Lifting strakes do they produce lift in the submerged sections

[Lynton Banks]

I would say the fact a portion of your rudders are above the "fast" water flow does not help. You would probably get an increase in rudder performance if the entire rudder blade was below the hull and not partially hidden. Just an observation from one who knows nothing about fast boats.

I believe Dag Pike wrote about Nelson 42s being used by the Italian coast guard, and said something along the lines of "judicious use of the throttles were required in certain conditions, due to the small surface area of the rudder blades".

Trying to work out if the rudder hight was deliberately positioned to use the air sucked down @ speed to reduce the drag effect of “fast water “ flow .
Recessing the P bracket or fairing it in would be very easy to do .
The attention to detail with the “ hook “ , that planing surface Fwds of the exhaust hole .That was added post mould pop.

Reasons I think is because he ( the designer ) previously used a system like this .Which lifts the rudders .
Unusual rudder placement https://www.boatdesign.net/threads/unusual-rudder-placement.67775/

 

[Lynton Banks]

The rudder cross-section shape used there is unsuitable for high speed where ventilation may occur. They may even cause a force reversal in certain conditions.

Thanks for the input really appreciate it .
It’s perfectly safe I have used them full over in extremis .

The rationale is complicated further his previous rudder irritation, that height adjustable rudder bar .
He went from that to this albeit in a narrower beam .The 46 with a 4.44 M beam to the 42/48 ( same hull mould ) 4.18 M beam .L more or less the same about 1/2 M difference in favour of the 46 , but don’t hold me to this .

This is not my boat so I opened up another thread .
Unusual rudder placement https://www.boatdesign.net/threads/unusual-rudder-placement.67775/


Could drag reduction be a common denominator in both cases ?

 

[skaraborgcraft]

Could drag reduction be a common denominator in both cases ?

Definitely, and i think looking at both set ups, extreme ways to achieve it, but perhaps top end speed figures is what sells these types of boat.

I have to wonder how an insurance assessor would read the line, "did not respond to helm input before the collision". Any actual builders plans for your hull and stern gear? Maybe "throttle steer" is more common on fast boats? Seems extreme to me.

 

[Lynton Banks]

Definitely, and i think looking at both set ups, extreme ways to achieve it, but perhaps top end speed figures is what sells these types of boat.

I have to wonder how an insurance assessor would read the line, "did not respond to helm input before the collision". Any actual builders plans for your hull and stern gear? Maybe "throttle steer" is more common on fast boats? Seems extreme to me.

I am on the boat tomorrow as it happens and there are some plans .I will have a look .

Upside is cruising 26-32 knots you need very little steering adjustments as the boat develops a set .Wake and aberrant waves are just absorbed the track remains unaltered.So it’s relaxing easy to drive .You are not making constant course corrections .Indeed once up and running on a course you can take your hands off the wheel ….a lot .


The underwater strakes dimensions and the 23 * dead rise , wide ish slightly inverted chine flats at the stern , those trim surfaces in front of the exhaust holes which are angled inline with the hull , as it heels the inside of the turn one obviously flattens out with the water surface and I think create more lift pushing it back up right = it stiff huge dynamic transverse resistance to heel over .

May be adding on my boat the 42/48 bigger blades on the rudder stocks ( easy to do in the wet R+D stage ) it only achieves added more drag , the extra turning they achieved obviously being greater surface area just couldn’t make much of a difference with the set .To over come the set to get real tight turns at 30 knots would need out of proportion blades adding far too much drag ??

Or accepting the blade size for drag reasons , fitting a a flush p bracket base reducing aeration of the rudder , getting more bite , grip again they have to overcome the massive set .Again the better water flow just is not enough.You have added drag back for little gain .

Now with the 46/54 s the movable rudder bar boats the helm vary the blade depth .
Those blades are a longer distance from the wheels , the wheels almost hanging off the transom .Presume low shaft angle gain .

I am not a NA just playing with ideas .Can’t calculate the difference 1 or so degree lower shaft angle , the amount of drag a blade makes, what effect distance from the wheel ( presume turbulence) has on the blade efficiency etc .
Can’t calculate the NET drag on my boat with or without the P bracket base streamlined. Or the heel angles , The transverse righting moments , how big a rudder is needed to turn tight @ 30 knots and what effect all this has the performance.

The boats very stiff gets very set running.
It turns at speed 27-32 but not very tight more arcs .Turning circle approaching a mile not boat Ls .
At extremis say 35-38 knots it just heels over very slightly on full rudder and maybe arcs very slightly.That heel can be overcome by trim tabs .If I do that then in effect the blades are just being dragged causing drag slowing it , not doing much turning .I think they can’t overcome the set the huge dynamic stability which I assume grows from 25 knots up .

Drooping it down ‘ throttle steering’ it’s fine under 25 knots .The slower it goes the tighter the turns .Planing i think starts high teens .It also increases it inward heel from 25 knots down .As if the transverse dynamic stability is now dropped low enough so the rudder forces can overcome the “ set “

 

[skaraborgcraft]

Beyond my knowledge base. I did find this chart, it was not the info i was looking for though, though it did mention as a boat gets faster it needs proportionally smaller rudders to be effective at high speed, due to the amount of water flow over the surface. I have done 80knots in a small offshore boat, and no more thankyou.

 

[fallguy]

Can't you at least measure and make sure the rudder height is on the center axis of the shaft and can't you verify toe in and out?

I'd at least want to know rudder toe if it were my boat, and also if the bottom of the rudders were in line with the shaft, or if a shaft extension made the rudders short an inch, etc.

When you get up to speed; a video of the rudders from the top might be fun and enlightenment.

 

[Lynton Banks]

Can't you at least measure and make sure the rudder height is on the center axis of the shaft and can't you verify toe in and out?

I'd at least want to know rudder toe if it were my boat, and also if the bottom of the rudders were in line with the shaft, or if a shaft extension made the rudders short an inch, etc.

When you get up to speed; a video of the rudders from the top might be fun and enlightenment.

Thx for the input .
This will have to wait until the summer .They appear straight and with I think a tiny toe in .

I will turn them full lock in the dock tomorrow and report back , to confirm the rudder gauge is accurate and the blades actually are moving full range , as I can see them easily.

 

[fallguy]

The range is not the question. But u would want to know if the rudders are deep enough and to also know the toe%.

Guys like baeckmo are super smart and really offers great advice on the mounts aerating the tip of rudder water, but the idea you don't even know your toe% to me is bothersome. Those boats are really not high speed boats, in my opinion, so it seems to me something is wrong if you lose rudder at below 40kts. And maybe the aeration is that bad, but then if you find a videographer; you ought to be able to see it..

I believe Gerr recommends 10% toe in. If you find out you are toed out; it may be enough to get rudder back at the top.

I'm a casual boat guy. I have no c.v. When I look at the boat you show; it seems an epic travesty you lose steering at such low speeds, and I doubt the designer set it up that way, so something simpler seems wrong like someone had the rudders off or maybe they never tuned them and just stayed below 30?

 

[Lynton Banks]

The range is not the question. But u would want to know if the rudders are deep enough and to also know the toe%.

Guys like baeckmo are super smart and really offers great advice on the mounts aerating the tip of rudder water, but the idea you don't even know your toe% to me is bothersome. Those boats are really not high speed boats, in my opinion, so it seems to me something is wrong if you lose rudder at below 40kts. And maybe the aeration is that bad, but then if you find a videographer; you ought to be able to see it..

I believe Gerr recommends 10% toe in. If you find out you are toed out; it may be enough to get rudder back at the top.

I'm a casual boat guy. I have no c.v. When I look at the boat you show; it seems an epic travesty you lose steering at such low speeds, and I doubt the designer set it up that way, so something simpler seems wrong like someone had the rudders off or maybe they never tuned them and just stayed below 30?

This sounds plausible.
Makes sense to get the toe specs and check / adjust as necessary.Tnx .

 

[Ad Hoc]

...I am not a NA just playing with ideas ....

Then play with this "idea".

Take a triangle, a right angled triangle. With the base, being horizontal, at say 75mm, with an angle of 23 degree (your deadrise).
What length is the hypotenuse?
What is the difference between them...?
Then how much lift you get...on the flat 75mm part....and then...how much lift you get on the hyponeuse part.
Thus...how much lift do you really "think" you are getting, with or without spray rails?

 

[skaraborgcraft]

I saw something about Hickman using an air cushion to reduce friction, I got to wondering if your exhaust gasses are pushing so much air into the water that it is reducing the rudders effectiveness?

 

[Lynton Banks]

I saw something about Hickman using an air cushion to reduce friction, I got to wondering if your exhaust gasses are pushing so much air into the water that it is reducing the rudders effectiveness?

The direction of flow due of water to the V will backwards and outwards at speed .The E gases will run along that rear flattish plane behind the rectangular hole back wards and displaced out wards not down towards the keel and rudders .
If anything the gases ( 1800 rpm 1,6 bar boost 12.8 L , 4 stroke , 6 cyl = you do the vol maths ) extra aeration of the outer rear sections near the chine flat will reduce friction a little .

How ever the lower pressure area than what would have been without underwater exhaust, on that section could speed up a more transverse water flow over that huge pronounced strike which indeed runs all the way to the transom.= get a bit more lift as water changes direction more crossing the sharp edge .

Basically a net drag reduction thing .
The “ hook “ infront acting as well as a planing surface .

The underwater exhaust on leisure boats was first implemented by Amarti , on his Itama .Of course it quieter running as the water muffles the roar .But primarily I suspect it was attention to detail to gain a minor performance advantage.

Yard walking others I have seen are just that circular holes or maybe tear drop at most modification wise on large Fly bridge shallow V boats whose underwater strakes are tiny and end around the shaft exists , ie are not taken fully back to the transom,Cruise 22 knots flat shallow V sterns for adequate lift and rudders well inboard , not hanging , and steeper shaft angles .

It only Itama that have rectangular holes with that flap anterior.The posterior edge is lower than the anterior edge plane btw .He’s not wasting the gases he is using them I suspect ?

My next move is to get the rudders / pumps / hydraulics serviced before the season to check for mechanical limited movement at the extremities .

It’s no biggie just curiosity on the handling characteristics and hull subtle differences compared to others i see in yards .
Trying to understand the whys and where falls of the hull form .