Prop hitting air in the water...

Vintec · 02 December 2008, 12:33

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....

jwalker · 02 December 2008, 12:43

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.

Vintec · 02 December 2008, 13:58

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.

PeterM · 02 December 2008, 14:26

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 !

Vintec · 02 December 2008, 15:09

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?

PeterM · 02 December 2008, 15:30

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 !

jyasaki · 02 December 2008, 16:31

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

BassBoy · 02 December 2008, 16:43

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.

Chris · 02 December 2008, 17:51

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

Vintec · 03 December 2008, 08:15

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

Chris · 03 December 2008, 09:11

I would trim the leg in as suggested above to avoid slip and then I would experiment with lifting the engine up a couple of inches on the transom as this will give you back some speed.

Ultimately if you don't have power trim then its all about finding a balance between speed and grip.

Chris

Poly · 03 December 2008, 09:30

Quote:

Originally Posted by Vintec

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?

the two numbers are (1) the pitch of the prop (2) the diameter of the prop. So I think you have a 10" diam x 10" pitch S/S prop and a 9(7/8)" diam x 12" pitch Aluminium prop. Going down in pitch (12-10") will lose you some top end speed. Going up in diameter 1/8" will probably not be obvious, but will be slightly harder to "turn" as will, the heavier s/steel so you might lose some revs. On the other hand your acceleration will improve.

PeterM · 03 December 2008, 09:30

Chris - wont raising the engine put the prop closer to the surface of the water ( as it runs off the hull at planning speed ) and risk pulling in more air ? I'm no expert on this as I've only had it happen a couple of times & its solved on my boat by trimming right in in the turns.

As you say will trimming in give the biggest improvement then upping the motor will give back the performance ?

Richard B · 03 December 2008, 09:37

Quote:

Originally Posted by Vintec

However, does anyone know why this happens and what I can do about it?

I'm sorry to say that you may not be able to do much about the problem with that hull. It's only medium-V and the tubes are mounted quite low. As a result, when you do a tight turn, the tube will be foced into the water on the inside of the turn, and the keel will lift out of the water. Sorry but I think you don't have the right sort of hull for what you want.

Vintec · 03 December 2008, 09:50

By the looks of what most of you are saying, it looks like a combination of factors.
The medium V hull is definately the case, however, when in plane it lifts quite a bit out of the water. But for sure not as much as bigger RIB's with much deeper V.

I've no powertrim so no way of trimming down when going into turns.
So will bring down the engine 1 hole, and see what this will do for turning, top end speed and overall handling.

The prop is not the best but good enough for the engine / boat combination I think. So for the little extra gain by going for a better prop, is not worth the money. Better save that for my next engine 60 hp or so....

Think I've summed it up correct like this.
Thanks everyone for your input!

If anyone has some other vision, please let me know!

Vincent

Bigmuz7 · 04 December 2008, 06:51

Quote:

Originally Posted by Richard B

I'm sorry to say that you may not be able to do much about the problem with that hull. It's only medium-V and the tubes are mounted quite low. As a result, when you do a tight turn, the tube will be foced into the water on the inside of the turn, and the keel will lift out of the water.

I think there is something in that too ,.. not so much the hull v depth,.. but the tube size in relation to the hull and stern weight, they look large and will provide a lot of lift in a turn , with an engine that doesnt weigh very much in relation to them. Your engine is well deep enough as it is IMO

Vintec · 04 December 2008, 09:16

Think there is a good point in that.
And the boat is slightly underpowered, aiming for 60hp next year or so. Depending on depth of wallet......
Hope with a heavier and more powerfull engine, the balance will be better and especially the lift during planing.

ps. Bigmuz, don't want to be a know it all, cause I sure am not, but with Honda's VTEC is spelled VTEC and not VTECH. Couldn't help myself seeing this in your personal info on the left of each post.

Locozodiac · 04 December 2008, 15:03

You could contact the rib manufacturer and ask him what is the factory deep V bottom/anticav plate height parameters best peformer recommended, maybe he has the light at the end of the tunnel.

Happy Ribbing

Chris · 04 December 2008, 17:32

Quote:

Originally Posted by Blackroady

Chris - wont raising the engine put the prop closer to the surface of the water ( as it runs off the hull at planning speed ) and risk pulling in more air ? I'm no expert on this as I've only had it happen a couple of times & its solved on my boat by trimming right in in the turns.

As you say will trimming in give the biggest improvement then upping the motor will give back the performance ?

When I read the post that Vintec placed saying....

"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"

The fact the vent plate is 1" below the transom at the front and above the transom at the back suggested to me that the engine is trimmed quite far out which would certainly explain the cavitation. So by trimming in this will resolve the cavitation but at the expense of a fair bit of speed. I would be looking to then lift the engine until the cav plate was level or slightly above the bottom of the transom in order to recover that speed and fuel efficiency.

Vintec · 04 December 2008, 17:58

There's just one slight issue in raising my engine. The engine is attached to the transom by the turnable screws at the front end of the bracket (don't know how to call these....), so no bolts are used.
I didn't use bolts yet because I didn't want make holes in the transom, since the transom is lowered for my shorthaft engine and I'll have it brought up to normal hight when I'll get a bigger engine.

So how can bring the engine up without drilling holes in the transom?
Or is it no problem to drill holes and later close them up again?