Lifting Strakes

Hello everyone,

I am just wondering if lifting strakes would be necessary for a 6m planing monohull with a sterndrive.

Looking at all the SeaRay's around I notice they only have one lifting strake on either side of the hull.

Will lifting strakes increase top speed? Will they increase low speed planing efficiency? How many is a good amount?

Thanks everyone

 

This may be of some help.

http://www.sponbergyachtdesign.com/Bluebill.htm

 

That's a nice article on the Sponberg YD site.

It also clearly shows what happens when one wants to push the boat too much beyond it's design envelope. I am refering to the owner's engine replacement with overpowering, not to Eric's subsequent well-done but essentially palliative cure.
It goes 22 kts now (up from 15 kts for which it was originally designed), but it can only be safely (?) steered from the flybridge, with such a pronounced bow-up attitude. Sometimes it is imho much wiser to accept the boat for what it is, or change the boat type if one wants to go faster.

Cheers

 

In my opinion, yes, lifting strakes add to the boat's lift and performance in the planing regime and should be included. If you are designing the boat yourself, put them in. If it is someone else's design, is the designer around who could advise you on shape and placement?

On a 6M boat, you really need only the chine strakes. Personally, I stay away from intermediate lifting strakes unless the client really asks for them. They are a pain in the *** to build, particularly in composite in a female mold, plus they create bubbles that lead directly to propeller ventilation in a lot of cases. You might notice a little bit of speed increase maybe a fraction of a knot or two, but then how much increase do you want? Generally, speed is determined mostly by boat weight and installed horsepower, colored of course by CG location and your propeller specs. Get those right first, and don't worry about more efficiency due to lifting strakes--it will be miniscule by comparison to those big factors under your control.

On Bluebill, the owner loved the boat a lot and did not want to give it up for another model. I think he enjoyed spending money on it, but the added weight of just plain stuff got to be too much. So what do you do in a situation like that--throw more money at it. It certainly gave me a very interesting design job to do, plus an IBEX lecture, a Professional Boatbuilder article, and a fair bit of other press in a few other magazines. The concept has been copied a few times by others as well--what is it they say: copying is the best form of flattery?!

I hope that helps.

Eric

 

Thank you everyone, this forum is amazing, everyone is willing to answer my, what probably seem common knowledge, questions

Thanks again!

 

Ditto with Eric on the bubble train into the prop. Although it doesn't scale properly it is really obvious in a tank test.

Forward they do serve the useful purpose of disconnecting water flowing up the hull before it gets up the side of the hull and into the air as spray. Down turned chines forward work great, but if you can get the water off even lower it seems to keep the boat drier.

 

Strakes on most boats are just strips along the bottom and do very little or do nothing at all so are just spray rails .
What part of the boat do you want to lift ?, in 99% of cases its the back where all the weight is so make them lift and make them wide !!anything less than 120mm on a 6m boat is as useful as **** on a bull and totally useless !!,
150 to 200mm wide or depending on the speed you can get to you will need even wider They are there to work for you !, they need to go from back of the transom edge to at least 2/3 the length of the bottom wetted area .
The bubble thing people are so hooked on is a load of cobblers .
If its an outboard boat with the motor mounted high maybe but it only if you have strakes close to the keel line to track into the path of the prop it could be a potentual problem when cornering maybe !, At 60mph plus and not to many people keep doing tight turns at that speed so its a no brainer and not worth even thinking about .
The strakes need to be at least 600mm to a 800 mm from the keel on each side 150mm to 200mm wide or even wider and going forward at least 1.5 to 2,0 metres long before narrowing gradually to disapat to nothing by the time it has got to the 2/3 of the wetted surface !
Its the back of the boat you are trying to lift not the front ,
At 60mph plus then they will be working well ! cornering will not be a problem and the bubble thing is just a myth !!
An added advantage also is they are a good place for your trailer rollers to sit on as well !
I have worked with strakes since i bought my first boat in 1972 and seen some horrible things done ! so many are just purerly decoration and a place on a glass hull that are weak points as the bottom panels move up and down and thats where it bends and eventually delaminates .
The flat of a strake produces lift the vee of the hull looses a % of it lift and as the vee increases so the %of lift reduces !!
The more the vee the less the lift ! a strake is there to regain and increase lift !!, the size/width is related to the speed of the boat as the speed gets higher so the strake width gets smaller !! a slow boat in the 40 mph needs a wide strake !! ,a 6o mph boat needs a narrower strake !
As an add onor add to existing strake i have always fitted solid wood epoxy glued to the surface sealed with resin and then taken the boat for a run if its to much its easy to simply router some off the width ,re seal and try again till you get what you want ,then finish them off properly .
Strakes used and placed properly can stop chine walking! but thats a whole new subject of its own !.