Setting up a strut on a small catamaran single drive hull

[Old School]

My son has a 650EP cat hull with a single drive flexshaft. As delivered from the factory the strut is positioned with a negative attitude. No amount of upward adjustment will even align the bottom of the strut in a neutral alignment. The forward web on the strut actually contacts the hull with the strut still negative.

The stuffing tube exits the hull with a downward angle all the way to the strut whereas I feel the stuffing tube should exit the hull in a smooth curve then level out and enter the strut in a neutral (horizontal) alignment, thus allowing for either positive or negative adjustment of the strut as necessary. The stuffing tube has been incorrectly fitted so that the strut is mounted too high in the hull and yet still has a negative attitude with no possibility of raising the strut to correct it.

Thus the stuffing tube requires replacement with a new tube that exits the hull and enters the strut in a neutral alignment. To set up the stuffing tube I need to know a neutral position to set up the strut to get the right curvature from under the hull. That is, what is the orientation of the strut in relation to the adjacent sponson stepped surface, please to set up a neutral attitude for the strut?

[CraigP]

Technically, you’re correct. The stuffing tube should be replaced. But a quick fix and one that might suffice is to take your Dremel tool and put a flat clearance on the section of web that is hitting the hull. You have all kinds of meat to work with, so no harm or foul...

[Old School]

Many thanks. We have decided to replace the stuffing tube as to set the strut at a basic neutral orientation with the current stuffing tube would have severely limited the choice of propellor as raising the strut would have brought the prop closer to the rudder support arm, limiting a prop to less than 30 mm diameter. The rudder support arm also was poorly positioned being about 3 mm too low and thus impacting on propellor choice. This will be positioned higher on the mount pad. Together with the new stuffing tube will give the clearance we need for a better prop selection.

As the TP Power 2940 motor has a 4 mm shaft we will maintain a 4 mm flexshaft throughout. My son wishes to retain a liner so 1/4" O.D. tube will be used. This will run through the strut in one piece as the stuffing tube and I will fabricate a rotating bushing using 7/32" tubing with 3/16" tubing internally to make up the difference to run a 4 mm shaft. The rotating bushing will be two thicknesses of tubing soldered together and cross drilled. The strut will still be removable as it will slide over the end of the stuffing tubing.

The stuffing tubing has been bent to position the strut level with the bottom of the hull sponson surface when placed on a flat surface (neutral). This will then allow a degree of positive and negative adjustment as needed. The factory install only allowed negative adjustment, even at its most upper point.

The factory strut was fitted with ball bearings which I have found is a very poor design from an engineering point of view. The outer flanged bearing is barely 2 mm thick. While running yesterday the bearing moved aft taking up the compression gap my son had left on the flexshaft. The shaft then was allowed to flop about causing the prop tips to contact the rudder support arm. Other than installation tolerance friction there was no method to retain the bearing insitu. This was a recipe for disaster. The rudder support arm needed simple blemishing to relieve any impact damage however, the lovingly balanced and polished prop looks like a total loss. When Kyosho made this hull no doubt it was a quality item, however, successive factories that have released the boat (Dragon Hobby and Exceed) seemed to have taken shortcomings in production to keep the price down.

[rol243]

sounds like that rudder support arm should not be positioned where it is now. firstly its way to close to the prop for decent selection of prop sizes and secondly the prop at rpm,s does like to unload its free water without any restrictions such as something above it.. take a look at the fastest r/c boats and note where the prop is and does it have any restrictions for water release. i would be moving the rudder assembly over more.

[martin]

A lot of these boats using the ball race struts are only intended to run a small gap between the flange bearing & the drive dog ( 1mm - 1.5mm max gap ) for the very reasons that you have found when using a larger gap. These boats are only intended to use the fairly low powered motors they come with so flex wind up is not an issue with only the small gap between the dog.

[martin]

Also a good starting height for the strut ( whether you are adding positive or negative to the strut ) is when the boat is sat on a flat table the bottom of the strut should be 1/8" off the table.

[martin]

I have the Dragon Hobby Saga 650ep cat ( same boat ) & the stuffing tube is not bonded into the hull, their is a short piece of tube bonded into the hull & the stuffing tube is slide into this outer tube. This allows the stuffing tube a bit of forward or back movement, so when you put positive or negative on the strut it allows the strut to pivot on the stuffing tube. If the stuffing tube didn't float like it does on mine it wouldn't allow the strut to pivot as easy.

[Old School]

Many thanks gentlemen. The boat is typical of the small mini-zonda type hull with a small rudder support arm mount pad, so relocation other than offset is very restrictive. My son wishes to retain the inline rudder location so a combination of raising the pad mount a further 3 mm plus proper alignment of the strut should give the prop clearance necessary. I have seen other boats the same and the rudder support arm is mounted higher on the mount pad on the transom. The combination of poor support arm mount location lower on the mounting pad, plus a poorly designed stuffing tube allowing negative strut attitude only, combined to limit both prop selection and strut adjustment.

As it is a specialised hull then alternative parts for the rudder mounting are simply not available. I have searched the usual suspects and unfortunately the pad mount surface area is minuscule and will not take the standard TFL type strut/rudder mounts commonly available. Any change will require manufacturing replacement parts to suit. We will continue with the modifications outlined above and if further alterations are necessary then the thinking cap will need to be used. The boat has plenty of power plus a tight turning circle so hopefully the inline rudder location will not need to be altered.

[Old School]

I have the Dragon Hobby Saga 650ep cat ( same boat ) & the stuffing tube is not bonded into the hull, their is a short piece of tube bonded into the hull & the stuffing tube is slide into this outer tube. This allows the stuffing tube a bit of forward or back movement, so when you put positive or negative on the strut it allows the strut to pivot on the stuffing tube. If the stuffing tube didn't float like it does on mine it wouldn't allow the strut to pivot as easy.

Martin,
the boat is basically the Dragon Hobby hull but made by Exceed. I do not think it has the quality of the Dragon Hobby boat. Yes, the stuffing tube was inserted into a "stuffing box" tubing in the hull, however, there was no movement there and the stuffing tube was shaped such that the strut was held in a severe negative attitude. Raising the strut to its highest level (contacting the hull) did not even bring the strut to anything approaching a neutral attitude plus bringing the prop tips dangerously close to the rudder support arm.

[martin]

I havnt played around with mine for a few years now but just got it out to look at & I can get good vertical movement on the strut & good Pos & neg movement. I do remember that when I first got the boat it had lots of negative angle & didn't move much or easy ( all the ones I have seen have also been like that ), I do remember playing around with this assembly to get it adjusting to what I wanted it to be.

[NativePaul]

Or there are always bearing retaining compounds, keeping bearings in place is the actual designed use for the Loctite 603,638,648,etc that many of us use to make our flexies and wire drives.

[martin]

I don't think the Dragon version is made any better judging by all the picks I have seen of the 650 cats, I think they may all be made in the same factory but with different paint jobs to suit the wholesaler.

[martin]

Yes Loctite 641 would be a better choice for fitting the bearing as its bearing specific fit & can be undone without to much force or heat. The 603,648 etc are high strength retainer compounds.

[Old School]

Thankyou

[Old School]

This has changed to a full reconfigure of the shaft assembly. A stuffing box tube has been epoxied in the hull. The stuffing tube will slide into this and is secured at the strut end. The stock 3 mm flexshaft (4 mm stubshaft) will be replaced by a 4 mm flexshaft. I have fabricated a brass rotating bush to suit the 4 mm shaft as the stock ball bearings caused too much grief.

A threaded aluminium mount plate was epoxied in the transom to assist with removal of the rudder support arm should the strut need removal in the future for maintenance.

The strut placement now is set at neutral and will accommodate several degrees of both negative and positive adjustment. The factory set up had several degrees of negative attitude with no way to obtain even a neutral attitude. All is corrected now. The Chinese seller (a competitor of Hobbyking) has shown no interest at all in the issues raised with this boat and has not replied since an initial email sent regarding missing/faulty parts on this hull. You get what you pay for.

[martin]

When I did my one 3 or 4 years ago I did re bend the stock stuffing tube to get a neutral strut angle which I can also get pos & neg angles on the strut.

[Old School]

Many thanks. Just awaiting a locking collet and tests can be carried out.