Why does she dig herself a hole!

[mike lewis]

 

Why is it that when my boat is going forward, the stern digs in?

The more power i put on, the more the stern digs in. It dam near covers my stern button!

My boat is 48 ft with a normal swim and propeller. Is this just happening to me?

We have tried putting more weight in the front, but that didn't make much difference.

 

Would a bow like a 'super tanker' stop the stern from digging itself a hole?

 

There must be a name for this. Even at low speeds, it feels like the boat is dragging the bottom.

 

Help!!!!

[Justme]

I am no expert but on my sea fishing boat if that happens (or the reverse when to bows get pushed under) It is because I have set the outboard at the wrong angle and or height to the bottom of the boat. If the prop does not go under the boat enough or its highly angled it does this. Thats on a 16 footer that weighs less than 1/2 ton so not sure if thats comparable to a NB.

 

Justme

[mike lewis]

I am no expert but on my sea fishing boat if that happens (or the reverse when to bows get pushed under) It is because I have set the outboard at the wrong angle and or height to the bottom of the boat. If the prop does not go under the boat enough or its highly angled it does this. Thats on a 16 footer that weighs less than 1/2 ton so not sure if thats comparable to a NB.

 

Justme

 

Its good to know its not just my boat. This happens to every craft! large or small.

Edited July 30, 2008 by mike lewis

[bottle]

Its good to know its not just my boat. This happens to every craft! large or small.

 

The depth of water will also exacerbate the problem, if it is shallow, just ease off on the 'throttle' an she will level out and travel faster.

Edited July 30, 2008 by bottle

[Gary Peacock]

Maybe the boat is falling into the hole the prop left?

[mike lewis]

Maybe the boat is falling into the hole the prop left?

 

Gary, I think your right.

 

But why is the propeller digging a hole????

 

The depth of water will also exacerbate the problem, if it is shallow, just ease off on the 'throttle' an she will level out and travel faster.

 

We have notice this and we do back off the throttle. But the underling problem is that the stern digs in.

It must be related to the propeller. Because when we broke down once and were towed the stern did not dig in.

[Guest axiom propellers]

The phenomenon that you describe is commonly known as 'Slump'

[Chris Pink]

 

Why is it that when my boat is going forward, the stern digs in?

The more power i put on, the more the stern digs in. It dam near covers my stern button!

 

All boats do it, which is logical if you think that the propellor is moving water and there will inevitable be a delay before the absence of moved water can be filled.

 

The working motor narrow boats from which all modern designs can be traced would sit with counters 2" or more out of the water for this very reason.

 

You have too much ballast sir.

 

on the other hand it does aid with cavitation so maybe simply raising your stern button is the answer. The deeper the water the faster your optimum speed (see recent topic) and the more your stern will drop.

[andywatson]

 

Why is it that when my boat is going forward, the stern digs in?

The more power i put on, the more the stern digs in. It dam near covers my stern button!

My boat is 48 ft with a normal swim and propeller. Is this just happening to me?

We have tried putting more weight in the front, but that didn't make much difference.

 

Would a bow like a 'super tanker' stop the stern from digging itself a hole?

 

There must be a name for this. Even at low speeds, it feels like the boat is dragging the bottom.

 

Help!!!!

 

As I see it the prop works by moving water from the front of the boat to behind.

This means that there is negative pressure in front of the prop (ie under the boat) compared to behind.

This negative pressure effectively sucks the boat down.

 

The effect of this will be reduced by

A slowing down

B better underwater design to get water to the prop - longer, better shaped swim

C deeper water

 

I'd say you're stuck with it on that boat.

Edited July 30, 2008 by andywatson

[LEO]

Hi,

 

Actually, I think it's the first stages of 'planing'. My 30 footer (with a BMC 1.5) used to do this in the winter when the front water tank was empty, try throttling back. More economy, goes just as fast, but you lose the front end lifting effect.

 

With a full 80 gallon tank mine stopped this.

 

I suspect it's the reason they changed the desing of the bows of modern cargo ships.

 

ALBI.

Edited July 30, 2008 by LEO

[WJM]

Something I see a lot on the Tideway when narrowboats are running fast in deep water is the counter baseplate above the water. That is, the side of the swim is open to the air, not usually along its whole length but often along the front half. I sometimes point it out the helmsman but I doubt the even understand what I am telling them. My guess is that they dont have enough ballast. It cant be a good thing because it allows the prop to suck air.

[John Orentas]

Most types of boats will do this, it is caused by the venturi effect as the water below the boat accelerates relative to the hull and the pressure drops. It was said at the inquest that the Herald of Free Enterprise pulled down a full four feet in shallower water.. A similar thing happens with the 'ground effect' on racing cars.

[mike lewis]

Something I see a lot on the Tideway when narrowboats are running fast in deep water is the counter baseplate above the water. That is, the side of the swim is open to the air, not usually along its whole length but often along the front half. I sometimes point it out the helmsman but I doubt the even understand what I am telling them. My guess is that they dont have enough ballast. It cant be a good thing because it allows the prop to suck air.

 

Yes, we have notice that the underside of the swim is visible and there are lots of bubbles in the water coming out of the propeller.

I'm getting a bad back moving all this ballast around, to try and cure what i now know is called slump.

[Grace and Favour]

And, indeed, is exactly the same thing, (but in reverse) that keeps airplanes in the air . .

 

Air is pulled over the 'longer' distance of the top curvature of the wing, (the underside surface distance being shorter) Thus creating a partial vacuum (or - 'thinner air') above the wing - which sucks the wing up into the area . . .

 

and that's a force against that of gravity of course.

 

What you're doing with NB Swifts & Swallows is encouraged by gravity as well

[mike lewis]

And, indeed, is exactly the same thing, (but in reverse) that keeps airplanes in the air . .

 

Air is pulled over the 'longer' distance of the top curvature of the wing, (the underside surface distance being shorter) Thus creating a partial vacuum (or - 'thinner air') above the wing - which sucks the wing up into the area . . .

 

and that's a force against that of gravity of course.

 

What you're doing with NB Swifts & Swallows is encouraged by gravity as well

 

I understand all of that, but. If the air is causing the problem. Why haven't boat designers put 'skirts' around the sides of the swim? To prevent air being dragged by the prop.

[bottle]

Mike

 

How does your boat 'sit' when moored?

 

This is not a criticism but either the boat is not ballasted properly or you are trying to travel to fast for the water (depth) you are on.

[howardang]

 

Why is it that when my boat is going forward, the stern digs in?

The more power i put on, the more the stern digs in. It dam near covers my stern button!

My boat is 48 ft with a normal swim and propeller. Is this just happening to me?

We have tried putting more weight in the front, but that didn't make much difference.

 

Would a bow like a 'super tanker' stop the stern from digging itself a hole?

 

There must be a name for this. Even at low speeds, it feels like the boat is dragging the bottom.

 

Help!!!!

Its proper name is "Squat" and it is caused by the flow of water into the propeller when the boat is in shallow water. When the water is pulled under the boat by the prop it is going at a faster speed than the water surrounding the boat and therefore the pressure under the boat is less than elsewhere. Because the pressure is less the boat then sinks slightly or "squats", increasing its draught. The answer to it is to reduce speed.

 

There have been many ship groundings caused by squat - one of the more famous ones was the QE2 which ran aground near Marthas Vineyard some years ago when going too fast over a reef, resulting in extensive damage to the bottom of the hull. The increase in draught in this case was 6 feet or so!

 

You might find this link interesting.

 

http://www.ntsb.gov/recs/letters/1993/M93_30_33.pdf

 

 

Howard

Edited July 30, 2008 by howardang

[Grace and Favour]

Brilliant stuff Howard . . and thank you for that . .

 

and the next time my loved one mentions she's going for 'a squat' - I shall slow down . . .

[Gary Peacock]

Someone I know had a bit of a "incident" involving a large fast boat (Sea plane tender), in a moment of non thinking he fully open the throttles on a canal and in a split second the water disappeared from beneath the stern resulting in the transom literally dropping onto the canal bottom followed by a inrush of water from the resulting damage!

Edited July 30, 2008 by Gary Peacock

[Keeping Up]

All boats squat down at the stern when you open the throttle. And as Graham (NB Alnwick) has shown in his photos, if you get up a good speed the front will also squat down so that the whole boat is lower in the water

 

You can use the effect to your advantage, if you've got enough courage. If you approach a bridge and find that it is a little to low to fit underneath - back off and try again, but as you approach the bridge just open the throttle to maximum. You'll either squat down enough to clear the bridge, or else hit it ten times as hard as you would otherwise have done, and make the insurance claim really worthwhile.

[alan_fincher]

In my experience the effect is generally much more noticeable if a narrowboat is ballasted much lower at the stern than the bow.

 

Ones that sit level in the water when not moving still do it, but generally less so, (in my opinion).

 

Additionally, most budget and mid range boats tend to be built by just pulling in the sides to form the bow shape, which then means the base-plate has to be bent upwards at the front to meet them. I don't know, but it seems to me that this will cause even a level ballasted boat to deflect water under the front more than is the case with a completely flat baseplate, and this is likely to raise the bows and lower the back.

[DHutch]

Additionally, most budget and mid range boats tend to be built by just pulling in the sides to form the bow shape, which then means the base-plate has to be bent upwards at the front to meet them. I don't know, but it seems to me that this will cause even a level ballasted boat to deflect water under the front more than is the case with a completely flat baseplate, and this is likely to raise the bows and lower the back.

And here you have the makings of a planing hull!!

[mike lewis]

Mike

 

How does your boat 'sit' when moored?

 

This is not a criticism but either the boat is not ballasted properly or you are trying to travel to fast for the water (depth) you are on.

 

She sits quite flat when moored, i've taken real care when ballasting the boat. I know what your getting at and it would have never been taken as a criticism.

Everybody has been fantastic with there help on this topic. I thank you to all.

[John Orentas]

There is another element to this issue that has not yet been mentioned.. It is generally accepted by boaters that longer bow and stern 'swims' are a good thing in boat design, there is one aspect however where they can be a disadvantage, almost by definition a long swim will give less buoyancy, when I built my second boat which was a very much sleeker design compared to my first one, the difference in performance was almost immediately apparent..

 

The boat needed about 1/2 a ton less ballast that the calculation would suggest, filling the water tank which of course is at the extreme 'front end' and even stepping onto the rear deck seemed to have an exaggerated effect.. But most of all, the stern 'squat' as someone called it was very pronounced as a result of having rather less flotation at the bow and stern.

 

Don't get me wrong, I would never have changed the design, the boat was very fast through the water and caused much less wash than other craft, it just had one or two idiosyncrasy's.

Edited July 31, 2008 by John Orentas

[bottle]

Hi Mike

 

Is your 'uxter plate' above the water level, if so she may need some more ballast.

 

All this will not stop her 'digging an hole' that is what boats do but it may reduce it.