planing pods/skis/amas

[sigurd]

I think some special speedsailing boats use skis that are free to pitch, and some like the yellow pages use high AR pods that are locked in pitch.
In which circumstances would you use which alternative?

There has been some discussion here about tris with vakas optimized for displacement speeds and amas that are optimised for planing. I guess that if only one high AR ama is used for each side of the tri, some means of pitch stability must be implemented, foils?
Or maybe use a fore and an aft ama on each side?
Otherwise I think the ama would be so long that it would have a lot of skin friction.

The yves parlier boat uses stepped hulls but these are compro/opti-mised for both displacement and planing.

[Doug Lord]

Foils on multihulls especially those with a trimaran configuration allow the amas' to be very small since they don't have to fly the main hull but more importantly they can be used to develop the righting moment for the boat, like the Rave, Hobie trifoiler and the 40'SCAT.
Planing ama's seem to have a lot to offer above a certain speed but it's hard to beat the virtually unlimited power to carry sail of the independent altitude control systems on Bradfields or Kettermans boats. The bigger the boat using these systems the more structural problems to engineer it right.
For ultimate speed planing hulls seem to be the way to go but "hybrids" like Parlier's boat may have the worst of both worlds since the stepped hull is draggy between 10 and 20 knots more or less. Because of the stepped hulls foils are required to control the pitching moment of the boat which limits top speed. On seaplanes with stepped hulls the elevator on the plane controls pitching-something has to because stepped hulls are unstable in pitch....

[sigurd]

Can you elaborate why stepped hulls are unstable in pitch?

I thought a stepped hull is where there are two planing surfaces, one fore and one aft, like the parlier cat? It seems to me it could be as stable as an unstepped one, since you can have one contact surface far forward, and one all the way aft.

What I also am curious about is whether there is any merit to free-pitching, planing units, (skis, pods, short hulls whatever), for instance used as "front legs" in a tri.

[Doug Lord]

As to stepped hulls being unstable in pitch one article I read was in Seahorse magazine earlier this year where a member of Parliers team used that fact to explain why the boat had rudder t-foils fitted(if I find the issue later I'll post it). Another good article on powerboat applications of stepped hulls is in Issue # 88 of Professional Boatbuilder which talks about a unique stepped hull solution to enhance controllability by using an aft "stabilizer". (idea by John Plum)
The idea of "free pitching" planing units might have some value depending on the specific application; what did you have in mind? More importantly, exactly what are you trying to achieve with a "free pitching " planing unit?
The Gougeon brothers designed a trimaran(Adrenaline) that allowed the ama to pivot in pitch and they said it worked well. The problem I see on a "free pitch" planing system is how do you keep the rest of the boat under control in pitch? If you use foils you've probably limited your top end speed and you've already affected the lower end by using planing hulls in the first place.
It all depends on what you're trying to achieve....
===============================
Here are a couple of excerpts from the article in Professional Boatbuilder by Eugene P. Clement:
"This interestng result shows an inevitable shortcoming of a stepped hull in which the needed lift aft is provided by a fixed afterbody rather than by an adjustable stabilizer. The designer in the former case must locate the afterbody in a compromise position, suitable only for some conditions of weight,CG location,
and speed. For most running conditions, however,the afterbody will inevitbly be in the "wrong" position, giving a running trim angle that results in less-than-optimum performance,and possibly porpoising."
---
" In short, the tests showed that a suitable stern stabilizer allowing operator control of the trim angle will not only offer minimum drag at different loading conditions, but will also overcome the porpoising problem so frequently encountered by stepped boats."
===============
The interesting thing about Mr. Plums stabilizer is that it was a planing section that was raised up and down(and remained level as the boat banked); the t-foil may work better on a hull like Parliers that has a large force that could tend to lift the transoms....

[sigurd]

Let's say I were to design a small ocean going triscaph - three short, wide planing hulls like Seaspider or Yellow Pages Endeavour. I could make them free in pitch or I could lock them relative to the frame. on flat water the pitch of the locked one would be relatively constant independent of speed. Not sure how the freely pitching hulls would/could behave though.
From powerboat experience I think that it would be desirable to have a low AoA to get planing - after that some increase (how much) in AoA gives a lower surface area and thus less resistance. I do not know whether such a trait could be built into a freely pitching hull - my powerboating experience tells me that the opposite generally happens.

Then there is the issue of waves. a locked triscaph ama would change its center of lift and "pitch relative to the water" while a pivoting one could be more constant in AoA and thus more efficient?

[tspeer]

Foils on multihulls especially those with a trimaran configuration allow the amas' to be very small since they don't have to fly the main hull but more importantly they can be used to develop the righting moment for the boat, like the Rave, Hobie trifoiler and the 40'SCAT....

This is a popular belief, but I think a mistaken one. Dave Keiper started out with small amas, and he capsized the boat three times - each time he was moving slowly and got hit by a gust. If you look at his book, over time the amas got bigger, and bigger.

There are four regimes to consider: light wind sailing hullborne, flying in moderate winds, sailing hullborne with the foils down in heavy conditions, lying to a parachute anchor in survival conditions. An ocean-going foiler needs to be able to cover all of these conditions. The foils allow you to optimize the amas more for stability in heavy conditions without being as concerned about performance in medium conditions. They allow the main hull to be optimized more for light wind conditions. And the hydrofoils can be deployed or retracted to allow the hull to "change gears" just like changing sails does for the rig.

But it's not a good idea to drastically reduce the size of the amas.

[Doug Lord]

I agree that on ocean going foilers serious consideration has to be given to how much ama size can be reduced but I think the size can be reduced from the size required to efficiently fly the main hull on a 60 footer(for instance)-but it definitely has to be looked at closely.
On smaller multi foilers I think the ama's can be reduced more drastically but they still have to be able to take a gust when the boat is off foils w/o an immediate capsize. I think Dr. Sam has got it about right on the Rave.
One of the most important factors here is overall beam which with a Rave type foiler can be substantially more than a conventional tri. The extra beam allows a smaller ama to have an effect on stability and gust responce off the foils comparable to a larger ama closer into the main hull.
On the rc foilers I've designed I found that an ama with a total buoyancy about 85% of the boats displacement works good in the gusty and shifty conditions prevalent on small lakes/ponds. Overall beam of the F3 is 72" with an LOA of 56".
So I think it's fair to say that a multi foiler can have smaller amas than tri's designed TO FLY THE MAIN HULL but their size has to be looked at carefully especially for ocean use.

[Phil Locker]

As to stepped hulls being unstable in pitch one article I read was in Seahorse magazine earlier this year where a member of Parliers team used that fact to explain why the boat had rudder t-foils fitted(if I find the issue later I'll post it). Another good article on powerboat applications of stepped hulls is in Issue # 88 of Professional Boatbuilder which talks about a unique stepped hull solution to enhance controllability by using an aft "stabilizer". (idea by John Plum)

The above article, plus the development of Yves Parlier's boat, and the growing popularity of T-foil rudders in the I14 class have all been rolling around in my head, leading me to wonder if a stepped hull with T-foil rudder might work on a small skiffy dinghy. The step would likely have to be aft of the daggerboard to prevent excessive ventilation.

I'm thinking in terms of a single hander about 11' long, while not aiming to fit in the box that defines a moth. I'd think this would lend itself to having your weight further forward (I14's are just about trapezing off the transom now) and a minimum wetted surface while planing. One question that comes to mind is - how small can the step be and still be effective? For the unfortunate times you're in displacement mode you'd want as long a waterline length as possible (such a short boat) but if built very light you will be drawing very little water, and hence the vertical height of the step will be limited by that alone. Just thinking out loud a bit here...

A bit of a doodle is attached.

[Matt Lingley]

Wow that looks cool! Can see how in very light winds it might even work well as you could sit right forward and lift the back section right out of the water to reduce wetted surface. Would the step really make much difference to waterline lenghth though? I would have thought that the biggest problem it would have would be very similar to having really bad transom drag at displacement speeds.
I guess being a wide boat it will have less of a hump to get over between displacement and planeing than Parlier's cat.

[Doug Lord]

Phil, very interesting. One of the things Mr. Plum proved in his early experiments referenced in the article above was that the step could be very small so your idea may be worth looking further into. I have experimented on models with the idea of pumping/pulling air into a openable slot at about the same position you show the step with the idea of ventilating the back end and reducing drag. No clear results but I know of experiments like this that have been tried on full size boats with good results.
It seems to me, though, that in the speed range
of around 10mph to 30+mph fully flying foils would be almost untouchable. Moths already fly in 7-8mph of wind and that speed(takeoff windspeed) is headed lower. Rohan Veal has been clocked at over 21mph on foils(11' Moth) and that speed is headed up.
Foils seem to allow a hull design that is optimized for lite air and I'm sure some future foiler incarnations will feature fully retractable main foils.
============================
Phil, speaking of foils , have you built any hydrofoils yet? Interested?

[gggGuest]

the Moth box is fairly big, unless you are going for big rigs or tunnel hulls its quite a challenge not to fit it!

As I remember there was at least one Moth back in the 60-s witha stepped hull, but only a small one, about 6 inches off transom or something...

[Doug Lord]

Phil, I found this in the article above:"The testing also demonstrated the important point that a small step(which is desirable for reduced drag at low speed) will give satisfactory separation of flow from the afterbody bottom when used in conjunction with a hullform whose width decreases appreciably between midlength and transom. This latter feature, by facilitating separation of the flow from the sides of the hull, results in ventilation of the afterbody bottom with even a very shallow step."
One of the succesfull steps they tested was 1/16"( 1.5mm) on a 10' model!
Again, the article is in issue number 88 of Professional Boatbuilder(article by Eugene P. Clement) and this info was on page 84.
I've been doing a lot of research on steps for a project and this one article has been a treasure trove-Plum was one of the unheralded early step pioneers in power boats..

[Phil Locker]

.... Would the step really make much difference to waterline lenghth though?

Figure all up weight of boat + sailor at something well under 300 pounds. I'd have to refine the model and see where it sits on its lines, but with it properly trimmed fore/aft so that the sheerline is parallel to the waterline, I'd like to see the hull submerged from the knuckle at the bow right through to the transom. Too high of a step (and too much rocker) and this won't happen.

One of the things Mr. Plum proved in his early experiments referenced in the article above was that the step could be very small so your idea may be worth looking further into. I have experimented on models with the idea of pumping/pulling air into a openable slot at about the same position you show the step with the idea of ventilating the back end and reducing drag

You're much further than me in researching this. But as I said, that article got me thinking. I wonder if too small a step will make the boat fussy on fore/aft trim when planing. You wouldn't want the boat dragging its transom. But then I haven't been thinking about this so much as ventilating the back end as simply having a planing pad amidships that takes advantage of the lift off the t-foil rudder to reduce wetted area of the hull without the need for trapezing too far aft (as the 14s are doing).

It seems to me, though, that in the speed range
of around 10mph to 30+mph fully flying foils would be almost untouchable..

No doubt. But fully foiling isn't the sandbox I want to play in.

the Moth box is fairly big, unless you are going for big rigs or tunnel hulls its quite a challenge not to fit it!

I wouldn't want to be restricted to the Moth rig limits. This is a blank sheet of paper approach just for fun. If I were to actually build this, I'd be flying a small a-sail off a short fixed sprit so I could play with the odd assortment of boats that come out to our Monday night "skiff" races.

Sorry all, I didn't mean to hijack this thread.
Phil

[Phil Locker]

Phil, speaking of foils , have you built any hydrofoils yet? Interested?

I've done a few sets of the lifting foils themselves for I-14 T-foil rudders... both fixed (where the rudder itself itself is raked fore/aft to adjust pitch, as in Ovington's approach) and to be fitted with axles (a la Bieker).

Haven't done a complete T-foil rudder yet, although one is in the queue.

The small hyrofoils are a bit tricky due to scale. Requires a bit of planning to support them well after flipping to machine the 2nd side.

If interested in pursuing, contact me offline.
Cheers
Phil

[sigurd]

Usually stepped (one step) hulls are ridden on both the stern and on the bow right? or did I misunderstand and the stern is sometimes lifted clear off the water?

Where do I start when deciding what AoA the planing surfaces should have?

[Doug Lord]

If I were you I'd get a copy of this artcle and the other Professional Boatbuilder articles on stepped hulls and/or contact Mr. Clement, the author, at Eclement5@aol.com
In the article Clement points out that the ideal running angle for the cambered hull surfaces used by Plum was 2-3 degrees modified to a higher AOA in folowing seas or rougher weather. Fascinating stuff!

[tspeer]

I like the deep foils, but I wonder about the step. Especially since the vast majority of my sailing has been done at sub-planing speeds. I think I'd be tempted to put in a flap that was flush with the hull but could be deployed to form a step when the winds made it worthwhile.

[Phil Locker]

I like the deep foils, but I wonder about the step. Especially since the vast majority of my sailing has been done at sub-planing speeds. I think I'd be tempted to put in a flap that was flush with the hull but could be deployed to form a step when the winds made it worthwhile.

I don't know how you'd accomplish that without quite a steep approach to the step / planing pad. Easier to have an actual hydrofoil that nests flush with the bottom of the hull and can be cantilevered down (carbon or G10 supports on each end of it). But then you're back into full foiling territory.

[Doug Lord]

Phil, if you get the article you'll see that a wedge shaped step would be fine. On a 14 footer it could be 3" in the F & A direction and made so the aft end pops up more or less 3/32"(2.4m)when it is deployed. There is an important sweep angle(outboard end aft of center)that would have to be used but the beauty is that it would be 100% retractable!
So you've got a pretty good idea and it seems to me that it could have applications on planing sailboat hulls in earlier planing and faster top end.
But I don't think it could beat a foiler but I'll never say never.....

[Skippy]

Easier to have an actual hydrofoil that nests flush with the bottom of the hull and can be cantilevered down (carbon or G10 supports on each end of it). But then you're back into full foiling territory.
If you had a low AR foil with highly swept leading edges, that should solve two common foil problems: deeper draft and structural weakness. And even if it's not adjustable, it does have more wetted surface, but wouldn't it still have a higher AR than the planing surface, and wouldn't the flow lines be cleaner?

[Phil Locker]

I wouldn't want to be restricted to the Moth rig limits. This is a blank sheet of paper approach just for fun. If I were to actually build this, I'd be flying a small a-sail off a short fixed sprit so I could play with the odd assortment of boats that come out to our Monday night "skiff" races.
Phil

What the heck, this is my slow season. The hull is under way. No planing pad until I get it on the water, pin down the exact daggerboard placement, and then will fabricate a few alternative "pads" of different heights and shapes to see what they do to performance in a breeze.

http://www.philsfoils.com/11ftr/11ftr.html

[Doug Lord]

Way to go , Phil! How much do you think it will weigh ready to sail?

[sigurd]

The problem as I have heard it, with adapting sailboard rigs, is that they are made for much less power than your skiff. A 10m board rig will be made for light wind, and designed to flex out the force from gusts/harder wind. so you may be a bit underpowered with all your RM, otherwise I think efficiency of the rigs are ok?

Maybe it would be a super solution with a stiffer mast than what the sailmaker intended?

[Phil Locker]

Way to go , Phil! How much do you think it will weigh ready to sail?

Way too early to tell. I'm trying to build it light, without being fanatical about a bit of extra epoxy here & there (build speed takes priority at this point). And I know I could have got away with a thinner core, especially in the wings (but thick foam in the wings is a subst. for aft floatation tanks).

It'll weight what it weighs... I'll report when its done.

[Phil Locker]

The problem as I have heard it, with adapting sailboard rigs, is that they are made for much less power than your skiff. A 10m board rig will be made for light wind, and designed to flex out the force from gusts/harder wind. so you may be a bit underpowered with all your RM, otherwise I think efficiency of the rigs are ok?

Maybe it would be a super solution with a stiffer mast than what the sailmaker intended?

I'm trying to solicite educated opinions on how to make it work, but it seems most people trying it haven't had much success. But as they say, cheap is a virtue overcoming many faults. Dropping in a modified sailboard mast & sail bought "off the shelf" is going to cost considerably less than a one-off solution. Shroud position, spreaders, prodder, are all under consideration.

[grob]

Phil,

If you take a look at our boat www.fourhulls.com, you will see that we use two 7m windsurfing rigs. Although I have heard that opinion many times I haven't encountered the problems that sigurd talks about. Yes the top will twist off, its designed to do that. But the rest of the sail still seems to hold its shape well.

I am still of the opinion that using windsurfing rigs is a good option. I am yet to find a more cost effective lightweight solution.

All the best

Gareth
Warob Marine

[alyne]

Phil,

I can also confirm that windsurfer rigs have potential. I have used an 8.1 m windsurfer rig on my beach cat, and have had a few fast sails on it. See link:

Pink Onion (http://www.itsteps.net/the_pink_onion.htm)

OK, I have learnt alot about rigging windsurf rigs since these initial pics (needed another foot of downhaul :D ) but I have generated plenty of power to get up on the plane. With this boat I just reinforced the botton 1 metre section of mast with fibreglass tape and had it free standing, this gave a little extra strength at the foot without distorting the sailshape

A much better solution I shall be using in my next boat is to attach shrouds directly under the boom, I believe this is what Gareth does. The rig is designed in a way to be stayed at this point.
I would recommend: By a sail, mast and boom off ebay...plenty to choose from, and devise a simple system to stay the mast below the boom.

Your build pics look great.
Hi there Gareth... have been following your design with interest

Andy

[grob]

Hi andy,

Thanks for the complement, we use a "sub mast" which is attached to the boat with stays, this is basicaly a strut that takes all the compression loads.

This sub mast can be seen clearly in this picture

http://www.fourhulls.com/assets/images/autogen/a_buildseq10.jpg

The windsurfing boom is then attached to the casting at the top of the sub mast via a clevis pin joint. The advantage of this system is that the mast is supported at the base and the boom, this allows the windsurfing mast to behave and bend as it was designed to.

Well done on your design, I have nothing but admiration for those that attempt these projects.

All the best

Gareth

[gggGuest]

...cost considerably less than a one-off solution.

This is of course an argument for using a Moth sail - the top Australian "sailmaker" gets batches made at a Far Eastern outfit to keep costs down.

[Doug Lord]

Phil, did you see the latest Prof. Boatbuilder with the article by Clement on stepped planing hulls? It would sure be cool if you could incorporate that shape(similar to the underside of a 63412 section) into your hull in some removable way. Then you could sail with and without it and see what happens. It seems to me that if the aft portion of the swept planing surface could be hinged or something it could allow use in every condition.
So how's the boat coming?

[tspeer]

...It would sure be cool if you could incorporate that shape(similar to the underside of a 63412 section) into your hull in some removable way. ...

The shape was in no way similar to a NACA 63412 section. From the article, "It's important to realize that the hydrodynamic pressure drag is also very low. The virtue of hte sophisticated Johnson camber shape is that it results in the needed amount of lift, together with a minimum amount of total drag."

The camber of the surface was a Virgil Johnson three-term section from:

Johnson, V. E., Jr., "Theoretical and Experimental Investigation of Supercavitating Hydrofoils Operating Near the Free Water Surface," NASA Technical Report R-93, 1961.

A planing surface is essentially a fully-ventilated hydrofoil with the upper cavity filled with boat instead of vapor. It's got nothing to do with a NACA airfoil shape intended for wings.

[Doug Lord]

My apologies; from the way I remembered it it seemed similar to the bottom of a 63412.
Just looking again at the article and the picture on page 170 & the ilustration on page 173 there is a certain similarity to me-at least in the last half. What caught my eye was the way the back end of the shape sort of drooped.I was trying just to evoke the shape as I remembered it NOT to suggest that a 63412 section could be used in any respect whatsoever..Poor choice of imagery... But if it were to be tried it would pay to do it right and maybe even get Mr. Clements help and advice.
The turned down(drooped) portion of the shape(like a partially deployed flap) might be able to be hinged reducing the step to zero. A removable version may or may not be able to be made but it seems like it might be worth a try. If it works then the back end of a final version might be hinged or whatever works. It sure seems to me to be worth investigating further...
The article is in issue #97 the October/ November 2005 issue p164 "The Evolution of the Dynaplane design".

[Phil Locker]

Interesting article. But I'm not sure how applicable the specifics of the planing step for a high speed V bottom hard chined powerboat would be to a soft chined sailing dinghy. Maybe the design translates perfectly, who knows. But I doubt that the control issues that appear at 70 knots are as critical at sailing speeds. The T-foil rudder will help lift the transom, as well as control porposing. Key seems to be to ensure that you ventilate behind the step, either via the chines or through the hull.

To answer Doug's question, I found a bit of time this week to laminate the inside skin of the hull. Hopefully can start adding internal structure soon. Pictures are updated on the website.

While a step that pops open like a giant Elvestrom bailer is interesting, it would add a lot of complexity. The point of the boat (for me) is to have something fun to plane around in when the wind is up, so any performance hit from a fixed step in light air is a non-issue. Experimenting with planing pad shapes will come once the basic boat is sorted out and sailing.