Thread: Keel angle

Where is the ventilation plate (often wrongly called a cavitation plate) located, compared to the bottom of the transom? Where is the bottom of the transom when the boat is at rest, compared to the LWL?

Hi Par,

Thank you for your reply.
The motor is a long shaft and is fitted to the correct hight, that is with the plate/cav plate just about level with the hull of the craft, that part is all ok, and I think that in my rebuild I have not got the clearance from the keel under the boau with enough clearance for the prop to get all the water if needs.
Mory

G'day Mory
Can you post pics, in profile and from astern, of the keel & motor in the down position?
regards
Alan

Outboards need a bit of boat in front of them, or the leg acts like a plow and the prop ventilates badly. Profile and stern photos with the engine down will reveal all.

Originally Posted by b.o.a.t.

G'day Mory
Can you post pics, in profile and from astern, of the keel & motor in the down position?
regards
Alan

Hello Alan, Thank you for your reply, and if I work out how to put the picture on here I will, but I always get lost in this.
I have taken three pictures, stern on with motor shown ti hull, and onew fron either side, motor in the lowest position, Mory

Originally Posted by b.o.a.t.

G'day Mory
Can you post pics, in profile and from astern, of the keel & motor in the down position?
regards
Alan

Hello Alan, Thank you for your reply, and if I can work out how to put the picture on here I will, but I always get lost in this.
I have taken three pictures, stern on with motor shown to hull, and one fron either side, motor in the lowest position, Mory

Hi Mory
I see you figured out the attachments.
Is that the normal operating position?
regards

Hi Mory - I too would like to know if the motor is in the normal operating position as I note in your post that you said it was in the "lowest poition". The normal motor trim angle is the important thing as the motor trim angle in the photo is clearly insufficient and would make the bow dig in whilst too much angle can make the transom drag. My Yamaha guide helpfully says the motor must be trimmed at the "correct angle" but finding that angle for most of us generally requires some trial and error - Par probably has a mathematical formula !

Originally Posted by bagman

Hi Mory - I too would like to know if the motor is in the normal operating position as I note in your post that you said it was in the "lowest poition". The normal motor trim angle is the important thing as the motor trim angle in the photo is clearly insufficient and would make the bow dig in whilst too much angle can make the transom drag. My Yamaha guide helpfully says the motor must be trimmed at the "correct angle" but finding that angle for most of us generally requires some trial and error - Par probably has a mathematical formula !

Sorry, you are all answering the wrong question, I fully understand about the motor position and trim.The query I would like to know, is I rebuilt this boat, and I think I do not have enough clearance ,from the keel, so I think I am getting a vortex and maybe cavitaion from the prop, as there is not enough water coming down to the prop. Why I think this, is the speedo does not work ( the pick up is in the leg on the 50hp Yamaha) , so there is no pressure to the back of the leg, therefore I think not enough water getting to the prop as the keel is to deep, as seen in the picture,I now think I should cut this back by about 60 t 70 cm or about two feet. This would give me an angle of clearance back to the prop. The motor is in the lowest postion as some one ask for that to be shown. Thanking you all Mory.

Hi Mory
PAR & I just asked for it in down position - should have specified "normal operating down"...

I'm not the expert on this - many years since I did any significant power boating.
Just want to clarify that by "keel" you mean the wide flat chunky plate holding the
bottom panels together rather than the narrow skeg extending below it.
If so, I think you are correct, and that at speed, the water hasn't enough time
to reconverge behind it before hitting the motor.

It MIGHT be possible to chamfer the aft corners of the keel plate back under
the boat about a foot to start the water convergence sooner, and without
compromising strength too much. Or not...

But I really want to defer on this to someone with current wide experience on
this, and with sound knowledge of structural integrity around a high stess
joint. As I say, my power boating is a lot of years ago, and with a limited range
of hulls, all alloy.
regards
Alan

Hi Alan,

Yes I think you are right, I have to take the keel angle back about 60-70 cm, [two foot,] under the boat, this will then line up with the motor in the working position, this will get the water to the prop at speed. Thanks everybody for your imput, will now to try to get the job done,
Mory at Blanchetown.

G'day Mory
I think you need to keep the depth of the keel at its centre, all the way to the
transom. otherwise you lose its reinforcing strength at the transom.
Only chamfer the corners, probably down to a triangular cross section at the transom.

Done like this, the water will already be reconverging behind the boat
when it leaves the aft lip of the keel, vs the current situation where it only
starts to move together after the boat has passed.
Inertia delay & all that other physics stuff forgotten soon after high school...
cheers

There could be a number of issues at play here. I think the prop is ventilating, not cavitating (big difference) and this could be from a number of things.

The easiest way to solve your issues is to just install an anti ventilation wing on the motor's anti ventilation plate. There are two types: one has a delta shape, often with vertical fins,and the other looks more wing like. The delta is good for high speed boats, while the wing types are better with boats going under 35 MPH. They both do the same thing and prevent the prop from sucking down air from the turbulent water clearing the transom. They also help maintain pressure on the low pressure side of the prop arc. I'll bet this will cure your problems and it's just a few bolts to install.

If you want to get into the real dynamics, we'll need pictures of the motor in action, showing the wake and turbulence from the transom, the ventilation plate height, etc.

Simply put, the ventilation plate likes to be right at the bottom of the boat. Some boats like it as much as an 1 1/2" (38 mm) deeper, while others like it as much as an 1" (25 mm) higher. This is usually a function of the shapes used in the boat.

The skeg could be cut down a bit, likely without any concern of the boat's structure (there appears to be a keel), but more importantly are the inboard runners, which seem to stop about 8" (200 mm) from the transom. The worst thing that can be done is to smooth them out and taper them in, as it appears has been done. What you really want is a crisp, squared off trailing edge, not rounded and tapered. I know this sounds crazy, but you want the water to "release" not reassemble on a powerboat. The water doesn't have time to reassemble into a smooth, clean flow, when bashing along on plane, so the best thing you can do is insure it exits the boat with a crisp edge, which will leave the turbulence with less aeration.

I designed a 700 HP race boat last fall, who's build is nearing completion now. The inboard lift strakes and the trailing edge of the transom have a section of aluminum angle stock molded into them, which makes a clean square edge for the water to release off of. To further this, the aluminum edges, which stick out about a 1/2" (13 mm) have a chisel cut on them, which will need to be "dressed" and sharpened from time to time. All this just to sneak a few extra HP from the frictional and parasitic drag, produced by the hull at the speeds it will travel at. Okay, this is a bit annal in regard to edge treatments, but it's a race boat and they want to win; a lot.

Also not commonly known is, you need some resistance in front of the outboard leg or it'll "self-ventilate". Any one that's tried to put and outboard between the hulls of a cat or pontoon boat, quickly learns this lesson. Lets see if I can explain this simply. As the hull move through the water up on plane, it's in a slightly bow up attitude. The stern is buried and the bow clear of the water. A well balanced boat will carry about 3 degrees of bow up attitude. This inclined plane, pressurizes the water as it moves aft, under the boat. Some of this pressure slides outboard, because of the V shape, but some remains pressurized as it reaches the transom. When the water releases from the transom, it tries to fill the void left by the boat passing through it and being under more pressure then normal, also climbs up the transom a tad and curls off.

Now, the prop lives in a weird environment, where the blades have to traverse denser and less dense water. This is because the bottom of the prop arc is deeper (hence more dense) than the top of the arc. The pressurized water coming off the transom is more dense close to the hull, which helps equalize the pressure on the blades, making it more efficient. Without a hull there to create this pressure, like on a cat or pontoon boat, the prop will suck down air from the turbulent, less dense surface water, as each blade comes around to the top of the prop arc. This is why engine manufactures put anti ventilation plates on the outboards. As designed, they are intended to work with a transom, so they're relatively small. In some cases (like I suspect with this boat) you need more pressure at the top of the prop arc, so a wing (DoleFin and Stingray are common brands) can easily solve this. This is the wing style.





On a cat or pontoon boat, you'll have a miniature transom, dangling down to: one hold the motor, but also to increase the pressure at the surface of the water, so the prop doesn't suck air. Wheew, maybe that made sense.

Lastly, the skeg isn't necessary on a powerboat. If you look at every new production built powerboat, made in the last 3 decades, you'll note there aren't any skegs, just lifting strakes and these are carefully shaped. Cut the whole thing off and chop off the inboard rub strips, so they are crisp edged and at least a couple of feet before the transom. Inboard strakes can cause eddies to flow into the prop, especially in turns. The lower portion of the outboard (the leg) will provide all the lateral stability you need at speed. Of course once you slow to displacement speeds, it'll wander around, but this is normal behavior for a small outboard powered boat.

Originally Posted by Mory

I have taken three pictures, stern on with motor shown to hull, and one fron either side, motor in the lowest position,

The cavitation plate looks too low for my liking.......it should be in line with the bottom of the transom thereby running at surface water level when on the plane.. The effect of the skegg/keel is irrelevant. It is important not to have the cavitation plate below the bottom of the transom. Other than building up the top of the transom (to raise the engine) you may find that the best option is to fit a hydrofoil (wing) to the cavitation plate. This should clean up the water to stop cavitation/ventilation at the prop however it will not be a 100% fix....... it may also have an effect on the boat handling particularly in turns. Just be aware that the stern may be a little 'twitchy' when turning at speed until you get used to it. You should also play with the trim to obtain optimum performance.

I notice in the photos that the fixing bolts are in the centre of the sloted holes. If this is the case for all of them then slacken them off and raise the motor on them....it may be all that is needed.