Stern Drive Power Trim

[cngarrod]

On newly acquired boat i have got adjustable trim tabs.. never had them before and they work a treat!

So... do i need to use the trim adjutment on the stern drive.. and what is it for?

My last boat had power trim on the drive.. but the broker told me never to use it unless i was putting the boat on a trailer...!

Guess i was suckered up on that one???

Cheers,

C

 

[ccscott49]

You should use both, the trim tabs and the tilt/trim, they do similar jobs, but you cannot trim the boat side to side with the power tilt/trim, only the trim tabs will do that. Use them both to get your best trim, speed, consumption and comfort. You need to experiment with them, to get the best out of them. Is there anybody near you with the same boat, that could show you, telling you here is not going to cut it!

 

[cngarrod]

ah ha... Cheers for that... i will have a play.. have got the tab kinda sorted... so assume the drive trim works the same principle...?

Will have a word with a couple of other guys in the marina and go from there!

Thanks,

Craig.

 

[ccscott49]

The power trim, will raise or lower the bow, can be used to stop or minimise prop ventilation and ventilation on turns, and assist cruising speed v consumption. All good stuff eh! By the way I've posted on your electrics question, please be careful! Is this a petrol boat, if so please be double careful with your electrics!

 

[ari]

Tabs work by "pushing the water down" to lift the stern, lifting only one side will of course tilt the boat port or starboard.

Power trim trims the angle of the propellor and thus the fore and aft trim. Useing the two outdrives seperately will allow a degree of effect on side to side trim, but because they are so close together you need quite a lot of difference to have much effect.

You can of course use the tabs to alter fore and aft trim, but because they push down into the water they will cause a certain amount of drag, and the outdrives are much more efficient for nose up/down, although because they are further apart the tabs are better for side to side trim.

Therefore, if you have both, keep the tabs right up, use the outdrives together to obtain ideal bow up/down trim, then when you are happy with that use minimal trim tabs to correct any side to side list.

Incidently, earlier outdrives such as Volvo 280 had a fixed running angle, but an electric motor to tilt the leg for beaching or trailering. Because of this the electric motor couldn't possibly trim the legs underway as it was not strong enough to push against the force of the props. If you had this type previously then your broker was absolutely right to tell you to keep them trimmed right down whilst using the boat and only use the tilt for beaching/trailering. Newer outdrive types such as the Volvo 290 (which I think came into being in about 1984, replacing the previous 280) have hydraulic rams, which do allow you to trim them under way.

So, outdrives for bow up/down, tabs for list port/starboard, otherwise tabs right up. The only exception to this is if trying to keep the boat on the plane at reletively low speed, when tabs right down and legs right in will help to lift the stern and keep the boat on the plane.

Hope that helps.

Ari.

 

[halcyon]

My understanding of trim tabs is that you put tabs down to hold the bow down ( minimise bow wave )to put boat on plane quicker. Once over the hump the tabs are backed of to the neutral, and trimed left or right to balance baot for non equal loading. The legs are then used to trim the boat on the plane, holding bow up or down for optimum performance.

Brian

 

[ari]

Depends on the power of the boat. You are absolutely right, having the tabs right down will help it get onto the plane, in the same way that they assist keeping it on the plane at lower speeds., but for the majority of planing boats these days it seems that they have sufficient power to push them quickly onto the plane anyway, particularly with the legs trimmed right in.

Actually, that has reminded me of another good point. It is good practice to always leave the boat with the legs trimmed right in and the tabs right up so as to expose the minimum of hydraulic ram to fouling by weed, barnacles etc (unless the boat is drying out, obviously!)

Ari

 

[oldgit]

Any body care to comment on, That boats that need trim tabs are a direct result of the hull designer being incapable of designing a decent hull?

Exercise is bunk.If you are healthy,you do not need it;if you are sick you should not take it.

 

[ari]

Planing boats need trim tabs (amongst other things) generally to correct a list created by a beam wind. The boat tends to lean into the wind as the bow gets blown off at planing speeds as it is out of the water. If you look at the helm indicator when holding a straight course in a beam wind you will see that you are actually steering into the wind slightly to compensate for the wind blowing the bow downwind. As the boat is therefore effectively in a slight turn into the wind to hold a straight course it will list that way (ie lean into the wind). the only way to correct this is with tabs.

If a designer designed the boat to be dead level in a starboard beam wind for example, it would lean when no wind or when running into or away from the wind (thus nulifying the effect of wind on the beam) and it would double the problem when you turned around and went the other way creating wind on the port beam!

Tabs are also useful for maintaining planing at lower speeds (by assisting with keeping the stern up) and also help in a heavy head sea by pushing more of the bow down so that you are cutting the waves with a sharper part of the V hull.

Tabs aren't there to correct bad design, they are to help compensate for wind and sea conditions on a planing boat.