Wayward slow speed 'bow-steer' on deep vee hulls?

rustybarge

rustybarge

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We have a local Botnia police boat on the river which is a medium vee planing hull: it tracks true and straight at slow speeds. I've seen lots of deep vee ribs and speed boats that track like drunken spiders at displacement speeds: goes to the right steer left, then goes left steer right ....

I was looking at an American aluminium boat kit from Specmar with 20° deadrise, it weighs about 3.5 tons and is 32' .

At what length/weight does a deep vee planing hull start to track straight at 6-8kts like big boats do?




 
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It might be a characteristic of having a single engine.
Neither of my twin engined boats wandered at low speed, but the singles snaked around as if the helmsman had been on the Capt Morgans.
It’s possible to anticipate the wander and counter-steer, but it takes practice. Many people’s initial attempts make it worse, so most just give up and let it wander.
 
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I thought it was single props that make a boat veer (more noticeable on inland waters) a pair of counter rotating props on the same drive should make her track true like a torpedo

If I didn’t have the iPad slow typing issue I’d have posted first :rolleyes:
 
Hell no. My twin out drive wanders like a drunk at low speeds unless you get the revs absolutely perfect on the synchroniser. Even then it has a slow wander. 34 foot lwl. Conversely she tracks straight as a die if you start pushing the limits of displacement speeds. And has a very slow wander at planing speeds, but that I think has more to do with trim tabs.

I think the clue w.r.t. Deep V may be ballast. My boat leans into a turn rather majestically and is great fun doing so. I think when the boat is leaning ever so slightly due to whatever ballast reason it is enough to start a turn. The flatter the bottom the less impact any ballast issues are going to make. My tender for instance can be set on a track and it does not deviate from it. Not unless wind or wave impact upon it.
 
Our 23 degree deadrise all up weight near 20 tons tracks like a torpedo so much so auto pilot is redundant.

There’s something about the centrifugal effect , weight of spinning mass s .
Let’s move away from the deadrise an focus on that .
Little ribs have Zilch spinning kg,s compared to me for example even though we be near enough share a common deadrise .

I think that’s the denominator centrifugal effect , coupled with as Bruce infers with little single engines craft being deflected by a relative sizeable wave to them .

Think gyros stab for rolling , ( vertical axis ) then have sufficient kgs spinning along the horizontal and this resists turning to port or stb .
Ribs don,t have the kg,s , we must have est over a ton or more + = props shaft gears , crank all spinning in the long axis of the boat .

Basically at 2000 rpm I can’t turn sharp I have to drop the rpm to 1500 ish to turn sharp .
It’s not the rudders they turn cause drag and the speed straight on drops .So when I drop the rpm ( spinning mass ) it starts to respond ,ie turn .
 
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FlowerPower said:
It might be a characteristic of having a single engine.
Neither of my twin engined boats wandered at low speed, but the singles snaked around as if the helmsman had been on the Capt Morgans.
It’s possible to anticipate the wander and counter-steer, but it takes practice. Many people’s initial attempts make it worse, so most just give up and let it wander.
Click to expand...

What length/weight were your twins and did you find that waves caught the bow at slow speeds and pushed the boat sideways?
 
BruceK said:
Hell no. My twin out drive wanders like a drunk at low speeds unless you get the revs absolutely perfect on the synchroniser. Even then it has a slow wander. 34 foot lwl. Conversely she tracks straight as a die if you start pushing the limits of displacement speeds. And has a very slow wander at planing speeds, but that I think has more to do with trim tabs.

I think the clue w.r.t. Deep V may be ballast. My boat leans into a turn rather majestically and is great fun doing so. I think when the boat is leaning ever so slightly due to whatever ballast reason it is enough to start a turn. The flatter the bottom the less impact any ballast issues are going to make. My tender for instance can be set on a track and it does not deviate from it. Not unless wind or wave impact upon it.
Click to expand...

That's my feeling too: the deep vee is never exactly vertical to the water surface so even a slight lean will cause the hull to turn one way or the other creating even more lean in the process. This will produce an ever increasing curve around the straight ahead position.

What was your max displacement speed ?
 
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rustybarge said:
What length/weight were your twins and did you find that waves caught the bow at slow speeds and pushed the boat sideways?
Click to expand...

First twin was 28ft 4400kg.
Current boat is 36ft and 7900kg.

Both had outdrives adjusted to have a fair amount of toe-in.
Waves don’t really move the bow around at slow speeds, although the current boat’s bow is blown by the wind at docking speed.

.
 
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Portofino said:
Our 23 degree deadrise all up weight near 20 tons tracks like a torpedo so much so auto pilot is redundant.

There’s something about the centrifugal effect , weight of spinning mass s .
Let’s move away from the deadrise an focus on that .
Little ribs have Zilch spinning kg,s compared to me for example even though we be near enough share a common deadrise .

I think that’s the denominator centrifugal effect , coupled with as Bruce infers with little single engines craft being deflected by a relative sizeable wave to them .

Think gyros stab for rolling , ( vertical axis ) then have sufficient kgs spinning along the horizontal and this resists turning to port or stb .
Ribs don,t have the kg,s , we must have est over a ton or more + = props shaft gears , crank all spinning in the long axis of the boat .

Basically at 2000 rpm I can’t turn sharp I have to drop the rpm to 1500 ish to turn sharp .
It’s not the rudders they turn cause drag and the speed straight on drops .So when I drop the rpm ( spinning mass ) it starts to respond ,ie turn .
Click to expand...

If centrifugal force was responsible, then perhaps a small light twin engined boat would have a lot more directional stability than a single. But taking the Botnia as an example; it has a low deadrise slow planing hull that can stay on the plane at 12 kts which is why it's so good in rough weather, but with a fine bow which slices though the waves.

A deep-vee boat also has a fine bow, but a very sharp deep stern which will not keep the boat perpendicular to the surface. Of course larger boats have massive wetted surfaces which help to give dynamic stability to the hull.

So perhaps it's a combination of narrow bow and stern which cause the unstable tacking as the boat literally falls from one side of the hull to the other?
 
FlowerPower said:
First twin was 28ft 4400kg.
Current boat is 36ft and 7900kg.

Both had outdrives adjusted to have a fair amount of toe-in.
Waves don’t really move the bow around at slow speeds, although the current boat’s bow is blown by the wind at docking speed.

.
Click to expand...

Methinks the Specmar boat is very light at 3.5 tons and 20° deadrise vs. your 7.9 tons. Perhaps that is the most important criteria, rather than deadrise.
 
For anyone interested in building your own boat from a kit of pre-cut parts I thought the $40k price was very reasonable for a 10mtr alloy boat. There would be freight to pay on top of that.

Does anyone know the approx cost of a professional coded welder for a boat of this size and complexity?

 
rustybarge said:
That's my feeling too: the deep vee is never exactly vertical to the water surface so even a slight lean will cause the hull to turn one way or the other creating even more lean in the process. This will produce an ever increasing curve around the straight ahead position.

What was your max displacement speed ?
Click to expand...

I said pushing it. Max displacement speed is about 6.5knts. Push it to 7.5 to 8 and she steadies the wander.
 
BruceK said:
I said pushing it. Max displacement speed is about 6.5knts. Push it to 7.5 to 8 and she steadies the wander.
Click to expand...

Maybe that's the solution : create a bow wave to help stabilise the hull and cushion and support the deep vee bow section.
 
Can I ask what your intentions are with a commercial crab boat?

At 3.5 tons she is incredibly light for her size. What thickness plate will be used?
 
BruceK said:
Can I ask what your intentions are with a commercial crab boat?

At 3.5 tons she is incredibly light for her size. What thickness plate will be used?
Click to expand...

6500lbs/2.9 tons rigged with single 300hp on empty fuel.

3.8 tons 140gal/4 pob/ gear.


 
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Hoping to use as a dayboat/sea angling/weekend camping on board type of boat to cruise the coast between Gib and Portugal.

The straights of Gibraltar are windy and have the reputation for steep waves and strong tidal flows.
 
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