Got an unusual handling question.
I have a 5.2 glass half cab, with a 90Hp 2 stroke, upgraded by a previous owner in the boats past(fyi, boat hull specs say up to 115hp), with a "vengeance" stainless steel prop upgrade, and a large hydrofoil fitted, and cable type steering.
I have this weird handling thing at around 30 knots, where the boat seems to tilt to the port/left side under power. Travelling in a straight line, but tilting/leaning sharply to the left.
It's happened a few times. If I slow down it goes away. Any ideas ?
Prop moves in a clockwise rotation, so the boat would be tilting anti clockwise if that makes sense ? Its not weight distribution, that was the fist thing I thought of, and most weight was on starboard if anything.
I measured and by my calculation the engine centre is about 10mm to the starboard, so 5mm off centre, cant see this causing the issue though.
Pics of the capitation plates below, its difficult to get the proper angle, but the bottom capitation plate seems to be slightly above the bottom of the boat, which per my research, is correct.
Pic of engine fully trimmed in.....
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and then out to level roughly
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Any suggestions of how to fix it ?
I took some better pics....
This is boat level, capitation plate level,.....
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And a view from the back......
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Sounds like you could do with some trim tabs...
Most boats with a decent vee will lean due to prop torque, prop selection and/or weight distribution...
Its often exacebated by wind/swell direction and the steeper the deadrise the more prone it is to happen..
Try it without the foil on. The outside edges look like they are way below the line of the hull, which is not what you want. maybe it is dragging the boat down, more so to one side if the boat isn't perfectly balanced?
Two things:
- A 5.2m hull shouldn't need a foil with a 90hp 2-stroke. It'll be creating excessive stern lift while on plane resulting in the boat's attitude being too high and flat in the water to remain hydrodynamically stable. As such it will want to fall onto one chine or the other. You need to drop the stern a bit at speed.
- As for always falling to the port side ... your transducer looks way too low in the water. Normally they should only be 3-4mm below the running surface of a glass hull and aligned parallel with the water surface at desired sounding speed. Yours looks to be about an inch lower than the strake at the leading edge and angles down from there, so it's effectively acting as a small trim tab causing lift on the starboard side (which will roll the boat towards the port chine). Plus prop torque doing the same thing.
I'd take the foil off, raise and level the transducer (or at least kick it up out of the way) then see how it performs. If it seems stern heavy at that point, look at weight redistribution before putting the foil back on - they have a decent use case for tiller outboards but are generally just a bandaid fix for weight distribution issues on larger boats with forward steering.
I have had a similar issue at times ....with a deep vee half cab....
1.I would take the foil off as suggested...you probably dont really need it..or its just complicating the issue anyway....
2..If it still happens I would first alter trim the motor to bring the nose up.....you need to get the boat running on the rear of the hull rather than "half side the hull and a chine".This is possibly where the foil isnt doing you any favours.I have generally found its a trim issue.What I dont understand is why it occurs at 30knots...which again points to the lift from the foil?
3.I ended up putting on those cheap spring actuated trim tabs ....which work fine ... but Im not doing the long runs in open water the guys in Qld do.
+1 for taking the foil off, I haven’t seen a boat yet that responded positively to these. Not saying it won’t help in some cases but I just haven’t seen it. I find that they lift the stern out of the water too far and that’s where your stability comes from, for the sake of 4 bolts just try it. My guess is that you will be more stable on the plane, experience lighter steering (especially with cable steer) and will likely cavitate less on a turn, though you might find it’s a little less responsive to trim.
delete me! - see next post
Very interesting, about the excessive stern lift may be causing the issue, and perhaps prop torque is tilting it to the port chine.Originally Posted by Mopheus
The transducer was mounted by the previous owner. Is the tranducer being directly inline with the strake anything to do with it? Is that a stupid or bad spot ? I understand turbulence can be an issue there, but doesn't seem to effect its display performance. I can twist it up level no probs, easy fix, but would it still have the handling issues ?
Also, because of your suggestion I read up a bit about how they are meant to be mounted, it should be an inch higher from what I can find.
However I'm very reluctant to move it and drill more holes under the waterline, unless it is really necessary, as its a really stupid place to put one ?
If I did move it, where would be the best spot ?
Also, would the upgraded propeller have anything to do with it ? Its a Stainless Steel Mercury Endurance 48-16986 16P I have no idea about props and what those numbers actually mean, but I understand they need to be matched to a hull/engine set up ?
Multiple quotes don't seem to be working so go with me on the formatting below
The transducer was mounted by the previous owner. Is the transducer being directly inline with the strake anything to do with it? Is that a stupid or bad spot?
It's generally recommended that you avoid installing them inline with strakes but almost everyone does because it's convenient and often the only place left after you allow for trim tabs and drain plugs. If it reads well then it's not an issue - avoiding the strakes is simply to avoid the turbulence that often accompanies them.
There's a decent chance though that it only reads well at speed in its current position because it's so deep in the water. Lifting it to correct depth in that position could alter its on-plane performance significantly.
However I'm very reluctant to move it and drill more holes under the waterline, unless it is really necessary, as its a really stupid place to put one?
Devil's advocate - the prior owner installed the transducer and arguably did a poor job of it ... d'ya really want to trust their workmanship on sealing the transom penetrations? Filling them with epoxy isn't hard to do - no need to match the gelcoat over because the transducer mount will cover it anyway.
The transducer is definitely the lesser of your issues though - removing the foil will likely solve your handling issues; moving the transducer will not. I mentioned it only because you were wondering why it fell onto the port side not starboard, and it's likely the transducer + prop torque together that are causing that. Dynamically stabilise the hull though by not imparting excessive lift to the stern and you should be golden irrespective of transducer position / depth.
If I did move it, where would be the best spot?
You can't go inwards due to the drain plug and proximity to the outboard, so move further starboard to achieve 2" minimum clearance between the edge of the strake and the transducer.
Also, would the upgraded propeller have anything to do with it?
Yes but no. Prop torque contributes to tendency to roll anti-clockwise but is unlikely the primary cause - just a contributing factor. Doubt you'd be able to resolve the instability by repropping, unless the re-prop dropped top end speed such that you no longer hit the dynamically unstable zone.
Also, hopefully, the transducer still has some movement up/down. Usually the brackets are slotted of around 2 cm with a covering plate sometimes. As Mopheus suggested it would be worthwhile checking it out as it looks to have been moved already according to the pictures.
Yea, good point, If I remember right, I think there was a little bit of play left to move up a bit.
I'm away at work at the moment, cant wait to get back, remove the hydrofoil, tidy up the transducer and take it out at full whack to test if it happens again.
Also, I emailed Steve of the propellorspecialist.com.au he called me up to discuss it, which was really good service considering. Anyway, I asked about my prop, he said there is nothing that stands out as wrong with the prop with the boat configuration, but the only way to know is the see what the revs are at top speed with a full load. But also said the hydrofoil sounds like the main culprit !
If the free options dont work, then I will look at that cheap trim tabs. Do you have a link to the one you bought ?
Also, how does she go in choppy open seas ? I would be concerned that if I was crossing rolling seas diagonally, one side would be digging in and tipping me over ?
I think Morph nailed it on the first attempt. It’s really easy to loosen the transducer nut, rotate it all the way out of the water. Then take it for a run. The faster you go the more that transducer is acting like a trim tab. If that works then you can take the foil off at the ramp and take it for another run. That foil may have been an attempted bandaid fix for what the transducer was doing.
My last transducer install I had it sitting in line with the hull and only about 2-3mm proud of the line and it worked great at speed without throwing up a rooster tail or effecting performance. As long as there’s clean water running over the surface at speed you should be ok.
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The smart tabs...were from Nauticus...although I see Bennetts are making them too nowadays.About $200. They are not as robust or controllable as the more expensive trims...but once you get the pressure settings right...you can basically forget them.
As for the transducer...I ended up mounting mine inside the hull where I had easy access to the hull.If you have that access I would recommend this for coastal use anyway..not for deep water use.Someone on this site did a post about how to set it up......its been brilliant.
Small tiller-steered tinnies are about the only place where I can see foils making sense, because even if you move your battery + tank to the front you still have outboard + captain at the stern. A bit of extra stern lift can make all the difference to getting on plane and there's no easy way to achieve it otherwise, assuming you don't want to fit trim tabs to your tinnie...
Unfortunately they've become a go-to bandaid fix for overweight sterns in larger boats where there are better options like moving weight forward. They do help get up on plane so are effective in that sense, but come with the potential for dynamic instability that people either don't notice or just put up with. Either way it tends to negatively impact the boat's ability to handle challenging driving or sea conditions.
Spring tabs are a pretty decent option if you always boat in calm waters. If there's a chance you might be coming home with a following sea though, they can be downright dangerous if you don't take the time to lock them up out of the way. Stern lift + following sea = bow steer + broach. Some hulls are more prone to it than others, but as a general rule you want to avoid stern lift in following seas.
I wouldn't fit them to anything but a lake / river / estuary boat myself, because I'm lazy and would get sick of locking them up when conditions say I should
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