prop sometimes hit's nothing anymore, well no water to be exact....
The engine will over rev, like when jumping.
Sometimes it's enough to quickly make the corner less tight, and sometimes I'll need to bring the trotle down and then steer in a straight line before it will go off again.

I'm a little worried that is has to do with the fact that my rib was designed for a longshaft and I fitted a shortshaft. Wait before you go and think I'm an complete idiot..... I did have the transom lowered in the middle where my engine is fitted. As you can see on the picture.....

However, does anyone know why this happens and what I can do about it?
Because it really takes the fun out of fast and tight cornering....
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Old 02 December 2008, 08:43   #2
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Your engine looks low enough so it's got to be something else. My guess is your prop is a general purpose device. Props with teddy bear's ears as blades are not great in turns and they don't shed weed easily either. The solution is simple...stop showing off and/or drive only in straight lines.
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Old 02 December 2008, 09:58   #3
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The point about the prop might indeed make sence, but I'll need some more information on that point then, since I'm not yet very well known with props and all their different specs and inlfuences on the way the boat handles.

Anyone....??

ps, I found out my problem not when "showing off".... it was the last two weeks when in holland nobody is on the water becuase it's cold.... Also no police on the water to stop me speeding, so the best time I can think of for testing things like this.
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Old 02 December 2008, 10:26   #4
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If its possible to move the engine down by 1 hole on the mount ( i think you can do this form the pic) is it worth a try to see if you are pulling air in during a turn. On a couple of my smaller boats you can get this (even though the engine is low enough) if you really turn fast or rev it too high - its just the way things work sometime !
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Old 02 December 2008, 11:09   #5
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Just to make sure I understand what you mean, moving down one hole on the mount would be getting the shaft of the engine one hole closer to the transom. So trim it down a bit?
That's one of the big disadvantanges of no powertrim....

But thinking that should also work, as I remember our console boat from 12 years ago, with trim down turning was a lot better.

Still wondering about the prop though, what is the way to go there?
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Old 02 December 2008, 11:30   #6
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Yes - either trim it in & see if it improves - you may end up running with the bow down more , or physically lower the engine vertically on the bolts through the transom. Both will put the prop into clearer water , but you may loose a bit from the top end of the speed range by doing both.

As for the prop - its a very complex bit of fluid dynamics as to what will happen depending on blade shape/ angle/ cup etc . I'll let someone else have a go at explaining as its a bit of dark art to me !
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Old 02 December 2008, 12:31   #7
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A hydrofoil (basically a contoured extension of the anti-ventilation plate) may help with your cornering at the expense of a bit of top end speed. Should also help climbing out of the hole. It works (in theory at least) by isolating the prop from the waters surface. Usually they're designed to give a bit of lift at speed as well, though that's a matter of some conjecture.

You will have to drill some holes in the anti-ventilation plate to mount the thing; some people don't like doing that.

You could also try no mods, and trim down before racking into a tight turn. Or, as has been mentioned, drop the engine height to get the prop deeper.

What's happening is that you are trimming for a straight line, and when you turn, the boat leans to the inside, effectively raising the prop towards the surface. If you're trimmed high enough on a straight line, the turn brings the prop close enough to suck air down into the prop's bite, and you get ventilation (which means no more thrust.)

Luck;

jky
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Old 02 December 2008, 12:43   #8
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bassboy

I agree with jyaski. Your anti vent plate looks low enough, but if you are cornering at high speed whilst trimmed up, it's no wonder your prop is losing grip.

The more waterline length you have and the 'deeper' the prop is in the water, the less likely it is to ventilate.

Very basically- trim bow up for straight line speed, trim bow down for tight planing turns.

You could also spend some more euros and get a stainless prop. It worked wonders for me.
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Old 02 December 2008, 13:51   #9
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Vintec,

Its difficult to tell from the picture but is your anti vent plate below the bottom of the transom or is it above if so how high is it above the bottom.

Also what does the edge of your prop feel like? if it has any nicks or roughness this will be enough to cause cavitation and you will loose grip much more easily. a file and some sandpaper to finish will sort out that problem.

Chris
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Old 03 December 2008, 04:15   #10
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Chris,

The anti vent plate is on the front of the shaft about 2 cm (little less then 1 inch) below the transom, with the current trim possition the end of the anti vent plate would be a little bit above the transom. It's trimmed on hole 5 out of 6 on the way up. Hope I said this correct....

The prop, I bought this new last June and has no damages, there is a fraction of paint missing on one of the blades, but that can hardly be discribed as damage...
So this should be ok.

However I do have an stainless prop at home, but this is a totaly different size, this is 10" x 10". The prop on the pic is an original yamaha prop in aluminium size 9 7/8" x 12".
Still not sure what each of the numbers exactly stand for, then the bigger the number the bigger the size.
Maybe somebody can also help me with that?

Vincent
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