temperature affecting trim?

[david h]

G'day,

Last week I put my Sjoormen in the tub in the morning and she was trimmed nice and level. The water was cold, (being winter at the moment)

This week i put her back in the water without adjusting anything and she was down at the bow. The water was significantly warmer, ( had been in sun for few hours) Could this be due to the temperature.

I know that temp affects bouyancy but would it have this much effect on Trim?

please comment..


David h

[bradv]

I've noticed that my Dumas

I've noticed that my Dumas Akula, being a dynamic diver, is quite sensitive to temperature changes. Often I take a while trimming it to run at different depths (fully surfaced, partially surfaced, scopes out, fully submerged) on the top 4 speed notches of my transmitter when I arrive at the lake at 8am. Particularly in winter, by the end of my second run ~11am this very specific trim has obviously changed. This will be due to the surface layer of the water warming over the day and thus being less dense, increasing the sub's propensity to dive.

I don't usually notice this in summer, probably because the lake is pretty warm anyway so there's little change in surface temperature over the day, as compared to winter when the lake starts off cold. Between the two seasons I usually have to add a sinker or two to my ballast for winter.


P.S. I'm not always that paranoid about setting the speed/depth trim It just means I can simply ignore the dive planes if I want to.

[PaulC]

Hi David,

I'm not sure about

Hi David,

I'm not sure about the temp effect, but there are a few things I have observed in the past.

Letting a boat sit in water over time has affected trim. Actually, letting it sit has revealed its true trim as opposed to the initial trim.

Also, I had a boat that would change trim between runs on the same day. Had me baffled. I'd run, come in due to a low battery, replace the battery, put it back in and it was nose heavy. Nothing was changed -- except the battery. The weight difference between two NiCad packs of different manufacturers threw it out of trim significantly.

The weight difference between a full and empty onboard propel bottle will alter things as well (set your trim using a full bottle).

Hope some of this helps!

[mylo]

I feel a little out

I feel a little out of line responding to a sub question, since I'm real new to this but....

...it would seem to me that if water changes in density as air does with temperature, then the boyancy of the model will change with it. In other words, put 'er in the lake in the A.M. when the water is cooler, it is going to trim differently than when she goes out to sea in the P.M. after the water has warmed up.

What I've learned from this is that it would be advisable to have a sub that can be trimmed (and re-trimmed) quite easily, to accomodate for the changing enviroment.

Myles.

[mylo]

David,

I did a little

[color=#000000]David,

I did a little experiment with a 1]

[mylo]

The experiment is conclusive.

Warmer water

[color=#000000]The experiment is conclusive.

Warmer water = less dense = less bouyancy.
Colder water = more dense = more bouyancy.

I was not overly surprised by these results, once I thought about it for a second. What did surprise me was how little change it took in temperature to affect the bouyancy of my 1]

[david h]

G'day all,

thankyou one and all

G'day all,

thankyou one and all for the feedback. However my querie was not really related to Bouyancy. I know that temperature effects this, my main querie was regarding TRIM.[B]

I really wanted to know how temperature would affect the boat sitting level or not in the water. Not so much how easily or not she would dive.

regards,

David H

[bradv]

Actually, the change in longitudional

Actually, the change in longitudional trim is something I've noticed with my Akula as well. Being a dynamic diver, I only have about 3mm of freeboard, so any change in this is fairly obvious as the water comes a lot further along the rounded bow & stern when the sub is sitting lower. Unusually though, the change in freeboard with temperature is most noticable at the bow, which sits obviously lower (sometimes requiring a retrim) when it the day warms up. The stern tends to stay fairly static no matter what the temperature - odd!

So yes, this does seem to correspond with what you're seeing. Maybe its something to do with the placement of the centre-of-gravity of the sub? I can't recall off-hand where mine is, but as I've got a lot of lead closer to the bow than to the stern, as well as the battery pack mounted fore, I suspect my COG is forward of the mid-point of the sub, possibly giving rise to this trim-change issue.

[mylo]

I would suspect without a

I would suspect without a doubt that if a boat is heavier at one end (COG not in the middle) then that end is going to be effected more by water temp because it demands more positive bouyancy to stay trimmed. I would have thought that a sub needs the COG to be, in fact, in the centre so that it doesn't rotate on the longitudinal axis. I thought that the whole idea behind the placement of lead counter weights is to get the COG in the middle so that trim problems like this don't occur.

Myles.

[Guest]

I'm afraid I have found

I'm afraid I have found this a constant problem over the years.

The only sensible solution, is that advised by Bruggen in his 'MST' book. That is to fit a moveable ballast system.

He suggests using the main battery as the movebale 'ballast'. Connecting the main battery to a servo of some description, and rigging the battery to a sliding tray.

This makes it simple to trim your boat everytime, without having to resort to that little bag of lead.

Andy



Edited By Sub culture on 1125005835

[JWLaRue]

This makes sense to me.

It

This makes sense to me.

It would depend on the specific locations of the ballast tank and flotation foam.....where there is flotation, the density of the water will affect the trim.

So if there's more flotation in the bow than in the stern, I'd expect the trim of the bow to change with the bow sitting higher in colder water than warm.

-Jeff

[scott t]

How about adding a poor

[color=#000000]How about adding a poor mans Piston tank. Put a suitable sized surenge in the bow and stern of the boat.
Attach them with hose to a water tight compartment you can draw air from. Have the surenge pushed all the way in on initial balancing.
As one end gets heavy during operating session, pull the heavy ends surenge to displace water at that end.
This might give enough boyancey to level the boat. ]