lots of yaw at high speed under power

[Crikey]

Dave, the best rule I use is to observe the hull waterline stripe and trim to level with two people sitting evenly in the cockpit and the proper load of fuel on board. Your outboard height and trim should then be adjusted correctly as well. Overall weight is another question. If still nonperforming you can move mass forward from there.
Because of what I said earlier, I think the keel on an should probably be fully retracted. Mastreb also stated an important point. These are transitional hulls, trying to achieve two different goals. Don't expect it to track like a ski boat with skegs.

[DaveC426913]

Dave, the best rule I use is to observe the hull waterline stripe and trim to level with two people sitting evenly in the cockpit and the proper load of fuel on board. Your outboard height and trim should then be adjusted correctly as well. Overall weight is another question. If still nonperforming you can move mass forward from there.

I will try that, thanks.

These are transitional hulls, trying to achieve two different goals. Don't expect it to track like a ski boat with skegs.

Agreed, but I'm not looking for performance tweaks here; I'm looking to not scare the bejeebus out of every boater within furlong, including those on my boat.

I know, I know:

"Easy! Stay under 7 knots."

[vkmaynard]

7-10 knots is an wallowing speed for the X. We absolutely use about 2-3" of centerboard line down even a higher speeds. The boat tracks better.

When you get above 11 mph tacking improves.

"Plane" on the X as about 21-22 mph. Not achievable with a 50 HP motor.

Victor

[mastreb]

Planing is defined by that speed at which the weight of a vessel is predominantly lofted by hydrodynamic force (lift) rather than hydrostatic force (buoyancy). It's not the speed at which water stays to the side, the position of the bow wave, the speed at which things are really "fun", or the speed at which the boat appears to be "out of the water". It's the speed at which buoyancy is no longer affecting lift. Many of these other effects occur at predictable points, but they're not the definitive characteristic.

That speed occurs on Macs at just around 12 knots, depending on how laden they are.

An obvious characteristic of planing is the mechanism of steerage: The boat rolls into turns. You can feel this in every turn. It's not controversial.

An obvious characteristic of displacing water is the mechanism of steerage: The boat rolls out of turns. You can feel this in every turn. It too is not controversial.

So, if when displacing the boat rolls out of a turn, and if when planing the boat rolls in to a turn, and if there's no single moment when this shift occurs, then there must be a transitionary period when the boat does neither (not possible), both (not possible), or oscillates between them (which is exactly what happens).

Leaving boards down increases drag, lengthens the period of transition, and increases resistance to rolling into a turn under plane. The factory recommends never operating with boards down at speeds above 7 knots because they know this. Magically, 7 knots is also just over displacement speed. Guess why?

While Your Mileage May Vary a little, it won't vary by much. You may do eight knots in displacement (boards down for extra drag), you may do 11 knots on a plane. But at 9 knots, all of these boats wallow with poor steerage. That's simply the physics of oscillating between modes.

I'm sorry to say that if your engine is between 15 and 40 hp, it's got unnecessary excess power that won't get it on a plane. My engine has 60 hp, gets my boat on a plane easily, and does so at an RPM that a 50 hp motor can easily match--I don't have to be doing WOT to get there. Given that an X planes more easily than an M, if you're having trouble planing with a 50 hp motor, I would check your engine output, the fairness of your hull, pull all boards up, and maybe check your prop, because while it won't get your boat to brochure speeds, 50 hp should easily get it above 12 knots.

I had barnacle growth on my boat once that kept me to 8 knots at WOT. 60hp could do nothing about it. While I knew I had a fairness problem, I thought I must also have an engine problem. Nope. Had it cleaned off and bottom painted, came out of the shipyard and viola--18 knots restored.

So check things out. 50 hp should plane no problem. Boards up, of course.

[raycarlson]

I would tend to agree with the above poster. On this board the act of planning an X-M will be argued into eternity and never will everyone reach any concrete conclusion because we are all such qualified experts on all things Mac. But Mstreb seems to pretty much have it nailed here. My M wallows around till the 10-11mph mark, then at 13mph 4200-4300rpm no boards everything smooths out and it starts steering like its on railroad tracks all the way to redline.I usually cruise at 45-4800rpm 14-15mph when in a hurry as it does suck gas at that power level. This is on a 50hp TLDI Nissan.I've had many bass and ski boats and once the boat is planning they all feel the same wheather ski,bass or Mac M.

[vkmaynard]

To quote an internet resource: "There is no exact definition, particularly after you enter the semi-planing stage. There is general agreement that at planing speeds the hull is "over the bow wave". I usually assume it is about three times the hull speed." http://www.boatdesign.net/forums/boat-d ... -2347.html

What you are describing is a "semi-planning stage".

A 50 will not do this. I used to own one. When the boat actually gets on plane at about 21-22 mph you can feel the bow fall down over the bow wave as the center of gravity move forward of the wave.

Victor

[mastreb]

Hey I'm also an internet resource, and I say it's planing Perhaps we can all agree to call it "Post-wallowing mode". I have no doubt that there's a smoother, faster version of planing above 20 knots that my boat will never reach, but my boat is very smooth at 16 knots, although obviously deeper in the water than a cigarette boat.

It matters not what we call the mode, what matters is the effect. 12+ knots is the point where the boat's steerage goes to the same mode as steering under plane and the boat operates hydrodynamically rather than hydrostatically. I say if it walks like a duck and it floats like a duck, it's a very fast duck.

I did a quick look around for an authoritative definition of planing, and I found this paper:
http://oa.upm.es/14340/2/Documentacion/ ... miento.pdf

While it's not authoritative, it's got lots of math symbols in it so I automatically presume it's correct. It defines displacement as a wavelength speed/length ratio as <1.34 (the commonly accepted number for calculating displacement speed) and the wavelength speed/length ratio for semi-planing as >2.8..6.0. Between those numbers is semi-planing.

This squares almost exactly with my definition for planing, as it puts the planing speed of Mac at 14 knots:

3.0 * sqrt(23 foot LWL) = 14.

So it appears that Vic is correct, I'm describing semi-planing. However, the paper predicts 28 knots for a Mac to be on a true plane, which only a very few of us have ever seen in that case.

So here are the definitions I'm going to use in my posts going forward:

Drift mode: 0..2 knots. Steerage is unreliable as there is not enough movement past the boards for reliable steerage. Idle speed on large motors is above this speed.

Displacement mode: 3..7 knots. Steerage with boards down is quite reliable. Steerage with motor skeg only is somewhat unreliable in currents or winds.

Semi-displacement mode: 8..12 knots. Boat oscillates between displacement mode and planing mode, exhibiting the well-known phenomenon of dynamic instability in semi-plane. Steerage is unreliable due to oscillation and will result in wallowing. Boards should be up--having them down increases the duration of this mode.

Semi-Planing mode: 13..20 knots. Boat reliably steers in planing mode with motor skeg only. Daggerboard or centerboard down is truly dangerous as it prevents proper roll-in while steering on a plane. Rudders down is beyond their rated strength and unnecessary. Buoyancy is still a factor in supporting the boat, and the boat is lower in the water than a hard-chined planing hull.

Planing mode: 21+ knots. Realm of high power motors most of us will never see except on YouTube. Boat is completely out of the water and obviously on a plane, completely supported by hydrodynamic lift. No transitional effects remain. Any boards in the water is dangerous.

[vkmaynard]

Great mode descriptions!

Mac wallow range is a little frustrating for sure. Too fast for rudder down too slow for good tracking.

The M seems to have much less problem tracking at lower power settings.

We were wake boarding the weekend before last and sailing in South Port NC this weekend. Too much fun for one boat.

Victor

[capncarp]

Drop the board 2-4 inches. I don't sail anymore. Only power. The board is the key. NOT TRIM TAB. That's only for countering torque. My X steers fine at high speed.
capncarp,
99

[RobertB]

I have a slightly different explanation of the modes, mainly experienced on larger bodies of water like the wider areas of the Chesapeake Bay with reasonable wave action (disregard for those days where there are no waves or wind to make it worth unpacking the sails):

0-1 knots: Bobbing cork mode, breakfast poised to escape
1-7 knots: Conversation possible mode, captain has company in the cockpit, snacks and drinks welcome
8-14 knots: Pretty dam wet and noisy mode, captain alone with crew below playing cards and/or holding on for dear life, everything tastes salty, need regular change of towels to keep glasses kind of dry (On a smooth day, 8-14 knots just is not as wet)

[Crikey]

(Mastreb wrote: However, the paper predicts 28 knots for a Mac to be on a true plane, which only a very few of us have ever seen in that case.)

Matt, that seems a little high when planing at 16 + seems like all the effect your going to get on the semi-displacement hull. Do you mean that's the speed no water is being displaced at all? I would think you'd be having to go about 90 to do that! There are also a lot of straked and stepped powerboats, with far more obvious keels than a Mac, that don't appreciably detract from turning under plane. An appropriately positioned skeg of little drag profile could help somewhat and may make an interesting mod.
R.

[RobertB]

I love it every time this topic reemerges. It is kind of like talking about drag racing with minivans. Occasionally one can configure their ride to perform to that level (look up http://www.turbominivan.com) but the great majority just try to make sure the engine runs and the stereo sounds good.

[mastreb]

(Mastreb wrote: However, the paper predicts 28 knots for a Mac to be on a true plane, which only a very few of us have ever seen in that case.)

Matt, that seems a little high when planing at 16 + seems like all the effect your going to get on the semi-displacement hull. Do you mean that's the speed no water is being displaced at all? I would think you'd be having to go about 90 to do that! There are also a lot of straked and stepped powerboats, with far more obvious keels than a Mac, that don't appreciably detract from turning under plane. An appropriately positioned skeg of little drag profile could help somewhat and may make an interesting mod.
R.

Mac owners with 90..115 hp outboards report that there's an "over the bow wave" mode (both M and X) where the boat is very flat on the water but only on the stern, and apparently extremely stable. This doesn't exactly square with planing theory, according to which the Mac's non-chined but flat stern should exhibit much higher drag and lower stability than a hard-chined powerboat, but which according to owner's reports does very well at these speeds. I don't know how much turning they're doing, whether they're mast-up at 25+ knots, or have any other experience to report, I just know that according to the theory, Mac's are supposed to develop 10X the drag above 15 knots compared to a fully chined hull, and experience reports that they obviously don't or they'd never get to these speeds with a small increment in power.

I do know that my next motor is a 90hp ETEC >and< a 5hp Tohatsu...

[Crikey]

Yeah, hearin that. But I think it's going to be a while before my Admiral authorizes those kinds of boat-bucks. I still think there could be some improvements to be had. When I put small skegs on brackets on my M I could easily steer better, with better pointing (under power of course...). I'd like to upsize 6X and angle out diagonally somewhat. Maybe 30 deg.

Based on what you just said - there's good scope for some 30% bubble reduction.

[BOAT]

Odd, I have not experienced the yaw or wobble. The boat just goes where I point it - but I would never have the daggerboard down over 7 mph. At speed the end of the board can create lift and turn the boat from side to side, even capsize you if you crank it to hard too fast with the board down. (At least that's what Mike told me). I don't want to risk it.

'boat' does go very fast WOT (well it seems fast to me) and we make pretty good turns but we also get soaked because the waves make the water splash on us in the wind. It's got a real nice feel with full ballast and WOT - real easy to maneuver and no pounding, the ballast makes the waves feel a little softer.

Is 5800 RPM good RPM?