MoBo and trimming !

[Refueler]

The Primor 24 I have with AQ170 power is a tricky one to trim ... basically I believe the previous owner who swapped out the 130hp for the 170hp lump made a mistake. The change from the 4cyl to 6cyl engine has put more weight in the stern along with the heavier sterndrive.
The original boat may not have had trim tabs ... someone fitted a Pneumatic system in the past and then previous owner swapped to an electro-hydraulic ... but without angle indicators ... its all guess work !!

When just myself and one other are in the boat ... both at fwd end of cockpit ... I find that trim keeps boat steady - but used sparingly ! and as speed increases past the 15kts - ease back on tabs ... at 22kts and over - it appears that I have little or no tabs down ..

The other day - a couple of guests on board for a river trip .... wife and they all sat on the aft bench seat ... and I had trouble trying to get the bow down with the tabs ... the boat was sitting stern down of course and this made her ultra sensitive to any helm action / trim tab use.
It also caused the speed to be seriously lower than normal - eating fuel at an alarming rate ...

ie ... I was getting 9kts when I should have been at about 14kts ... fuel consumption at about 12 - 14ltrs / hr ... but more like 20ltrs /hr ... Trying to use trim tabs to get bow down and get her start to plane .. was not so easy ... as the tabs went down .. she would veer of to one side or another .. but occasionally she would stay straight and as bow came down - the speed would increase quite quickly - but then I needed to pull throttle back a touch .. which put her back again ...

I had to let the boat run as is .. the guests were first time on a boat and when it veered - they were visibly concerned ...

The question for me - is it worth the expense of modifying the trim system to automatic ? Is automatic really that good ?

Note : Give me a set of sails and I'm in my element ... but this MoBo lark is still giving me moments !!

 

[julians]

I wouldnt bother with auto trim tabs, they're unlikely to give you much improvement, you'll get used to working out how much trim tab you need in what situation.

Trim tabs also need a bit of speed to work, they get more effective/have more effect the faster you go, but if you can't get up to a speed where they start to work, then they're not going to do much.

Can you experiment with adding some ballast/weight right at the front to even out the weight distribution a bit? That might give you a bit more consistency.

Small boats are very sensitive to weight distribution, and sometimes cant be rectified with trim tabs alone,, so sometimes people might not be able to sit where they want, you might have to ask people to sit in specific locations to enable the boat to run better. If you can't do that/the people won't do that, then you'll probably just have to go slow when you have that many people aboard - if you can't improve things with ballast.

I reckon a small amount of ballast in the right place will improve things a lot, even if it's just the gear (bags and other stuff) people bring, if it's in the right (or wrong) place it can make a big difference in a small boat

You might not have to actually add any weight, just move some existing gear you carry into a different place, eg if you keep a tool kit, spare parts, lines, fenders etc at the rear, try putting them at the bow instead, essentially move some weight to the front.

 

[Davy_S]

Be worth a try if you could borrow an Atwood hydrofoil or doel fin just to give it a try, I have had two attwood foils on a 15ft boat and a 22ft Cranchi out drive, they worked very well giving increased lift and planed earlier than before.

 

[Refueler]

All points read and taken .... weight distribution is the key I think ...

One problem is not so many mobo users here with Trim Tabs to talk to ...

 

[Refueler]

I have been considering the Mentes Marine ACS control ... no need to change anything apart from swap the manual control for the ACS auto control pad.

But considering its a bit pricey for a pad based on MEMS .... MEMS are very cheap - I know because I use them in various models etc.

 

[Seastoke]

Put them on a diet or mounjaro

 

[stelican]

Why not fit larger trim tabs

 

[Refueler]

Why not fit larger trim tabs

Now don't be daft ... !!

 

[stelican]

Now don't be daft ... !!

You said the boat is slow to plane and worse when loaded.
Have I missed something?

 

[Refueler]

You said the boat is slow to plane and worse when loaded.
Have I missed something?

Not so much miseed something - but possibly misunderstood ...

The boat is fine when not loaded by stern ... touchy - but tabs work ok ... just needs a delicate touch on the buttons.

Please go back and read post #1 ....

 

[ss2016]

The Primor 24 I have with AQ170 power is a tricky one to trim ... basically I believe the previous owner who swapped out the 130hp for the 170hp lump made a mistake. The change from the 4cyl to 6cyl engine has put more weight in the stern along with the heavier sterndrive.
The original boat may not have had trim tabs ... someone fitted a Pneumatic system in the past and then previous owner swapped to an electro-hydraulic ... but without angle indicators ... its all guess work !!

When just myself and one other are in the boat ... both at fwd end of cockpit ... I find that trim keeps boat steady - but used sparingly ! and as speed increases past the 15kts - ease back on tabs ... at 22kts and over - it appears that I have little or no tabs down ..

The other day - a couple of guests on board for a river trip .... wife and they all sat on the aft bench seat ... and I had trouble trying to get the bow down with the tabs ... the boat was sitting stern down of course and this made her ultra sensitive to any helm action / trim tab use.
It also caused the speed to be seriously lower than normal - eating fuel at an alarming rate ...

ie ... I was getting 9kts when I should have been at about 14kts ... fuel consumption at about 12 - 14ltrs / hr ... but more like 20ltrs /hr ... Trying to use trim tabs to get bow down and get her start to plane .. was not so easy ... as the tabs went down .. she would veer of to one side or another .. but occasionally she would stay straight and as bow came down - the speed would increase quite quickly - but then I needed to pull throttle back a touch .. which put her back again ...

I had to let the boat run as is .. the guests were first time on a boat and when it veered - they were visibly concerned ...

The question for me - is it worth the expense of modifying the trim system to automatic ? Is automatic really that good ?

Note : Give me a set of sails and I'm in my element ... but this MoBo lark is still giving me moments !!

I came from sail to the darkside about 5 years ago. Tabs are therefore relatively new to me. ie I am no expert. Since you have so few answers I will write my thoughts.
My boat is a 400hp 31'Sargo, so not directly comparable. About 5 tons plus water fuel and people plus the inevitable junk.
I use the tabs and while they are quite useful in roll they have only a small effect in pitch. The point which has not been mentioned (I think) is the sterndrive. Its position is vastly more powerful than the tabs. If I were in your shoes I might try experimenting with that. Full down will lift the back of the boat somewhat. I don't do this because it gets on the plane anyway. Once on the plane I adjust it for maximum speed for a given rpm. I think this occurs when the thrust is absolutely parallel to the horizontal. I do then fiddle with the tabs sometimes but the effect is minimal.
My boat is basically a two speed boat. 7kts is very economical. If you want to go faster it needs to planing properly for economy (miles per litre) That occurs around 22Kts and 2900rpm. Depending on fouling about 1.9 to 2.4 l/Nmile. Faster or slower it gets very thirsty.
PS I don't think the tabs do most of the lifting but I might be wrong. What they do do I suspect is by impeding the water flow they increase the pressure of the water in front of them on the underside of the hull.

 

[Refueler]

The 280 sterndrive I have is advised NOT to use while running ... to leave in down position. The angle of its down is set by a pin - similar to an outboard.
The lifting mechanism is as I am advised only for occasional use in shallow waters / beaching ... in fact I am advised NOT to raise the leg when moored - but to leave the leg down due to raising causes the bellows / pipes to be bent and can lead to cracking over time.

The tab's do in fact bring that bow down - so much so that they have to be used carefully ...

If there is just myself or two of use and we are both sitting in fwd section of cockpit ... the tabs bring bow down as speed increases - but once we get over about 18kts - start to reduce tabs as boat starts to plane on her own.
The problem I highlighted in the opening post - was that the trip in question had 3 people sitting aft in the cockpit ... causing stern to sit lower .. bow higher ... trying to use trim tabs to correct was difficult and bow if not careful would veer off to side with boat heeling .... NOT best way to introduce new people to boating !!
I should have got people to move .. spread the weight better ... but I didn't.

Its interesting that I have a high powered RC Bat Boat ....



This exhibits exact same habit of bow veering off at speed ... I have trim tabs to fit that to try and cure it ... at present - I set CoG more fwd than its designed for - but that helps prevent it.

 

[jrudge]

At the end of the day it is a small boat and loading will make a difference.

If you put all the weight at the back it is basically out of trim before you start.

The veering off will be chine walk which I had on a cranchi 33 and was frightening when it happened.

The boat needs to be loaded properly. Planes have formal c of g limits for similar reasons as there is only so much that trim can do and after that point it becomes fundamentally unstable or liable to upset.

 

[Refueler]

At the end of the day it is a small boat and loading will make a difference.

If you put all the weight at the back it is basically out of trim before you start.

The veering off will be chine walk which I had on a cranchi 33 and was frightening when it happened.

The boat needs to be loaded properly. Planes have formal c of g limits for similar reasons as there is only so much that trim can do and after that point it becomes fundamentally unstable or liable to upset.

My point in original post ...

And the question as to whether auto-trim tabs might help ....

Interesting that when I had 5 guests on the boat ... 1 in cabin .. 1 on fwd passenger seat .. 3 on the transom bench ... she ran quite happy ... in fact I don't remember any veering etc.

 

[jrudge]

Auto trim tabs are just trim tabs that work automatically. They are not magic. I had them ( funnily enough on the same cranchi 33 ) many moons ago. They won't react fast enough to make a boat that has been made unstable stable.

The boat ran ok with 3 on the back and two others more forward as it was within reasonable trim. The weight at the front compensated for the weight at the back and moved the centre of gravity forwards.

 

[Refueler]

Understood ... my thoughts are obvioulsy 'load' the boat better .... have it stable and then let auto tabs look after it as speeds change etc.

The "Mentes Marine ACS control" seemed to react quickly - it ought to if using MEMS ....

Video » Mente Marine

I appreciate that they are not going to install and video a boat that is not balanced !!

 

[julians]

Understood ... my thoughts are obvioulsy 'load' the boat better .... have it stable and then let auto tabs look after it as speeds change etc.

The "Mentes Marine ACS control" seemed to react quickly - it ought to if using MEMS ....

Video » Mente Marine

I appreciate that they are not going to install and video a boat that is not balanced !!

It doesn't matter how quick the "brain" works if the tabs themselves take several seconds to move from fully retracted to fully extended, and they can only acheive small window of correction anyway. Auto trim tabs are not the solution to the problem of a significant fore/aft weight imbalance, they're the solution to the problem of a achieving a level boat without having to do it yourself. If you can't do it yourself, automating what you have will also not work.

 

[aerobat]

On a 24 footer the weight of several adults will have a significant impact on handling and performance .

Initially , loaded with quests at the aft bench , you start with an ass heavy condition and try to correct it by forcing the bow down . By doing this you create a condition of "bowsurfing" where she looses stability and veers off / lists suddenly over . My 2006 240searay sundancer did the same extremely .

All you can do is to accept the ass heavy condition and accept she will plane at a higher minimum speed . Do not force the bow down ! A 24 footer is not a boat size where you can say "it does not care" if there are lets say 6 adults more or less onboard .

The really good sign : your aq170 is obviously juicy enough to make you plane out at all .

I saw many boats this size who struggle to plane with several guests all sitting aft and ending up plowing thru water with the ass buried in the sea until the skipper shamefully asks some to move forward while he with a red angry face tries to milk the last bit out of the throttle lever .

So your aq170 is good ! The 130 most probably would not fully plane at all .

 

[ylop]

Any option to move internal weight forward? Anchor/chain/spare fuel/water/batteries/beer rather than using the tabs? I used to use a MoBo which had fuel tanks forward and the handling changed noticeably between full/empty, we improved that by moving stuff around.

 

[Refueler]

On a 24 footer the weight of several adults will have a significant impact on handling and performance .

Initially , loaded with quests at the aft bench , you start with an ass heavy condition and try to correct it by forcing the bow down . By doing this you create a condition of "bowsurfing" where she looses stability and veers off / lists suddenly over . My 2006 240searay sundancer did the same extremely .

All you can do is to accept the ass heavy condition and accept she will plane at a higher minimum speed . Do not force the bow down ! A 24 footer is not a boat size where you can say "it does not care" if there are lets say 6 adults more or less onboard .

The really good sign : your aq170 is obviously juicy enough to make you plane out at all .

I saw many boats this size who struggle to plane with several guests all sitting aft and ending up plowing thru water with the ass buried in the sea until the skipper shamefully asks some to move forward while he with a red angry face tries to milk the last bit out of the throttle lever .

So your aq170 is good ! The 130 most probably would not fully plane at all .

I understand that 24ft is not a size to allow much leeway in weight distribution. I was just surprised at the extreme that day caused ...

I was at helm .. stbd fwd cockpit and 3 people sat on the aft cockpit bench seat.

Alongside - she did not look so bad - so did not ask for people to move. Only one would have anyway as only one more seat opposite me fwd.

When we got out of my channel .. she felt sensitive - which I realised was the 'arse down' syndrome. I tried to delicately get the bow down even a few degrees to try help ... but anything other than a quick dab at either tab - had her veering off course .. and heeling.
Engine RPM was much higher for the speed she was making .. she was averaging 8.5 - 9kts where she should have been 15 - 16kts at least .. fuel consumption was excessive.

My thoughts on Auto .. were based on the boat being sensitive even when not badly loaded ... I can get her to 25kts - but that takes a lot of fine dabbing of tabs .. first down to help her get on plane ... then as speed starts rapidly rising .. dabbing them up again to allow the hull to work ...
Each outing is different ....