Chine walking Searider 5.4

Zubenelgenubi · 12 October 2009, 15:22

Does anyone have problems with their SR chine walking? Mine has a Mercury 90hp 2 stroke.
In calm water I can go full ahead (about 40/41kn) without any problems, the engine needs to be well trimmed up to achieve this speed & it appears that the only contact with the water is the engine leg & the aft 2 feet of the hull. Any disturbance on the water will induce chine walking which gets worse with each roll, I guess it is similar to parametric rolling on ships.
The chine walking can be stopped by reducing the rpm & not adjusting the trim or by trimming the engine down & leaving the rpm unadjusted. Obviously both have the effect of reducing the top speed (to about 35 kn).
Is there a solution to this problem while still maintaining full rpm?
The cavitation plate on the engine is just in line with the bottom of the keel.

Stoo · 12 October 2009, 16:26

Zub, you might want to have a look at this thread...

http://rib.net/forum/showthread.php?t=32415

Turbodiesel · 12 October 2009, 21:27

They all do it with 90's on the back!!

Zubenelgenubi · 12 October 2009, 21:32

So do you reckon there is any point in lifting the engine up a notch or two then or do SR's with 90's have a "critical speed" above which they will chine walk due to hull design?

Nos4r2 · 12 October 2009, 22:09

I reckon they all do it once you trim them out too far in search of 40 knots

matt h · 13 October 2009, 08:39

if you can hit 41kts then you have achieved what the hull was designed for!! leave it at that.

HUMBER P4VWL · 13 October 2009, 10:49

Quote:

Originally Posted by matt h

if you can hit 41kts then you have achieved what the hull was designed for!! leave it at that.

I'd agree, if you want more speed than that in a boat that size, get a different boat! Our 5.4 with a 90 opti did the same. Pushing an SR any faster would be madness. Ribs are mainly for seaworthiness, if you want flat out speed on calm water, perhaps the rib isn't the boat for you? Personally, 41 knots is plenty for all but racing circumstances.

JSP · 13 October 2009, 11:10

My SR4 chine walked like nothing else. Not had the 5.4 do it yet.

ollyit · 13 October 2009, 11:35

Quote:

Originally Posted by matt h

if you can hit 41kts then you have achieved what the hull was designed for!! leave it at that.

I'd be happy with that

Zubenelgenubi · 13 October 2009, 14:43

Im more than happy with the speed & am not looking for it to go any faster,38-40kn is more than enough in the SR.
The issue is not being able to maintain the speed even in a slight chop without the boat chine walking, it seems strange that a boat designed to carry a 90 hp 2 stroke engine & fitted with a 90hp 2 stroke engine (engine weighs 135kg) chine walks & from what I can gather it seems to be a characteristic of this boat??

Jimboat · 14 October 2009, 02:30

Zubenelgenubi - chine walk is pretty common on performance hulls like yours.

As the hull accelerates, lift increases and the wetted running surfaces that are required to support the hull are reduced (more Speed = more Lift = less Surface). As the speed increases throughout the velocity range, the hull often gets to a point where the lifting surfaces become very much reduced and the hull is now "balancing" on a small area of the vee-portion or the "vee-pad" of the hull. When that surface becomes sufficiently small, it becomes very tricky to "balance" the hull on its vee or pad. The result is a rocking of the hull from side-to-side. This rocking can tend to get a little more extreme with each motion, and so the "balancing" must then be provided by additional driver (steering/throttle/trim) input in order to maintain the hull in a balanced state.

With some driver input, the condition can be stabilized. With changes to hull setup and/or changes to the driving technique the condition can be alleviated.

Chine walking is predominantly characteristic of vee-hulls with deep Vees (more deadrise), hulls with deep or narrow running pads and hulls, with a Veed pad or no pad (straight vee). These bottom designs are just more inherently difficult to balance at higher speeds. Another contribution to chine walking can be seen at higher speeds from "propeller slap" (usually seen more with props of fewer blades). As the prop turns, each blade enters the water and another exits the water. This irregular in/out of every blade, changes the dynamic forces at the location of the propshaft, repeatedly putting an imbalance on a hull that is trying desperately to balance on it's vee or pad, and ultimately initiates chine walking.
Setup of your hull and driver "seat-time" are the best solutions to the problem. Usually an alteration to the hull, or hull setup and/or modification to your driving methods (read seat-time) will improve the problem.

Some established steps toward minimizing chine walking include:
1. Check & adjust steering
2. Use solid mounts
3. Clean Hull Lines
4. Weight balance of hull
5. Motor height
6. Propeller selection
7. Seat time (experience)
8. Minimize Trim Angle

Summary: Optimize Hull Setup and Driver "seat-time"!

check out article on chine walk for more details.

mdt · 14 October 2009, 11:40

Ah Jimboat, that all makes a lot of sense. On the 5.4SR the chines on the hull run out to nothing towards the stern and there is next to no “planning pad” so there is not a lot of hull to balance the boat on.

AVONMON · 14 October 2009, 12:46

Hi,try a set of Nauticus Smart Trim Tabs (www.nauticusinc.com) gas ram non metalic components automatic adjustment with the gas rams.Better hole shot,eliminates chine walking and reduces planning speed.It did it for me.

Zubenelgenubi · 14 October 2009, 16:00

Thanks for all the information, the explanation is completely logical!
Isnt it a wonder that on the SR hull which has been around for years & years that Avon have not tried to solve the inherent chine walking problem with some type of fixed tabs / extension aft of the transom to improve high speed stability?
Ill have to experiment further to find a happy medium between trim v's speed v's chine walking.

Poly · 14 October 2009, 19:18

Quote:

Originally Posted by Zubenelgenubi

Isnt it a wonder that on the SR hull which has been around for years & years that Avon have not tried to solve the inherent chine walking problem with some type of fixed tabs / extension aft of the transom to improve high speed stability?

I doubt that the typical new buyer of a SR is using it at maximum speed very often. Its not sold for its performance - but rather its commercial attributes.

Zubenelgenubi · 22 October 2009, 20:32

Quote:

Originally Posted by AVONMON

Hi,try a set of Nauticus Smart Trim Tabs (www.nauticusinc.com) gas ram non metalic components automatic adjustment with the gas rams.Better hole shot,eliminates chine walking and reduces planning speed.It did it for me.

Looked at nauticusinc.com website. The tabs are something of a passive damping system.
Has anyone installed trim tabs on their SR?
As far as I can see, the trim tabs have the effect of increasing the water plane area of the hull beyond the transom, however if they are installed at the trailing edge of the hull, they will just increase the hull length, ie the WPA but will be at the same angle as the hull & I cant see how they prevent chine walking. If they are trimmed down to angle X they will have the effect of lifting the stern & lowering the bow & consequently increasing the WPA of the hull proper, the same effect as just trimming the engine down?
Thoughts / experiences?