Dorado 30 High speed stability

[Austin]

If you're not familiar with the dorado 30 it is a west coats of Florida style boat with a very sharp bow entry that flattens out in the back. The boat was designed to powered by an inboard with and outdrive, anywhere from 210hp up to 300hp. My personal boat was converted to twin 200hp outboards.

The boat handles well at speeds below 27 statue mph, it has a tendency to bow steer but that is common with this style of boat. The issue I would like to resolve is at speeds faster than 27 SMPH, The boat likes to wonder around and does not stay pointed straight, even in calm waters.

My though was to extend the center strake all the way to the rear of the boat to increase stability and prevent the back from sliding. Just wanted to know what yalls thoughts were.

 

[kapnD]

Trimming the motors out at higher speeds may help, but as you mentioned, this hull type does have limitations.
Be sure to wear your kill switch lanyard when you’re up on plane!

 

[Mr Efficiency]

That centre skeg truncates where, at the moment ? I assume at least a couple of feet forward of the transom, and you have a pod beyond that. It probably wouldn't hurt if it was extended to the transom, but what effect that might have on your problem, is another matter, it may even worsen it. I am struggling to fathom how it wanders around in completely flat water. Are the two engines counter-rotating ?

 

[Ad Hoc]

The photos seem to suggest that at higher speed you're not getting flow separation to aid dynamic stability.

Here:

When i see this, there does not appear to be sufficient encouragement for the water flow to separate.

So as you go farther aft, you have this:

Nice wide flat region - perfect for planning - gives lot of nice lift.

But, the water flow can literally go anywhere... it has no real direction, in that sense (I am over simplifying here for sake of brevity).

So it would suggest that you need chine lifting strakes along the bottom to create a more directional flow and so the strakes act like little rudders gripping the water.
Something roughly like this:-

It wouldn't hurt to make the chines a bit better at doing this aft as well.

EDIT
Found an image of typical hull stakes, as an example:

 

[Mr Efficiency]

Well, it could be a host of things, even slackness in the steering, causing it to wander. I would seek out owners of such boats for their experience.

 

[Austin]

The engines are counter rotating, I have bled the steering, made sure everything is tight, tried multiple engine trims and trim tabs, and I have tried different toe in and out with the motors to no avail.

Most all the Dorado 30 owners i have talked to agree the boats get loose at higher speeds.

 

[Mr Efficiency]

Well, if it is an inherent hull trait, that does rule a lot of possibilities out. When you say "calm", do you mean dead flat, not even a barely perceptible swell ? Is the boat porpoising at all, at the speed it starts wandering ?

 

[Mr Efficiency]

Ad Hoc is right there isn't much boat in the water at speed, as it rises up on the gently curved, near flat aft sections, it is a bit like skating with little grip on the water, it wouldn't take much of a differential in forces applied to the boat, to have it shift course. Is it possible to fiddle with engine height at all ?

 

[Austin]

If the water is dead calm the boat will go straight, but any little wind or wake will cause instability. I have piloted a lot of boats and never experienced one like this, at speeds around 32-35 mph you can feel the boat start sliding but is still going straight, almost like crab walking. I don think these hull were designed for these types of power and speed, that is why I am here to possibly come up with a solution to modify the hull design.

It is very difficult to get the boat to porpoise, the faster the boat goes the more it tends to nose down.

Engine height can be adjusted, they are on a bracket about 3 ft off the transom, looking at them while planing and the water line is right at the cavitation plate.

 

[Mr Efficiency]

I might be inclined to try lifting the engines, that will put your centre of lateral resistance further forward, and anything that is pushing your hull off course, pivots around that centre, obviously if you get ventilation of the props, it will have to go down a hole. Your boat, if the same as this one, at speed would have not a lot of boat in the water, and mainly well aft, and a lot of boat in the air forward, setting up the possibility of being easily pushed off course, by little puffs of breeze, on the beam.

 

[Mr Efficiency]

Raising the engines will also make you line of thrust higher, pushing the nose down a little, but if you have found in-trim ineffective in altering the situation, that probably isn't going to be a help. But two outboard legs way aft, is certainly shifting the CLR back, compared to transom hung.

 

[Mr Efficiency]

Raising the engines is easy enough, I'd do that before trying hull alterations, and the more I think about it, I would not extend the centre skeg.

 

[Austin]

I can raise the engines, but I feel they are very close to cavitation already.

 

[Mr Efficiency]

They are stainless props, I assume ? You might get away with it. Is that pod really that long ? ( 3 feet) That is a lot, which makes me think the CLR is too far aft. Does the boat answer the helm strongly at speed ?

 

[Austin]

Yes the steering is very sensitive.

Most of the outboard conversions are done this way, with a bracket or Porta-Bracket off the back.