Motor trim tab

[waternwaves]

Moe, Dimitri, Frank, chip et al.....

Come to think of it, has anyone ever seen a mac underway or moored that wasnt too stern heavy???,


The only time I can think is when I saw 50's on the transoms of brand new 96X's, that hadn't been sold yet.

[Mark Prouty]

IMHO, they look level at anchor. It is underway that they are bow high.
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[Chip Hindes]

Agree w/Mark. On the Bimini and Dry Tortugas Cruises, I have observed quite a number of Xs in all states of loading, at rest and in motion. Using the boot stripe as reference, every one is very close to level. Big motors versus small, lots of fuel versus not so much, whether you use the bunk for storage or as a bunk, doesn't seem to matter much.

Underway, except at "no wake" speeds, of course they squat at the stern -just like every other boat I've ever seen. All the way up to planing speed, the bow is up; once they begin to go on plane, the bow comes back down, again just like every other planing boat. I'm not going to argue about whether or not a Mac at a particular speed is on "true" plane; it's a bogus argument.

I have seen Billy's 29x, 140 Suzuki, fully loaded for cruising, with the stick and rags as he calls them, up (though of course furled) on a true plane. I believe it was nearly 30 mph. At that speed, his rides pretty flat, of course bow slightly high, just like every other planing boat at that speed. And BTW, he has a lot of stuff in his boat: radar, microwave, A/C, 2 or 3 monster batteries, monster inverter, don't think I noticed a TV but it wouldn't surprise me. At rest it's not substantially different in waterline than any other Mac.

[Moe]

At speeds lower than hull speed, there are many bow waves rippling along the waterline. The height of these, depends, in part, on the pitch attitude of the boat. Greater pitch, greater height. As speed increases, so does height, but primarily, so does wavelength. At hull speed, the boat is supported by two bow waves, the first up forward, and the second bow wave, at the stern, aka the stern wave.

Once speed increases much past hull speed, the second bow wave moves past the stern, which drops without its support, leaving the boat to climb its one and only bow wave. The height of that bow wave also depends on the boat's pitch attitude.

Boats pulling wakeboards trim up to increase that attitude, which increases the wake size, and prevents planing (i.e. raises the planing speed), which would reduce the wake. Some wakeboard boats even have a convex "rocker" designed into the keel to help increase the attitude and wake size, and prevent planing. Does the 26X's "belly" do this as well? I don't know, but it is much further forward than wake boat rocker.

Many fishing folks run boats with motors half the size the boat was designed for. I think that was more common when I was growing up than today. In most cases, the smaller motor has no more trouble getting the boat up on plane than the larger motor, it just does it differently. With much less weight on the transom, the pitch attitude is lower, the bow wave is lower, and the boat smoothly, if more slowly, rises up on plane, with less bow rise, and hence less bow fall once on plane. The same boat with the larger motor gets on plane more dramatically. With more weight on the transom and greater pitch attitude, there's a larger bow wave, and more bow rise and fall. Of course, with more horsepower, this boat literally "jumps" on plane, climbing over the larger bow wave more quickly. Aka, "holeshot."

The more lightly powered boat operates in the "no man's land" between hull speed and planing with less bow rise and plowing than the same boat more heavily powered.

I feel that the 26X, with 50HP carbureted two-stroke, was originally in the situation of the lightly-powered fishing boat. But what we've done over the years, with much heavier four-strokes, and to a bit lesser extent, direct-injected two-strokes, is install that heavier motor, but without the extra horsepower to get over the larger bow wave.

Once the boat is on plane, the bow should drop, and it shouldn't need down-trim to achieve maximum RPM and speed, unless it's stern-heavy.

So while Jack might be correct that the problem is that the boat isn't fully on plane (sorry Dimitri ), that doesn't mean it isn't stern-heavy (or that it is for that matter). It could actually be the stern weight keeping it from getting fully on plane.

--
Moe

[Newell]

I disagree with the previous 2 posts. The boat in the pic. looks stern heavy to me and it's not clear from the picture if the ballast is in or not. I have seen all sorts of Xs with different relationships to the boot line at rest. The most common is stern heavy. I equate these to captains that are more into powerboating or camping than sailing. Several captains that I have raced against and who are more at home with the sails up and the motor off (some with smaller motors) have balanced their boat with a noticable bow down attitude.

I have not had enough observation with the M to notice how they are being balanced. Though I have heard the factory compensated more for a heavy engine with fixed ballast.

It is easier to have a stern heavy X than a bow heavy one. Putting the large engine on the back is the first step. Large gas tanks and rear berth storage along with crew in the cockpit complete the balance plan. If you see a bow down boat it is only because the owner has made a very conscious effort to move weight forward. Whether or not a bow heavy boat sails faster is open for debate but don't be surprized if one beats you, I'm not.

[Duane Dunn, Allegro]

Here's even a better picture with yet more stuff on board. The one above was on the San Juan rendezvous in 2003 and it has about a 1" aft down attitude. The ballast tanks are empty in the picture above.

Since then we have added more stuff, lots to the rear of the boat but the trim has gotten better. This includes 24 gallons of fuel in 4 - 6 gallon tanks. We were loaded for our 23 day Canada trip in the picture below and are tied to a bouy at Sucia Is. about 2-1/2 weeks into the trip and we had 3-1/2 tanks of fuel on board. The ballast is full in this picture. We had everything along but the kitchen sink.

Here's all the stuff we had in the boat, not a light load by any definition. Filled to the brinm for cruising not racing. It takes a lot of stuff for 5 people to live on a Mac for a 500 mile trip. Still level as can be both front to back and side to side. The only things missing were the 75 lb dinghy and it's 57 lb motor which we were in when the picture was taken. The motor goes on the transom bracket and the dinghy goes up on the bow just behind the hatch. They off set each other and do not change the trim.

[Tony D-26X_SusieQ]

WOW! You can sure tell sailing season is over. What about you warm weather folks. Do some sailing to tell us about. This stuff is to technical for me. As long as it hasn't fallen off that tab is doing it's job, just let it sleep.