TOPIC: Glasspar del Mar "sliding?"

I have heard from a couple sources that del Mars have a tendency to slide sideways in a turn at speed. Is this true? If so, has anyone ever added a "skeg" or two to the hull bottom? (some arrangement similar to the photo below) It would seem to me that it could substantially improve the "bite." Now that I have the deck and stringers out, I'm considering adding one right down the centerline about 4" high tapering to 1" x 4' long or maybe a couple about 3/4"x2"x36" centered with the keel about 2' apart...or maybe both. I figure it can't hurt and just might improve handling. Any thoughts?

Nautilus,
I believe the Delmar has a reasonable V-section forward/midships, running to a quite flat bottom at the stern.
One often hears about a flat-bottomed boat ‘skidding’ in turns. A flat-bottomed planning boat turns around a point somewhere between 1/3 and ½ of the length from the bow, the skeg of the outboard motor locates the stern. Think of a car travelling backwards, where the steering wheels are at the back (like a rudder) and the car actually turns around the (fixed) wheels at the front.
Here then, is where you might find a skeg is most effectively placed. Racing skiffs had their ‘sharks-fin’ skegs in this position, and many flat-bottomed racing boats in the 40s/50s had a skeg just inside the chine on the turning side, about midships or aft.
Now imagine that this backwards running car has a very short wheelbase, of say 1’6” (equivalent to a skeg close to the steering outboard/rudder) and the steering effect would be almost nil.
I believe it is unlikely that a skeg or fin at the stern will make much difference (you already have the rudder of the outboard) but could introduce unwanted turbulence or air just ahead of the prop, during a turn.

I think that what could be happening with the DelMar is that the pronounced V forward is acting like a large skeg and digging in when banking, thus creating ‘oversteer’, but giving the feel of the back sliding sideways. I expect many will disagree with this possibility.

The sailing hull you have as an example shows that a drop keel is used (probably just aft of the mast), and the long /shallow aft keel would assist in directional stability when sailing in a cross wind.

Unfortunately, trial & error will be the best way to find out - have fun! Hopefully another DelMar owner has experienced and solved the problem and will give you the solution. Good luck.
Ken.

That handling is very common to Larson Falls Flyers too. It really makes you jump when it does catch and turn, or the when it does decide to slide after starting to bank and turn. After a while you learn the speeds and how sharp to turn to get it to do, or not do, what you want.

My 1957 Endura Craft does the same. it turns great up to 23 mph after that it slides out on truns. I have been thlnking of taking 2 2x2x4 and split them at a 45 and this would make 4 peices. Now I was going to put 2 or each side of the keel about 8ins apart. And was hoping this would stop the sliding out in truns.
Mike aka the pirate
PS
I have had 35 to 75hp motors and its the same on truns
This is a ruff idea of what I mean.

To Downunder...I agree that a skeg in the stern would have little effect if/when the boat is going forward. However, it seems to me that when the outboard is turned, it is not anchoring the stern in place but is instead pushing the stern sideways...and that flat stern has nothing to grip the water. I've decided to install a removable skeg for testing purposes. If there's an appreciable difference, I'll post the results.

Anybody else got any coments on how to stop a boat from sliding out in corners
Mike aka the pirate

If it were me, I would do some research. Find some boats about the same size as your Glasspar. Go boating with those people and see how those boats handle. If you like what you feel, have a look at the hull and see what is different.

I had my OMC Playmate out last weekend. I made a sharp turn and it was like it was a slot car!

99% of vintage hulls slip in turns. This is because of the generally flat bottoms. To increase traction, run a cupped propeller and run the lower unit deeper. If the hull us hooked at all, it's gonna perform even worse. Hooked glassics are a dime a dozen. I'd just accept it as the way it is. Would you expect a 53' chevy truck to turn on a dime? Didn't think so. Want better performance - get a deep V.

I disagree. Many non-purists have done modifications to their '53 Chevys that allow them to perform to modern expectations. That's all I'm looking to do...improve performance. This thread was started to garner/discuss ideas about how to do that, not solicit opinions about what other boat I should own.

But I did give you a couple ideas Nautilus - You might want to try them. Good luck.

Some (or all) of the inboard ski boats have a small keel, like a shark fin. I'm sure they calculate the balance point of thrust and position the keel accordingly. You could try that. Those boats have a very flat bottom and would likely slide allover without the fin keel.

I have toyed with the idea of adding some small keels to the front half of the bottoms of my parent's pontoon to keep it from drifting in the wind while I attempt to dock.

An old, but excellent naval architecture book I have reads “ …Improperly balanced hulls, such as those which have their C of G aft of the true centre of lateral plane (wetted area) are frequently subject to making snap turns.” … “The flat skid, or snap turn if completed, is the result of improperly related forces in a precarious state of equilibrium, allowing the stern to be flung about as in a bad yaw which winds up in the boat making sternway. ie, the boat “skids out in the stern”. This drawing from the book illustrates how a boat turns.

I've been giving this a bit of thought.
Consider a flat bottomed outboard boat, on the plane, going straight ahead. The wetted surface area is symmetrical and all forces are balanced, the boat happily goes where it is pushed. See “A” on the drawing.
Now consider this same flat bottom boat with the motor swung to make a right turn (“B”), consider the activity in relation to the motor: the prop/rudder is (effectively always) going forward, and the planing surface now has a new leading edge (which is the left chine) and the wetted surface is a strange non-symmetrical shape. The left chine is lifting slightly, (making the boat bank). In fact, relative to the prop/rudder the boat is still travelling ahead, but certain forces and shapes create resistance and the hull ‘fights’ the water’s resistance. Now, where to place a skeg?
A skeg close to the rudder will add considerable resistance (and probably contribute air the prop (C”), whereas if placed further forward it will create a turning point (“D”).
With accurate plans the position of the forward fin skeg could probably be calculated if the initial turning wet area was accurately known (but don't ask me to do it).
Ken.

Ahhhh, now we're talking hydrodynamic theory! Excellent.

Damed good feedback, Downunder. Now THAT makes sense to me. It would seem that the only variables left would be the length and depth of the skeg, which I suspect could only be determined by trial and error. For that reason, my idea of a removable (and replaceable) skeg could come in handy.

Nautilus wrote:

Damed good feedback, Downunder. Now THAT makes sense to me. It would seem that the only variables left would be the length and depth of the skeg, which I suspect could only be determined by trial and error. For that reason, my idea of a removable (and replaceable) skeg could come in handy.

That way you can tailor the fin so you get mediocre performance.

Nautilus,

cal/email the chairman's office of the nearest college of engineering & tell them what you'd like to know & i'll bet that a polite question will soon get the CORRECT answer on how/where to put the fins/runners/????

that's how i found out how to BETTER distribute the smoke in a HUGE commercial BBQ pit that i once owned. - it took a grad student about 2 days to come up with the solution.

yours, satx

I still think my idea of the 2x2x4 split at a 45 degree angle will work. It will give the last 4 feet of the flat bottom something to bit on in the trun. And if it don't this is not a performance boat any way.
Mike aka the pirate
PS
ONly one way to find out

Just look at the Jillions of tournament ski boats out there with "skegs" "turning fins" "tracking fins", whatever you'd like to call them.

Here's a good example:

My HydroCraft Fantasy has a skeg. I can get some pics and measurements of its location and the size if you would like. It is a very flat bottom with sponsons, different breed altogether, but if it helps let me know.
Neil

Early DeMars had sea-unfriendly hulls. Late Del Mars with the Seafair hull handled beautifully. Not much you can do about a round-chined hull. They are what they are.