A boat(39)s (34)attitude(34) on the water - what should it be?-gctid815380

[Dadrock33]

Question of the day: Admiral was really upset last weekend when we went out (1998 Avanti 3685) and she said that from the port side passenger seat, she couldn't see over the bow to see if there was anything in the water. I've just installed the Bennett EIC5000 trim tab system so that I can accurately tell how much "tab" I'm using to get up on plane, to run with for better fuel efficiency, etc . . . . but this brings up the question:

Just exactly where and how is THIS boat supposed to "sit" [or is that "run"]?? . . . :S

I know she's not a planning hull so she's not going to scoot across the wave tops and she'll bounce around a bit due to the flat aft section which is part of a planning hull. So my Avanti is not a planning hull; it's a deep-V hull for larger waves and "bigger waters" and she's heavy to help go through all those afore-mentioned items. In all the documentation though, I don't see a suggested "angle" for riding attack or even a picture, "You're boat should be riding like this" . . . . I think I'll have a better understanding of the whole concept when the flo-scans are installed and I can trim her out to get the best out of the fuel efficiency and engine load and then can decide on the trim tab settings (if any) . . . . . what do you think? Inquiring minds want to know . . . . .. inch:

[watchthis]

Are you sure that isn't a planning hull? I believe you two should be able to see the water rather well in front of you at cruise if trimmed out properly.

[Dirtyface]

You can determine the best running angle by just watching your gps, nose too high it's slow down, nose too low it'll slow down..you have to find the sweet spot which will change depending on speed and load. The style of boat you have will not have the same visibility as say a sedan (the helm on a cruiser is typically farther back so you're looking across the bow), compound this with the running angle and you cannot see directly in front of the boat for some distance. I once got a ride on a searay 540 and when underway I swear you could not see anything in front less than 100yds away. Hope this helps. Brad

[CptCrunchie]

You have two primary options for boat speed, and they will dictate most of your boat 'attitude'. First, there is hull speed. If you take the square root of the length of your boat at the waterline and multiply it by 1.34, that's roughly the speed in knots where you will get the best fuel economy. You will be able to tweak it a little once you reach that speed. At hull speed, your trim tabs will do minimal, because your boat will basically be sitting flat to the water.

I'm guessing that your admiral can't see because you are not on plane. Indeed, visibility would be limited if you are plowing water. Suggest you increase the speed until the bow drops. Within moments, your fuel economy will increase as will your speed. A boat fighting to get on plane uses a lot of fuel, because it is trying to push the water out of the way rather than climb on top of it.

Your other option is to start with your trim tabs all the way down. This will get you on plane the quickest, but you still have to add enough power to get it on plane. Once on plane, you can gradually back off your throttle, but the moment you feel the bow rise, increase the speed.

[smitty477]

"[quote]"CptCrunchie" post=815419 wrote:
You have two primary options for boat speed, and they will dictate most of your boat 'attitude'. First, there is hull speed. If you take the square root of the length of your boat at the waterline and multiply it by 1.34, that's roughly the speed in knots where you will get the best fuel economy. You will be able to tweak it a little once you reach that speed. At hull speed, your trim tabs will do minimal, because your boat will basically be sitting flat to the water."

Your best fuel economy will be much lower than the formulae of square root of the waterline length X 1.34. It will be close to the square root of the waterline length itself.

I have some notes on the Avanti 3685 at home form a long while back. I believe that was Bayliners second largest express boat back then and had a ton of weight out back. Is this one of them that also had diesel power and the larger genset? I will check at home but I believe these boats will run flatter and better at higher rates of speed assuming they are propped correctly and have a good tune - if memory serves me well they do not like the lower speeds in the teens. Please advise if this is a diesel boat and if it is heavily loaded.... thanks.

[SomeSailor]

A 36' Avante is most certainly a planing hull. You need to get those drives down, tabs down and throttle her out of the hole and then she'll scoot along on the water and you'll be able to see over the bow.

[smitty477]

"SomeSailor" post=815472 wrote:
A 36' Avante is most certainly a planing hull. You need to get those drives down, tabs down and throttle her out of the hole and then she'll scoot along on the water and you'll be able to see over the bow.

yes - I believe SomeSailor has nailed it.

[Solandri]

"CptCrunchie" post=815419 wrote:
You have two primary options for boat speed, and they will dictate most of your boat 'attitude'. First, there is hull speed. If you take the square root of the length of your boat at the waterline and multiply it by 1.34, that's roughly the speed in knots where you will get the best fuel economy.

Hull speed is when your mileage will start to drop quickly. It's basically the fastest you should push your boat in displacement mode if you want to keep fuel consumption reasonable. As noted above, best economy happens significantly below hull speed. The diagrams in this post I made explain what's going on.

http://www.baylinerownersclub.org/in...l-range#810697

As to OP's question, the "correct" attitude depends on speed, hull shape, and weight distribution. For a displacement hull, you probably want it level with the waterline that's painted on (that's how the marine engineer designed it to run, so you gotta have faith that he did his homework right), although as noted in the diagram if you exceed hull speed this will be impossible. For a planing hull, you want to trim it to just before it starts porpoising.

http://www.boatus.com/magazine/2014/...ng-in-trim.asp

If you're still unhappy with the attitude, redistributing weight is the next step. Try to keep weight low (helps stability). Fore or aft obviously affects pitch. Not so obvious, close to the centerline shortens roll period, further from the centerline lengthens roll period. So if you've got a boat that tends to roll a lot you can try moving weight towards or away from the center to modify its natural roll period to differ from the waves you're encountering. Likewise, close to the middle of the boat shortens pitch period, close to the bow and stern lengthens pitch period. (These are general rules of thumb. There's a lot more going on here since inertia is a tensor (a 3x3 matrix) - e.g. I've noticed a slight yaw oscillation in my boat with a period of about 30 seconds, which I suspect is due to an asymmetrical inertia tensor.)

[Knot_Happy]

As Smitty477 and SomeSailor have noted the 36 Avanti is a planning hull. As they can be a bit heavy aft it will run a little bow up when on a plane, but you should still have good visibility. Cruise on plane should be around 15knots or 16 to 17 mph.

It is a deep vee at 14 ft deadrise, and that will make it run smoother in big water. The formula given for displacement speed by Smitty477 is a reference and represents the maximum you will achieve before the boat starts to rise up and try to plane, fuel economy is significantly better below that speed, for reference Bayliner labels their boat models typically by deck length - 36 ft is the Avanti approximate deck length- the waterline is shorter a guess is maybe 33ft, overall length includes swimstep and pulpit and is about 39ft. So squareroot 33 x 1.34 = about 7.7 knot, or about 8.5 mph, this is max displacement speed so more likely about 8 mph . Between 8.5 mph and about 15 mph you will plow water, get very poor economy and ride nose up and have poor forward visibility. Most planning hulls have very little difference in economy for different speeds once on plane. Running at a lower speed on plane does not save much fuel vs running at a higher speed. The primary decision for speed once on plane is typically rougher water conditions merit a slower speed and top speed should not over stress the engines. If you do buy fuel flow Gage's you will find the burn rate per mile is very flat across your planing speed ranges. That is unless you have a very high power boat. ( I have a friend with an 80 mph 42 ft offshore and the above rules don't apply, he burns more fuel per hour than I do all season) :huh:

General practice is off plane the trim tabs may be some help to level side to side but have no real benefit for and aft. You likely know all this, but for any new owners following this topic many planing hulls are very heavy aft, the best practice is typically trim full nose down give full throttle until the boat gets on plane. This will lift the back of the boat the fastest. once on plane , about 15mph for the Avanti, slowly trim nose up until the boat gains speed and adjust power and trim for desired cruise. The actual trim angle varies significantly due to weather, water conditions, cruise speed, loading-distribution of gear and persons on board, fuel levels, water tank levels, wind, a whole lot of variables. The best indicator I have found is once on plane pick a power setting and make small adjustments to the trim to get the best speed and handling.

As for the best attitude that's the one that makes you and your crew smile the most:P :P

[6104696]

That is most definitely a planing hull. I would suggest that you find an experience skipper to take you out and show you how to properly trim the boat for various sea conditions.

[Tabman]

You may find this "Trim Tabs 101" I wrote a while back helpful.

If you are new to using Trim Tabs you may find this helpful in learning how to use them to achieve the best results. All boats react differently to Trim Tabs and the best way to find out what works best for yours is to experiment, and remember, use short bursts of the controls and let the boat settle down between corrections. Using them will soon become second nature.

Also some boats that suffer from "wandering" at low speeds (particularly I/Os) will benefit from having the Tabs fully down at no wake speeds. They sort of act like feathers on an arrow and make steering a little easier.

Tom McGow

Bennett Marine

Getting and Staying Trimmed

All boats assume different fore to aft attitudes at different throttle settings and vary in sensitivity to lateral weight distribution.

A boat's optimum running attitude is determined by the operator. While some people may define optimum running attitude as the highest possible speed for a given amount of engine RPMs, others desire the best possible fuel economy, yet others may be trimming the boat to get just the right mix of speed and wake (such as for waterskiing.)

Optimum running attitude is when the boat is running to the operator's satisfaction for the given operating conditions. There are as many optimum running attitudes as there are boats and boat owners

A good way to determine a boats optimum running angle is to run the boat lightly loaded at full speed in calm water. During this test observe the boat's bow in relation to the horizon. Most boats run at or near their optimum attitude under these conditions. This should give you a feel for the appearance of the wake and bow spray when running at an efficient attitude. Note that not all boats will achieve their optimum running attitude under these conditions. Some boats will benefit from extra lift even when running at their maximum throttle settings. If you feel the boat will benefit from added bow down trim when running at speed start with the trim tabs fully up and deflect the trim tabs in short bursts. Be alert to changes in the boats handling, as you bring the bow down. Observe any changes in RPMs and/or speed. Adjust power trim if applicable.

Indications of Running Untrimmed

When a boat is running untrimmed the bow spray will exit the sides of the boat far aft. The stern wave (wake) is high and curling like a breaker on the beach. The rooster tail is high and close to the stern. The engine is laboring and the ride tends to be less smooth.

Indications of Running Trimmed

The bow spray moves forward and is flung not as far from the boat. The wake diminishes in height, as the rooster tail flattens out and moves away from the boat. The engine is operating under less load as evidenced by the tachometer and speed as well as sounding "less strained".

One Step at a Time

The key to obtaining optimal results from trim tabs is to operate them in short "bursts" and let the boat react before making another adjustment. The amount of time between corrections is influenced by the size of the trim tabs and the boat's speed. This will help avoid overtrimming or ending up with one tab too far down when correcting lateral trim. You will quickly become acquainted with a boat's particular traits.

Take Off

Properly sized trim tabs can significantly reduce the time needed to get up on plane. They also allow a boat to keep its bow down and stay on plane at lower speeds.

As the throttle is advanced the stern of the boat begins to squat, lifting the bow. As the boat accelerates, push the bow down position of the helm control in short bursts. The boat reacts by the stern lifting, the bow coming down, speed increasing, and reduced engine laboring. If you over do it and deflect the tabs too far the boat will end up overtrimmed. When over trimmed, the steering becomes "over sensitive" and wants to pull off course to port or starboard. If this occurs, operate the control "bow up" until the desired attitude is established.

Getting the Most from Power Trim

Adjust the trim tabs to achieve the desired running attitude. Then use the power trim to position the propeller thrust parallel to the water flow. If necessary, re-adjust the trim tabs to fine tune the attitude. By observing the boat's speed and engine RPMs the best combination of trim tabs and power trim will be apparent. Trim tab angle indicators and a power trim angle indicator are particularly useful in duplicating effective settings.

Trimming to Sea Conditions

When running into a head sea you want to trim the bow down so the sharp forward sections of the boat do their work cleaving the waves. This provides the most comfortable ride and minimizes stress on the boat (and passengers). In a following sea the tabs should be fully retracted for maximum steering response.

Correction of a List

The normal control setup for trim tabs operates in relation to the desired changes in trim and not the actual movement of the tabs. Therefore, do not think about what the tabs are doing, but rather on the control and what you want the boat to do. As above, make the corrections in bursts and allow the boat to settle to the new settings. You may find it easier to correct the boat's fore and aft attitude before you correct the side to side trim.

Correction of Porpoising

Operate the tabs in very short bursts of about half a second. Continue until porpoising subsides. The objective is to have only a very slight amount of tab deflection, just the amount needed to cure the up and down motion of the bow.

[green650]

Visibility depends a lot on the boat design too. Bayliner seems to have put style over function for most of their designs. The raked back windshields, high bows, trying to make the boats look fast and aerodynamic. Aerodynamics really don't come into play at 20mph were most boats cruise. Get on a 38xx and check out the view from the lower helm. Not very good. Once underway the bow rises and makes everything worse. Now look at a Grand Banks or other traditional trawler. Flat, almost vertical windshields. And even cruising at 15mph they are not trying to get up out of the water.

As for the Bayliner planing boats, most of them simply do not have enough power to keep the boat level. If you cruise at barely over planing speed, the nose is still in the air and the rear is about to sink in. Any faster and you will be near redline on that single engine. Not so on performance boats that have doubles, triples, quads, etc.

But, that's why Bayliner's are more affordable to the masses, compromises.

[SomeSailor]

"green650" post=815577 wrote:
Now look at a Grand Banks or other traditional trawler. ... And even cruising at 15mph they are not trying to get up out of the water.

Yeah right... No problem there.

True displacement trawlers don't have to worry much about visibility (or hitting 15kts)

[johnrupp]

What angle did it have during the sea trials? Could you see over dash then?

[Dadrock33]

"SomeSailor" post=815472 wrote:
A 36' Avante is most certainly a planing hull. You need to get those drives down, tabs down and throttle her out of the hole and then she'll scoot along on the water and you'll be able to see over the bow.

. . . no drives to put down . . . she's got inboards . . . . it's not that bad for me but the Admiral is complaining so thought I'd better learn some more nautical engineering from y'all . . . . I was just wondering what "angle" she should normally run at? . . . you say the 3685 Avanti is a planning hull? . . . had me there . . . :S