Handling, steering problems

I have designed and built a 33 ft GRP boat. I have been dealing with a steering problem occurred during the sea trials. I would appreciate any suggestions you may have based on your experience, solving the problem.

The boat's main characteristics are the following:

Hull length 9.98m
Hull beam 3.70m
Waterline length 8.26m
Weight approx. 6500 kg
Hullform V shaped planning hull with 17 degrees transom deadrise, hard chines and spray rails.
Material GRP
Engines 2 x Yanmar 315 hp
Propulsion Twin shaft drives
Steering system Hydraulic steering system (single pump, single cylinder), twin rudders transom mounted

Steering cylinder Vetus 125 kgf
Rudders transom mounted. Blade area 0.1084 m2 each

Speed 22 knots at 3000 RPM (cruising speed) , 30 knots at max RPM

When the boat is cruising at trawling speeds or at low speeds (up to 19 knots) steering is quite effective. When the speed exceeds 19-20 knots, the boat cannot be steered. More specifically when one turns the steering wheel right, the boat leans to the starboard side but never turns, or turns after quite a while.

Do you think the problem is because of the transom mounted steering blades? Maybe the area of the blades is too much for the speed of the boat and the size of the steering cylinder? Maybe the hullform
hides a terrible design mistake causing the steering problems?


Again I would appreciate if you could suggest a solution based on you experience or with other similar cases.

I have attached some photos of the hull to get an idea.

Best Regards
Vassilis

 

Similar symptoms

I found this thread which is about repowering and converting a Luhrs 29 from inboard to outboard. The bad handling this guy describes on his Luhrs is what I am dealing with my boat except my boat runs straight like an arrow (it does not lean on its right or left side like the Luhrs). Besides this handling is awful.


http://www.boatdesign.net/forums/showthread.php?t=5005&highlight=outboard to inboard conversion

 

i have experience the same problem before,basically you must check your hydraulic hose connection. if everything turn our correct,there is a possibility that your steering cylinder need to be upgrade to load capacity,you may consult your supplier for proper calculation. i am not an expert on this subject but just to give you my point of view,hope it help.

Cheers!!

 

Had the same problem on a 32 ft Fibercraft converted to 2 250 hp Yamahas. Boat had a permanent "squat board " at the transom . I added trim tabs and the problem moved from 25 knots to 40, way faster than the hull was designed to go.

 

Rudders are foils that develop positive pressure on one side and negative pressure on the other side when you apply lock. More than likely you are drawing air down into the low pressure section of the blade from the section of the shaft that protrudes below the line of the transom. The shaft will have a trough behind it capable of feeding air into the blade. This will reduce effectiveness to around 30% of the unaerated blade.

You need to devise a way to cut off the air supply to the blade. Something similar to the so called cavitation plate on an outboard. In fact, if my view is correct, the cause is identical to what occurs when a prop aerates and loses grip. If you have ever experienced this you will know how ineffective and aerated foil becomes.

I doubt if you can actually see the air getting down into the rudder in a turn but if you can listen over the engine noise and turbulence you might detect something. You may hear the suction - unlikely though. You may be able to pick up bubbling in the wake.

I am assuming the hydraulics are turning the blade in response to the helm and are not bypassing on pressure relief. You should be able to readily determine the blade angle by observation.

The stern hung rudders you see on smaller sailing boats have to be much larger than if the rubber was mounted beneath the hull. They are compromise for ease of beaching or trailering so do not provide a good example of a rudder. You have a small blade area and it needs to be working in water not in aerated water.

Rick W

 

so what you suggest is either increase the blade area or install through-hull rudders instead of transom-mounted.

 

No. I am suggesting a plate say 300mm long and 200mm wide that is a neat fit around the shaft. It needs to be faired to the transom. It is like a rigid trim tab but in line with the bottom of the hull. The objective is to ensure streamline flow under the plate and exclude air from running down into the blade from at the wake of the shaft.

The plate should work by forcing the high pressure water at the front of the shaft into the low pressure zone at the sides and behind the shaft. In simple terms there is no way a trough can form behind the shaft that allows air to connect with the the rudder blade.

Another solution could be in very tiny detail about fairing in front and behind the exposed shaft. As you have a "V" hull the top edge of the rudder is lower than the transom so the forward part of the blade does not interfere with the hull. This means there is a small section of the shaft that acts as a bluff body above the blade but exposed to water flow. There is high water pressure on the front and low pressure (a trough) behind the shaft. If there was a fixed fairing piece in front of the shaft and behind to ensure attached flow then this will also exclude air. The sectional shape of the fixed fairing could be similar to the rudder blade section.

This shaft fairing might work but the plate is probably easier and more effective.

One thing you should be able to observe is the trough in the water behind the shaft and you might see the aeration occur as you apply helm.

Rick W