Trim plate trailing edge cup ?

I have nearly completed fabrication of a new set of trim plates for my ski boat and have been contemplating the addition of a cup on the trailing edge like the propeller has!

Have been running the plates shown below for the last 2 seasons and they are working well to stop crashing when crossing other boats wakes, in rough water, excessive bow lift and at higher speeds to stop porpising (spelling?).

I am replacing them with plates near double the width and no fold on the sides, to decrease the downward angle necessary in these conditions and the excessive drag caused. Also hoping they produce a flatter wake in rough conditions than the old narrower ones.

From my way of thinking the cup on the prop gives it more grab and efficiency and a cup on the trailing edge of the trim tabs should also make the tabs more efficient at a lower angle.

Thaughts and advice please!


This picture is original tabs being built.

 

The cup on the propeller will not affect the trim tabs. Trim tabs have folds on the sides for stiffness. What do the new ones have? Is the total surface of the trim tabs different?

 

New tabs are the same depth but nearly twice as wide, with no folds on the edges like the old ones (they are plenty strong enough) the pitot spedo intake is being moved as far right as possible to accomodate the new tabs, so yes double the surface area. I built the originals and the new ones myself so can bend/strengthen wherever!

The reference to the propeller cup was the idea that putting a cup onto the trailing edge of the trim plate (like the prop has) would help by not having to put as much downward angle on the trim plate and therfore would cause less drag.

 

In my opinion, adding a cup to the trailing edge (TE) of a trim tab would not achieve the benefits that you wish. The reason to add cup to a propeller blade is to allow for a shift of pressure distribution further toward the TE of the blade. This also comes with a reduction in angle of attack (i.e., pitch) to maintain the same lift. You can think if cup as localized camber.

One misnomer about cup is that it offers higher propeller efficiency. This only happens if cavitation breakdown would be present otherwise, or if the particular loading requirement for the propeller blade would benefit from additional camber. In the absence of this, cup reduces efficiency. Generally not much, I grant you, but some.

Having said that, cup on a trim tab might function a lot like cup on a high performance outboard propeller, that may be fully "ventilated" on the suction side due to cavitation. In other words, it is functioning like a super-cavitating (or surface drive) propeller. Camber and cup is used here as a practical way to match up chordwise pressure distribution with blade thickness. Thinner is better, but the "ventilated" blade cavity has a wedge shape, so the blade itself will want to follow that wedge shape. Increasing thrust by increasing angle of attack (i.e., pitch) would put too much of the pressure toward the thin nose, so thrust is increased with camber, some of which is localized at the TE as cup. The local shape of cup also promotes separation, which is beneficial for drag reduction.

It would seem that you would also lose some tab angle sensitivity by adding cup. (You even might have some lift with negative tab angle.) The pressure is also shifted further aft, so you might need more ram pressure (?).

In summary, the only benefit that I can see to adding a cup is in promoting separation - but there would seem to be better ways to do this.

Don MacPherson
HydroComp

 

For a trim tab made from sheet metal with a relatively square edge wouldn't separation from the bottom surface be reasonably clean at any speeds faster than dead slow?

 

I would think so. The higher pressure with a cup would simply shed more effectively.

 

Coming back to one of the original observations, if you have two tabs of equal area the tab with the most width will provide the most lift.

So by increasing your width you will increase the lift. Increasing chord (fore to aft dimension) will provide additional lift, but not as much as the same increase in width.

Steve

 

Thanks for your explaination Don.

I will leave them flat underneath and have a look how they go.

I have slowly been working on correcting the excessive bow rise and porpising over the last couple of years. Initially by angling the cavitation plate downward and then adding the trim tabs. fitted a new propeller late this season with a lot more aft rake and this has made considerable improvements, at 50mph it requires no trim, over this trim needs to be added to stop porpising!

 

The most important thing is smooth flowing lines with a square clean cut end sharp as you can get . No cupping .
Its amazing what a differance shapre edges make ! With F1 and tunnel boats we spent hours getting edges sharp and straight !!clean water flow where it leaves the Boats hull surface .

 

Is the truth not that the prop shaft angle/weight distdribution and prop positon are the problem and that the tabs are an additon to try to correct another problem

 

Trim tabs can indeed "fix" some of the problems you mention, but trim tabs themselves are not an indication there is something "wrong" with a boat.

Boats operate in a dynamic environment. If the boat was trimmed perfectly and no one ever moved, the waves and wind never changed, fuel never got used and ice never melted keeping refreshing beverages icy cold then trim tabs would never be useful.

Trim Tabs are seldom "necessary," most boats will typically float and move through the water without them. They do however allow the operator to keep the boat trimmed for optimum efficiency and / or comfort in a wide variety of conditions and loadings. They enhance the performance and comfort of a boat in a very cost effective way.

Steve

 

Tunnels, I'm keeping the edges as square/sharp as possible but have to make it safe as there is a swim deck mounted above the tabs and the boat is used as a family ski boat so lots of activity in and around this area!

"Is the truth not that the prop shaft angle/weight distdribution and prop positon are the problem and that the tabs are an additon to try to correct another problem"

Problems, "mate"!!!!!
The boat was built around 59 to 62 (it took a while!).
It is used as a ski, tube, kneeboard, wakeboard, barefoot, swim platform and sometimes fishing boat.
Regularly the dam is very rough and we usually travel 15 to 20k's to quiet flat bays. Without tabs this was impossible and damaging. The tabs have been an excellent addition and transformed the boat.
The hull has about 50mm/2" of dead rise, so basicaly flat. Flat wake as well, which I want.
The boat has some rocker which after some studying of the forums gives the boat slow speed manoeuverability, but can be an issue with the porpising at speeds. The manoeuverability is required to compensate for no gearbox or dog box, it is direct drive.
Prop angle is 10deg so not too bad and as mentioned above the new prop with more aft rake has made a significant difference at minimising bow rise.
Been trying to get hold of the original plans, which a copy of exist, but due to circumstances beyond my control I will probably have to wait until someone passes away before getting hold of them!
I also want it to go 500mph and use no fuel.

 

When you get that worked out, I have a Patent lawyer I would like to introduce you to, for 50% of the revenue of course

 

I went to the Sea Work exhibition at Southampton this week and was interested to note that the Kamewa water jets now have an interceptor fitted. Can someone here explain the hydrodynamics of this and the similarity between it and the cupping of the trailing edge of a trim tab. It struck me that there may be a similarity of purpose.

 

Just had to do a search and find out what an "interceptor" on a jet was, found a good picture which explained it for me!

http://www.rolls-royce.com/Images/RR Kamawa A3_0908_tcm92-8662.pdf

Completed/fitted the new tabs so will post some pictures soon. Going to be 4 weeks before they have a test run though. Considering temporarily tack welding on a small cup just to see if it changes anything.

"When you get that worked out, I have a Patent lawyer I would like to introduce you to, for 50% of the revenue of course"

I'm actually getting close, and am currently looking for a long downhill section in the local dam to finally get it all to work!