Strange behaviour at high speed

Refueler

Refueler

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The Primor 24 I have ... 170HP Volvo Penta via 280 outdrive.

Top speed 25kts.

Note that boat was originally fitted with 130HP VP and I believe 250/270 outdrive.

Scenario with the 170HP + 280 drive :

At low to medium speed (16kts ~) .. its easy to trim and she purrs along nicely ...

As throttle is opened more - trim buttons are tapped to keep her running nice ... but as throttle opened and hits 20kts she can veer off to stbd and heels over hard ... such that throttle needs to be brought back ..... Its quite sudden and cannot be countered without throttling back ...
 
My experience of chine riding is that boat flops from side to side, rather than serious change of course.

Sounds daft perhaps but is the hull symmetrical and prop shaft truly on the centreline ? It sounds as if at the transition over 20kts, that the propwash establishes itself at an angle to the centreline possibly taking effect between the prop and the transom. May be a function of handedness of the prop ,? Is it left handed ?
 
Refueler said:
As throttle is opened more - trim buttons are tapped to keep her running nice ... but as throttle opened and hits 20kts she can veer off to stbd and heels over hard ... such that throttle needs to be brought back ..... Its quite sudden and cannot be countered without throttling back ...
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Could you give a little more information about the 'trim button tapping' please? Are you talking about the legs or the tabs, and are you trimming in or out? If you're putting the leg all the way in and particularly if also putting the tabs down, that might be your problem - you're burying the nose and the boat is 'tripping over it'.

It can be helpful to do this when getting the boat onto the plane (especially if a little underpowered) but once planing at speed you want the nose up a bit (trim out on the leg) with as little tab as possible. (There are nuances to this, for example trimming in the leg and dropping the tabs can help keep the nose down allowing the boat to plane at lower speeds, but it appears your issues are at higher speeds).
 
I had exactly the same on my 2010 searay 240 sundancer . She starts bowsurfing and beers off / lists hard over when not trimmed up very carefully on speed .

You need to trim up to raise the bow .

Bad news : its a s**i and tippy hull design , very sensitive to drive trim .
 
The outdrive is a Volvo 280 and I've been advised by a number of people and reading online - that this drive is not suited to trimming while running. Its advised to be full down and you set the trim angle (full down position) by the stop pin in the drive mount itself.

The Trim Tabs are manual hydraulic via a small control pad set into the console :

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The old disconnected pneumatic controls are still there ... right side halfway down ..

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Unfortunately there is no indication of amount tab is angled ... I cannot identify the system - if I could I would like to change to a displayed system so I know what angle my tabs are. Maybe its not so critical what manufacturer ?

I tend to use the safety rail that runs from cockpit to bow and round back to cockpit as the 'level' ... another motor boat guy with me one day reckoned to keep that rail level with horizon and bow down enough to be able to have reasonable view. I can get to about 17kts with that .. then I need to tap the buttons as speed goes higher ...
I did notice at end of last season - was first season with the boat .. that over 20kts - I was reducing the use of tabs as the stern seemed to rise and bow dip as the stern was riding the water and lift.

The boat as I said was originally equipped with a 130HP 4cyl VP and previous owner swapped it out for the 170HP 6cyl VP .... which I tend to think may be part of the problem. Too much power and torque adding to the stern now sitting deeper in the water.

If I start of and do not use tabs .. the bow very quickly pitches up and I lose a fair distance of water visual in front of the boat .. trimming bow down regains that visual ...
 
stelican said:
Try moving the leg trim pin out one hole ' Use the Trim tabbs to assist getting boat into plane then when at cruising speed retract trim tabbs
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It crossed my mind to move the pin - then I noted its already out from vertical in second or third hole - cant remember which ...

Had a run out today playing again ... hit 19.5kts and she was steady ... even took wide turns without getting upset ...

My problem is I'm primarily a Sailboat guy and my times with motor boats has been limited. Any with trim tabs had auto tabs except for one or two - then owner usually set the boat up ...

If I was in UK with the boat - of course there would be plenty of people at hand ... but here - I'm literally on my own with it ... lots of passenger friends .. but none with the 'knowledge;.
 
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Refueler said:
It crossed my mind to move the pin - then I noted its already out from vertical in second or third hole - cant remember which ...

Had a run out today playing again ... hit 19.5kts and she was steady ... even took wide turns without getting upset ...

My problem is I'm primarily a Sailboat guy and my times with motor boats has been limited. Any with trim tabs had auto tabs except for one or two - then owner usually set the boat up ...

If I was in UK with the boat - of course there would be plenty of people at hand ... but here - I'm literally on my own with it ... lots of passenger friends .. but none with the 'knowledge;.
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Move the pin. You can always move it back. My fast RIB had a renowned hull shape, yet could still be unsettled by poor trim. I can assure you it’s even more scary at over 40kn, but easily cured in that case. You’ll probably find you need more tab at lower speed of course.
 
Chiara’s slave said:
Move the pin. You can always move it back. My fast RIB had a renowned hull shape, yet could still be unsettled by poor trim. I can assure you it’s even more scary at over 40kn, but easily cured in that case. You’ll probably find you need more tab at lower speed of course.
Click to expand...

I plan to lift her out soon - once I get agreement with yard to replace the cover ... I will trailer her to the yard - near 1hrs drive - so I don't have to wait weather ... its still bl**** cold out there on the water !

Will look at that again .. also talk to the yard ... put all info into the pot ... from here - them ... see what comes out !!
 
stelican said:
Try moving the leg trim pin out one hole ' Use the Trim tabbs to assist getting boat into plane then when at cruising speed retract trim tabbs
Click to expand...
We had a lot smaller Shetland with a 60hp on the back, it would run well into a head sea, but if we were in a following sea we would have to stop and raise the engine out one hole, otherwise it would bury the bow and quicky veer off to one side (it could be any side) it could be very unerving until you had it sussed.
 
This one veers off to stbd .. which is why I suspect torque has something to do with it as well ...

Been looking online at possible control change ... the Bennet Auto runs out at about 700 euros for the MEMS control pad ...

Makes me wonder if I could use a MEMS board from my models ... my pal is a dab hand with this sort of stuff ... will see what he says ..
 
OK ... yesterday - May Day ..... had boat on the river and once clear of others - we opened up the throttle .... and she was as before - but better ... 19.5kts was steady ...
After fishing and run home - I decided to test the effects of NO trim tabs ..... as I have a suspicion that she may have been built without originally.

22.5kts and steady as a rock ... no trim tabs at all .... just fully up. On initial - the bow rises high but soon starts to come down and the stern lifts at speed ...
 
Each hull is different, but you can tell visually where the forward point of contact with the water is by looking over the side and observing where the wash wave is coming from: it shouldn’t be too far forward on a small fast boat.
 
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