Dagger boards and Planning hulls

Having had semi-displacement shaft drive boats, I now have an outboard powered boat, with a planning hull. It's very good save for the fact that it is tricky in a tight harbour especially with a good wind.

The problem I find with a planning hull is that the boat “skids” in a turn, unlike a yacht that seems to “pivot” around its keel.

It therefore occurred to me that some form of dagger board that could be lowered when entering and leaving harbour and then raised before accelerating onto the plane might be a neat solution to the slow speed handling problems of a planning hull.

I came across a link to a folding keel for a small boat designed (I think) by Stuart Roy. He in turn said that he had seen drawings of a dagger board employed on a powerboat very successfully for the same reasons. The boat was designed by Eivind Amble and Tor Stokke of Norway.

Does any one have any contact details or any other details about a dagger board for a powerboat?

Thanks in advance
Rob

 

No doubt such things work but are unnecessarily complicated most all the time. A simple skid fin is available from several sources like GlenL that simply bolts on to the boat bottom.

 

A centerboard or dagger board is like a pivot point. It helps mitigate leeway but if there is unbalanced, above the waterline, lateral area, exposed to the wind, fore or aft of the board, the boat will respond by trying to steer with or away from the the wind direction.

Another downside is that the center or dagger board will need a slot in the bottom of the boat. With a planing boat the slot will deliver a vertical geyser of substantial proportions when on a plane.

A pair of runners attached to the bottom will do the job nicely. A vertical fin will work only if it is in the right place with respect to the boat and the wind direction. Runners are the simplest and most practical option. Not only that, but well placed runners will help keep down spray and also give your boat a bit of protection when you run aground or intentionally beach the boat.

 

The problem you're having is typical of full plane craft running at displacement (slow) speeds. It's just the nature of the beast. The usual solution is a skeg, though at plane speed, this is just extra drag, so most just put up with the poor low speed handling and keep their top speed potential. A retractable board is a leak waiting to happen. Runners can work, though if very shallow, not so much at very low speeds.

 

Years ago there was a "shark fin" attachment accessory for transom mount troll motors. It worked well at the low speeds but tended to be a bit twitchy at highest speed. Maybe something like that would work with outboards, if there was a way to control the fin's surface area below the surface. Perhaps raising or tilting the outboard to control the amount of fin exposed would be possibilities....

Porta

 

You have opened two identical topics about this. The other one is: http://www.boatdesign.net/forums/powerboats/powerboat-dagger-boards-50349.html
Not too good idea for the readability of the forum and for tracking of the replies.
Cheers

 

This is what we use on a 1/4 scale crackerbox model to make a seriously ill handling planing hull turn. That setup is for right turns, but a fin on both sides of the transom allows turns in both directions. The same setup is used on deep v monos. The second picture shows the left side fin on a mono. It has a similar fin on the right and turns both ways equally well. The last picture shows one style of crackerbox central fin. That isn't needed on deep v hulls.

Lohring Miller

 

If your problem is only at very low speed and only in close quarters couldn't you just hang a lee board down as needed? My thought is that you could even hang it from the bow eye so it can't screw up your paint. Be sure it is not down when you speed up.

 

As a sailor I can't tell you how many times I have crashed my Boston Whaler at the fuel dock.

 

So it's a common problem and not a lack of experience. Do you think there is a market for the solution I describe in my previous post? My first thought was that the bow was too far forward, but then I thought that the farther forward the smaller and lower force the fin can be. A zero swing bow is easiest to maneuver. The added benefit is the ability to protect the bow from docking damage (common) and find shallows cheaply.

 

Gents, Many thanks for your replies. I did wonder if my topic might be ridiculed, but I guess people interested in boat design have a more open mind! Apologies for the dual post... was not sure which section was most appropriate.

My use of the term "dagger board" was the best way I could think of describing my idea - obviously there is no way you would have a dinghy type dagger board (slot open to the sea) on a planning boat :!:

I was thinking more along the lines of retractable (a/c undercarriage) or telescopic type fin. The suggestion of a fixed fin would be the simplest, but would be a non starter if you wanted to haul the boat out on a trailer.

Given that the general consensus of opinion is that the slow speed handling of a planning hull could be improved if it could be helped from sliding, the next logical question is what sort of sized / shape (let's call it a fin) fin might be required for a given size of boat?

Would a fin 2ft deep, but only 6" long be better than one 2ft long but only 1ft deep?

Obviously I'm not crazy enough to start chopping holes in my boat, but the suggestion by Skyak for the use of a Lee Board over the side, made me think that this could have a similar effect on the handling and allow one to test different shapes, sizes and positions along the hull.

I know some people will think "Why" and say that it's the nature of the beast that planning hulls, which are so good at speed, can be very tricky in a tight harbour.... but I look at these things and think "Why not?" can I make my boat better?

Thanks again for your input

 

If you could mount a twin outboard, you would resolve all your maneuverability problems without the hassle of creating holes, strange mechanisms for the daggerboard etc.

 

Agree... or I could fit a bow thruster.... I'm simply trying to think slightly differently about a problem.

 

I think this is more of a matter of getting used to the handling peculiarities of the boat, after being used to one that handles differently. The daggerboard idea, or similar appendages, seems a bit silly to me, and probably will to the OP as well when he gets more used to the boat, after all there are probably a million boats out there that are similar, but remain sufficiently manageable to avoid consumer revolt.

 

So right. Either the simple skegs or the single fin "skidfin" are the simplest solutions and neither need interfere with a trailer, assuming the trailer is rigged to handle them. Both are mainly for high speed maneuvering and to prevent really bad skids into things tougher than your boat. Low speed maneuvering around docks is best handled by experience since anything that will really affect that very much will be a problem at speed. Larger low-deadrise boats with high windage topsides are much more difficult to handle around docks and the answer is always practice until the boat is big enough for thrusters or dual motors.