Making a displacement rowing dinghy plane

I've seen those plates you talk about on canoe sterned fishing boats in the Med, and the south coast of England too. Often they have an adjustable angle courtesy of a pair of old yacht stainless or bronze bottlescrews.
Have you heard about interceptors? Interceptors do the same thing, but they are a vertical plate that enters the flow at the transom. It acts like a trim tab as the stationary water in front of it effectively builds up into a wedge, that the oncoming stream is deflected downwards by. They are largely in the boundary layer- so not as draggy as they might sound. Much easier to build than any plates, trim tabs etc. I'm going to do a pair in 5mm aluminium on wingnuts so I can experiment with the height- see if it works. If it doesn't work- just 4 holes to fill and no worries!
I don't yet know how much of a curved afterbody interceptors would be able to fix though.
If you want to see what an interceptor looks like- the main manufacturer is Humphree. They are used a lot on pilot boats. I had a drive of a pilot boat with them on it in Estonia a few years ago and had a play with them. Incredibly effective considering it's just a very small sheet of vertical metal! What was most noticeable and telling is to keep a straight course, if you put say, the port interceptor down at 25 knots (they are hydraulically actuated on big boats) the boat would turn to starboard, (and roll) to starboard and speed up very slightly. (That's mainly what they are for in pilot boats- to lean the boat to keep it level in a strong sidewind for example). But, it certainly is not a brake if it speeds up the side of the hull its applied to! How much of the turning was due to the rolling and vice versa is all a bit chicken and egg, but I was surprised to have to steer towards the lowered interceptor to keep a straight course, not away from it!

I also have a Swift 18 (1980s British trailersailer), that (now) has a big gennaker and pole but will not plane and is even fairly reluctant to surf- due, I'm guessing, to too much stern tuck. Its lack of performance considering its relatively light weight and large sails is frankly astonishing- but it's nice inside so I've kept it! If my interceptor experiments work favourably on the multicolour monstrosity, I might try to carry the 'technology' across! It's got a slightly retrousse stern so will be slightly harder to achieve ( Interceptors have to be perpendicular to flow I think) - but not beyond the wit of a man with a West B-Pack.

 

About interceptors:
http://sassdesign.net/Interceptors in theory and practice.pdf
JS

 

I've been watching this thread with some interest and I have to wonder if it might be a good deal simpler to laminate perhaps 3 thicknesses of 4mm ply to the approximate transverse curvature of the bottom and to attach the resulting piece to the transom with some fillets of thicker ply.I would guess that a downward deflection of perhaps 12 degrees to the waterline and a fore and aft dimension of say 140mm would be a starting point. Its a simple enough concept and won't require the creation of holes through the bottom in the manner that interceptors will and if the geometry seems wrong first time,it isn't such a major project to vary things a bit.The thought also crossed my mind that even centreboard cases can splash water upwards and into the boat when rowing,an interceptor at greater speed has to be prone to the same thing.

 

A centreboard case is a (flat) tube open to the air at the top. Interceptors are bolted hard to the transom.

Sub planing speed, some water might recover upwards behind an interceptor as a rooster tail behind the boat, I imagine, but I doubt it would ever be an upwards jet like a centreboard case can produce because it's not a tube of any sort.

I can achieve adjustable depth interceptors similarly without holes in my transom, but I think I'll go for the stronger and easier holes/ bolts, inner hardwood pads option.

I think your suggestion is an under hull trim tab. More wood, so heavier than interceptors which would be cut from a sheet of 12 mm plywood or 5 mm aluminium about the size of 4 playing cards. I think I'll go big throttle stick, sitting forward in boat followed by interceptors, followed if necessary by add on sub hull reshaping pieces only if and when required.

Last summer I spotted a young drunken man tearing up and down the river here at I guess about 12 or 13 knots with the same 5 hp mariner (tohatsu) 4 stroke engine I have in a very similar looking boat. Bow was right in the air so he couldn't see over it, but it was planing, albeit mainly through swimmers. It had to be planing at that speed. White water behind etc.
He had no shirt. I felt that at no point was he in the mood to discuss afterbody hull shapes of 1970s GRP rowing tenders. I was mainly just glad it wasn't my Mariner 5. He was (eventually) escorted from the river.

Mine might perform the same as his though, which would be a great place to start from if it does.

 

I'm familiar with the idea of interceptors, but assumed that they created lots of drag so was intrigued by your experience with them. Thanks to Sassdesign for that useful reference, I will bookmark it!

 

No, interceptors do not create drag, or not overall to the whole system. I guess they might create a lot of drag in the last foot of boat, but remove more drag, cumulatively, in the other feet of boat that are trimmed better! ( Although we can't really talk about hulls in that way)
Yacht racer Mike Golding used them on the transom of an open 60 in the Vendée Globe race some years back. Made a tremendous difference he claimed. Unfortunately, because it was Mike Golding, ( Mike has ' nearly won the Vendée Globe' more times than anyone alive!) he was either dismasted or lost his keel shortly afterwards and that became a bigger story than the flaps on the transom. The race organisers changed the rules counting them as moveable appendages, so effectively outlawing their future use. It wasn't the interceptors per se they were outlawing, more a general, foil related rule change that took them out by chance.
There is a good PDF somewhere about the interceptors on that boat, and they also reduced drag at sub planing speed I was surprised to read. Ecover4 I think the boat was. It went like stink until the mast or keel fell off!

 

If you want to read more about the strange world of surfaces perpendicular to the flow direction also Google 'Gurney Flap'.
Not exactly the same but dispels any thoughts that such appendages carry a massive drag penalty. They aren't used on racing cars any more, but they are used on the trailing edges of the wings of some aerobatics aircraft.

 

Fascinating document Jurgen!
You doubtless know the answer to my next questions!
1) My transom is not vertical, it is the usual 10 degrees or so off vertical. Do I need to construct pads at the transom to build it out so my interceptors are vertical, or build out even more so they are perpendicular to the bottom of the boat? How critical is the perpendicularity?
2) My GRP boat has come out of a mould, so it does not have a sharp edge between the bottom and the transom- it has a curve here of about 10mm radius. For correct interceptor performance should I square off this radius in the position of the interceptor with filler?
3) For a 10 foot boat with 5 hp, what (transom) interceptor depth and width would you suggest starting out with? I will be building them with adjustable depth, but any suggestions on a sensible starting depth would be gratefully received!

Your document shows images with an air injection tube behind the interceptor. But it does not speak about this. Have you done much testing with the air? Is it ambient pressure or have you tried pumping it in? 36v Cordless leaf blower springs to mind!

 

Actually,they are used on racing cars,as seen on this Red bull from last year.their function in this type of use is to drag the separation point on the underside a bit further aft.



My comment about interceptors was based on the only examples I have ever been at sea with,which were adjustable and installed in a kind of transverse centreboard case.I would have been hard pressed to notice any significant change of trim or top speed regardless of their extension.A basic version on the transom is a simple solution,but may not work wonders.

 

Ha!, well there we are.
I like it that it's an L-moulding, I'm guessing taped on with double sided tape with another bit of clear anti chafe tape stuck over the top, just to stop it peeling off from the front. Good to see my level of fabrication taking place in a $multi M formula one team!

 

1) That the transom tilts a little is probably not that important.

2) The lower edge of the transom can perhaps be putty up so that it becomes sharp and seals against the interceptor. The lower edge of the interceptor must be as sharp as possible.

3) Five horsepower for a small boat like this is more than enough to planing the boat. Depending on the total weight, the speed and the position of the center of gravity, the depth of the interceptor will be between five and fifteen mm.

To be sure that it all works as desired, the bottom of the boat must be limited with longitudinal spray strips. I consider this to be an absolute requirement for a planing boat

In order to achieve high speeds, the pitch of the propeller must be adapted to the desired speed. From what I can tell from the limited data you provided the boat should do fifteen to eighteen knots. But it depends entirely on the performance of the propeller.

Regarding the aeration aft of the interceptor, it is only for the lowest speeds before aft bottom is completely dry. No additional pressure is required.

JS

 

Thanks for taking the time to address all my questions! I'll report back here with the results and photos. Because I am doing transom interceptors I will set them up with adjustable height ( on stainless wing nuts) so testing at a range of different heights will be very easy to do, once the weather warms up enough for working on boats outdoors.